the impact of lamb genotype on carcass composition and the ... · the financial value of a carcass...

The impact of lamb genotype on carcass

composition and the relationship with

intramuscular fat.

This thesis is presented for the degree of

Doctor of Philosophy of Murdoch University

by

Fiona Anderson

BVSc

MANZCVS (Equine Medicine)

Diplomate, ACVIM (Large Animal Internal Medicine)

June 2015

III

Declaration

I hereby declare that this thesis is my own account of my research and contains as its

main content, work which has not previously been submitted for a degree at any tertiary

education institution.

Signed Date

Fiona Anderson

IV

Summary

The financial value of a carcass is influenced by lean meat yield percentage (LMY%),

which represents the proportion of the carcass that is lean meat (muscle). Australian

lamb producers can select for this trait indirectly via three existing Australian Sheep

Breeding Values (ASBVs), namely post-weaning weight (PWWT), c-site fat depth

(PFAT) and eye muscle depth (PEMD). Previous research assessing the effect of these

breeding values on carcass composition has focussed on individual point measurements

of tissue depth or the weights of a small number of indicator muscles. In contrast, this

PhD has used computed tomography (CT) scanning to assess the impact of ASBVs on

composition at a whole carcass level.

Chapters 3 and 4 of this thesis focus on the change in body composition (fat, lean and

bone), in the fore, saddle (middle) and hind sections of the carcass as measured by CT.

An allometric approach was adopted in the analysis of the data from 1,665 lamb

carcasses, which allowed a robust interpretation of the impact of the carcass breeding

values and production factors on carcass composition. It was discovered that increasing

sire PEMD and reducing sire PFAT increased LMY% (7.7% and 9.5% units) across the

range of these breeding values. Furthermore, lean was redistributed to the saddle region

(3.8% units for PEMD and 3.7% units for PFAT). Additionally both breeding values

reduced carcass fatness across the sire range by 24.7% and 16.6% for PFAT and PEMD

respectively. Increasing sire PWWT had minimal impact on LMY% and carcass fat.

Carcass bone was most influenced by reducing sire PFAT, with the other breeding

values having little (PWWT) or no (PEMD) effect.

Chapter 5 uses further analysis of data collected in Chapters 3 and 4 to report on the

financial implications of using ASBVs to select for improved LMY%. The use of CT to

V

report the financial improvement to carcass lean value achieved through genetic

selection has not previously been comprehensively reported. Lambs were compared at a

standard carcass weight (23kg) and age (269 days), which allowed the influence of the

breeding values on both LMY% and HCWT to be accounted for in the calculation of

carcass lean value. Reducing sire PFAT had the greatest impact on carcass lean value

followed by PEMD and PWWT.

The eating quality and nutritive value of lamb are essential traits for consumer

satisfaction. Selection for leanness has been shown to reduce intramuscular fat (IMF) in

the loin (longissimus lumborum) muscle and so have a detrimental impact on eating

quality. For this reason, selection to improve LMY% must be balanced against the

consumer focused traits. The measurement of IMF% in relation to genetic selection is

undertaken in the loin muscle, with little known about the other muscles of the carcass.

Chapters 6 and 7 detail the correlation of IMF% between 5 muscles located in different

carcass regions (fore, saddle and hind). These correlations were generally found to be

strong, particularly for that of the loin IMF% with the fore and hind section muscles.

From an industry perspective this is a significant outcome, as it implies that genetic

selection for IMF% in the loin will also cause correlated changes in the other muscles.

The relationship between LMY% and IMF% in each of these muscles was also

assessed, and found to be consistently negative. Lastly, CT was assessed for its

effectiveness to predict IMF%. Although a negative correlation was found between

average pixel density and IMF%, the ability of CT scanning to predict IMF%

demonstrated relatively poor precision.

In summary this work has utilised CT to quantify the impact of ASBVs on carcass

composition in lamb. It details the financial impact of these effects due to changes in

VI

lean % and its distribution throughout the carcass. Additionally it explores the

relationship between IMF% in different regions of the carcass and the impact of

selection for LMY%. Finally it assesses the ability of CT to predict IMF% which would

be of benefit to industry in the event that CT measurement becomes mainstream within

commercial abattoirs.

VII

Acknowledgements

It is only when you reach the stage of writing the acknowledgements section of a thesis

that you fully realise just how many people have helped you along your way! It is really

important that these people are acknowledged and I thank them for their contributions.

My amazing supervisors Associate Professor Graham Gardner and Professor David

Pethick have invested an enormous amount of time and energy in developing my skills

as a researcher. They have provided me with a solid foundation in research design and

data analysis which has allowed me to explore the topics within my thesis. Both Dave

and Graham are passionate researchers, with their enthusiasm for all things sheep, beef

and research infectious. Dave and Graham have taught me the importance of delivering

a cohesive message to both the scientific community and the industry which both

supports and benefits from the research. They have given me some amazing

opportunities to deliver results of my experiments both in Australia and abroad which

my thesis and associated papers have benefitted from. Importantly they have made the

experience of completing a PhD most enjoyable and a career as a researcher something

to aspire to.

Of course a PhD journey always starts somewhere, and for me that was a chat with

Associate Professor John Bolton, who sent me off in the right direction to meet my

supervisors and has been ever encouraging along the way.

The Sheep CRC and Murdoch have provided the essential financial support necessary

for the completion of my PhD, for which I am very grateful. The CRC for Sheep

Industry Innovation is supported by the Australian Government’s Cooperative Research

Centre Program, Australian Wool Innovation Ltd. and Meat & Livestock Australia. The

VIII

Sheep CRC has facilitated events that allowed me to meet other scientists within

Australia and abroad which enabled my work to be exposed to critique, which has

ultimately made me a more robust scientist. I gratefully acknowledge the contributions

of staff and resources provided at each site for the INF: NSW Department of Primary

Industries, University of New England, Department of Primary Industries Victoria, SA

Research & Development Institute and the Department of Agriculture and Food WA.

The Meat program also benefits from the contribution of staff employed by CSIRO and

Murdoch University. Essential funding was also obtained from the Australian Meat

Processors Corporation through the Australian Bureau of Agricultural and Resource

Economics and Sciences awards which was a great program to be associated with.

‘The team’ at Murdoch has been essential to my project and consists of a great bunch of

people who have made this PhD achievable and enjoyable all at the same time. Our

coffee mornings provide answers to problems, people to sound ideas off, an avenue to

vent frustrations and of great importance to a PhD, plenty of laughs and enjoyable

times. Without the help of Andrew Williams and his computer skills in particular I

would still be half way through my analysis and could not have achieved such a volume

of work. The laboratory and support staff were an essential part of my PhD and include

Malcom Boyce, Rini Margwani, Ken Chong and Di Pethick who helped me wade

through sample analysis. Rosie Deveny, Kirsty Marshall and Serina Finlayson were all

fantastic assistants to the laboratory staff with sample analysis. The rest of the team,

who helped with sample collection and processing and who were also a great personal

support include: Honor Calnan, Sarah Stewart, Sarah Bonny, Sandra Corbett, Cameron

Jose and Peter McGilchrist. Lisolotte Pannier has been made important contributions to

preparation of some chapters/manuscripts and of course as part of the team with sample

collection and processing. A separate mention should go to my contemporary PhD

IX

colleagues Sarah John and Khama Kelman who have shared this journey with me.

Fortunately as a trio we were able to share the responsibilities of being the positive

influence when the others were not having such a great run!

My family have been an amazing support along the protracted path of completing a PhD

and deserve special thanks from me. My husband Lachlan has always been supportive

and encouraging of my work and understands how important it is to me. He has helped

me through late nights, early mornings and stressful times. My parents have been

helping and encouraging me for many decades to work hard and offer the appropriate

comfort when things are not going so smoothly. They have had such a strong influence

on me that my success should also be considered theirs. Although it couldn’t be said

that baby Hannah has been a ‘help’ we certainly did welcome her birth during the final

stages of writing up - she was good enough to let me get plenty of work done in that

final campaign to get everything finalised and published.

X

Publications

Journal publications:

Anderson F., Pannier L., Pethick D.W., Gardner G.E. (2015). Intramuscular fat

in lamb muscle and the impact of selection for improved carcass lean meat yield.

Animal, 9(6); 1081-1090.

Anderson F., Pethick D.W., Gardner G.E. (2015). The correlation of

intramuscular fat content between muscles of the lamb carcass and the use of

computed tomography to predict intramuscular fat percentage in lambs. Animal,

9(7):1239-1249.

Anderson F., Williams A., Pannier L., Pethick D.W., Gardner G.E. (2015). Sire

carcass breeding values affect body composition in lambs – 1. Effects on lean

weight and its distribution within the carcass as measured by computed

tomography. Meat Science, 108, 145-154.

Journal publications submitted:

Anderson F., Williams A., Pannier L., Pethick D.W., Gardner G.E. (2015). Sire

carcass breeding values affect body composition in lambs – 2. Effects on fat and

bone weight and their distribution within the carcass as measured by computed

tomography. (Meat Science - submitted)

Anderson F., Pethick D.W., Gardner G.E (2015). The impact of genetics on

retail meat value in Australian lamb carcass. (Meat Science - submitted)

Conference proceedings:

Anderson F., Williams A., Pannier L., Pethick D.W., Gardner G.E., Australian

Sheep Breeding Values increase % carcass lean and redistribute lean tissue to

the saddle region. Proceedings of the 59th

International Congress of Meat

Science and Technology, Izmir, 2013, O-1: available on line

at: file:///Volumes/ÿCOMST%202013/ebook_files/papers/oral/O-1.pdf

Anderson F., Williams A, Pannier L., Pethick D.W., Gardner G.E. Altering the

Carcase Plus Index has weakened its impact on lean meat yield %. In

XI

Proceedings of the 64th Annual Meeting of the European Federation of Animal

Production Nantes, (Ed. JF Hocquette), Book of Abstracts No.19, 103

(Wageningen Academic Publishers), 2013.

Anderson F., Pannier L., Pethick D.W., Gardner G.E., PEMD delivers

increased carcase lean and redistribution of lean to the saddle region in lambs.

In Proceedings of the 63rd

Annual Meeting of the European Association for

Animal Production Bratislava, Book of Abstracts No.18, 96 (Ed. JF Hocquette),

Wageningen Academic Publishers, 2012.

Anderson F., Williams A., Pannier L., Pethick D.W., Gardner G.E., (2012)

Altering the Carcase Plus Index has weakened its impact on lean meat yield %.

‘In ‘Proceedings of Sheep CRC Postgraduate Student Conference’. Coffs

Harbour, New South Wales.

Gardner, G.E., Anderson, F, Williams, A, Kelman, K.R., Pannier, L, Pethick,

D.W., Growth breeding value redistributes weight to the saddle region of lamb

carcasses. In Proceedings of the 63rd Annual Meeting of the European

Association for Animal Production Bratislava, Book of abstracts No18., pp 302.

(Ed. JF Hocquette), (Wageningen Academic Publishers), 2012.

Gardner, G.E., Anderson, F., Williams, A., Ball, A.J., Hancock, B. and Pethick,

D.W. (2012) Systems for determining carcass lean meat yield% in beef and

lamb. Proceedings of the 63rd

Annual Meeting of the European Association for

Animal Production Bratislava, Book of Abstracts No.18, p301 (Ed. JF

Hocquette), Wageningen Academic Publishers, 2012.

Gardner, G.E., Pannier, L, Anderson, F, Kelman, K.R., Williams, A, Jacob,

R.H., Ball, A.J., Pethick, D.W., The impact of selection for leanness on lamb

carcase composition, intramuscular fat, and muscle metabolic type, is not

influenced by nutritional variation within Australian production systems. In

Proceedings 8th

International Symposium on the Nutrition of Herbivores

Advances in Animal Biosciences, Aberystwyth, UK, 2, 406, 2011.

Anderson F., Williams A., Pannier L., Pethick D.W., Gardner G.E., (2011)

PEMD delivers an increase in carcase lean and redistribution of lean to the loin

region in lambs. ‘In ‘Proceedings of Sheep CRC Postgraduate Student

Conference’. Coffs Harbour, New South Wales.

Anderson F., Williams A., Pannier L., Pethick D.W., Gardner G.E., (2010) The

impact of carcass Australian Sheep Breeding Values on muscle distribution. ‘In

XII

‘Proceedings of Sheep CRC Postgraduate Student Conference’. Coffs Harbour,

New South Wales.

Anderson F., Pannier L., Pethick D.W., Gardner G.E, Australian Sheep

Breeding Values for carcass traits may alter muscle distribution in lamb

carcasses. Animal Production Science 50, Vii-Vii, (CSIRO publishing) 2010.

XIII

Abbreviations

ASBV Australian Sheep Breeding Value

C5 fat depth Measure of fat depth at the 5th rib (mm)

c-site fat depth Measure of fat depth at the 12th rib, 45 mm from the midline (mm)

CT Computed tomography

DXA Dual energy X ray absorptiometry

GR tissue depth Total tissue depth (mm) at the 12th rib, 110mm from the midline

HCWT Hot carcass weight

IMF Intramuscular fat

LMY Lean meat yield

MSA Meat Standards Australia

NIR Near infrared

PEMD Post weaning eye muscle depth measured at the 12th rib, 45 mm from

the midline

PFAT Post weaning fat depth measured over the 12th rib, 45 mm from the

midline

PWWT Post weaning growth assessed by live weight at approximately 240

days

XIV

Enzymes

Citrate synthase EC 2.3.3.1

Isocitrate dehydrogenase EC 1.1.1.41

Lactate dehydrogenase EC 1.1.1.27

XV

Table of Contents

SUMMARY ........................................................................................................................................ IV

ACKNOWLEDGEMENTS ............................................................................................................... VII

PUBLICATIONS ................................................................................................................................. X

ABBREVIATIONS ........................................................................................................................... XIII

ENZYMES ........................................................................................................................................ XIV

TABLE OF CONTENTS ................................................................................................................... XV

LIST OF FIGURES .......................................................................................................................... XXI

LIST OF TABLES .......................................................................................................................... XXIII

CHAPTER 1. INTRODUCTION ........................................................................................................ 1

CHAPTER 2. LITERATURE REVIEW............................................................................................. 6

2.1 BIOLOGY OF LAMB GROWTH ............................................................................................................... 6

2.1.1 Growth impetus and maturity .................................................................................................... 6

2.1.2 Methods of describing carcass growth and composition ........................................................... 9

2.1.3 Growth and development of the carcass tissues (lean, fat and bone) ...................................... 10

2.1.4 Mature size and comparing composition of lambs .................................................................. 14

2.1.5 Other factors that influence carcass composition .................................................................... 20

2.2 EATING QUALITY AND NUTRITIONAL VALUE OF LAMB ..................................................................... 33

2.2.1 Intramuscular Fat .................................................................................................................... 33

2.3 MEASUREMENT TECHNOLOGIES FOR CARCASS COMPOSITION AND EATING QUALITY ....................... 40

2.3.1 Subjective measurements of carcass composition ................................................................... 41

2.3.2 Site measurements ................................................................................................................... 41

2.3.3 Ultrasound ............................................................................................................................... 42

2.3.4 Probe technologies .................................................................................................................. 43

2.3.5 Computed tomography ............................................................................................................. 44

2.3.6 Radiographs and DXA ............................................................................................................. 46

XVI

2.3.7 Video image analysis systems (VIAS) .......................................................................................47

2.3.8 Near infrared spectroscopy (NIR) ............................................................................................47

2.4 HYPOTHESES .....................................................................................................................................48

CHAPTER 3. SIRE CARCASS BREEDING VALUES AFFECT BODY COMPOSITION IN

LAMBS - 1. EFFECTS ON LEAN WEIGHT AND ITS DISTRIBUTION WITHIN THE

CARCASS AS MEASURED BY COMPUTED TOMOGRAPHY. .................................................. 54

3.1 ABSTRACT ........................................................................................................................................54

3.2 INTRODUCTION .................................................................................................................................55

3.3 MATERIAL AND METHODS .................................................................................................................57

3.3.1 Experimental design and slaughter details ..............................................................................57

3.3.2 Slaughter protocol ....................................................................................................................58

3.3.3 Computed tomography scanning ..............................................................................................60

3.3.4 Data used .................................................................................................................................61

3.3.5 Statistical analyses ...................................................................................................................63

3.4 RESULTS ...........................................................................................................................................68

3.4.1 Production and management effects ........................................................................................68

3.4.2 Impact of genetics.....................................................................................................................72

3.4.3 Allometric (b) coefficients ........................................................................................................77

3.5 DISCUSSION ......................................................................................................................................78

3.5.1 Allometric (b) coefficient ..........................................................................................................78

3.5.2 Genetic influences on carcass lean tissue ................................................................................78

3.5.3 Production and management effects on carcass lean...............................................................83

3.5.4 Comparison of effects ...............................................................................................................85

3.6 CONCLUSIONS ...................................................................................................................................86

CHAPTER 4. SIRE CARCASS BREEDING VALUES AFFECT BODY COMPOSITION IN

LAMBS - 2. EFFECT ON FAT AND BONE WEIGHT AND ITS DISTRIBUTION WITHIN THE

CARCASS AS MEASURED BY COMPUTED TOMOGRAPHY. .................................................. 87

4.1 ABSTRACT ........................................................................................................................................87

4.2 INTRODUCTION .................................................................................................................................88

XVII

4.3 MATERIAL AND METHODS ................................................................................................................ 90

4.3.1 Experimental design and slaughter details .............................................................................. 90

4.3.2 Computed tomography scanning ............................................................................................. 91

4.3.3 Data used ................................................................................................................................. 92

4.3.4 Statistical analyses................................................................................................................... 93

4.4 RESULTS ........................................................................................................................................... 96

4.4.1 Impact of production and management on CT fat and bone % ............................................... 96

4.4.2 Impact of genetics on carcass fat and bone ........................................................................... 101

4.4.3 Allometric (b) coefficients ...................................................................................................... 108

4.5 DISCUSSION .................................................................................................................................... 109

4.5.1 Genetic influences on carcass fat and bone ........................................................................... 109

4.5.2 Production factors affecting carcass fat and bone ................................................................ 113

4.5.3 Allometric coefficient ............................................................................................................. 115

4.5.4 Comparison of effects ............................................................................................................ 116

4.6 CONCLUSIONS ................................................................................................................................ 118

CHAPTER 5. THE IMPACT OF GENETICS ON RETAIL MEAT VALUE IN AUSTRALIAN

LAMB. .............................................................................................................................................. 120

5.1 ABSTRACT ...................................................................................................................................... 120

5.2 INTRODUCTION ............................................................................................................................... 121

5.3 MATERIALS AND METHODS ............................................................................................................ 123

5.3.1 Experimental design and slaughter details. ........................................................................... 123

5.3.2 Data used ............................................................................................................................... 124

5.3.3 Data transformation .............................................................................................................. 127

5.3.4 Establishing models for predicting section lean weights. ...................................................... 128

5.3.5 Estimating weights of lean tissue within sections .................................................................. 131

5.3.6 Calculating lean value ........................................................................................................... 132

5.4 RESULTS ......................................................................................................................................... 134

5.4.1 Impact of sex on carcass composition and lean value ........................................................... 134

5.4.2 Impact of genetics on carcass composition and lean value in the carcass ............................ 139

XVIII

5.5 DISCUSSION ................................................................................................................................... 149

5.5.1 The impact of genetics on the value of the lamb carcass ...................................................... 149

5.5.2 The impact of sex on the value of the lamb carcass .............................................................. 153

5.5.3 Comparison of effects ............................................................................................................ 154

5.5.4 Limitations and future work .................................................................................................. 155

5.6 CONCLUSION.................................................................................................................................. 157

CHAPTER 6. INTRAMUSCULAR FAT IN LAMB MUSCLE AND THE IMPACT OF

SELECTION FOR IMPROVED CARCASS LEAN MEAT YIELD.............................................. 158

6.1 ABSTRACT ..................................................................................................................................... 158

6.2 INTRODUCTION .............................................................................................................................. 160

6.3 MATERIAL AND METHODS .............................................................................................................. 162

6.3.1 Experimental design and slaughter details ........................................................................... 162

6.3.2 Sample collection and measurements .................................................................................... 163

6.3.3 Statistical analyses ................................................................................................................ 164

6.4 RESULTS ........................................................................................................................................ 168

6.4.1 Effect of non-genetic factors.................................................................................................. 168

6.4.2 Effect of genetic factors ......................................................................................................... 172

6.4.3 Effect of hot carcass weight and lean meat yield percentage ................................................ 177

6.5 DISCUSSION ................................................................................................................................... 178

6.5.1 Genetic influence on intramuscular fat ................................................................................. 180

6.5.2 The impact of lean meat yield percentage and hot carcass weight on intramuscular fat ...... 183

6.5.3 Production and management effects on intramuscular fat .................................................... 184

6.6 CONCLUSION.................................................................................................................................. 185

CHAPTER 7. THE CORRELATION OF INTRAMUSCULAR FAT CONTENT BETWEEN

MUSCLES OF THE LAMB CARCASS AND THE USE OF COMPUTED TOMOGRAPHY TO

PREDICT INTRAMUSCULAR FAT PERCENTAGE IN LAMBS. .............................................. 187

7.1 ABSTRACT ..................................................................................................................................... 187

7.2 INTRODUCTION .............................................................................................................................. 189

7.3 MATERIAL AND METHODS .............................................................................................................. 191

XIX

7.3.1 Experimental design and slaughter details ............................................................................ 191

7.3.2 Sample collection and measurements .................................................................................... 193

7.3.3 Computed tomography scanning ........................................................................................... 195

7.3.4 Raw Data Description ........................................................................................................... 196

7.3.5 Statistical analyses................................................................................................................. 201

7.4 RESULTS ......................................................................................................................................... 202

7.4.1 Prediction of IMF% in lamb using average CT pixel density and muscle type ..................... 203

7.4.2 Prediction of IMF% in lamb using average CT pixel density, muscle type, and other carcass

measures ......................................................................................................................................... 206

7.4.3 Prediction of IMF% in lamb using average CT pixel density, CT fat%, CT lean%, muscle type,

and on-farm information................................................................................................................. 207

7.4.4 Prediction of IMF% within each muscle (m longissimus lumborum, m semimembranosus, m.

semitendinosus, m. supraspinatus, m. infraspinatus) using CT density, CT Fat% and the CT density

of the m. longissimus lumborum ..................................................................................................... 207

7.5 DISCUSSION .................................................................................................................................... 209

7.5.1 Correlation of IMF% between the muscles ........................................................................... 209

7.5.2 Prediction of IMF% based on CT density and muscle ........................................................... 210

7.5.3 Incorporating additional information for predicting IMF% ................................................. 212

7.6 CONCLUSION .................................................................................................................................. 215

CHAPTER 8. GENERAL DISCUSSION ........................................................................................ 217

8.1 SELECTION FOR LEAN MEAT YIELD AND IMPLICATIONS FOR THE AUSTRALIAN SHEEP INDUSTRY .. 217

8.2 USE OF THE ALLOMETRIC ANALYSIS ............................................................................................... 219

8.3 CHANGES IN CARCASS COMPOSITION DUE TO GENETIC AND NON-GENETIC FACTORS...................... 221

8.3.1 Impact of genetics on carcass composition and value ........................................................... 221

8.3.2 Non genetic effects ................................................................................................................. 232

8.4 INVESTIGATIONS INTO INTRAMUSCULAR FAT % IN MUSCLES ACROSS THE LAMB CARCASS. ........... 233

8.4.1 IMF% of muscles from different regions across the Australian lamb carcass and correlations

with the m. longissimus lumborum ................................................................................................. 233

8.4.2 Prediction using CT ............................................................................................................... 234

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8.4.3 The impact of lean meat yield on intramuscular fat throughout the lamb carcass. .............. 236

8.5 ALTERATIONS TO EXPERIMENTAL DESIGN AND FUTURE STUDIES ................................................... 237

8.5.1 Alterations to experimental design ........................................................................................ 237

8.5.2 Future studies ........................................................................................................................ 239

8.6 SUMMARY ...................................................................................................................................... 242

REFERENCES ................................................................................................................................. 244

XXI

List of Figures

Figure 2-1 The progress of carcass tissues (fat, lean and bone) as a portion of maturity as they relate to

live weight as a proportion of maturity. (Adapted from Butterfield et al (1984b)) ............................ 8

Figure 2-2 Growth path of two steers to different mature sizes (adapted from Butterfield et al (1988) ) .. 15

Figure 2-3 The change in carcass composition of fat, lean and bone as an animal reaches maturity.

(adapted from Butterfield et al (1988)) ............................................................................................. 15

Figure 2-4 Comparison of carcass composition of large and small strains of animals when compared at

the same carcass weight. (adapted from Butterfield et al (1988) ) ................................................... 16

Figure 3-1 The relationship between lamb sire estimates for percentage of lean and post-weaning eye

muscle depth (PEMD). Symbols represent sire estimates plus the least squared means for (o)

Maternal, (×) Merino (∆) Terminal sired lambs and are obtained from the ASBV model in which

PEMD was removed. The line represents least squared means (± SE as dashed lines) for PEMD

from the model containing the Australian Sheep Breeding Values. ................................................. 77

Figure 6-1 The relationship between intramuscular fat % in lamb and sire post weaning weight (PWWT)

Australian Sheep Breeding Value, for lambs born as multiples and raised as singles. Symbols (●)

represent residuals for each lamb as deviations from the predicted means for intramuscular fat%.

Line represents least square means (±SE as dashed lines) across the PWWT range. ..................... 174

Figure 6-2 The relationship between intramuscular fat % in lamb and sire post weaning c-site fat depth

(PFAT) Australian Sheep Breeding Value, for the M. semimembranosus. Symbols (●) represent

residuals as deviations from the predicted means for intramuscular fat%. Line represents least

square means (±SE as dashed lines) across the PFAT range. ......................................................... 175

Figure 6-3 The relationship between intramuscular fat% in lamb and sire post weaning eye muscle depth

(PEMD) Australian Sheep Breeding Value, for lambs born as multiples and raised as singles.

Symbols (●) represent residuals as deviations from the predicted means for intramuscular fat%.

Line represents least square means (±SE as dashed lines) across the PEMD range (±SE). ............ 176

Figure 6-4 The relationship between intramuscular fat % in lamb and the percentage of lean in the

carcass, as measured by computed tomography (CT). Symbols (o) represent residuals as deviations

from the predicted means for intramuscular fat%. Line represents least square means (±SE as

dotted lines) across the range of CT lean %. .................................................................................. 178

XXII

Figure 7-1 Raw data of the intramuscular fat % in lamb of the m. semimembranosus, m. semitendinosus,

m. supraspinatus, m. infraspinatus and m. longissimus lumborum as it relates to average computed

tomography pixel density (Hu) of the fat and muscle pixels. ......................................................... 200

Figure 7-2 The relationship between intramuscular fat % in lamb and computed tomography pixel density

(Hu) (model 3) for the m. infraspinatus (slope = -0.02±0.006, intercept=4.60), m. longissimus

lumborum (slope = -0.07±0.013, intercept=8.50), m. semimembranosus (slope = -0.04±0.013,

intercept=5.92), m. supraspinatus (slope = 0.06±0.008, intercept=7.29) and m. semitendinosus

(slope = -0.03±0.007, intercept=5.89). Lines represent least square means (±SE as dotted lines)

across the range of average computed tomography pixels (Hu). .................................................... 204

XXIII

List of tables

Table 3-1 Total number of lamb carcasses scanned using computed tomography at each site. ................. 59

Table 3-2 Mean ± standard deviation, (minimum and maximum) values in lamb for the weight (kg) of fat,

lean, bone and all components combined within the whole carcass, and the fore, saddle and hind

sections. ............................................................................................................................................ 62

Table 3-3 Number of lamb sires and mean (min, max) for the Australian Sheep Breeding Values for post

weaning weight (PWWT), post weaning c-site fat depth (PFAT) and post weaning eye muscle

depth (PEMD) for each sire type. ..................................................................................................... 63

Table 3-4 Number of lambs analysed in the base model according to sex, sire type, birthing and rearing

type and dam breed. .......................................................................................................................... 67

Table 3-5 F-values, and numerator and denominator degrees of freedom of factors affecting lean weight

of lamb in the whole carcass and the distribution of lean in the fore, saddle and hind sections of the

lamb carcass. ..................................................................................................................................... 70

Table 3-6 Relative change (% Change in weight) for site-year, sex dam breed(sire type) and birth-type

rear-type on lamb carcass lean weight and the lean distribution between the fore, saddle and hind

sections of the lamb carcass. ............................................................................................................. 71

Table 3-7. F-values, and numerator and denominator degrees of freedom for Australian Sheep Breeding

Values affecting lamb lean weight in the whole carcass and lean distribution between the fore,

saddle and hind sections of the lamb carcass. ................................................................................... 75

Table 3-8 Percentage change in weight per unit of Australia Sheep Breeding Values on lamb carcass lean

weight and lean distribution between the fore, saddle and hind sections of the lamb carcass. ......... 76

Table 4-1 F-values and numerator and denominator degrees of freedom of factors affecting fat and bone

weight of lamb in the whole carcass and the distribution of fat and bone between the fore, saddle

and hind sections of the lamb carcass. .............................................................................................. 97

Table 4-2 The relative change (% change in weight) for site-year, sex dam breed(sire type) and birth-type

rear-type’s impact on lamb carcass fat weight and the distribution of fat between the fore, saddle

and hind sections of the lamb carcass. .............................................................................................. 98

Table 4-3 The relative change (% change in weight) for site-year, sex dam breed(sire type) and birth-type

rear-type’s impact on carcass bone weight of lamb and the distribution of bone between the fore,

saddle and hind sections of the lamb carcass. ................................................................................... 99

XXIV

Table 4-4. . F-values and numerator and denominator degrees of freedom in lambs for Australian Sheep

Breeding Values affecting whole carcass lamb fat and bone weight and distribution of fat and bone

between the fore, saddle and hind sections of the lamb carcass. .................................................... 104

Table 4-5 Percentage change in weight per unit of Australian Sheep Breeding Value on lamb carcass fat

weight of lamb and fat distribution between the fore, saddle and hind sections of the lamb carcass.

....................................................................................................................................................... 105

Table 4-6 Percentage change per unit of Australia Sheep Breeding Value on lamb carcass bone weight

and bone distribution between the fore, saddle and hind sections of the lamb carcass. ................. 106

Table 5-1 Average age of lambs at slaughter and number of carcasses scanned using computed

tomography in each lamb kill group at each site. ........................................................................... 126

Table 5-2 Number of lamb sires and mean (min, max) for the Australian Sheep Breeding Values for post

weaning weight (PWWT), post weaning c-site fat depth (PFAT) and post weaning eye muscle

depth (PEMD) and Carcass Plus Index for each sire type of lamb carcasses undergoing computed

tomography. ................................................................................................................................... 126

Table 5-3 Number of lambs analysed in the base model according to sex, sire type, birthing and rearing

type and dam breed. ....................................................................................................................... 129

Table 5-4 Predicted hot carcass weights (kg) ± SE of the Maternal, Merino and Terminal sired wether

lambs born to Merino dams at an average age of 280 days. ........................................................... 133

Table 5-5 F-values and degrees of freedom for the numerator (NDF) and denominator (DDF) for factors

affecting the proportions of fat, lean and bone in the fore, saddle and hind sections of the lamb

carcass. ........................................................................................................................................... 135

Table 5-6 The relative change (% change in weight) for fat, lean and bone in the fore, saddle and hind

sections of the carcass due to sex, sire types, dam breeds and Australian Sheep Breeding Value

effects for lambs slaughtered at 23kg. ............................................................................................ 136

Table 5-7 Weight (kg) and value ($) of lean in the fore, saddle and hind sections of the lamb carcass at the

same weight (23 kg) and the same age (280 days). ........................................................................ 138

Table 5-8 Predicted carcass value for the fore, saddle and hind sections of carcass for the Maternal,

Merino and Terminal sired lambs for the range of post weaning weight (PWWT), c-site fat depth

(PFAT), c-site eye muscle depth (PEMD) and Carcass Plus Index values – wether progeny of

Merino dams at 23kg carcass weight. ............................................................................................ 141

XXV

Table 5-9 Predicted carcass value for the fore, saddle and hind sections of carcass for the Maternal,

Merino and Terminal sired lambs for the range of post weaning weight (PWWT), c-site fat depth

(PFAT), c-site eye muscle depth (PEMD) and Carcass Plus Index values – wether progeny of

Merino dams at 280 days of age. .................................................................................................... 147

Table 6-1 Number of lamb sires and mean (min, max) of Australian Sheep Breeding Values for each sire

type. ................................................................................................................................................ 163

Table 6-2 Number of lambs analysed in the base model according to sire type, sex, birthing and rearing

type, dam breed and kill group. ...................................................................................................... 165

Table 6-3 Lamb hot carcass weight (kg) and carcass lean percentage as measured by computed

tomography displaying raw mean ± SD (min, max). ...................................................................... 167

Table 6-4 F values, and numerator and denominator degrees of freedom, for the effects of the base linear

mixed effects model, corrected for hot carcass weight (HCWT), computed tomography lean % and

Australian Sheep Breeding Values on intramuscular fat % of lamb muscles (Muscularis

semimembranosus, Muscularis semitendinosus, Muscularis supraspinatus, Muscularis infraspinatus

and Muscularis longissimus lumborum). ........................................................................................ 169

Table 6-5 Lamb intramuscular fat percentage for sex, dam breed, sire type and kill groups within the

Muscularis semimembranosus, Muscularis semitendinosus, Muscularis supraspinatus, Muscularis

infraspinatus and Muscularis longissimus lumborum (not corrected for hot carcass weight). ....... 171

Table 6-6 Coefficients ± standard error (SE) for the Muscularis semimembranosus, Muscularis

semitendinosus, Muscularis supraspinatus, Muscularis infraspinatus and Muscularis longissimus

lumborum in the model corrected for hot carcass weight. .............................................................. 177

Table 7-1 Number of lambs used according to sire type, sex, birthing and rearing type, dam breed and kill

group. .............................................................................................................................................. 192

Table 7-2 Showing for each muscle (m. semimembranosus, m. semitendinosus, m. supraspinatus m.

infraspinatus and m. longissimus lumborum) the number available for analysis, intramuscular fat %

as measured by near infrared spectroscopy and average pixel density (Hu). ................................. 194

Table 7-3 Lamb age (days), hot carcass weight (kg), carcass lean and fat percentage as measured by

computed tomography, eye muscle depth (mm), c-site fat depth (mm) and GR tissue depth (mm)

displaying raw mean ± SD (min, max). .......................................................................................... 198

Table 7-4 Partial correlation coefficients (above the diagonal) and simple correlation coefficients (below

the diagonal) of the IMF% and computed tomography derived % of fat (CT fat%) in lamb between

XXVI

the m. semimembranosus (SM), m. semitendinosus (ST), m. supraspinatus (SS) m. infraspinatus (IS)

and m. longissimus lumborum(LL). ................................................................................................ 203

Table 7-5 F-values, coefficient of determination (R-square), and root mean square error (RMSE) for

models predicting intramuscular fat % in lamb using muscle type, average computed tomography

pixel density of fat and muscle (CT density), hot carcass weight, GR Tissue depth (mm), eye

muscle depth (mm), c-site fat depth (mm) and computed tomography derived % of fat and/or lean

tissue (CT Lean % or Fat%). .......................................................................................................... 205

Table 7-6 F-values, coefficient of determination (R-square), and root mean square error (RMSE) for

models predicting intramuscular fat % in lamb using muscle type, average computed tomography

pixel density (CT density) of fat and lean, % computed tomography fat and lean (CT Lean% or CT

Fat%), on-farm information and carcass measurements. ............................................................... 206

Table 7-7 F-values, coefficient of determination (R2), and root mean square error (RMSE) for models

predicting intramuscular fat % in the m. semimembranosus, m. semitendinosus, m. supraspinatus m.

infraspinatus and m. longissimus lumborum in lamb using average computed tomography pixel

density (CT density) of fat and lean, computed tomography derived % of fat (CT Fat%), and CT

density of the m. longissimus lumborum. ....................................................................................... 208

Chapter 1 - Introduction

1

Chapter 1. Introduction

Lean meat yield is the amount of meat that can be obtained from a carcass, and this is a

significant driver of profitability for meat supply chains (Pethick, Ball, Banks et al.

2010). The amount of saleable meat is assessed by the determination of carcass weight

in combination with an estimate of lean meat yield percent (LMY%), which is defined

as the proportion of lean meat (muscle) in the carcass. As such, the LMY% of a carcass

is maximised when there are large amounts of muscle with diminishing proportions of

fat and bone.

Improving LMY% in lamb is of value at multiple points throughout the lamb supply

chain, creating value for consumers, retailers, processors, and producers. Consumers

have a preference for lamb cuts which are larger and leaner and have less trim-able

(Banks 2002, Laville, Bouix, Sayd et al. 2004, Williams and Droulez 2010) which is a

stronger marker driver for the production of lamb carcasses that meet these preferences.

Knowledge of LMY% can better enable retailers to guarantee the consumer they will

receive their desired cut preferences. Retailers also benefit from high yielding lambs as

carcasses have more salable meat and if this muscle is located in the more valuable

regions of the carcass, such as the loin, ultimately they improve profits through

increased carcass value. Additionally, if retailers deliver cuts which meet consumer

preferences, then consumer confidence grows which may increase sales. For processors,

higher yielding carcasses are more valuable as they require less trimming of fat and

bone, resulting in less waste and reduced labour at bone-out (Hopkins, Wotton, Gamble

et al. 1995a). Additionally, processors will likely have paid for lambs based on their hot

carcass weight, therefore less saleable meat from low yielding carcasses results in less

return on the carcass purchase price. For the producer, increasing LMY% of lambs is


2

advantageous as they can be finished to heavier weights without lambs becoming too

fat. Overly fat or lean carcasses receive price penalties at processing in Australia, based

upon palpated GR tissue depth thus penalties are incurred for GR >25mm, or GR

<5mm. However, for lambs within these ranges there is little price incentive to

encourage producers to optimise LMY%, in part a reflection of the processors need to

compete for a declining total sheep slaughter (Pethick et al. 2010).

In order to attract producers to turn off lambs that meet market specifications there must

be suitable price incentives. A key to a LMY% based pricing scheme is the ability to

accurately and rapidly assess LMY% in the processing plant. Previously systems to

estimate LMY% have included measurement of carcass weight and GR tissue depth,

however this is not an accurate assessment of LMY%, especially in commercial

abattoirs with a high through-put (Hopkins, Safari, Thompson et al. 2004). The use of

Video Image Analysis (VIA) systems have been shown to improve the accuracy and

speed of predicting carcass LMY% (Hopkins et al. 2004) and has been used on a

limited commercial basis in Australia. Development of technologies, such as dual

energy x-ray absorptiometry (DXA), is underway and will enable more accurate, rapid

and cost effective measurement of LMY% in the processing plant. Computed

tomography (CT) has been used to measure carcass composition in lamb (Gardner,

Williams, Siddell et al. 2010, Lambe, Navajas, McLean et al. 2007), however currently

remains too expensive and slow to be incorporated into Australian processing plants,

though advances in technology may see their more widespread commercial use in the

future.

One of the biggest market drivers is the desire of processors to receive lambs of high

LMY% to reduce costs associated with carcass trimming. Consumers also reflect this


3

same desire for lean lambs (Williams and Droulez 2010), with the outcome being a

drive the lamb industry to preferentially select for lambs which are high yielding and

lean. Australian lamb producers use genetic selection to manipulate LMY%, which

historically has been the focus in the terminal sire breeds (Banks 2003). It is now

increasingly important for all sectors of the Australian lamb industry to produce lambs

that have improved muscling and reduced salvage fat to provide valuable income to

wool or mixed production systems. As such there is a drive to improve the LMY% not

only in Terminal sired lambs but also the Merino and Maternal breeds of sheep.

Therefore Australian lamb producers are looking to utilise genetics that will improve

LMY% in breeds of sheep that were previously used for the production of wool alone.

In the modern day Australian lamb industry, genetic improvement in a range of traits is

facilitated by using Australian Sheep Breeding Values (ASBVs) which estimate an

animals’ breeding value for a wide range of production and carcass traits (Brown,

Huisman, Swan et al. 2007). The values are based on both the animals’ pedigree and

performance. The carcass breeding values include growth and weight information:

weight (pre weaning measured at 100 days (WWT) or post weaning measured at 200

days (PWWT)). There are also measurements taken at the c-site (located 45mm from

the midline at the 12th

rib) of live animals (using ultrasound) to produce post weaning

fat depth (mm) (PFAT) and eye muscle depth (mm) (PEMD) breeding values.

With producers increasingly using genetic selection and ASBVs to increase HCWT and

LMY%, it is important to assess the magnitude of the improvements and translate them

to financial gains. The HCWT of lambs has been shown to increase in response to

selection for increased sire PWWT and PEMD and reduced sire PFAT (Gardner,

Williams, Ball et al. 2015). However, the impact of ASBVs on LMY%, its distribution


4

within the carcass, and the financial implications of genetic selection for improved

LMY% have not been well documented.

The ASBVS PWWT, PEMD and PFAT can be combined into indexes such Carcass

Plus which place different weightings on certain breeding values to give a single value

which selects for a desired trait, for example carcass weight (HCWT) and LMY%. The

current weightings for the carcass plus index are: 30% weaning weight (WWT), 35%

PWWT, 30% PEMD and 5% on PFAT. The Carcass Plus index is dominated by the

selection for growth (PWWT and WWT), with a specific aim of promoting high growth

to achieve slaughter weights quickly. The incorporation of PEMD and PFAT is

specifically to improve muscling and reduce carcass fat in the carcass. The 5% inclusion

of PFAT in the carcass plus index is a smaller than earlier forms of the index, which

was to address the negative impact that heavy selection for reduced sire PFAT has on

intramuscular fat and eating quality (Pannier, Gardner, Pearce et al. 2014a, Pannier,

Pethick, Geesink et al. 2014c). There was also a perception that producers were finding

it challenging to finish lambs to an adequate level of back fat depth at the point of

slaughter. This change has also been shown to impact on the LMY% and carcass value

of lambs (Anderson, Williams, Pannier et al. 2013a) , therefore further investigation

into the impact of ASBVs on yield and carcass value is warranted.

The use of ASBVs to improve LMY% has also been shown to impact negatively on

eating quality. This is important because consumers have been shown to purchase and

pay more for lamb of a higher eating quality (Pethick, Banks, Hales et al. 2006a). This

negative effect on eating quality is delivered in part by causing a decrease in

intramuscular fat % (Pannier et al. 2014a, Pannier et al. 2014c). Intramuscular fat

influences eating quality through its impact on flavour, juiciness and tenderness and is


5

linked to overall liking (Neely, Lorenzen, Miller et al. 1998, Pannier et al. 2014a,

Pannier et al. 2014c, Thompson 2004a, Wheeler, Cundiff and Koch 1994). None-the-

less, the impact of LMY% has only previously been demonstrated in the loin (Pannier et

al. 2014c). To date this effect has not been shown in other regions of the carcass.

Due to the importance of IMF% on eating quality, a rapid non-destructive method for

measuring IMF% would potentially allow cuts of low IMF% to be identified prior to

sale. One method of identifying IMF% in lamb is computed tomography (CT)

(Clelland, Bunger, McLean et al. 2014, Lambe, McLean, Macfarlane et al. 2010),

however this research has focussed on its use in the longissimus. If CT is used for

determination of carcass composition, then it would also be important to know the

accuracy of CT at predicting IMF% in other regions of the carcass.

This thesis provides important industry relevant information about the impacts of

selection for increased LMY% through the use of ASBVs on the composition of the

carcass. It utilises computed tomography (CT) to accurately determine the composition

of fat, lean and bone in three regions of the carcass from approximately 1700 lambs and

provides important information on the financial implications of genetic selection on

carcass lean value. Additionally, the impact of selection using ASBVs and LMY% on

IMF% throughout different regions of the carcass is investigated. Finally the use of CT

to predict IMF% in different regions of the carcass is determined and the correlation of

IMF% in the loin, the most commonly measured muscle, with other muscles throughout

the carcass was determined. The results of this thesis will provide the Australian Sheep

Industry with valuable information that will directly influence the use of genetics to

select for LMY% and the development of new breeding values for IMF%.

Chapter 2 – Literature review

6

Chapter 2. Literature Review

2.1 Biology of lamb growth

Carcass composition is defined as the proportion of the major tissues bone, muscle and

fat (Berg and Butterfield 1968), with these tissues comprising approximately 45 to 50%

of sheep live weight (Butterfield 1988, Gardner et al. 2015). Fat, lean and bone develop

at different rates to each other and to the weight of the carcass as a whole (Berg and

Butterfield 1968, Butterfield 1988). As an animal grows, the organs and carcass tissues

will comprise varying proportions of the animals’ body weight until the animal reaches

its mature body weight. Throughout growth and at maturity, carcass composition and

distribution of the three major tissue types is impacted by a number of factors,

including: lamb age, proportion of maturity, mature size, genetics (including sire type

and dam breed), sex, nutrition, carcass weight.

2.1.1 Growth impetus and maturity

Growth impetus refers to the relative rate of growth of different body parts and was first

described by Mc Meekan (1940) and Wallace (1948b). It is challenging to define when

an animal has reached maturity. Thompson et al (1985b) suggested that maturity for an

ad lib feeding program was “...when it had reached at least 0.85 of its asymptote for the

exponential relationship between body weight and cumulative food consumed, and the

average weekly increment in body weight for at least 10 weeks prior to slaughter was

not significantly different from zero (P<0.05).” Mature weight can be defined as the

state of anatomical equilibrium achieved when an animal has ceased to grow

(Butterfield, Griffiths, Thompson et al. 1983a), and all tissues have reached their mature

weights. However given fat may continue to be deposited after the bone and muscle

components have reached their point of maturity the definition of maturity can be


7

extrapolated to mean that the point of maturity is reached when the muscle and bone are

no longer being deposited. As such, maturity was defined by Berg and Butterfield

(1966) as “..the stage in which the non-lipid components of the body reach a steady

state..”.

Growth patterns of the three main carcass tissues (fat, lean and bone) can be determined

by the serial assessment of the tissues from birth until maturity. This allows the rate of

development of each of these tissues as they relate to the growth of the carcass as a

whole to be evaluated. Within each of these tissues, it is also possible to determine the

regional rate of development of these tissues as they relate to the total weight of the

respective tissue.

The growth impetus of the three main carcass tissues is roughly similar between

species, however the magnitudes of the impetus will vary within and between species

according to different age, weight, breed and nutrition (Berg and Butterfield 1968).

Early maturation of a tissue means the tissue comprises a higher proportion of the

carcass weight at birth than it does later in life (Butterfield et al. 1983a). The

relationship in sheep of tissue weight as it relates to mature body weight is shown in

Figure 2-1. Investigations into the relative growth of the three main carcass tissues show

bone as the earliest maturing tissue (i.e. a low growth impetus), followed by muscle and

then fat, which has a high growth impetus (Berg and Butterfield 1968, Butterfield et al.

1983a). This means that bone is relatively early maturing and as the animal progresses

to maturity the weight of bone as a portion of the whole carcass weight decreases. Fat

has the highest growth impetus, becoming a larger portion of the carcass weight as the

animal proceeds to maturity. Muscle develops at a similar rate to that of the whole


8

carcass but as the animal reaches maturity the muscle weight does become a decreasing

portion of the carcass weight.

Figure 2-1 The progress of carcass tissues (fat, lean and bone) as a portion of maturity

as they relate to live weight as a proportion of maturity. (Adapted from Butterfield et al

(1984b))

It is important to describe the conditions under which an animal has been grown if

comments are to be made about its progression to mature size. Optimal nutrition for

example is necessary for maximal expression of genetic characteristics (Butterfield

1988). It is important to remember that a mature weight may be influenced by many

non-genetic factors. Taylor (1985a) listed the following as factors likely to influence

mature size: nutrition, disease, physical environment, activity, social environment, and

age.


9

2.1.2 Methods of describing carcass growth and composition

2.1.2.1 Allometry

Allometry has been used to describe the changes in size or dimension of one part of an

organism as they relate to the overall change in size or dimension of the whole organism

(Gayon 2000). Allometry was originally used to describe the relative changes in the

proportions of the body tissues as they related to the overall size of the animal by

Huxley and Teissier (Huxley and Teissier 1936). They initially described a relationship

between the whole body (y) and its components (x) in the equation y = bxα which was

altered to y = axb . Where y is the dependent variable and is used to describe the relative

rate of growth of a component (x) in relation to the growth of the whole. The b term in

this equation has commonly been called the growth coefficient. The equation can be

analysed in the log-log form log y = log a + b log x, which allows the b coefficient to be

determined from the slope of the linear equation. Using this technique, growth impetus

is described as “high” when the b coefficient is greater than 1. This means the

component being measured is becoming an increasing proportion of the total weight.

For example fat with a high growth impetus is likely to have a b co-efficient >1

compared to bone which has a low growth impetus (b<1). An “average” impetus means

the ‘b’ coefficient is close to 1 and therefore the component being measured is growing

at the same rate as the whole.

The principles of allometry have been used to describe the rate of growth of the main

carcass tissues in cattle (Berg and Butterfield 1966, Berg and Butterfield 1968), and

sheep (Butterfield et al. 1983a). Similarity, the allometric equation can be adapted to

examine the relative growth of muscles in relation to whole carcass musculature

(Thompson, Atkins and Gilmour 1979c). Additionally, fat depots within the carcass can

be ranked as being of high, low or average growth impetus (Thompson, Atkins and


10

Gilmour 1979a, Thompson et al. 1979c). The same approach was also used for bone

(Thompson et al. 1979c).

2.1.2.2 Quadratic equation

Butterfield (1983a) adapted the Huxley equation to describe relative growth in terms of

a quadratic equation but the meaning of high, average and low growth impetus remained

the same. The pathway followed by tissues as they progress to maturity was specified

by a single value (Butterfield 1988) ‘q’. This has similar meaning to the b coefficient in

Huxley’s equation, however if <1 then the tissue has a high growth impetus and >1 a

low growth impetus. At a q of 1, the component is considered to be growing at the same

rate as the animal or carcass, which has the same meaning as when in Huxley’s equation

the b co-efficient is equal to1.

2.1.3 Growth and development of the carcass tissues (lean, fat and

bone)

A number of experiments have been performed which examine the relative rates of

maturation, mature composition and distribution of the three main carcass tissues in

lamb (Butterfield et al. 1983a, Butterfield and Thompson 1983, Butterfield, Zamora,

James et al. 1983b, Butterfield, Zamora, James et al. 1983c). These studies utilise data

from serial slaughter and dissection of Merino rams over a range of carcass weights.

Carcass composition has also been studied in other sheep breeds, and sexes (Butterfield,

Reddacliff, Thompson et al. 1984a, Butterfield, Thompson and Reddacliff 1985,

Butterfield et al. 1984b, Thompson, Atkins and Gilmour 1979b, Thompson et al. 1979c,

Thompson and Parks 1983).

2.1.3.1 Development of lean tissue

As described previously, the musculature is described as being relatively early

maturing, when compared to the growth of the carcass as a whole (Berg and Butterfield


11

1968, Butterfield 1988). The number of myofibres in muscle is mostly determined prior

to birth (Greenwood, Hunt, Hermanson et al. 2000, Nissen, Danielsen, Jorgensen et al.

2003). In lamb, primary myogenesis begins at day 32 of gestation, secondary

myogenesis at day 38, with myogenesis generally completed by day 110 of gestation

(Greenwood, Slepetis, Bell et al. 1999, Maier, McEwan, Dodds et al. 1992, Wilson,

McEwan, Sheard et al. 1992). Muscle mainly develops through postnatal hypertrophy

(Greenwood et al. 2000, Zhu, Ford, Nathanielsz et al. 2004, Zhu, Ford, Means et al.

2006) but also through hyperplasia in some species (Picard, Lefaucheur, Berri et al.

2002).

The growth of muscles within the carcass have been described for lamb (Butterfield et

al. 1983c) and beef (Butterfield and Berg 1966a, b). Similar to the growth of the carcass

tissues, muscles can be defined as either early or late maturing when compared to the

total weight of muscle within the sheep (Butterfield et al. 1983c). A study by Thompson

et al (Thompson et al. 1979c) showed no difference in the distribution of muscle

between lamb breeds, in contrast to studies by Seebeck (1968) who showed some small

differences.

Butterfield et al (1983c) examined the dissected weights of many of the muscles of the

lamb carcass in small and large mature size Merinos. The experiment divided these

muscles into 9 main groups: proximal hind limb, distal hind limb, surrounding spinal

column, abdominal wall, proximal forelimb, distal fore limb, thorax to fore limb, neck

to fore limb, neck and thorax. Muscles within these groups largely matured at the same

rate, although there are some inconsistencies within a group of muscles. The results of

the study by Butterfield et al (1983c) are described below.


12

Group 1 (proximal hind limb) were considered late maturing, and are poorly developed

at birth. Group 2 (distal hind limb) were early maturing compared to the growth of the

musculature as a whole making up a large proportion of the muscle weight at birth, with

this proportion declining as the sheep approached maturity. Group 3 muscles include the

muscles surrounding the spinal column and in in the study of Butterfield et al (1983c)

were described as early maturing. This is in contrast to other studies (Hammond 1932)

which were discussed by Pomeroy (1978) where the spinal musculature was shown to

be a late maturing tissue. Group 4 (abdominal musculature) grew at the same rate as the

total musculature and was considered average in its rate of maturation. Group 5 and 6

consisted of the proximal distal fore limb musculature. Although generally classified as

being early maturing there was considerable variation between individual muscles.

Groups 7-9 are comprised of the muscles of the cranial aspect of the sheep which are

not related to forelimb function. Overall these groups were classified as being late

maturing, with the most extreme being the splenius muscle, which in rams is especially

well developed, likely as a mechanism for fighting with other rams at sexual maturity.

2.1.3.2 Development of the fat depots

In general fat is considered a late maturing tissue (Berg and Butterfield 1968,

Butterfield et al. 1985, Butterfield et al. 1984b, Ponnampalam, Butler, Hopkins et al.

2008) when compared to the growth of the carcass. Lambs are comprised of a number

of fat depots: subcutaneous, intramuscular, intermuscular, and mesenteric. Greater focus

has been on the carcass fat depots (subcutaneous and inter-muscular fat) as they will

influence the amount of saleable meat in the carcass. Intramuscular fat (IMF) has also

received specific attention due to its influence on juiciness and tenderness (Pethick,

Barendse, Hocquette et al. 9-13 September, 2007).


13

In Merino rams, the allometric coefficients (b) for all fat depots were not significantly

different from 1 indicating that they developed at the same rate as total carcass fat

(Butterfield and Thompson 1983). In comparison, in Dorset Horn rams and wethers,

the intermuscular and thoracic fat depots are considered early maturing and omental fat

late maturing (Butterfield et al. 1985). Castration of Dorset Horn rams resulted in

wethers having a greater portion of subcutaneous fat and less intermuscular fat when

compared to rams (Butterfield et al. 1985).

The development of IMF is discussed in greater detail in 2.2.1.3 Factors affecting

intramuscular fat.

2.1.3.3 Development of bone

Bone is considered an early maturing tissue (Berg and Butterfield 1968, Butterfield

1988). The limb bones of sheep are relatively early maturing (Butterfield 1988), which

implies that their proportion of total bone weight decreases over time. The limb bones

of sheep have been shown to have a disto-proximal growth (Thompson et al. 1979c),

similar to that described in pigs (Davies 1975) and cattle (Jones, Price and Berg 1978).

This implies the distal bones mature earlier than the more proximal bones, with this

effect more pronounced in the forelimbs. Cake et al (2007) discovered contrasting

results with the humerus. The growth of bone weight is partially dependent on other

tissues in the body. For example the influence of body weight on limb load and bone

modelling (Cake, Gardner, Boyce et al. 2006a).

Cake et al (2007) showed that Poll Dorset x Merino lambs selected for increased

muscling had lower bone weights relative to HCWT compared to other genotypes. This

same study by Cake et al (2007) showed pure Merino lambs to have similar total bone

lengths to other breeds but with shorter proximal limb bones and longer distal limb


14

bones. Additionally, bones of pure Merino lambs were heavier at a given carcass

weight, which supports the notion of Hopkins and Fogarty (1998b) that Merinos have

higher bone trim.

2.1.4 Mature size and comparing composition of lambs

Due to the differences in the rates of development of the tissues (fat, lean, bone and

organs), the composition of a carcass with respect to proportions of these tissues will

vary as the animal matures. Depending on the nature of the information required to be

obtained, it may be appropriate to compare lambs at the same live weight, carcass

weight, lean weight, fat depths, stage of maturity or age. In the past sheep carcass

composition was often compared based on age. Due to the differences in the tissue

maturation rates this can give misleading information about the effect that different

genetics, sex or breeds have on the composition of the sheep. Furthermore, comparing

lambs at the same age does not necessarily compare them at the same stage of maturity

if their mature size is different. If market specifications are important then comparisons

of lamb composition at the same carcass weight are justified.

2.1.4.1 Influence of mature size on carcass composition

Animals have a predetermined ‘mature size’ which is programmed from the early

embryonic stage of an animal’s life (Taylor 1985b). The mature size and composition of

an animal at maturity is often unknown. Figure 2-2 shows the growth of two steers on

their way to different mature sizes. Regardless of their mature size they will follow a

similar growth path to maturity that has been described by Brody (Brody 1945) with the

time taken for two animals of different mature sizes to reach their mature weights

differing. An animal of large mature size takes a longer time to reach its mature weight

than the animal of small mature size.


15

Figure 2-2 Growth path of two steers to different mature sizes (adapted from Butterfield

et al (1988) )

Given the differing rates of maturation of the three carcass tissues (fat, lean and bone)

the relative proportions these tissues exist in will vary as the animal approaches

maturity (Berg and Butterfield 1968) Figure 2-3. Muscle and bone are relatively early

maturing tissues when compared to the growth rate of the carcass and are expected to

make up a high proportion of carcass weight at an early stage of maturity, with this

proportion declining as the animal approaches maturity. The converse is true of fat.

Figure 2-3 The change in carcass composition of fat, lean and bone as an animal reaches

maturity. (adapted from Butterfield et al (1988))


16

When two animals that are programmed to grow to different mature sizes are compared

at the same carcass weight, then it is expected they will have different proportions of

fat, lean and bone as they will be at different stages of maturity Figure 2-4.

Figure 2-4 Comparison of carcass composition of large and small strains of animals

when compared at the same carcass weight. (adapted from Butterfield et al (1988) )

It was determined by Butterfield (1983a) that maturity coefficients for fat, lean and bone

were very similar between large and small mature sized rams. As expected, based on the

rates of maturation of the three tissues, when lambs were compared at the same live

weight, the large mature sized rams had increased proportions of bone and less fat when

compared to the small mature sized lambs due to the early maturation of bone when

compared to fat. When compared at the same proportion of maturity there were few

differences in their composition, although the large mature sized rams had slightly more

fat (Butterfield et al. 1983a) compared to the small mature sized rams. When compared

at maturity, the larger sized sheep had higher weights of muscle, fat and bone but as a

percentage of the total carcass weight these proportions were very similar between the

two groups (Butterfield et al. 1983a).


17

This also applies to the situation where the distribution of tissue types within a carcass

is being investigated. In animals of different mature size, tissue depots that mature at a

rate different to that of the carcass will comprise different portion of the carcass weight

at different stages of maturity. The rate of maturation of the bones was similar between

the two strains of lamb with limb bones considered early maturing (Cake et al. 2006a).

When compared at the same live weight, the larger stains of animals appear to have a

larger proportion of bone weight in the limbs, compared with smaller strains

(Butterfield 1988) due to them being at an earlier stage of maturity.

The maturation rate of muscles from large and small mature sized rams was also shown

to be very similar (Butterfield et al. 1983c). When the two strains of rams were

compared at the same total muscle weight, the large strain of ram had a higher

proportion of their muscle mass comprised of the early maturing muscles compared to

the later maturing muscles (Butterfield et al. 1983c). When the two strains were

compared at the same proportion of their mature muscle weight these differences largely

disappeared which indicates that mature size accounted for muscle of the differences in

muscle distribution of the rams.

It can be difficult to extrapolate the results of early studies in mature size to the modern

day lamb as much of this early work was performed in Merino lambs (Butterfield et al.

1983a, Butterfield and Thompson 1983, Butterfield et al. 1983b, Butterfield et al.

1983c, Thompson 1983, Thompson, Butterfield and Perry 1985a, 1987, Thompson et al.

1985b), which were the predominant breed at this time. The impact of mature size in

other breeds has been investigated (Butterfield et al. 1984b, Perry, Thompson and

Butterfield 1992a, Thompson and Parks 1983) but less extensively and it is difficult to

know if these breeds respond differently to selection pressure for increased


18

growth/mature size. Therefore the mature size and pattern of growth of a breed/strain of

sheep is not always known, so it can be difficult to determine if changes in composition

at the same weight or age are due to mature size, mature composition or growth of the

carcass tissues. Additionally these early experiments regarding mature size and serial

measurement of composition to maturity have not been repeated in the current

Australian Sheep Industry.

The rate of maturation of fat did not differ between high and low mature sized merinos

(Butterfield and Thompson 1983). Animals selected for high and low weaning weight

produced different maturation patterns of fat relative to body weight despite mature fat

proportions remaining the same (Thompson and Parks 1983).

2.1.4.2 Measures of carcass maturity

The rate of maturation of tissues can vary between sexes and breeds of lamb (Butterfield

1988). Additionally, differences in mature size and composition can account for the

variation in composition when lambs are compared at the same weight or age. If lambs

are to be compared at the same stage of maturity then it is important to be able to assess

or measure maturity.

The muscle to bone ratio has been used to measure the stage of maturity of sheep

(Butterfield 1988), cattle (Berg and Butterfield 1966) and pigs (Davies 1974, Davies

and Kallweit 1979). As a result of the relative maturation rates of muscle and bone it is

expected that when an animal is immature it will have a low muscle:bone ratio, but that

this ratio will increase as the animal matures. For example at birth a Merino ram has

been shown to have a muscle:bone ratio of 2:1, which increases to 3:1 at 10% of

maturity and 4:1 at 60% maturity (Butterfield 1988).


19

Muscle to bone ratios may be less accurate in modern lamb breeding systems where

terminal sires are often selected for extremes of muscling (Cake et al. 2006a). In this

instance, bone indices alone have been suggested to provide more accurate assessment

of maturity (Cake et al. 2006a). The thickness and appearance of the bone physes have

been used to assess skeletal maturity through a break-joint score (USDA 1982).

Problems associated with this method has been that longitudinal growth can cease well

before the closure of the physes (Oberbauer, Currie, Krook et al. 1989, Oberbauer,

Krook, Hogue et al. 1988).

Bone mineral content may also be been used to indicate stage of maturity, with an

increase in calcium:phosphorus (Grynpas 1993), and a decline in magnesium (Cake et

al. 2006a, Ravaglioli, Krajewski, Celotti et al. 1996) reported with increase animal age.

Cake et al (2006a) described the allometric growth of lamb forelimb bones and

suggested that forelimb growth may be a useful indicator of maturity.

The muscle:fat ratio is less often used but it may provide important information with

respect to composition of economic importance. Excessive levels of fat are not desired

by consumers and processors. In contrast to bone, the ratio of muscle:fat will decrease

as the animal reaches maturity.

The eruption of teeth is a crude measure in lamb to differentiate lambs from hoggets,

and offers rapid assessment in the processing plant. In lamb there are breed effects with

Border Leicester cross lambs showing earlier eruption of the first molar (Hopkins,

Stanley, Martin et al. 2007a). The eruption of dentition in cattle has been poorly

correlated to other indices such as ossification (Lawrence, Whatley, Montgomery et al.

2001).


20

2.1.5 Other factors that influence carcass composition

2.1.5.1 Influence of sex on carcass composition

A difficulty encountered when examining the literature is the variation in study design

that has been used to analyse sex differences in composition and tissue maturation.

Some studies examine differences between rams and wethers (Butterfield et al. 1984b),

others examine wethers versus ewes, with some looking at cryptorchids, wethers and

ewes (Kirton, Clarke and Hickey 1982). It would be ideal to compare in the same study,

the difference between rams, ewes and wethers of a number of sheep breeds.

2.1.5.1.1 Weight and growth rate

Within breeds, the males have been shown to have a greater mature size and faster

growth rate when compared to ewes (Butler-Hogg, Francombe and Dransfield 1984,

Hopkins et al. 2007a, Kirton et al. 1982, Thompson et al. 1985b). There was no

difference in the mature weight of Dorset Horn rams and wethers (Butterfield et al.

1984a) however this study compared only 5 rams to 7 wethers. Lee et al (1990) looked

at the effect of sex on growth and carcass composition on second cross lambs that were

under commercial grazing conditions. The results of this study show that entire animals

(rams and cryptorchids) grew faster than ewes and wethers.

2.1.5.1.2 Composition at maturity

McClellend, Bonaiti and Taylor (1976) looked at ewes and rams at a range of 40 to 70

% of their mature weights. When they compared these animals at the same degree of

maturity they found that there was little difference in their carcass composition. Based

on this information they hypothesised that at maturity the tissue composition of ewes

and rams would be similar, but were concerned about extrapolation to predict

composition that was well beyond their study. Thompson (1983) and (1985a) examined

maturation patterns of sexes by comparing ewes and rams to maturity. In contrast to


21

McClellend’s hypothesis, Thompson showed that at maturity rams had significantly

more muscle (lean) and bone but less fat than the ewes. Fourie (1970) also obtained data

from mature rams and ewes similar to that of Thompson. At maturity, rams have been

shown to have a higher proportion of live weight as bone and muscle compared to the

wethers, with the wethers showing increased proportions of fat (Butterfield et al.

1984a).

2.1.5.1.3 Variations in tissue maturation and carcass composition between sexes

Some studies show no impact of sex on the rate of maturation of carcass tissues as they

relate to carcass weight (Fourie et al. 1970, Thompson et al. 1979a, Tulloh 1963). In

contrast, it has also been shown to differ between sexes (Butterfield 1988). Thompson et

al. (1983) show a cross over effect where at very early ages the rams had more fat and

less lean and bone than the females, with this situation reversed as the animals approach

maturity. At approximately 50% of mature weight, the composition of males and

females is relatively similar which is similar to the results of McClelland (1976). The

maturation of fat, lean and bone were similar for Dorset Horn rams and wethers

(Butterfield et al. 1984b).

When lambs from a range of genotypes were compared at the same age using dual

energy X-ray absorptiometry (DXA), the wether lambs had more lean% and less fat% at

all ages examined (4, 8, 14, 22 months) (Ponnampalam, Hopkins, Dunshea et al.

2007b). This was a similar result to Lee (1990) showed cryptorchid and rams to be

leaner than both wethers and ewes, as measured by GR tissue depth.

Across breeds (Southdown and Romney), Fourie (1970) showed that both rams and

ewes both showed increasing proportions of fat and corresponding decreases in lean and

bone. The three breeds studied showed ewes consistently had more fat and less bone


22

and lean in the 5-30kg carcass weight range the animals were studied over. Thompson

et al. (1979c) showed no difference in the distribution of subcutaneous or intermuscular

fat in cross bred lambs of various breed cross over a 34-54 kg range.

The distribution of musculature between sexes in Merinos has been examined using 9

‘standard muscle groups’ (Perry, Thompson and Butterfield 1988). In this study they

found a difference in muscle distribution between rams and ewes in 7 out of the 9

standard muscle groups. In this study it was shown that ewes were heavier than rams in

all limb groupings but lighter than rams in three groups associated with the neck

musculature. The sex differences between the muscle development post puberty may be

a reflection of hormonal influences, behavioural and physiological differences (Perry et

al. 1988). Butterfield and Berg (1972) and Loshe (1973) suggested males required a

larger muscle mass in the neck region to support a larger head and horns and exert their

dominance resulting in the increased weight of splenius muscle observed in rams.

Thompson (1983) showed in Merinos, that the ratio of muscle to bone (muscle:bone)

was very similar between rams and ewes as they progress to maturity. In contrast

Butler-Hogg et al. (1984) and Fourie (1970) showed that ewes were superior with a

higher muscle to bone ratio compared to males. Compared at the same carcass weight,

wether lambs were shown to have higher bone % than ewe lambs (Thompson et al.

1979a). Thompson et al. (1979c) showed wethers had more forelimb and less hind limb

bone than ewe lambs when compared at the same bone weight. Mature rams were

shown to have greater bone weight in the axial skeleton and less limb bone when

compared to ewes (Perry, Thompson and Butterfield 1992b). Differences in limb bone

length between ewes and wethers are minimal (Cake et al. 2006a) and can be adjusted,

making this index of maturity potentially more reliable than others.


23

2.1.5.2 Influence of nutrition on carcass composition

Maternal nutrition has been shown to impact lamb growth and carcass composition

(Greenwood et al. 2000, Harding and Johnston 1995, Muhlhausler, Duffield and

McMillen 2007, Zhu et al. 2004, Zhu et al. 2006). The impact of maternal nutrition is

likely to be severe and prolonged before it will impact on the carcass composition of the

progeny (Kenyon and Blair 2014). A study of nutritional restriction in first cross ewes

showed that severe feed shortage in early pregnancy will not impact significantly on

lamb production, proving there is adequate nutrition supplied in late pregnancy and

during postnatal growth (Krausgrill, Tulloh, Shorthose et al. 1999). This result was

supported by a more recent study of Greenwood and Thompson (2007). However, the

expression of the genetic potential for growth has been shown to be reduced when

lambs are underfed during lactation (Hegarty, Shands, Marchant et al. 2006c),

demonstrating the importance of maternal nutrition for lamb growth.

Ewes provide the majority of the lamb’s nutrition pre-weaning nutrition through their

milk, with pasture intake increasing as ewe lactation declines (Langlands 1972). Mellor

and Murray (1985) showed in Scottish Blackfaced sheep that under nutrition in late

gestation reduces udder development and manufacture of colostrum, which has the

potential to influence postnatal lamb growth. Additionally, compared to mature ewes,

maiden ewes lose more weight under nutrition restriction which impacts their ability to

produce milk and may impose more severe nutritional restriction on their lambs

(Hegarty et al. 2006c).

There has been some debate about the effect that nutrition has on fat deposition in

lambs. A study by Muhlhausler et al. (2007) show that increased maternal nutrition in

the prenatal period can influence the development of adipose tissue in early postnatal


24

life. Measures of carcass fatness like GR tissue depth have been shown to be reduced in

lambs on low nutrition (Hegarty et al. 2006c). In this study, these differences were

reduced when lambs were adjusted for weaning live weight. Other studies have

indicated that rapidly growing lambs are fatter than slower growing lambs

(Ponnampalam et al. 2008). In this study, the authors suggested the result may be due to

variations in grazing by the lambs where the fast growing lambs had increased feed

intake Similarly, when compared at the same carcass weight, Hall et al. (2001) showed

that fast growing lambs were fatter, as measured by GR and c-site fat depth, than the

slow growing lambs. This result was true for both the cryptorchid and ewe lambs.

Similar results were shown by Lee (1990). It has been shown that when energy intake is

greater than that required for maximal bone and lean accretion that the excess energy is

used for fat accretion (Black 1983). Conversely it has been shown that slow growth will

result in leaner lambs (Chestnutt 1994, Murphy, Loerch, McClure et al. 1994, Thatcher

and Gaunt 1992).

Hegarty et al. (1999) showed over a range of live weights (33-63kg) that deposition of

fat was most influenced by energy intake with less impact of protein. Alternatively, it

has been shown that high protein diets can result in increased growth rates and fatter

carcasses (Agricultural Research Council 1980) . However ‘leaner’ live weight gain

have been obtained with increased by pass protein (Ørskov, McDonald, Grubb et al.

1976). In contrast, other studies have shown little effect of nutrition/growth rate on

carcass fat (Kirton, Bennett, Dobbie et al. 1995b, Lee 1986), although in these studies

growth rates were quite modest (<200g/day).

It has been shown that maternal restriction during gestation can reduce foetal muscle

fibre numbers (Greenwood et al. 2000, Quigley, Kleemann, Kakar et al. 2005, Zhu et


25

al. 2004) and therefore may detrimentally affect postnatal skeletal muscle growth.

Greenwood et al. (2000) showed that there was variation in the impact maternal

nutrition has on muscular growth in different regions of the lamb carcass. This same

study showed that the impact of postnatal growth was most severe in the smaller, slow

growing lambs. The duration of the maternal nutrition restriction has also been shown to

be a factor in determining the degree of reduction in body size in pigs (Widdowson

1974) which may also be relevant to maternal nutritional restriction in sheep.

Bone growth is generally genetically predetermined, however nutrition can have an

impact. Effects of under nutrition on lamb growth were initially studied by Wallace

(1948a) and Palsson (1952). They established that early maturing tissues such as bone

are less affected by nutritional restriction than late maturing tissues (e.g. fat).

2.1.5.3 Influence of genetics on carcass composition

Genetic selection is used to improve a wide range of traits including carcass traits such

as LMY%. Few studies compare the differences due to breed type with existing studies

often comparing a small number of breeds or breed types, with small numbers of

animals in each category. There is evidence that breed can influence tissue maturation

and carcass composition (Thompson et al. 1979b, Wolf, Smith and Sales 1980, Wood,

MacFie, Pomeroy et al. 1980, Wynn and Thwaites 1981). However some studies

showed little variation in the composition of lamb carcasses across breeds (Arnold,

Gharaybeh, Dudzinski et al. 1969), though these comparisons were made in lambs of

very low live weights. McClelland et al. (1976) and showed there was little difference

in the proportions of fat, lean and bone between different breeds of sheep when

compared at the same degree of maturity. Cross bred lambs were compared at a range of

slaughter weights (34-54kg) with Dorset Horn sires producing lambs that were fast


26

growing and reached slaughter weights earlier than others (Atkins and Thompson

1979).

2.1.5.3.1 Sire type

The impact of sire type on lean tissue has previously been studied (Ponnampalam,

Hopkins, Butler et al. 2007a). This experiment showed Merino sired lambs to have

lower values for loin weight, eye muscle area and depth compared to the Maternal

(Border Leicester) and Terminal (Poll Dorset) sired lambs at the same age. In a related

study, lambs sired by Terminal sires had more muscle and less bone and fat than

Maternal sired lambs (Ponnampalam et al. 2007b). Most Terminal breeds have been

selected for lean growth which results in progeny that grow faster than Merinos

(Hopkins, Stanley, Martin et al. 2007b) and deposit more lean tissue than Maternal and

Merino sired lambs.

The partitioning of fat between different compartments in sheep was shown to differ

between ‘meat-sire types’ and ‘maternal-sire type’ breeds by Kempster et al. (1981). A

comparison between mature Merino and Dorset Horn rams revealed Dorset Horns to

have a larger proportion of carcass fat (Butterfield and Thompson 1983). In a

comparison of Terminal (Dorset Horn) and Maternal (Border Leicester) rams, the

Maternal sired breeds like the Border Leicester have produced lambs with more fat and

less muscle than Terminal sire breeds (Dorset Horns) when compared at the same

carcass weight. Additionally the Terminal sires produced lambs that increased fat

measures more slowly (Atkins and Thompson 1979). In a related study, the Border

Leicester sires produced lambs that had more subcutaneous and intermuscular fat and

more bone than those sired by Dorset Horns (Thompson et al. 1979a). More recent

studies also show Maternal sired lambs to have increased carcass fat (Ponnampalam et

al. 2008). Other studies have indicated that maternal breeds have larger depots of


27

internal fat, likely as a reserve for their increased milk production (Wood et al. 1980). In

a DXA experiment, Maternal sired lambs (Border Leicester) had the most fat compared

to the Terminal (Poll Dorset) and Merino sired lambs, with the converse true of lean

tissue (Ponnampalam et al. 2007b). Ferguson et al. (Ferguson, Young, Kearney et al.

2010) demonstrated a link between fatness and reproductive capacity. Given that

Maternal breeds have been selected for improved reproductive performance, it is likely

that this has been linked to increased fatness within these animals, which will also be

evident within Maternal sired lambs.

Sire type also impacts the weight of bone in the lamb carcass, with Cake et al. (2007)

showing Merino sired lambs to have heavier limb bone weights compared to Terminal

and Maternal sired lambs. Additionally, Thompson et al. (1979a) demonstrated

Maternal sired lambs (Border Leicester) had more carcass bone than Terminal (Dorset

Horns).

2.1.5.3.2 Dam breed

Atkins and Thompson (1979) showed that the Merino ewes produced lambs that were

slower growing than other ewe breeds, with progeny of Border Leicester x Merino ewes

being the fastest. The progeny of the Merino ewes were also slower to accrete fat (as

measured at the c-site) than the Border Leicester x Merino ewes. Some studies have

shown only a small impact of dam breed on carcass composition (Thompson et al.

1979a), and others no effect (Arnold et al. 1969), although the latter study was of lambs

at 14.8kg.


28

2.1.5.3.3 Australian Sheep Breeding Values

Sheep Genetics Australia produces Australian Sheep Breeding Values (ASBVs) for

many economically significant wool and carcass traits. These ASBVs are based on the

measurement of phenotypic information of sheep and their progeny and estimate the

potential for these sheep to deliver genetic improvement for the desired trait to their

progeny. Australian lamb producers indirectly select for yield via three existing ASBVs:

post weaning weight (PWWT), C-site fat depth (PFAT) and eye muscle depth (PEMD).

They are used to select for growth, leanness and muscling respectively. Both PFAT and

PEMD are both measured on live animals by ultrasound at the 12th

rib and PWWT is

calculated from a post weaning weight at 240 days of age. These breeding values, with

the addition of weaning weight (WWT) can be combined into an index called Carcass

Plus: 30% weaning weight (WWT), 35% PWWT, 30% on PEMD and 5% on PFAT.

The Carcass Plus index is designed for use by Terminal sired breeders with a specific

aim of promoting high growth and muscling, while keeping carcasses lean.

The impact of selection using these breeding values to improve lean meat yield has been

investigated using indicators like muscle and fat depths (Gardner et al. 2010, Hegarty

et al. 2006c, Hopkins et al. 2007b, Ponnampalam et al. 2007a), bone lengths and

weights (Cake et al. 2007, Cake, Gardner, Hegarty et al. 2006b) and cut weights

(Gardner et al. 2010). The use of technology such as DXA (Ponnampalam et al. 2007b)

and computed tomography (CT) scanning (Gardner et al. 2010) enables more detailed

assessment of carcass composition in response to genetic selection using the carcass

ASBVs, with these technologies discussed in a later section.


29

Post weaning weight (PWWT)

It has been shown that lambs growing at a faster rate are proportionately leaner when

compared at the same carcass weight, largely due to an increase in mature weight

(Bennett, Kirton, Johnson et al. 1991, Butterfield 1988). There is evidence for PWWT

to increase growth rate and mature size (Huisman and Brown 2008, Thompson et al.

1985b), therefore when lambs of high PWWT are compared to lambs of low PWWT

they would be less physiologically mature. Bone and lean are considered early maturing

tissues (Berg and Butterfield 1968, Butterfield et al. 1983a), and it would be expected

they comprise a larger proportion of carcass weight if the lamb is less physiologically

mature. Therefore at the same carcass weight, high PWWT lambs would be expected to

contain a greater proportion of lean and bone compared to low PWWT lambs. The

progeny of sires with increased PWWT ASBVs have been shown to have increased

weight at slaughter and a concurrent increase in HSCW (Gardner et al. 2015, Gardner et

al. 2010), but no increase in lean meat yield % (Gardner et al. 2010).

Hegarty in (2006a) showed there was little impact of the PWWT ASBV on carcass

composition apart from an increase in the amount of bone, consistent with these animals

reaching a larger mature size. Gardner et al. (2010) showed that with a 10 unit increase

in sire PWWT the weight of the hind limb bone increased by 10.5g (1.2%) and whole

carcass CT bone was increased by 0.26 units. In the same study, increasing sire PWWT

resulted in an 8% decrease in C-site fat depth and 0.40 unit decrease in CT fat%, which

would be consistent with a less physiologically mature lamb. A small increase in muscle

weight was detected in the topside.

Animals selected for high growth (PWWT) have been shown to be negatively impacted

by low nutrition (Hall, Gilmour, Fogarty et al. 2002, Hegarty et al. 2006c). The


30

expression of PWWT was shown to be reduced in lambs at low nutrition for pre-

weaning live weight gain, weaning weight and slaughter weight (Hegarty et al. 2006c).

Post weaning eye muscle depth (PEMD)

Numerous studies have shown that selection for PEMD impacts at its site of

measurement, the C-site (Hall et al. 2002, Hegarty et al. 2006a, Hegarty et al. 2006c).

Hegarty et al. (2006c) reported that for each millimetre increase in sire PEMD resulted

in an increase in eye muscle depth of 0.61mm, similar to the increases observed by Hall

et al. (2002). The study of Hegarty et al. (2006a) showed that sires of increased PEMD

produced lambs with increased loin weights and some increase in weights of hind limb

muscles (topside), indicating a more whole carcass effect on muscularity. Increasing

sire PEMD also reduced carcass C site fat depth more than can be attributed to that of

the PFAT ASBV (Hegarty et al. 2006c). Gardner et al. (2010) has shown that

increasing sire PEMD increased eye muscle area as well as the weight of the eye of the

short loin. This same study revealed a decreased C-site fat depth which was very site

specific as the total weight of short loin fat was not altered. Therefore the reported

increase in GR tissue depth in the study of Gardner et al. (2010) was thought to be

associated with increased muscularity or a very localised redistribution of fat away from

the C-site measurement site. This same study showed using CT that PEMD had little

impact on the overall lean meat yield % of the carcass which suggests redistribution of

muscle from other muscles in the carcass to the loin.

Hegarty et al. (2006c) have highlighted the importance of understanding how nutrition

and genetics interact in order to manipulate lamb growth and composition. It was shown

that the genetic potential for muscling was not affected by nutrition as both high and

low levels of nutrition allowed for maximal expression of the animals potential for


31

muscling. This was shown by a consistent increase in the depth of the loin associated

with increasing the PEMD ASBV over both high and low planes of nutrition. In

response to high nutrition, lambs selected for muscling (increasing PEMD ASBV) did

not increase total fatness and this result was not explained by their corresponding PFAT

ASBV.

Post weaning c-site fat depth (PFAT)

Selecting for lean tissue growth has been shown to increase the depth of the

ultrasonographic eye muscle measurement and decrease fat depth at the C-site (Nsoso,

Young and Beatson 2004). Work in pigs has shown that selection for decreasing back

fat can alter the distribution of fat (Trezona-Murray 2008, Wood, Whelehan, Ellis et al.

1983), decreasing it at the site of measurement and redistributing it other subcutaneous

fat depots. The PFAT ASBV has been shown to impact on muscle at the site of its

measurement, the c-site (Hall et al. 2002, Hegarty et al. 2006c). Hegarty et al. (2006c)

showed that both GR tissue depth and c-site fat depth were both reduced in lambs

whose sires were of low PFAT values. Gardner et al. (2010) showed a decrease in all fat

depth indicators: loin fat weight, C-site fat depth, GR tissue depth and C5 fat depth.

Additionally, the same study revealed a decrease in whole carcass fat % as measured by

CT. Based on this information it is likely that selection for decreasing sire PFAT, will

lead to a decrease in total carcass fat, but with the strongest effect in the saddle region.

In this same experiment, Gardner et al. (2010), showed that selection for reduced sire

PFAT, increased carcass bone % of lamb, and weight of hind limb bone. Low nutrition

impacted on the expression of the PFATs influence on carcass fat measures, as when on

low nutrition, the lambs accreted less fat per millimetre increase in sire PFAT (2006c).


32

2.1.5.3.4 The impact of carcass index breeding values on composition.

The carcass related indexes are used by industry which combine a number of ASBVs,

each with a specific weighting to provide breeders with a single number to select for

increased weight and muscling. The Carcass Plus Index at the time of this analysis was

made up of: PWWT 35%, WWT 30%, PEMD 30% and PFAT 5%. Therefore we expect

the impact of the Carcass Plus index to largely reflect the combined impact of PWWT,

WWT and PEMD, with a heavy influence delivered by growth (PWWT and WWT)

given its 65% representation in the Index. The current inclusion of breeding values was

altered from the original inclusion of PFAT at 20% to the current 5% as the extreme

selection for leanness was negatively impacting intramuscular fat% and eating quality

(Pannier, Gardner, Pearce et al. 2013, Pannier et al. 2014c). There was also a perception

that producers were finding it challenging to finish lambs to an adequate level of back

fat depth at the point of slaughter. The impact this has had on the financial value of the

carcass has been preliminarily explored (Anderson et al. 2013a)however further

investigation into its impact on carcass composition and retail value is warranted. The

Carcass Plus index is not used commercially by Merino breeders as it does not

incorporate selection for wool traits, with index selection in Maternal sires having more

emphasis on traits such as number of lambs weaned and ewe resilience, including

maintaining fat to optimise fertility. The impact the breeding values and Carcass Plus

index have on carcass composition of Merino and Maternal sired lambs is still important

to determine as their progeny is similarly slaughtered for commercial consumption.

Carcass 2020 is structured with some differences: BWT (body weight) 8%, WWT

(weaning weight) 24%, PWWT 25%, PFAT 9%, PEMD 22%, PWEC (post weaning

worm egg count) 12%. The Carcass 2020 Index has a 90% correlation with Carcass Plus


33

due the similar high weighting of growth, fat and muscle and therefore its effects on

LMY% should be similar, although less in magnitude.

2.2 Eating quality and nutritional value of lamb

The assessment of meat quality includes factors such as nutritional value, meat colour,

water holding capacity and palatability (Hopkins and Geesink 2009, Warner,

Greenwood, Pethick et al. 2010a). Consumers demand a high quality lamb product and

good value for money, (Pethick, Davidson, Hopkins et al. 2005a) but have also

identified a willingness to pay a premium for a product of high quality (Pethick et al.

2006a). Lamb consumers rate their satisfaction with lamb based on flavour/odour,

tenderness and juiciness (Pethick, Pleasants, Gee et al. 2006b), in contrast to beef where

tenderness is considered more important (Watson, Gee, Polkinghorne et al. 2008).

There is good evidence to suggest that improvements to leanness (LMY%) may affect

the eating quality and nutritional value of lamb (Hopkins, Hegarty and Farrell 2005,

Karamichou, Richardson, Nute et al. 2006), through impacts on tenderness, juiciness

(Pannier et al. 2014a, Pannier et al. 2014c), zinc and iron content (Kelman, Pannier,

Pethick et al. 2014b, Pannier, Pethick, Boyce et al. 2014b). Additionally, fibre type of

muscles may impact on eating quality and nutritive value through impacts on colour,

tenderness and intramuscular fat (IMF) % (Lee, Joo and Ryu 2010). The following

sections explore the factors that affect IMF% and fibre type in lamb and the potential

impact that increasing LMY% may have on them and in turn eating quality and the

nutritive value of lamb

2.2.1 Intramuscular Fat

2.2.1.1 Impact of intramuscular fat on eating quality

IMF is the amount of fat contained within muscles, in contrast to intermuscular fat

which is the fat that is located between the muscles. Microscopically, IMF is made up of


34

spherical cells which are generally smaller than adipocytes from other fat depots (Lee,

Lee, Kim et al. 2000). IMF is present to varying degrees in all skeletal muscles

(Brackebrush, McKeith, Carr et al. 1991), with IMF% is a key determinant of eating

quality in red meat (Harper and Pethick 2004a). It is well accepted that IMF% has a

positive impact on flavour, juiciness and tenderness (Hopkins, Hegarty, Walker et al.

2006a, Murray, Pommier, Gibson et al. 2004, Pannier et al. 2014a, Thompson 2004b)

and has been shown to account for approximately 10 to 15% of the variation in the

palatability of beef (Dikeman 1987). IMF improves tenderness by disrupting alignment

of the surrounding muscle fibres (Warner et al. 2010a). Marbling is a term used by the

beef industry to describe the white fleck or streaks of IMF between muscle fibres

(Hocquette, Gondret, Baéza et al. 2010, Watson et al. 2008) and can be given a visual

score in cattle.

In lamb it is thought that a minimum of 4-5 IMF% is required for consumer satisfaction

with regard to palatability (Hopkins et al. 2006a). Accordingly, the IMF% of the M.

longissimus lumborum (short loin) has been identified as a key factor for maintaining

premium eating quality of lamb (Pannier et al. 2014a, Pannier et al. 2014c). There is

concern in the Australian sheep industry that extremely low IMF may result in dry,

tasteless meat (McPhee, Hopkins and Pethick 2008). With meat quality is becoming

more important to consumers (Pethick et al. 2006a) and given IMF contributes to eating

quality, it is important to consider IMF throughout the carcass and the impact that

current genetic selection has on its content within muscles.

Most research on IMF% in lamb has been conducted on the M. longissimus lumborum

(Hopkins, Hegarty, Walker et al. 2006b, McPhee et al. 2008, Pannier et al. 2014c), with

few studies comparing IMF content of this muscle with others (Craigie, Lambe,


35

Richardson et al. 2012). Furthermore, there is currently no research that investigates the

IMF% of muscles of the forelimb in lamb.

2.2.1.2 Measurement of intramuscular fat

IMF can be measured in muscle by chemical determination (ether or chloroform

extraction) (Perry, Shorthose, Ferguson et al. 2001). Chemical determination of IMF

includes the total of phospholipids, triglycerides and cholesterol (Hocquette et al. 2010).

This is considered highly accurate but is also time consuming and destructive to the

meat sample being examined. There is increasing pressure to develop rapid, non-

destructive and accurate methods for determination of IMF% in carcasses at slaughter.

This may enable the grading of carcasses based on their IMF content and potential

eating quality. Other technologies used to determine IMF% include real time ultrasound

(Mörlein, Rosner, Brand et al. 2005), near infrared spectroscopy (NIR) (Prevolnik,

Candek-Potokar, Skorjanc et al. 2005), impedance spectroscopy (Altmann and Pliquett

2006), CT (Clelland et al. 2014, Kongsro and Gjerlaug-Enger 2013, Lambe et al. 2010,

Macfarlane, Young, Lewis et al. 2005) and hyperspectral imaging (Barbin, Elmasry,

Sun et al. 2012). Some of these technologies offer non-destructive methods for

determination of IMF% and can be used on the live animal or carcass.

2.2.1.3 Factors affecting intramuscular fat

2.2.1.3.1 Impact of species and muscle on intramuscular fat

Muscle tends to be similar in its chemical composition, with the exception being fat

content which varies between and within species. For example pork and poultry have

very low IMF%, compared to beef and sheep, where IMF% is greater (Hocquette et al.

2010).


36

Levels of IMF have been shown to vary with the location of the muscle in beef

(Dikeman 1987) and sheep (Warner, Jacob, Edwards et al. 2010b). The reason for some

of these differences is related to muscle function and therefore fibre type (Hocquette,

Cassar-Malek, Jurie et al. 2012). Hocquette et al (2012) showed that more oxidative

fibres contain more phospholipids and triglycerides. Additionally, Picard et al (2002)

showed that muscles responsible for the maintenance of posture tend to be more

oxidative and comprised of Type 1 muscle fibres with a propensity for higher levels of

IMF%. Conversely, muscles with higher glycolytic activity have lower IMF%

(Hocquette et al. 2010). This is not always true, with one study showing that bison,

which have more oxidative muscles can have low IMF% (Agabriel, Bony and Micol

1998). Similarly in pigs it has been observed that within the same muscle, the redder

and therefore likely more oxidative Type 1 fibres can contain less IMF than the in the

white (less oxidative) Type 1 fibres (Hocquette et al. 2010). These two examples show

that although fibre type has some correlation with IMF content of the muscle it is not a

sound predictive tool.

Brackebrush et al. (1991) has shown in cattle that there is a high correlation in cattle

between the M. longissimus thoracis et lumborum and other muscles. This indicates

measurement in one muscle can be predictive of IMF content of other muscles. A

similar study in sheep showing correlations between a wide range of muscles has not

been performed.

2.2.1.3.2 Impact of age and maturity on intramuscular fat

The development of muscle and fat of animals is linked to the prenatal development of

these tissues (Picard et al. 2002). In addition to the development and differentiation of

muscle fibres, the fatty acid metabolism are important for the final expression of IMF

within a muscle (Cagnazzo, Te Pas, Priem et al. 2006). The prenatal development of


37

IMF is poorly characterised but it is thought that both hyperplasia and hypertrophy of

adipocytes plays a role in the development of IMF (Albrecht, Teuscher, Ender et al.

2006). In cattle, in utero restriction of nutrition has been shown to have an impact on

the development of IMF (Greenwood and Cafe 2007, Greenwood, Cafe, Hearnshaw et

al. 2006a).

In the postnatal period, age impacts on the IMF content of red meat and generally

increases with age in cattle (Cianzio, Topel, Whitehurst et al. 1985, Hood and Allen

1973) and sheep (Hopkins, Stanley, Martin et al. 2007c, McPhee et al. 2008, Okeudo

and Moss 2007, Pannier et al. 2014c, Pethick, Hopkins, D'Souza et al. 2005b). This

does not imply that IMF is a late maturing tissue, however due to the decrease and

cessation of growth of the lean tissue the relative concentrations of the two tissues

means that IMF% within the muscle will increase. IMF in the M. longissimus lumborum

has been shown in lamb to be early maturing (McPhee et al. 2008) compared to other

fat depots. This finding agrees with earlier work on IMF in lamb by Johnson et al.

(1972) who showed that in relation to total carcass fat depots, IMF was accreted early in

development of the lamb. Ponnampalam et al. (2008) reported fat depots in sheep to

increase relative to lean tissue as the accretion of protein decreased. Therefore the

concentration of fat within the muscle will increase later in the animals life, which may

lead to the assumption by some that IMF% is a late maturing tissue (Cianzio et al. 1985,

Hood and Allen 1973).

With IMF being an early maturing fat depot and given that other carcass fat depots are

later maturing and continue to develop once an animal reaches maturity it is unlikely

that retention of lambs for prolonged periods will have minimal impact on increasing

IMF content of muscles and may result in the acquisition of penalties for over fat


38

carcasses (McPhee et al. 2008). Additionally in cattle, Aoki et al. (2001) showed that

beyond a certain carcass weight in cattle that there was no further increase in IMF%

suggesting a ‘maximum’ IMF content may exist.

2.2.1.3.3 Impact of sex on intramuscular fat

The impact of sex on IMF in lamb has been shown to be variable. Most recently,

Pannier et al (2014c) showed that ewe lambs had more IMF than wether lambs. A

similar finding was reported by Craigie et al. (2012), although this study compared

ewes with rams. Contrasting these findings, other studies have found no difference in

IMF between the sexes (Okeudo and Moss 2007, Tejeda, Peña and Andrés 2008).

2.2.1.3.4 Impact of nutrition on intramuscular fat

Compared to other influence, nutrition has a relatively small impact in IMF content of

muscles compared to the genetic impacts (McPhee et al. 2008, Pannier et al. 2014c).

However some impact of nutrition has been observed in sheep, with nutritional

restriction reducing deposition of IMF (Hegarty et al. 2006c). Another study in sheep

showed that slow growth that resulted in decreased carcass fat and reduced IMF

(Murphy et al. 1994). Pethick et al. (2004) showed in cattle, that compared to grass,

grain was more effective at finishing cattle with regards to increasing IMF due to the

superior energy intake.

2.2.1.3.5 Impact of genetics and the selection for muscularity/lean meat yield on

intramuscular fat

Intramuscular fat content of muscle is impacted by genetic selection (Albrecht et al.

2006, McPhee et al. 2008, Pannier et al. 2014c) Pethick et al. (9-13 September, 2007)

showed some breeds of cattle displayed high levels of IMF% at low levels of carcass

fat. In this instance the cattle had been selected for increased intramuscular fat over a

long period of time which highlights the potential benefits of genetic selection to


39

manipulate IMF. In lamb, IMF has been shown to have high heritability (Mortimer, van

der Werf, Jacob et al. 2014) and therefore provides a good means of selecting for

improved or maintained levels of IMF.

In lamb, the IMF% of the loin has been shown to be higher in Maternal (Border

Leicester) sired lambs compared to other breeds (McPhee et al. 2008). A study by

Pannier et al. (2014c) showed no impact of sire type on IMF% however lambs from

Border-Leicester x Merino dams had more IMF% than those from Merino dams. These

results are in line with Border Leicesters having higher levels of overall carcass fat

(Hopkins and Fogarty 1998a, Hopkins et al. 2007c, Ponnampalam et al. 2007b).

Although use of Border Leicester genetics may positively impact on IMF% and

subsequent eating quality, these benefits must be weighed up against the potential

increase in other fat depots and carcass wastage due to the necessity to trim carcass fat.

The meat from double muscled cattle and cuts of meat from modern pig genotypes has

been shown to have reduced intramuscular fat content (Albrecht et al. 2006, Channon,

Reynolds and Baud 2001, Hocquette et al. 2010). This is in part related to increased

glycolytic activity of the muscle but also due to a ‘dilutional’ effect of increased muscle.

In lambs, a key factor driving reduced IMF% in the M. longissimus lumborum is

selection for lean growth (Pannier et al. 2014c). Pannier et al. (2014c) demonstrated an

association between various carcass indicators of fatness and IMF% in the M.

longissimus lumborum, whilst Gardner et al. (2010) reported a negative phenotypic (-

0.24) and genetic correlation (-0.46) between IMF% and lean meat yield percentage

(LMY%). Pannier et al. (2014c) demonstrated that both PFAT in Terminal sired lambs

and PEMD in Merino, Maternal and Terminal sired lambs reduced IMF% in the M.


40

longissimus lumborum. The impact of selection for reduced carcass fat and increased

muscling in muscles other than the M. longissimus lumborum of the lamb carcass is not

well reported.

Sires with high PEMD and/or low PFAT breeding values had increased weight of the

M. longissimus lumborum, located in the saddle section (Anderson, Williams, Pannier et

al. 2013b, Gardner et al. 2010) and had proportionately more lean weight in the saddle

(loin) section than in other regions of the carcass. Therefore it seems likely that the

impact of these breeding values on IMF%, delivered through their effects on muscle

hypertrophy (Hocquette et al. 2010) and so dilution of IMF%, will be greater in the

saddle musculature.

2.3 Measurement technologies for carcass composition and

eating quality

Currently in Australia producers are paid based on hot carcass weight, fat depths and

GR tissue depths. Within the lamb industry it will become increasingly important to use

and develop technologies that assist the rapid and accurate determination of carcass

composition (Hopkins, Anderson, Morgan et al. 1995b). Inexpensive methods that can

be used to rapidly predict composition are useful in the processing plant and for

selection of breeding animals (Stanford, Jones and Price 1998). For research purposes it

is more useful to utilise accurate and precise technologies, with cost less of a factor

(Stanford et al. 1998) Additionally, methods that can predict eating quality will enable

processors to provide feedback to producers and a better price based payment scheme so

that there are rewards for lean meat yield and eating quality. The following review

explores technologies currently being used in the Australian Lamb Industry for

commercial and experimental purposes as well as some which are currently being used

overseas or are in the experimental phases.


41

2.3.1 Subjective measurements of carcass composition

Visual assessment of composition in the live animal and visual assessment of the

carcass has been used to assess carcass composition (Jones, Jeremiah, Tong et al. 1992,

Kempster, Avis, Cuthbertson et al. 1976, Kempster, Croston and Jones 1981). This has

been more formally described in the European Unions’ EUROP carcass grading system

(Chevalier, Leclere, Plusa et al. 1993). This system utilises measurements of the carcass

weight, carcass conformation and carcass fat. The conformation classes describe the

profile of the carcass using the 5 letters E, U, R, O, and P with P the least desirable

shape and E the best. Within each letter score the carcass is able to be graded in 3 sub

group (e.g. E+, E and E-). Fat is described based on the subcutaneous and internal fat in

5 classes (1 the least and 5 the most carcass fat).

2.3.2 Site measurements

Linear measurements of an animals’ height, girth, body length and other carcass

measures has been used for a long time in an attempt to predict carcass composition

(Orme 1963). These methods are difficult to adapt to lambs of various sex, age and

breed type and are unable to distinguish between fat and lean depots which limits their

use. More recently the Australian lamb industry has assessed composition using point

measurements (C-site fat depth, C5 fat depth, GR tissue depth, eye muscle depth)

(Gardner et al. 2010, Hegarty et al. 2006a, Hopkins, Hall and Luff 1996, Hopkins et al.

2007b) and weights of cuts (short loin, round, topside) (Gardner et al. 2010, Hegarty et

al. 2006a).

GR tissue depth is a measure of total tissue depth at the 12th

rib, 110 mm from the

midline and is measured post slaughter in the processing plant (Kirton and Johnson

1979). This measurement can be made manually using a ruler (Kirton, Woods and

Duganzich 1984) or using optical probes (Hopkins and Roberts 1995, Kirton, Mercer,


42

Duganzich et al. 1995a) (also see section on probe technologies). In conjunction with

HCWT it has been used to predict lean yield in sheep (Hopkins 1994). The

measurement of subcutaneous fat depth at the 5th

rib (C5) has been used to assist

prediction of LMY in sheep (Safari, Hopkins and Fogarty 2001). The measurement of

the fat depth at the 12th

rib 45 mm from the midline (C-site fat depth) has been used to

predict and assess carcass composition (Atkins, Murray, Gilmour et al. 1991, Atkins

and Thompson 1979). The C-site measurement also forms the basis for the Australian

Sheep Breeding Value PFAT and is measured via ultrasound in the live animal (Brown

et al. 2007).

Measurement of the M. longissimus lumborum length, width and depth, plus various fat

measurements have historically been used (Palsson 1939) to predict carcass

composition. It continues to be useful, in combination with other post-mortem measures

to predict carcass composition (Gardner et al. 2015, Gardner et al. 2010).

2.3.3 Ultrasound

A b-mode image is a cross section of the tissues within the body. The image is

generated by the reflection of ultrasound waves at the various tissue boundaries

(Houghton and Turlington 1992). The brightness of each echo at each point in the image

corresponds to the strength of the echo: this gives rise to the name B-mode (brightness

mode) (Martin 2010). This technology has allowed the direct measurement of tissues

such as fat and muscle depots in a number of species (Stanford, Bailey, Jones et al.

2001, Stanford, Clark and Jones 1995, Stanford et al. 1998, Wilson 1992). An

advantage of ultrasound is that many of the sites measured at slaughter can be assessed

in the live animal and incorporated into breeding programs (Brash, Fogarty, Gilmour et

al. 1992). Ultrasound technology is relatively low cost and easily used with wide


43

adoption into many sheep industries worldwide (Brash et al. 1992, Stanford et al. 1998),

including Australia.

Ultrasound measurements that have been used to predict lean meat in the lamb carcass

include: width of the longissimus; maximum depth of the longissimus; depth of the

subcutaneous over the longissimus and the cross sectional area of the longissimus

(Stanford et al. 2001). The depth of the longissimus and the fat depth covering the

longissimus have been used to predict the saleable meat of the carcass (Hopkins et al.

1996, Stanford et al. 1995). Ultrasound has been used to assist in the selection of sires

with low fat depth at the C-site (Hall et al. 2002). A concern over using this technology

to select for leaner carcasses is that as a point measurement, it may not effectively select

for whole body composition changes. For example there is evidence in pigs that

selection for reduced back fat at the P2 site has resulted in a decrease in fat only at this

site without affected fat depositions elsewhere in the carcass (Trezona-Murray 2008).

Ultrasound has also been used for a number of decades to predict IMF in beef (Font-i-

Furnols, Brun, Tous et al. 2013, Herring, Kriese, Bertrand et al. 1998, Prieto, Navajas,

Richardson et al. 2010, Sapp, Bertrand, Pringle et al. 2002, Whittaker, Park, Thane et

al. 1992) and more recently in pigs (Mörlein et al. 2005, Newcom, Baas and Lampe

2002) and lamb (Lambe, Navajas, Fisher et al. 2009).

2.3.4 Probe technologies

Optical probes can be used to objectively measure fat and muscle depths. Probes that

have been developed include the Hennessy Grading Probe (Henessy Grading System,

Auckland, NZ), the AUS-Meat Sheep Probe (SASTEK, Hamilton, Queensland,

Australia), the Swedish FTC lamb probe (FTC Sweden, Upplans, Vasby, Sweden) and

the Ruakura GR Lamb Probe (Hamilton, NZ) (Kirton et al. 1995a). Work continues in


44

the area of probe technology to rapidly and more accurately measure fat and muscle

depths in sheep carcasses.

The AUS-Meat sheep probe can be used to measure GR tissue depth at chain speed of

8-9 carcasses per minute and is able to measure GR tissue depth to within 2 mm of GR

knife measurements (Hopkins et al. 1995b). Operator training is considered important in

order to optimise its success. The Hennessy Grading Probe measures the GR tissue

depth by measuring reflectivity of light by fat and muscle. The Hennessy probe when

used in lamb has not given reliable results which may be due to operator errors and the

low subcutaneous fat depths in lamb compared to beef (Hopkins, Toohey, Boyce et al.

2013).

2.3.5 Computed tomography

CT was originally designed for use in human medicine but has become a non-invasive

and accurate method of accurately determining body composition. The amount of

radiation transmitted through each tissue is dependent on the attenuation of the x-rays,

which will vary depending on the density of the tissues (Bunger, Macfarlane, Lambe et

al. 2011). The densities of the radiation are recorded in Hounsfield units (Hu). These

densities allow differentiation of carcass into its components: fat, lean and bone.

Some techniques involve the use of several cross sectional CT scans taken at specific

points on the carcass to measure fat, lean and bone at these points, which is then used to

predict tissue composition at the whole carcass level (Young, Simm and Glasbey 2001).

This technique was developed to minimise the time taken to scan live animals, but also

to minimise the associated costs. Predication equations were based on calibration of the

CT scanning against carcass dissection weights to provide a breed specific equation for

prediction of carcass composition.


45

Alternatively the Cavaleri method was developed which uses a larger number of cross

sectional images from the entire carcass. The volume of each tissue (fat, lean or bone) is

then calculated by the area of each tissue in each individual scan multiplied by the

distance between scans (Roberts, Cruz-orive, Reid et al. 1993). This technique is

accurate (Macfarlane, Lewis, Emmans et al. 2006) and does not require different

prediction equations for individual breeds but is more time consuming and therefore

more costly.

This enables generation of a 3D representation of the subject matter at higher resolution.

The spiral CT scanning is considered the ‘gold standard’ for determination of carcass

composition (Bunger et al. 2011) and therefore to benchmark other methods of carcass

evaluation. Spiral CT scanning generates an image in a similar way to other scanning

techniques however is able to generate many cross sectional images at small intervals.

Compared to the earlier methods of CT, where the x-ray source rotated around the objet

to create a single slice, the spiral scanners have an x-ray tube that is on a rotating gantry,

which continually rotates in the one direction while the table moves through the gantry.

CT is an expensive technology that is at present unable to be integrated into the routine

processing of carcasses. In a research capacity, CT has been used successfully to predict

carcass composition in lamb (Gardner et al. 2010, Lambe et al. 2007, Macfarlane et al.

2006, Macfarlane, Lewis, Emmans et al. 2009, Macfarlane et al. 2005, Navajas,

Glasbey, McLean et al. 2006, Navajas, Lambe, McLean et al. 2007), beef (Navajas,

Richardson, Fisher et al. 2010, Prieto et al. 2010) and pigs (Giles, Eamens, Arthur et al.

2009, Szabo, Babinszky, Verstegen et al. 1999). It has also been used in commercial


46

sheep (Bunger et al. 2011, Nicoll, Jopson and McEwan 2002) and pig (Kongsro 2011)

breeding programs worldwide

CT has the additional bonus of being able to be used to assess other carcass

characteristics in lamb such as IMF and therefore to some degree eating quality

(Anderson, Pethick and Gardner 2015b, Clelland et al. 2014, Karamichou et al. 2006,

Lambe, Navajas, Schofield et al. 2008, Macfarlane et al. 2005, Navajas et al. 2007,

Young et al. 2001). The precision of IMF% in lamb using CT scanning has had variable

results which can differ based on the CT scanning equipment used and the prediction

equation generated. For example much of the variation in IMF% can be accounted for

my also measuring carcass fat % (Clelland et al. 2014). Most work on the prediction of

IMF% using CT has been based on the M. longissimus lumborum with limited

information on the ability of CT to predict IMF elsewhere in the carcass.

2.3.6 Radiographs and DXA

There are few studies to date that examine the use of radiography to predict carcass

composition and yield. This technology has largely been superseded by technologies

such as dual energy x-ray absorptiometry (DXA) which provides a non-invasive, low

radiation method of determining whole carcass composition of fat, lean and bone. DXA

uses the principle that dual energy X-rays can be used to determine the mass and

composition of any two known materials (Laskey 1996). Studies in pigs have shown

good levels of precision (Suster, Leury, Ostrowska et al. 2003, Suster, Wark, Kerton et

al. 2000) and it has also been used to study the composition of lamb (Dunshea, Suster,

Eason et al. 2007, Pearce, Ferguson, Gardner et al. 2009, Ponnampalam et al. 2007b).


47

DXA has the ability to accurately estimate whole body composition, however is limited

by its ability to partition the information into different fat compartments and identify

individual muscles to retrieve more specific information about the carcass. The

relatively low cost and speed of DXA in comparison to technologies such as CT make it

an attractive technology for use in commercial ventures.

2.3.7 Video image analysis systems (VIAS)

VIA systems have been introduced into processing plants in a number of countries to

provide a rapid, objective, automated and non-invasive measure of carcass

characteristics at chain speed (Hopkins et al. 2004, Rius-Vilarrasa, Bünger, Maltin et al.

2009). This fulfils the desire to estimate LMY% and other carcass characteristics and

allow sorting of carcasses to meet specifications. The use of VIA systems in a pricing

scheme has also been investigated (Brady, Belk, LeValley et al. 2003, Cunha, Belk,

Scanga et al. 2004).

To improve prediction of carcass composition it is necessary to calibrate the device.

Previously this required expensive and time consuming bone out procedures, however

with the increasing access to technologies such as CT scanning this is no longer

necessary and the VIA can be trained to be more accurate with comparative ease.

2.3.8 Near infrared spectroscopy (NIR)

Near infrared spectroscopy (NIR) offers the ability to accurately determine the moisture,

fat and protein content of meat and meat products (Prevolnik, Candek-Potokar and

Skorjanc 2004). The ability of NIR to predict sensory information about meat and its

products is useful but less reliable. A review of Preito et al.(2009) discussed a number

of studies in lamb, beef and pork that have utilised NIR for the prediction of carcass

composition, IMF, colour and eating quality parameters. The method of calibration for


48

the sensory traits is important for accuracy, with studies reporting that a significant, but

low amount of variation in the variability in eating quality of lamb can be described by

NIR (Andrés, Murray, Navajas et al. 2007).

2.4 Hypotheses

This thesis explores the impact that genetic selection for improved LMY% in lamb has

on the composition of fat, lean and bone, and the distribution of these tissues throughout

the carcass. The impacts of improved LMY% on lamb eating quality and nutritional

value as measured by IMF% are also investigated. The correlation of IMF% between 5

muscles of the carcass and the ability of CT to predict IMF% are also reported. The

following hypotheses are explored in the following chapters.

Chapter 3: Sire carcass breeding values affect body composition in lambs - 1. Effects

on lean weight and its distribution within the carcass as measured by computed

tomography.

This chapter uses CT to explore the impact that genetic selection for improved LMY%

using Australian Sheep Breeding Values for PWWT, PFAT and PEMD have on the lean

composition of the Australia lamb carcass. Additionally it examines the change in the

distribution of lean tissue between the fore, saddle and hind sections that occurs with

such genetic selection. This chapter also reports on the production and management

factors that impact lean tissue within the lamb carcass. The specific hypothesis tested

for were:

Lambs from sires of increased PWWT ASBV will have no impact on lean

weight within the carcass.


49

Lambs from sires of reduced sire PFAT ASBV will have increased lean weight

in the carcass.

Increasing sire PEMD will increase the weight of the saddle region but have no

impact on whole carcass lean.

The Terminal sired lambs will have more lean when compared to those sired by

Maternal and Merinos.

This chapter was accepted for publication in Meat Science with the version accepted for

publication included in this thesis.

Anderson F., Williams A., Pannier L., Pethick D.W., Gardner G.E. (2015). Sire carcass

breeding values affect body composition in lambs – 1. Effects on lean weight and its

distribution within the carcass as measured by computed tomography. Meat Science,

108, 145-154.

Chapter 4: Sire carcass breeding values affect body composition in lambs - 2. Effects

on fat and bone weight and their distribution within the carcass as measured by

computed tomography.

This chapter uses CT to explore the impact that genetic selection for improved LMY%

using Australian Sheep Breeding Values for PWWT, PFAT and PEMD have on the fat

and bone composition of the Australia lamb carcass. Additionally it examines the

change in the distribution of these two tissues between the fore saddle and hind sections

that occurs with such genetic selection. This chapter also reports on the production and

management factors that impact fat and bone within the lamb carcass. The specific

hypotheses tested were:

At the same carcass weight, selection for decreasing sire PFAT will result in

decreased carcass fat across all carcass regions.


50

At the same carcass weight, selection for decreasing sire PFAT will result in an

increase in carcass bone weight, with a preferential increase in hind section bone

weight.

Increasing sire PEMD will decrease carcass fat in the saddle region only and

have no impact on bone weight within the carcass.

Increasing sire PWWT will decrease whole carcass fat and increase carcass bone

weight.

Maternal sired lambs are expected to have greater carcass fat when compared to

the Terminal and Merino sired lambs, when lambs are compared at the same

carcass weight.

Merino sired lambs are expected to have a higher proportion of bone compared

to the Terminal and Maternal sired lambs when lambs are compared at the same

carcass weight.

This Chapter was submitted for publication in Meat Science in June 2015. The version

submitted for publication is included in this thesis.

Chapter 5: The impact of genetics on retail meat value in Australian lamb.

The three carcass breeding values (PWWT, PEMD and PFAT) are aimed at improving

growth, muscling and leanness. The impacts of these breeding values on carcass

composition was explored in chapters 3 and 4, however the economic benefits of these

composition changes has not been established. This chapter explores the economic

implications of the carcass ASBVs (PWWT, PFAT and PEMD) and the Carcass Plus

Index on carcass lean value.

The specific hypotheses tested in this chapter were:


51

At a given carcass weight, the progeny of lambs from increased PWWT ASBVs

and PEMD ASBVs or reduced PFAT ASBVs will have increased carcass lean

value.

When lambs are compared at the same carcass weight, decreasing sire PFAT

will have the greatest impact on carcass value.

When assessing the impact of genetics at either the same carcass weight or age,

the carcass value will be greatest in the Terminal sired lambs.

Due to the increased magnitude of effect PWWT has on the HCWT, we

hypothesise that when compared at the same age, this breeding value will have

the greatest impact on carcass value.

This chapter was submitted for publication in Meat Science in June 2015. The version

submitted for publication is included in this thesis.

Chapter 6: Intramuscular fat in lamb muscle and the impact of selection for improved

carcass lean meat yield.

This chapter examines the factors that affect the IMF% of three regions of the lamb

carcass (fore, saddle and hind) and 5 muscles from these regions (M. longissimus

lumborum, M. supraspinatus, M. infraspinatus, M. semimembranosus and M.

semitendinosus). It also examines the impact that genetic selection improvements to

LMY% has on the IMF% of these 5 muscles and focusses on comparing and contrasting

the impacts that have been observed in the M. longissimus lumborum. This work

provides essential information regarding the impact of carcass breeding values on

intramuscular fat % and eating quality, facilitating the management of these detrimental

effects. The specific hypotheses tested in this chapter were:


52

Lambs from high PEMD sires or from low PFAT sires will have reduced IMF%

in the short loin, but to a lesser extent in the hind and fore sections.

Increasing sire PWWT will have no impact on IMF% in the carcass.

The IMF% of muscles in postural regions of the carcass will have greater IMF%

than muscles in locomotive regions.

This chapter was submitted to Animal and accepted for publication with the version

accepted for publication included in this thesis.

Anderson, F., Pannier, L., Pethick, D.W., Gardner, G.E. (2015). Intramuscular fat in

lamb muscle and the impact of selection for improved carcass lean meat yield. Animal

9(6): 1081-1090.

Chapter 7: The correlation of intramuscular fat content between muscles of the lamb

carcass and the use of computed tomography to predict intramuscular fat percentage in

lambs.

This chapter examines the correlation of intramuscular fat between 5 muscles of three

sections of the carcass (fore, saddle and hind sections). It explores the predictive

capabilities of computed tomography as it relates to IMF in 5 muscles from these three

sections (m. longissimus lumborum, m. supraspinatus, m. infraspinatus, m

semimembranosus and M. semitendinosus)

The specific hypotheses tested in this chapter were:

The IMF% of the m. longissimus lumborum muscle will be correlated with the

IMF% of the m. supraspinatus, m. infraspinatus, m. semimembranosus and m.

semitendinosus.

The CT pixel density will adequately predict the IMF% of CT scanned muscles,

allowing non-destructive rapid determination of IMF%.


53

This chapter was submitted to Animal and accepted for publication with the version

accepted for publication included in this thesis.

Anderson F., Pethick D.W., Gardner G.E. (2015). The correlation of intramuscular fat

content between muscles of the lamb carcass and the use of computed tomography to

predict intramuscular fat percentage in lambs. Animal, 9(7):1239-1249.

Within this thesis, Chapters 3-7 have been accepted or submitted to journals as

publications, therefore they contain reference to each other as Anderson et al (2015a-d)

in these chapters. However, in order to provide easy cross reference between the thesis

chapters the formal references are also followed by the chapter number in brackets. The

exception to this is Chapter 8 (General discussion) which references only the chapter

numbers.

Chapter 3 – The impact of breeding values on carcass composition (lean)

54

Chapter 3. Sire carcass breeding values affect body

composition in lambs - 1. Effects on lean weight and its

distribution within the carcass as measured by


The following chapter is the version that was accepted for publication:

Anderson, F., Williams, A., Pannier, L., Pethick, D.W., Gardner, G.E. (2015). Meat

Science, 108, 145-154.

3.1 Abstract

Data are obtained from computed tomography scanning of 1665 lambs at locations

around Australia. Lambs were progeny of Terminal, Maternal and Merino sires with

known Australian Sheep Breeding Values for post weaning c-site eye muscle depth

(mm; PEMD) and fat depth (mm; PFAT), and post weaning weight (kg; PWWT).

Across the 7.8 unit range of sire PEMD, carcass lean weight increased by 7.7%. This

lean was distributed to the saddle section (mid-section) where lean became 3.8%

heavier, with fore section lean becoming 3.5% lighter. Reducing sire PFAT across its

5.1 unit range increased carcass lean weight by 9.5%, and distributed lean to the saddle

section which was 3.7% heavier. Increasing sire PWWT increased lean at some sites in

some years, and on average increased saddle lean by 4% across the 24.7 unit PWWT

range. Changes in lean weight and distribution due to selection for carcass breeding

values will increase carcass value, particularly through increased weight of high value

loin cuts.


55

3.2 Introduction

The financial value of a carcass is influenced by its lean meat yield percentage, which

represents the proportion of the carcass that is lean meat (muscle). Consumer

preferences in domestic and international markets drive the industry to produce meat

cuts that are larger and leaner (Banks 2002, Hall, Kelf, Fogarty et al. 2000, Laville et al.

2004). To achieve this goal, Australian lamb producers currently select for lean meat

yield percentage indirectly via three existing Australian Sheep Breeding Values

(ASBVs) for post-weaning weight (PWWT), c-site fat depth (PFAT) and eye muscle

depth (PEMD), which are used to select for improved growth, leanness and muscling

respectively. The effects of selection using these ASBVs have previously been

investigated using indicators like muscle depths (Hopkins et al. 2007b), and weights of

specific cuts (Gardner et al. 2010), however have not been quantified in terms of the

change in whole carcass lean composition or distribution of lean tissues carcass regions.

Whilst there is evidence that a strong emphasis on PWWT ASBV will increase growth

rate and mature size (Huisman and Brown 2008), there is little data showing the effects

of this ASBV on carcass composition. It has been shown that lambs growing at a faster

rate are proportionately leaner when compared at the same weight, largely due to a

correlated increase in mature weight (Bennett et al. 1991, Butterfield 1988). However,

Hegarty et al. (2006a) showed that the genetic potential for growth (or increased

PWWT) did not impact significantly on the proportion of lean in the carcass. Likewise,

Gardner et al. (2010) showed no effect of PWWT on carcass composition as assessed

by computed tomography (CT), in spite of the increased live weight and hot carcass

weight (HCWT) at slaughter.


56

Numerous studies have shown that selection for increased sire PEMD impacts on

muscle depth at its site of measurement, the c-site (Hall et al. 2002, Hegarty et al.

2006a). Likewise, Gardner et al. (2010) also demonstrated an increased weight of the

M. longissimus lumborum in response to increasing PEMD, although in this case there

was little impact on other muscle weights or the percentage of lean in the carcass.

Alternatively, Hegarty et al. (2006a) showed that lambs selected for increased PEMD

increased the mass and dimensions of the loin muscle and had a small increase in four

different hind limb muscles, suggesting a carcass wide effect. However the latter study

only utilised nine sires compared to 93 sires used in the Gardner et al. (2010) study.

Hence we can expect that increasing sire PEMD will increase the proportion of carcass

lean, with this effect predominantly focused in the saddle region.

Decreasing sire PFAT ASBV has been shown to increase eye muscle depth at the c-site

(Nsoso et al. 2004). Gardner et al. (2010) showed that progeny of sires with low PFAT

had increased weight and dimensions of muscles within the saddle (loin weight and eye

muscle area) and hind (round) sections of the carcass, indicating a more widespread

impact of PFAT on muscle. However in this case there was no increase in the

proportion of carcass lean.

Lastly, sire type has also been shown to impact on carcass lean tissue (Ponnampalam et

al. 2007a). When compared at the same age, Merino sired lambs had lower values for

loin weight, eye muscle area and depth compared to Maternal (Border Leicester) and

Terminal (Poll Dorset) sired lambs. Compared at the same weight, the Terminal sired

lambs had a greater proportion of carcass lean than Maternal sired and pure Merino

lambs (Ponnampalam et al. 2008). Other studies highlight the Poll Dorset and Texel

cross lambs as having a higher muscle to bone ratio than Maternal (Border Leicester) or


57

Merino sired lambs (Atkins and Thompson 1979, Hopkins and Fogarty 1998b). On this

basis it is expected that the amount of carcass lean will be higher in Terminal sired

lambs when compared at the same weight.

This paper describes the association of factors such as site (research station), birth year,

sex, birth type (litter size), rearing type, dam breed, and sire type on CT lean, plus the

impact of genetic selection using PWWT, PFAT and PEMD ASBVs. Preliminary

results of parts of this experiment have been previously published (Anderson et al.

2013b), with the results for fat and bone composition published in Anderson et al.

(2015d) (Chapter 4). We hypothesised that when lambs are compared at the same

carcass weight, decreasing sire PFAT will increase the weight of carcass lean, whereas

increasing sire PWWT will have no effect on the weight of carcass lean. In addition we

expect that increasing sire PEMD will increase the weight of lean in the saddle region

but have no impact on the proportion of lean in the whole carcass. We also hypothesised

that the Terminal sired lambs will have a greater proportion of carcass lean than the

Maternal and Merino sired lambs.

3.3 Material and methods

3.3.1 Experimental design and slaughter details

The Australian Cooperative Research Centre (CRC) for Sheep Industry Innovation

established an Information Nucleus Flock (INF) in 2007, with details of the design of

the flock presented by Fogarty et al. (2007). Some of the objectives were to measure a

diverse range of phenotypic traits, including CT lean and to assess the impact of genetic

selection on these traits. From 2007 to 2010, approximately 6000 lambs were born and

raised at one of six research sites across Australia (Katanning WA, Kirby NSW, Stuan

SA, Turretfield SA, Hamilton Vic. and Rutherglen Vic), with these sites representing a

broad cross-section of the sheep producing regions of Australia. These lambs were


58

produced from Merino or Border-Leicester x Merino dams which were artificially

inseminated using semen from 100 sires per year, representing the major sheep breeds

used in the Australian sheep industry. Individual sires were chosen as they were

representative of a full range of ASBVs for key traits within each sire type. The sire

types included Terminal sires (Hampshire Down, Ile De France, Poll Dorset,

Southdown, Suffolk, Texel, White Suffolk), Maternal sires (Bond, Booroola Leicester,

Border Leicester, Coopworth, Corriedale, Dohne Merino, East Friesian, Prime South

African Meat Merino (Prime SAAM), White Dorper), and Merino sires (Merino, Poll

Merino). Within each site, the aim of selection of lambs for CT was to include at least

two progeny from each sire used at the site, selected across a live weight strata. Lambs

were grazed under extensive pasture conditions and supplemented with grain, hay or

pellets when pasture was limited which varied between sites (Ponnampalam, Butler,

Jacob et al. 2014).

3.3.2 Slaughter protocol

Within each year, at each of the six research stations, lambs were divided into groups

based on live weights, with each group killed separately (kill groups) at a target carcass

weight of 23 kg, with a total of 1665 lambs slaughtered. Lambs within kill groups were

on average within 5 days of age of each other and within a year there was an attempt to

represent all sire types in each kill group. Across the 9 site-year combinations in this

experiment there was a total of 25 kill groups, with the average age within a slaughter

groups ranging from 168 to 420 days of age and the number of lambs within each kill

group ranging from 20 to 99 lambs (Table 3-1).


59

Table 3-1 Total number of lamb carcasses scanned using computed tomography at each

site.

Site-Birth Year

Kill

group

number

Average age

at slaughter

(days)

Number

of lambs

Kirby 2007 1 235 72

2 270 63

3 352 96

Kirby 2008 1 269 97

2 345 99

3 408 99

4 420 96

Rutherglen 2010 1 198 55

2 254 59

Hamilton 2009 1 229 53

Struan 2010 1 260 67

2 287 67

3 322 27

Turretfield 2009 1 235 58

2 262 63

3 310 29

Katanning 2007 1 177 59

2 248 52

Katanning 2008 1 235 20

2 242 29

3 319 28

Katanning 2011 1 168 87

2 238 96

3 280 99

4 355 95

Total 25 - 1665

At all INF sites, lambs were yarded within 48 hours before slaughter, maintained off-

feed for at least 6 hours, and then weighed to determine pre-slaughter live weight. They

were then transported for 0.5-6 hours via truck to one of 5 commercial abattoirs, held in

lairage at the abattoir for between 1 and 12 hours, and then slaughtered.

All carcasses were electrically stimulated and trimmed according to AUSMEAT

standards (Anonymous 2005) and HCWT was then measured within 40 minutes of

slaughter. All lambs were measured and sampled for a wide range of carcass, meat and

growth traits.


60

3.3.3 Computed tomography scanning

Carcasses were transported for CT scanning to either Murdoch University (Picker PQ

5000 spiral CT scanner) or the University of New England (Picker, Bavaria, Germany)

within 72 hours of slaughter to determine the proportions of fat, lean and bone. Prior to

scanning the carcasses were split into three primal components to enable more rapid

post scanning processing of the CT images for the distribution analysis: fore-section,

saddle and hind section. The fore section was separated from the saddle by a cut

between the fourth and fifth ribs. The hind section was separated from the saddle by a

cut through the mid-length of the sixth lumbar vertebrae. In both cases the spiral

abdomen protocol was selected with settings: pilot scan length of 512 mm, field of view

set at 480mm, Index 20, kV 110, mA 150, revs 40, pitch 1.5 and standard algorithm. At

Murdoch University, the carcasses were scanned in 10 mm slice widths, with each slice

taken 10 mm apart. The University of New England used similar settings with some

differences: field of view set at 450 mm, kV 130, mA 100, 5 mm slice width and

distance between images of 15 mm.

The images produced from the CT scan were edited to remove non-carcass image

artefacts and were partitioned into bone, muscle and fat components (Image J version

1.37v, National Institutes of Health, Bethesda, MD, USA, used in conjunction with

Microsoft Excel). The discrimination point to identify the Hounsfield barriers for

associating pixels with fat, muscle and bone were –235 to 2.3 for fat, 2.4 to 164.3 for

lean and >164.3 for bone. An estimate of volume using Cavalieri’s method (Gundersen

and Jensen 1987, Gundersen, Bendtsen, Korbo et al. 1988) was calculated as follows:

m

VolumeCav = d × Σ areag - t × areamax

g=1


61

in which m is the number of CT scans taken and d is the distance between cross-

sectional CT scans, in this case 10 mm. The value of t is the thickness of each slice (g),

in this example 10 mm, and area max is the maximum area of any of the m scans.

The average of the Hounsfield units of the pixels of each component was then

determined and converted into density (kg/L) using a linear transformation (Mull 1984).

This was then used along with the volume of each component to determine the weight

of fat, lean and bone, which was then expressed as a percentage of total carcass weight

at the time of scanning. Given the density of the marrow tissue, it is classified as either

fat or lean using the boundary discrimination method described above. Additional

editing within Image J enabled the isolation of the marrow component of bone within

all images. Thus the above procedures could be repeated on the ‘marrow only’ images.

This enabled back correction for these pixels, reallocating them as bone and removing

their associated volumes from the lean and fat components of the first iteration of image

analysis. Thus using the CT scans it is possible to determine the percentage of fat, lean

and bone within each carcass.

3.3.4 Data used

CT scanning data from a total of 1665 animals from the 9 site-year combinations was

available for analysis of lean composition within the carcass (Table 3-1). The 111

animals from Katanning in 2007 were not scanned in sections, therefore when analysing

distribution of lean between carcass sections, 1554 lambs were included in the analysis.

The mean carcass weight of the slaughtered lambs was 23.3kg. The mean, maximum

and minimum weights of the fat, lean and bone for each of the sections is reported in

Table 3-2.

.

62

Table 3-2 Mean ± standard deviation, (minimum and maximum) values in lamb for the weight (kg) of fat, lean, bone and all components

combined within the whole carcass, and the fore, saddle and hind sections.

All components Fat Lean Bone

Mean ± SD Min-Max

Mean ± SD Min-Max

Mean ± SD Min-Max

Mean ± SD Min-Max

Whole carcass weight (kg) 23.3±4.39 13.3-40.0

6.3±2.19 2.1-15.3

13.3±2.05 7.4-20.8

3.8±0.56 2.4-5.7

Fore section weight (kg) 7.7±1.39 4.4-13.1

1.9±0.57 0.8-4.4

4.3±0.74 2.3-7.1

1.5±0.23 0.9-2.4

Saddle section weight (kg) 7.5±1.82 3.7-14.0

2.9±1.24 0.8-8.1

4.3±0.74 2.3-7.1

0.9±0.19 0.4-1.5

Hind section weight (kg) 8.0±1.34 4.8-13.2

1.4±0.45 0.4-3.3

5.3±0.84 2.9-8.6

1.4±0.18 0.9-2.0


63

Of the 85 Maternal, 119 Merino and 155 Terminal sires, 70, 109 and 154 had ASBV

values for PWWT, PEMD, and PFAT available (Table 3-3). The breeding values for

PEMD and PFAT are based upon live ultrasound measurements at the c-site (12th

ribs

45 mm from the midline), and PWWT is based upon live weight, all measured at the

post weaning time point (about 240 days of age). The sires used were representative of a

range of the ASBVs used for selection for lean meat yield. A percentage of sires

selected in a year were used in the subsequent year to provide linkage between years.

These ASBV values were all sourced from Sheep Genetics, which is Australia’s

national genetic evaluation database for sheep (Brown et al. 2007). The sire breeding

values and index estimates were generated within 3 separate data-bases for Terminal,

Maternal, and Merino sired progeny and were from an analysis completed in April

2013. Some of the youngest sires used in this experiment lacked industry records and

therefore did not have ASBVs available. Therefore when the ASBV’s were included in

the model, only 1612 animals were used in the analysis.

Table 3-3 Number of lamb sires and mean (min, max) for the Australian Sheep

Breeding Values for post weaning weight (PWWT), post weaning c-site fat depth

(PFAT) and post weaning eye muscle depth (PEMD) for each sire type.

Sire type No. of sires PWWT (kg) PFAT (mm) PEMD (mm)

Maternal 70 4.7 (-6.1, 12.4) -0.3 (-2.1, 2.6) 0.1 (-2.5, 1.8)

Merino 109 1.9 (-5.0, 10.8) -0.2 (-1.9, 1.9) 0.0 (-2.6, 2.6)

Terminal 154 12.7 (5.3, 18.6) -0.7 (-2.5, 2.3) 1.2 (-2.8, 5)

3.3.5 Statistical analyses

3.3.5.1 Data transformation

All data was converted to natural logarithms in order to utilise Huxley’s allometric

equation (y = axb) (Huxley and Teissier 1936). Where x is the independent variable, a is

the proportionality coefficient and b the growth coefficient of y relative to x. The value


64

of the b coefficient describes, the relative growth rate of the component (y) to either the

whole carcass weight or the weight of lean and will be either: early maturing (b<1), late

maturing (b>1) or maturing at the same rate as that of x (b=1).

By transforming all of the values to natural logarithms, (loge y = loge a + b.loge x )

the data is linearised and solved by least squares regression. A significant advantage of

using the log form of the equation is that it homogenises the variance over the entire

range of sample data. It also allows for the direct comparison of the differences in loge y

values as percent differences (Cole 2000) and it is on this basis that the data in this

paper has been interpreted. In this paper, the b coefficient describes the rate of

development of either the whole carcass lean, or the section weight of lean as a

component of either carcass weight, or total weight of lean. In both instances the b term

was examined with relevant first order interaction with the core terms of sire type, sex

within sire type, dam breed within sire type, birth type-rear type, site year, site year

within kill group. A similar approach to the analysis of the fat and bone composition of

the same lambs was taken so that the model could be constrained, meaning in the

analysis of whole carcass lean, reported increases or decreases in lean are offset by

similar changes in either fat or bone. Additionally, the allometric approach allows the

relative rates of development of carcass tissue types to be determined.

3.3.5.2 Linear mixed effects models

The loge transformed CT data were analysed using linear mixed effects models in SAS

(SAS version 9.0, SAS Institute, Cary, NC, USA). The examination of the data was

divided into two parts. Firstly whole body composition was assessed using the loge total

weight of carcass fat, lean or bone as the dependent variable, and loge carcass weight at

the time of CT scanning as the covariate. These 3 models were constrained such that

they maintained the exact same form across all three tissue types, enabling the addition


65

of differences in fat, lean, and bone to approximately off-set each other and thus the

addition of the tissues can still approximate carcass weight (results for carcass fat and

bone are reported in Anderson et al. (2015d) (Chapter 4). Secondly, to assess the

distribution of lean between the fore section, saddle, and hind section, the loge weight of

lean within each of the 3 carcass regions was used as the dependent variable, and the

loge total lean weight within the carcass was used as the covariate. Again, these 3

models were constrained such that they maintained the exact same form across all three

carcass regions, enabling the addition of differences in fore, saddle, and hind sections of

lean to approximately off-set each other and thus the addition of lean across the 3

carcass regions can still approximate total lean weight. The same approach was also

used to analyse fat and bone distribution across the three carcass sections with results

reported in Anderson et al. (2015d) (Chapter 4).

For all of the models described above, we tested a standard set of fixed effects, and

random terms which denotes the base model. The fixed effects included site-year

(combined effect of site and year of lamb birth: Katanning (2007, 2008, 2011), Kirby

(2007, 2008,) Hamilton 2009, Turretfield 2009 and Struan 2010); birth type and rearing

type (combined effect of animals born as single, twin or triplet and reared as single,

twin or triplet); sire type (Maternal, Merino and Terminal); sex within sire type (wether

Merino, wether Maternal, female Terminal, wether Terminal); dam breed within sire

type (Merino x Merino, Maternal x Merino, Terminal x Merino, Terminal x Border

Leicester-Merino) and kill group within site-year. The random terms included sire and

dam identification by lamb birth year. All appropriate first and second order interactions

between fixed effects and covariates were tested. Non-significant (P>0.05) terms were

removed in a step-wise manner resulting in a base model, constrained to be the same for

the 3 models within the whole body analysis (i.e. across fat, lean and bone), or for the 3


66

models within the lean distribution quarter analysis (i.e. across fore, saddle, and hind

section). Of the total number of carcasses undergoing CT scanning, 1665 had entries

for sex, sire type, birth-type rear-type, dam breed, and kill group and were included in

the base model (Table 3-4).

The base models described above were also tested with sire ASBVs for PWWT, PEMD

and PFAT in the model. Initially, all 3 ASBVs were included simultaneously as

covariates in the model, as well as their first order interactions with other terms. Non-

significant (P > 0.05) terms were removed in a stepwise manner. Due to the correlations

that exist between these breeding values in this data set (PWWT vs PEMD = 0.3;

PWWT vs PFAT = 0.3; PEMD vs PFAT = 0.1) this process was repeated with the

breeding values included one at a time to test the independence of their effects.

Table 3-4 Number of lambs analysed in the base model according to sex, sire type, birthing and rearing type and dam breed.

Sex Birth type-rearing type Dam breed

Single

born and

raised

Born as

twin-

raised as

single

Born as

twin-

raised as

twin

Born as

triplet-

raised as

single

Born as

triplet-

raised as

twin

Born and

raised as

triplet

Female Wether Merino BLM

Maternal 0 373 172 31 155 2 8 5 373 0

Merino 0 251 129 38 79 1 0 4 251 0

Terminal 507 534 413 96 472 8 30 22 527 514

Total 507 1158 714 165 706 11 38 31 1151 514

BLM: Border- Leicester x Merino

67


68

3.4 Results

3.4.1 Production and management effects

Between site-years the proportion of lean tissue within the carcass varied (P<0.01,

Table 3-5) by as much as 8.8% (Table 3-6), with Rutherglen 2010 having the least and

Kirby 2007 the most. The distribution of this lean was also different between site-years.

At a constant lean weight the Kirby 2007 lambs had the most fore section lean, the least

saddle section lean and one of the lowest weights of hind section lean. By contrast,

lambs from Turretfield 2009 had the least fore section lean, one of the most saddle

section lean and the greatest hind section lean, which compared to Kirby 2007 was

about 12% less in the fore, 8.7% more in the saddle and 4.2% more in the hind section.

Within each of these site-years there was also considerable variation between kill

groups (P<0.01, Table 3-5) the greatest difference occurring at Katanning in 2011

which varied in carcass lean by as much as 5.57% (data of individual kill groups not

shown). At the majority of the 8 site-year combinations where kill group comparisons

were possible, within a year there was a greater proportion of carcass lean in the earlier

kill group, compared to the oldest kill group (P<0.05). Within each site, the amount of

lean in a section (fore, saddle and hind) varied significantly between each kill groups.

The greatest variation was at Turretfield in 2009 where the amount of lean varied by

6.89% and 8.38% in the fore and saddle sections. Within a site-year there was a trend

for the earlier kill groups to have less fore section lean and greater saddle section lean

than the later kill groups, with this occurring at 6 of the 7 site-year combinations

(P<0.05). There was no consistent pattern in the change in proportion of hind lean.

A comparison between sexes was only able to be made within the Terminal sired lambs.

There were marked differences between wethers and ewes in both in the amount of lean


69

within the carcass (P<0.01,Table 3-5) and in the distribution of lean between sections

(P<0.01, Table 3-5). Wether lambs from Border Leicester x Merino and Merino dams

had 2.76% and 2.15% more lean than female lambs (Table 3-6). Wethers from Border

Leicester-Merino and Merino dams had 2.13% and 1.93% more lean (P<0.01) in the

fore section. In the saddle section wethers had 1.06% less lean than the ewe lambs

(P<0.05, Table 3-6), however this was only for lambs born to Border Leicester-Merino

dams. The Merino and Maternal sired lambs were all wethers, therefore sex

comparisons were unable to be explored in these animals.

The birth-type rear-type did not impact on the proportion of lean in the carcass but did

impact on the distribution of lean (P<0.05, Table 3-5). Lambs born as triplets and reared

as triplets, had about 2.2% less lean tissue in the hind section when compared to lambs

not born as triplets (Table 3-6). This was offset by non-significant increases in lean

weight in the fore and saddle sections.


70

Table 3-5 F-values, and numerator and denominator degrees of freedom of factors

affecting lean weight of lamb in the whole carcass and the distribution of lean in the

fore, saddle and hind sections of the lamb carcass.

Whole carcass

lean Lean distribution between sections (F-values)

Effect NDF,

DDF

F-value

NDF,

DDF

Fore-

section

Saddle-

section

Hind-

section

site-year 8,177 30.0**

7,170 124.8** 59.3** 29.7**

sex (sire type) 1,177 115.4**

1,170 47.0** 4.7* 23.2**

birth-type rear-type

NA

NA

5,170 1.8 0.9 2.9*

Sire type 2,177 22.1**

2,170 34.2** 6.6** 13.4**

kill group(site-year) 16,177 12.0**

15,170 8.2** 9.1** 3.8**

dam breed(sire type) 1,177 11.6**

1,170 0.15 0.1 0.0

site-year x sire type 13,177 2.8** 12,170 2.0* 1.7 1.5

site-year x dam breed (sire

type) 5,177 4.3**

5,170 3.1* 1.2 1.4

log x 1

1,177 8660.7*

* 1,170 6966.0** 4846.2** 11412**

NDF, DDF: numerator and denominator degrees of freedom.

NA: term was not retained in the final model as was not significant for any component (fat, lean, bone) 1

Where carcass lean was analysed, carcass weight was the covariate (x) and when lean distribution was

analysed whole carcass lean was the covariate (x)

* P<0.05, **P<0.01


71

Table 3-6 Relative change (% Change in weight) for site-year, sex dam breed(sire type)

and birth-type rear-type on lamb carcass lean weight and the lean distribution between

the fore, saddle and hind sections of the lamb carcass.

Carcass

lean

weight

Section lean weight

(Lean distribution between

sections)

Fore-

section

Saddle-

section

Hind-

section

Factor Level %

Change

in weight

% Change

in weight

% Change

in weight

% Change

in weight

Site year 1 Kirby 2007 1.80

f

11.47f -9.83

a -2.97

a

Kirby 2008 -3.02b

6.25e -7.60

b 0.24

c

Rutherglen 2010 -6.98a

6.21e -2.19

cd -3.51

a

Hamilton 2009 0.24e

3.55d -3.23

c -0.41

c

Struan 2010 -2.26bc

1.54c -0.97

def -0.58

c

Turretfield 2009 -0.10de

-0.62a -1.10

de 1.21

d

Katanning 2007 -1.29cd

NA NA NA

Katanning 2008 -0.16de

1.08bc

1.06f -1.67

b

Katanning 2011 0.00e

0.00ab

0.00def 0.00

c

Sex Dambreed Maternal x Merino M

-4.41i

2.29k -1.68

i -0.76

ij

(sire type)2 Merino x Merino M -2.83

jk

2.59k -1.24

ij -1.28

i

Terminal x BLM F

-3.61jk

-1.68i 0.76

l 0.78

k

Terminal x Merino F -2.15k

-1.93i 0.53

kl 1.18

k

Terminal x BLM M -0.85l

0.45j -0.30

jk -0.17

k

Terminal x Merino M 0.00l

0.00j 0.00

kl 0.00

k

Birth-type Born and raised as single

-1.39 -1.51 2.20y

rear-type 3 Born twin, raised single

-1.75 -1.05 2.16y

Born and raised as twin

-1.64 -1.27 2.20y

Born triplet, raised single

-1.94 -0.87 1.99yz

Born triplet, raised twin

-0.09 -1.78 1.16yz

Born and raised as triplet

0.00 0.00 0.00z

b coefficient (± SE)

log x

4

0.86

±0.009

1.003

±0.012

1.045

±0.015

0.967

±0.009

% Change in weight: the difference in logy values for each fixed effect compared to the fixed effect with

coefficient 0.00 expressed as a percentage.

M = wether; F = ewe; BLM: Border Leicester x Merino 1

a–f Within columns for site year, % Change in weight values without a common superscript differ

significantly at P < 0.05. 2

i-l Within columns for sexdambreed(sire type), % Change in weight values without a common

superscript differ significantly at P < 0.05. 3

w-y

Within columns for birth-type rear-type, % Change in weight values without a common superscript

differ significantly at P < 0.05. 4

Where carcass lean weight was analysed, carcass weight was the covariate (x) and when lean

distribution was analysed whole carcass lean was the covariate (x)

Significant effects are in bold (P<0.05)


72

3.4.2 Impact of genetics

3.4.2.1 Sire, sire type and dam breed

When evaluating whole carcass lean in the base model there were marked differences

between sires (P<0.01), with 95% of the sire estimates for all three sire types lying

within ± 3.3% at any given carcass weight. The distribution of lean between carcass

sections also differed between sires (P<0.01). The sire estimates for lean within a

section varied, with 95% of the sire estimates from the fore, saddle and hind lying

within ±1.7%, ±3.0% and ±1.4% at any given weight of whole carcass lean when

analysed in the base model.

Sire type comparisons could be made between wether lambs born to Merino dams

(Table 3-6). The amount of lean in the carcass differed between sire types (P<0.01,

Table 3-5) with the Terminal sired lambs having 2.8% and 4.4% more lean than the

Merino and Maternal sired lambs (Table 3-6). The distribution of lean tissue also

differed between the sire types (P<0.01, Table 3-5). The Terminal sired lambs had the

most lean in the saddle and hind sections but the least lean in the fore section compared

to the Merino and Maternal sired lambs (Table 3-6).

There were also differences between dam breeds in the proportion of lean in the carcass

(P<0.01,Table 3-5), with comparisons only possible within Terminal sired lambs. In the

female lambs the progeny of Merino dams had 1.5% more lean than those from BLM

dams, and similarly in wether lambs the progeny of Merino dams had 0.9% more lean

than those from the BLM dams (Table 3-6).


73

3.4.2.2 Australian Sheep Breeding Value Effects

Sire PWWT ASBV was associated with both the amount and distribution of carcass

lean (P<0.05, Table 3-7), but only at some sites on some years. Sire PWWT increased

carcass lean at Rutherglen in 2010 by 6.2% across the increasing 24.7 unit range of sire

PWWT (kg) (Table 3-8). Within sites the biggest variation was seen at Kirby in 2008,

where increasing PWWT across the 24.7 unit range, increased the mean weight by as

much as 6.9% in one kill group and reduced it by 4% in another (data of individual kill

groups not shown).

Increasing sire PWWT increased saddle lean on average by 4.0% (Table 3-8) across the

24.7 unit range. This effect varied between sites (P<0.05, Table 3-7 ) resulting in as

much as 10.9% more saddle lean across the PWWT range at the Katanning site in 2008

(Table 3-8). On average, the increases in saddle lean were offset by reduced fore-section

lean (P<0.05, Table 3-7).

Decreasing sire PFAT resulted in an increase in carcass lean and redistribution of lean

to the saddle section (P<0.01, Table 3-7). Across the 5.1 unit range (mm) of decreasing

PFAT, whole carcass lean increased by 9.5% (Table 3-8). Within the carcass lean tissue

there was a 3.7% increase in saddle lean across the decreasing PFAT range (Table 3-8),

which was offset by reduction in lean of the fore section.

Increasing the sire PEMD ASBV resulted in an increase in carcass lean and

redistribution of lean from the fore section to the saddle region (P<0.01, Table 3-7).

Over the 7.8 unit PEMD (mm) range there was an increase in carcass lean of 7.7%

(Table 3-8 and Figure 3-1). When animals were compared at the same lean weight an


74

increasing sire PEMD ASBV (mm) resulted in a 3.8% increase in saddle lean, offset by

a 3.5% reduction in fore section lean (Table 3-8).

Table 3-7. F-values, and numerator and denominator degrees of freedom for Australian Sheep Breeding Values affecting lamb

lean weight in the whole carcass and lean distribution between the fore, saddle and hind sections of the lamb carcass.

Whole carcass lean

Lean distribution between sections (F-

values)

Effect NDF,

DDF F-value

NDF,

DDF

Fore-

section

Saddle-

section

Hind-

section

PWWT

1,156 3.56

1,166 4.29* 5.4* 0.18

PWWT x site-year 8,156 2.3*

7,166 1.41 2.32* 1.45

PWWT x kill group(site-year) 16,156 2.14**

NA

NA NA NA

PFAT

1,156 96.6**

1,166 3.89* 7.57** 1.54

PEMD

1,156 38.02**

1,166 12.24** 8.37** 0.14

PEMD x sire type1 2,156 2.83 NA NA NA NA


NA: term not retained in the final model as was not significant for any section

PWWT: post weaning weight; PFAT: post weaning c-site fat depth; PEMD: post weaning eye muscle depth 1 This interaction was included in the model as it was significant in the analysis of carcass fat

* P<0.05, **P<0.01.

75


76

Table 3-8 Percentage change in weight per unit of Australia Sheep Breeding Values on

lamb carcass lean weight and lean distribution between the fore, saddle and hind

sections of the lamb carcass.

Carcass

lean

weight

Section lean weight

(Lean distribution between

sections)

Fore-

section

Saddle-

section

Hind-

section

Effect Level % Change

in weight

% Change

in weight

% Change

in weight

% Change

in weight

PWWT

0.08

-0.11 0.16 -0.02

PWWT* Kirby 2007 0.27

-0.14 0.08 0.10

site year Kirby 2008 0.06

-0.01 0.01 0.01

Rutherglen 2010 0.25

-0.12 0.07 0.06

Hamilton 2009 -0.12

-0.04 0.03 0.00

Struan 2010 0.00

-0.20 0.25 -0.01

Turretfield 2009 -0.07

-0.16 0.38 -0.15

Katanning 2007 0.06

NA NA NA

Katanning 2008 0.23

-0.23 0.44 -0.10

Katanning 2011 0.08

0.03 0.02 -0.04

PFAT

-1.86

0.40 -0.73 0.19

PEMD

0.99

-0.45 0.49 0.04

% Change in weight is per unit of the relevant Australian Sheep Breeding Value

NA: term was not retained in the final model as was not significant for any section (fore, saddle, hind)

PWWT: post weaning weight; PFAT: post weaning c-site fat depth; PEMD: post weaning eye muscle

depth

Significant effects are in bold (P<0.05)


77

Figure 3-1 The relationship between lamb sire estimates for percentage of lean and post-

weaning eye muscle depth (PEMD). Symbols represent sire estimates plus the least

squared means for (o) Maternal, (×) Merino (∆) Terminal sired lambs and are obtained

from the ASBV model in which PEMD was removed. The line represents least squared

means (± SE as dashed lines) for PEMD from the model containing the Australian

Sheep Breeding Values.

3.4.3 Allometric (b) coefficients

There were no significant interactions of the fixed effects (site-year, sex, sire type, dam

breed and kill group) with the b coefficient. Compared to the growth rate of the whole

carcass, lean tissue was relatively early maturing (P<0.01, Table 3-5), with an

allometric coefficient of 0.86 (Table 3-6). The rate of maturation of the lean tissue was

similar between all sections of the carcass (P<0.01, Table 3-5) with the allometric

coefficients of the fore, saddle and hind section being 1.0, 1.05 and 0.97 respectively

(Table 3-6).


78

3.5 Discussion

3.5.1 Allometric (b) coefficient

Relative to whole carcass weight lean tissue was early maturing, with an allometric

coefficient of 0.86 which is similar to that reported in previous studies (Butterfield et al.

1983a, Fourie et al. 1970). Alternatively, the growth coefficient of lean in the fore,

saddle and hind sections was close to 1 indicating that musculature across these regions

developed at a similar rate. This is contrary to earlier work that showed that individual

muscles within the carcass have different rates of development (Butterfield et al.

1984a). In particular the ‘spinal muscles’ were reported to be early maturing, with

significant growth in the early postnatal period, reaching a peak of their portion of lean

weight in the carcass at about 20% maturity, then slowly declining as the lamb

approaches maturity (Butterfield 1988). However, the lean growth coefficients from this

earlier work can be difficult to compare to ours as the points of dissection and types of

data analysis has varied. Given that the saddle region in our study describes more than

simply the loin musculature and at a time point of approximately 50% of mature size,

this may explain why our results do not match the results of this earlier work. Hence

while these results align with the well-known trend for lambs to reduce the proportional

lean composition of their carcass as they mature, we can also conclude that this effect

will occur at a relatively consistent rate within the fore, saddle and hind sections of the

carcass. As such slaughter end-points which try to optimise the weight of lean in the

saddle region relative to other carcass sections will be relatively in-effective.

3.5.2 Genetic influences on carcass lean tissue

3.5.2.1 Impact of sire type and dam breed

The wether progeny of Terminal sired lambs had the greatest proportion of lean when

compared to the wether progeny of Maternal and Merino sired lambs. Based on the

relative rate of maturation of lean tissue it is likely that the Terminal sired animals were


79

less mature when compared at the same weight or age, resulting in proportionately more

lean tissue in the carcass (Berg and Butterfield 1968). This is in contrast to

Ponnampalam et al. (2007b) who showed that Merinos had a greater lean % than Poll

Dorset sired lambs at the same age, however this observation was based on only 4 sires.

The other key difference between sire types was the distribution of lean tissue. At the

same lean weight the Terminal sired lambs had a greater proportion of lean tissue in the

saddle and hind sections compared to the Merino and Maternal sired animals. A similar

result has previously been suggested through assessment of indicator muscles such as

the loin and M. semitendinosus (Ponnampalam et al. 2007a), however in this study

lambs were compared at the same age, rather than the same lean weight. The reason for

altered lean distribution is unclear though may be associated with the emphasis on

selection for muscling in the Terminal sires. This selection has been on-going for

generations using both visual methods, and in recent years using a site specific

measurement of muscle depth in the saddle region which may account for the observed

redistribution.

The dam breed results align well with the sire type results with the lambs born to

Merino dams having more lean than those from the Border Leicester x Merino dams.

This was offset by Border Leicester x Merino dams producing lambs with a greater

portion of carcass fat (Anderson et al. 2015d) (Chapter 4) and intramuscular fat

(Hopkins et al. 2007c, Pannier et al. 2014c). This aligns well with this breeds improved

reproductive capacity, which is linked to whole carcass fatness (Ferguson et al. 2010).

Alternatively, this may be a reflection of their relative maturation rates, with Merinos

dams producing lambs that mature slower (Hopkins et al. 2007b).


80

3.5.2.2 PWWT ASBV

To partially support our hypothesis, increasing sire PWWT had no main effect on

carcass lean tissue when adjusted to the same carcass weight, however at some sites in

some years an increase in lean was observed. This contradicts previous findings from

smaller studies where PWWT had no effect (Gardner et al. 2010, Hegarty et al. 2006a,

Ponnampalam et al. 2007b), though it does support the general principle that lambs

selected for high post weaning weights would have a larger mature size (Huisman and

Brown 2008) and therefore they will be physiologically less mature when compared at

the same weight (Berg and Butterfield 1968, Butterfield et al. 1983a). Hegarty et al.

(2006c) demonstrated a reduced PWWT response when lambs were on depressed

nutrition, potentially explaining the lack of response at low nutrition sites. Therefore

the variation in response to increasing sire PWWT across different sites and years may

be due to variation in nutrition which has previously been documented for this

experiment (Ponnampalam et al. 2014).

An unexpected finding was that PWWT resulted in increased lean in the saddle, which

was largely offset by a reduction in fore section lean. This redistribution effect may be

explained by maturity as spinal musculature is relatively early maturing (Butterfield et

al. 1984a), yet contrary to this assertion the b coefficients in our analysis indicate that

lean tissue in all sections developed at a similar rate, suggesting that maturity is unlikely

to explain the redistribution of lean to the saddle. Our findings are in contrast to those of

Gardner et al. (2010) who showed that with increasing sire PWWT, the topside muscle

weight increased, with no impact of this breeding value on muscles of the saddle

section.


81

3.5.2.3 PEMD ASBV

In support of our initial hypothesis, more lean had been distributed to the saddle region

of the carcass. This appears to align with suggestions made in previous studies, where

selection for increased PEMD resulted in an increase in loin depth and weight (Gardner

et al. 2010, Hegarty et al. 2006c, Hopkins et al. 2007b), and little effect elsewhere in

the carcass. Alternatively, these previous experiments were unable to clearly

demonstrate redistribution of lean to the saddle as fore section lean was not recorded

and only a small number of indicator muscles (e.g. Round and top side) were collected

from the hind section. The redistribution of lean may be due to a change in muscle fibre

type, with a shift towards more type IIX muscle fibres within the saddle region leading

to an increased cross-sectional area and muscle hypertrophy (Greenwood, Gardner and

Hegarty 2006d). In support of this notion, there is evidence of a decrease in isocitrate

dehydrogenase activity in the M. longissimus lumborum associated with selection for

increased sire PEMD (Kelman et al. 2014b). Further experiments detailing oxidative

capacity on the fore and hind sections may elicit the cause of lean redistribution effects.

Contrary to our expectations, selecting for increasing sire PEMD increased whole

carcass lean, the first time that this effect has been reported across Merino and Maternal

sire types. This finding is at odds with a previous analysis of a much smaller subset of

the data used in this experiment (Gardner et al. 2010), however it aligns well with work

by Hopkins et al. (2007b) who reported an increase in lean % with an increase in sire

PEMD. None-the-less the magnitude of the effect in the Hopkins et al. (2007b) study

was markedly smaller than that reported here, and pertained only to a small number of

Poll Dorset sires (n=20). One explanation for this result could be associated with a

larger mature size, leading to proportionately more muscle when compared at the same

weight (Berg and Butterfield 1968, Butterfield et al. 1983a). However if maturity was


82

implicated then there should also have been proportionately more bone (Berg and

Butterfield 1968, Butterfield et al. 1984b, Fourie et al. 1970), an observation not

consistent with the animals in this study (Anderson et al. 2015d)(Chapter 4).

Furthermore, Huisman & Brown (Huisman and Brown 2008) have demonstrated no

phenotypic or genetic correlation between PEMD and mature weight. Therefore the

more muscular composition appears to be independent of maturity.

In contrast to the PWWT effects, the impact of PEMD on muscle did not vary between

sites, suggesting that this breeding value is less affected by nutritional variation. This

would support work by Hegarty et al. (2006c) which showed that the depth of loin

muscle from animals with high PEMD was similar under high and low nutritional

regimes. Of course under a “non-industry relevant” severe starvation scenario, it is

likely that the magnitude of the PEMD effect would eventually be affected. But clearly

the effects of this ASBV on composition appear to be more environmentally resilient

compared to the PWWT ASBV.

3.5.2.4 PFAT ASBV

Consistent with our hypothesis, selection for reduced PFAT increased lean weight

within the carcass. Other analyses of this data set have demonstrated that the increase in

lean is roughly offset by an equivalent decrease in fat (Anderson et al. 2015d)(Chapter

4). This change in body composition may be suggestive of low PFAT animals having a

larger mature size, however, previous research has demonstrated little genetic or

phenotypic correlation between sire PFAT and adult weight (Huisman and Brown 2008)

or growth rate (Kelman, Alston, W. et al. 2014a). Thus it seems likely that these

compositional differences will still be present when the animals reach their mature size.

These results are consistent with previous studies. Hopkins et al. (2007b) showed an

association between reducing PFAT and an increase in lean within the carcass, as


83

measured by dual energy x-ray absorbiometry. However this was only demonstrated in

the progeny of Poll Dorset sires, and the magnitude of the effect was smaller. Similarly,

Gardner et al. (2010) showed increased weights of specific muscles in the saddle (loin)

and hind section (round) in response to reducing PFAT, however in contrast to this

study there was no increase in whole carcass lean measured using CT.

The redistribution of lean to the saddle section from the fore section as sire PFAT

decreased was an unexpected and previously unreported finding. This may be

attributable to the PFAT measurement being taken from a single site in the saddle

section, placing more emphasis on reducing fat (offset by increasing lean) in this region

as suggested by Gardner et al. (2010). However, other analyses of this data set have

indicated that the impact of PFAT on fat reduction is consistent across all regions of the

carcass and is not focused on one site (Anderson et al. 2015d) (Chapter 4).

3.5.3 Production and management effects on carcass lean.

In support of our hypothesis, at any given carcass weight, Terminal sired wether

lambs had more lean than ewes. This aligns well with previous studies (Butterfield et al.

1985, Fourie et al. 1970) and partly reflects that rams and wethers grow to a larger mature

size than ewes (Thompson et al. 1985b), and at the same carcass weight will be leaner. A

similar result was observed by Ponnampalam et al. (2007b) where wethers had increased

lean % compared to ewes, even though these lambs were compared at the same age rather

than the same carcass weight. In terms of lean distribution, Terminal sired wether lambs

had more lean in the fore section compared to the ewes, a result that is similar to work

performed by Perry et al. (1988) in Merino rams and ewes. In the latter study, the rams

had increased neck and forelimb musculature, speculated to be consistent with the need

for males to enforce dominant behaviour and support a larger head and horns. This

reasoning may still be valid despite the males in the current study being castrated.


84

There was no difference in the rate of development of lean between wethers and ewes

(i.e. no sex interaction with the b coefficient), which is in contrast to other studies who

showed rams to have a higher impetus for muscle growth compared to ewes (Fourie et

al. 1970, Thompson 1983). McClelland et al. (1976) showed that at approximately 50 to

60% of maturity ram and female composition was similar which may account for the

lack of difference between sexes seen in our analysis. This result was emulated in

another analysis (Thompson et al. 1985a), although in this case a cross-over effect was

seen in tissue maturation rate between rams and ewes which may explain the variability

seen in some of the studies.

The birth-type and rear-type had no impact on carcass lean and little impact on the

distribution of carcass lean. The lambs born and raised as triplets had least hind section

lean which was significantly less that the single born and raised, the twin born and

raised or twin born single raised lambs. The reason for this difference is difficult to

explain with the current data.

Although this study was not designed to assess nutritional impacts on carcass

composition, the differences between sites, and to a lesser degree kill groups, are likely to

be reflective of nutritional variation. However the between-site variations in the amount

of carcass lean and the distribution of lean between the three sections (fore, saddle and

hind) may be due to differences in the dam genetics between sites. Alternative

explanations could be associated with variation in nutrition (Ponnampalam et al. 2014)

associated with ewe milk output and pasture conditions, or worm burdens (ewe and lamb

impacts). Within a site, kill group reflects to some extent lamb age. The increased

proportion of carcass lean in the earlier kill groups may be a reflection of lamb maturity


85

and the early maturation of lean (allometric coefficient < 1). A maturity explanation is

difficult to ascribe to the changing proportion of lean between kill groups, with all

sections tending to develop at the same rate (allometric coefficient = 1).

3.5.4 Comparison of effects

Compared to the non-genetic effects, the impact of the ASBVs on lean tissue is much

more profound. PFAT demonstrated the greatest impact on total carcass lean an effect

that was 20% greater in magnitude than PEMD. The impact of PWWT was lower than

both PFAT and PEMD and was heavily influenced by the different sites making its role

for improvements to lean percentage within the carcass more limited.

The results of this study provide a better understanding of the compositional changes

caused by the carcass breeding values, allowing more accurate assessment of their

impact on carcass value. The weighting of breeding values in industry indexes are

performed on the basis of their economic impact. In particular, this may be

underestimating the value of PEMD because of its substantial impact on the high value

loin musculature. Hence the data from this paper will enable improved precision for

arriving at more accurate economic weightings for the carcass.

The differences between sites and kill groups had the greatest impact of the non-genetic

effects on total carcass lean tissue and distribution. There was about 2.5 times the

variation in lean between the sites and kill groups when compared to the impact of sex

and birth-type rear-type. This demonstrates the substantial influence that nutrition and

other environmental factors can have on carcass composition. Some caution is required

in this interpretation given that we cannot separate the influence of dam genetics from

the observed site differences. Additionally, we cannot discount the fact that differences

between sites may in part be due to different CT scanners being used.


86

3.6 Conclusions

CT scanning of the 1665 lambs in this study has provided precise whole-body

quantification of the impact of decreasing sire PFAT and increasing sire PEMD on the

weight of lean in the carcass, which increased by 9.5% and 7.7% across the range of

these ASBVs when lambs were compared at the same carcass weight. Additionally,

these breeding values have been shown to redistribute lean from the fore section to the

saddle section, which will likely increase carcass value as the saddle region is more

highly valued. Similarly, PWWT ASBV increased the weight of lean in the carcass,

however this effect was not consistent across different sites/environments. PWWT also

redistributed lean to the more valued saddle region, without decreasing hind section

lean. These results indicate targeted selection for sires of decreased PFAT and

increased PEMD, have the ability to improve the value of the lamb carcass through the

manipulation of lean tissue. This experiment has for the first time allowed these

breeding value effects to be quantified at a whole body level, rather than just using cut

weights and carcass tissue depths to indicate these trends. The results of this experiment

will be used to determine the financial gains of using carcass ASBVs to improve carcass

composition in addition to the improvements in hot carcass weight that they provide

(Gardner et al. 2015).

Chapter 4 – The impact of breeding values on carcass composition (fat and bone)

87

Chapter 4. Sire carcass breeding values affect body

composition in lambs - 2. Effect on fat and bone weight

and its distribution within the carcass as measured by


The following chapter is the version submitted for publication:

Anderson, F., Williams, A., Pannier, L., Pethick, D.W., Gardner, G.E. (2015). Meat

Science.

4.1 Abstract

This study assessed the effect of paternal Australian Sheep Breeding Values for post

weaning c-site eye muscle depth (PEMD) and fat depth (PFAT), and post weaning

weight (PWWT) on the composition of lamb carcasses. Composition was measured

using computed tomography scans of 1665 lambs which were progeny of 85 Maternal,

115 Merino and 155 Terminal sires. Reducing sire PFAT decreased carcass fat weight

by 4.8% and increased carcass bone by 1.3% per unit of PFAT (range 5.1mm).

Increasing sire PEMD reduced carcass fat weight by 3.8% in Maternal and 2% in

Terminal sired lambs per unit of PEMD (range 7.8mm), with no impact on bone.

Increasing sire PWWT reduced carcass fat weight, but only at some experimental

locations. Differences in composition varied between sire types with Maternal sired

lambs having the most fat and Merino sired lambs the greatest bone weight. Genetic

effects on fatness were greater than the environmental or production factor effects, with

the converse true of bone.


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4.2 Introduction

A high proportion of saleable meat in the carcass is an important determinant of carcass

value, as it reduces the processing costs associated with fat and bone trim (Hopkins

1989, Jones, Simm and Young 2002) and meets consumer preferences for leaner cuts of

meat (Banks 2002, Pethick et al. 2005b). Producers can select for these carcass types

indirectly using three Australian Sheep Breeding Values (ASBVs): increased post

weaning weight (PWWT) and eye muscle depth (PEMD), and reduced post weaning fat

depth (PFAT). Previous studies have shown the impact of these ASBVs on carcass

fatness, but have relied on fat depths (Hopkins et al. 2007b), or the weight of specific

fat depots (Gardner et al. 2010) to “indicate” these effects. The use of indicator

measurements is inaccurate and importantly may be biased in circumstances where

tissue has been redistributed within the carcass, particularly due to genetic selection.

Previous studies have assessed whole carcass composition using technologies such as

dual energy absorbtiometry (Dunshea et al. 2007, Pearce et al. 2009, Ponnampalam et

al. 2007b) or computed tomography (CT) (Bunger et al. 2011, Gardner et al. 2010,

Young et al. 2001). This study has used CT technology to investigate the impact of

ASBVs on carcass weights of fat and bone and their distribution within the carcass.

Selecting sires for low PFAT breeding values has been shown to reduce fatness at the c-

site of measurement in their offspring (45 mm from the midline over the 12th rib) (Hall

et al. 2002, Hegarty et al. 2006a). Work in pigs has shown that selection for decreasing

back fat can alter the distribution of fat (Trezona-Murray 2008, Wood et al. 1983),

decreasing it at the site of measurement and partially redistributing it to other

subcutaneous fat depots. However, studies in sheep have demonstrated that reducing

sire PFAT has a more uniform impact across the carcass (Gardner et al. 2010, Hopkins

et al. 2007b, Kadim, Purchas, Rae et al. 1989). This reduced fatness is likely to be


89

offset by increased proportions of bone tissue, as demonstrated by Gardner et al. (2010)

who showed an increase in carcass hind limb bone weight and carcass bone %.

Increasing sire PEMD has been shown to produce lambs with reduced c-site fat (Hall et

al. 2002, Hegarty et al. 2006a) and less trimmable carcass fat (Hegarty et al. 2006a).

Yet contrary to these studies Gardner et al. (2010) found that the PEMD breeding value

reduced fat depths at the c-site but did not change the proportion of whole carcass fat.

The result of Gardner et al. (2010) is possibly more reliable than these earlier studies, as

the experiment contained 5-fold more sires and was measured using CT. The impact of

PEMD on bone is less well described, however, Gardner et al. (2010) also

demonstrated that PEMD had no impact on whole carcass bone %. Therefore the impact

of increasing sire PEMD is likely to reduce carcass fat, but only in the region around its

point of measurement, and have little impact on carcass bone.

In sheep, the PWWT breeding value exerts its influence on carcass composition through

its impact on mature size (Huisman and Brown 2008), resulting in lambs of faster

growth (Kelman pers comm). Given carcass fat is late maturing and bone early maturing

(Berg and Butterfield 1968, Butterfield et al. 1983a), sires of high PWWT should

produce lambs that are less mature at the same slaughter weight, containing less fat and

more bone. Work by Gardner et al. (2010) supported this notion showing that lambs

from high PWWT sires had decreased CT fat % and increased bone % indicating a

whole carcass effect on these tissues. There is no evidence of PWWT causing

redistribution of fat or bone tissue within the carcass.

Sire type has been shown to impact on the proportion of fat and bone. Lambs sired by

maternal breeds such as the Border Leicester have more carcass fat and less muscle


90

than Terminal sire breeds (e.g. Dorset Horns) when compared at the same carcass

weight (Fogarty, Hopkins and van der Ven 2000, Thompson et al. 1979c).

Ponnampalam et al. (2007b) demonstrated a similar effect comparing Border Leicester

sired lambs to Poll Dorset and Merinos. Sire type has also been shown to impact on

bone composition, with Merino sired lambs having heavier bone weights than Maternal

and Terminal sired lambs (Cake et al. 2007). Based on these studies it is expected that

lambs sired by Maternal breeds will have the greatest proportion of fat, while Merino

sires will produce lambs with an increased proportion of bone. The impact of sire type

on the distribution of these two tissues throughout the lamb carcass has not previously

been reported.

This paper describes the association of non-genetic and genetic factors on the carcass

composition of fat and bone in lambs as measured by CT, with the results of these

factors on lean described in Anderson et al.(2015c) (Chapter 3). We hypothesised that

selection for decreasing sire PFAT will result in decreased carcass fat across all carcass

regions, and increased carcass bone, with a preferential increase in hind section bone.

Increasing sire PEMD will have a site specific effect on fat, decreasing it in the saddle

region only, while having no impact on bone. Additionally, through its impact on

mature size, we hypothesised that increasing sire PWWT will decrease whole carcass

fat and increase carcass bone weight. Lastly, Maternal sired lambs are expected to have

greater carcass fat % and the Merino sired lambs an increase in bone % compared to

Terminal sired lambs.



Complete details of the experimental design, slaughter details are presented in Anderson

et al. (2015c)(Chapter 3) and more briefly in this chapter. The Australian Cooperative


91

Research Centre (CRC) for Sheep Industry Innovation established an Information

Nucleus Flock (INF) in 2007, with details of the design of the flock presented by

Fogarty, Banks, van de Werf, Ball and Gibson (2007). Within each year, at each of the

six research stations, a subset of lambs were chosen from 1-2 sites each year for CT

scanning of their carcasses following slaughter. The lambs were divided into groups

based on live weights, with each group killed separately (kill groups) at a target carcass

weight of 23 kg.

The sire types included Terminal sires (Hampshire Down, Ile De France, Poll Dorset,

Southdown, Suffolk, Texel, White Suffolk), Maternal sires (Bond, Booroola Leicester,

Border Leicester, Coopworth, Corriedale, Dohne Merino, East Friesian, Prime South

African Meat Merino (Prime SAAM), White Dorper), and Merino sires (Merino, Poll

Merino).Within each site, the aim of selection of lambs for CT was to include at least

two progeny from each sire used at the site, selected across a live weight strata. Lambs

within kill groups were on average within 5 days of age of each other and within a year

there was an attempt to represent all sire types in each kill group. Across the 9 site-year

combinations in this experiment there were a total of 25 kill groups, with the average

age within a slaughter groups ranging from 168 to 420 days of age and the number of

lambs within each kill group ranging from 20 to 99 lambs. For details of kill groups

refer to Chapter 3, Table 3-1. Lambs were grazed under extensive pasture conditions

and supplemented with grain, hay or pellets when pasture was limited which varied

between sites (Ponnampalam et al. 2014).


Carcasses were transported for CT scanning to either Murdoch University (Picker PQ

5000 spiral CT scanner) or the University of New England (Picker, Bavaria, Germany)

within 72 hours of slaughter to determine the proportions of fat, lean and bone. Prior to


92

scanning the carcasses were split into three primal components to enable more rapid

post scanning processing of the CT images for the distribution analysis: fore-section,

saddle and hind section. The fore section was separated from the saddle by a cut

between the fourth and fifth rib. The hind section was separated from the saddle by a cut

through the mid-length of the sixth lumbar vertebrae. The method used for

determination of muscle, fat and bone was similar to that described by Gardner et al.

(2010) with the discrimination between fat, lean and bone adapted from work by Alston

et al. (2005). Precision for predicting dissectable fat (R2=0.718; RMSE=0.713), and

bone (R2=0.789; RMSE=0.439) has previously been reported by Gardner, Pearce and

Smith (2007). A more detailed description of the CT scanning protocol and image

analysis are presented in Anderson et al. (2015c)(Chapter 3.2.3).

4.3.3 Data used

CT scanning data from a total of 1665 animals from the 9 site-year combinations was

available for analysis of fat and bone composition within the carcass (Chapter 3, Table

3-1). The 111 animals from Katanning in 2007 were not scanned in sections, therefore

when analysing distribution of lean between carcass sections, 1554 lambs were included

in the analysis. The mean weight (and range) of the lamb carcasses in this experiment

was 23.3kg (13.3-40.0 kg), with weights (and range) of fat, lean and bone, 6.3kg (2.1-

15.3), 13.3kg (7.4-20.8), and 3.8kg (2.4-5.7). The full description of carcass weights of

fat lean and bone across the three sections are reported in Anderson et al.

(2015c)(Chapter 3).

Of the 85 Maternal, 119 Merino and 155 Terminal sires, 70, 109 and 154 had ASBV

values for PWWT, PEMD, and PFAT available, with the mean and range of these sire

breeding values shown in Anderson et al. (2015c) (Chapter 3, Table 3-3). The breeding

values for PEMD and PFAT are based upon live ultrasound measurements at the c-site,


93

located at the 12th rib, 45mm from the midline, and PWWT is based upon live weight,

all measured at the post weaning time point (about 240 days of age). The ASBV values

were all sourced from Sheep Genetics, Australia’s national sheep genetic evaluation

database (Brown et al. 2007). The sire breeding values were generated within 3 separate

data-bases for Terminal, Maternal, and Merino sired progeny and were from an analysis

completed in April 2013.


4.3.4.1 Data transformation

An allometric approach allows the relative rates of development of carcass tissue types

to be determined. All data are converted to natural logarithms in order to utilise

Huxley’s allometric equation, y = axb in its loge linearised form (Huxley and Teissier

1936). By transforming all of the values to natural logarithms, (loge y = loge a + b.loge x

) the data are linearised and solved by least squares regression. A significant advantage

of using the loge form of the equation is that it homogenises the variance over the entire

range of sample data. It also allows for the direct comparison of the differences in loge

y values as percent differences (Cole 2000) and it is on this basis that the data in this

paper has been interpreted.

In the above allometric equation x is the independent variable, a is the proportionality

coefficient and b the growth coefficient of y relative to x. The value of the b coefficient

describes the relative growth rate of the component (y) to either the whole carcass

weight or the weight of fat (or bone) and will be either: early maturing (b<1), late

maturing (b>1) or maturing at the same rate as that of x (b=1). In this paper, the b

coefficient describes the rate of development of either fat (or bone) in the whole carcass

or the development of fat (or bone) within each section of the carcass (fore, saddle and

hind) relative to the whole carcass weight of fat (or bone). In both instances the b term


94

was examined with relevant first order interaction with the core terms of sire type, sex


within kill group.

A similar approach to the analysis of lean tissue in the same set of lambs was taken

(Anderson et al. 2015c)(Chapter 3) so that the model could be constrained, meaning in

the analysis of whole carcass composition, reported increases or decreases in fat, lean

and bone are offset by each other.

4.3.4.2 Linear mixed effects models

The loge transformed CT data were analysed using linear mixed effects models in SAS

(SAS version 9.0, SAS Institute, Cary, NC, USA). When whole body composition was

assessed, the loge weight of carcass fat or bone was used as the dependent variable, and

loge carcass weight was the covariate. A similar analysis was performed for lean tissue

(Anderson et al. 2015c) (Chapter 3) with the models constrained such that they

maintained the exact same form across all tissue types, enabling the addition of

differences in fat, bone and lean to approximately off-set each other and thus the

addition of the tissues can still approximate carcass weight. To assess the distribution of

fat or bone between the fore, saddle, and hind sections, the loge weight of fat (or bone)

within each of the 3 carcass regions was used as the dependent variable, and the loge

total fat (or bone) weight within the carcass was used as the covariate. Within a tissue

type (fat or bone) these models were constrained such that they maintained the exact

same form across all three carcass sections (fore, saddle and hind), enabling the addition

of differences in section fat (or bone) to approximately off-set each other and thus the

addition across the 3 carcass regions can still approximate total fat (or bone) weight.

The results for carcass composition and distribution of lean is reported in Anderson et

al. (2015c)(Chapter 3).


95

For all the models described above, we tested a standard set of fixed effects, and

random terms which denotes the base model. Fixed effects included site-year (combined

effect of site and year of lamb birth: Katanning (2007, 2008, 2011), Kirby (2007, 2008),

Hamilton 2009, Turretfield 2009 and Struan 2010), birth type and rearing type

(combined effect of animals born as single, twin or triplet and reared as single, twin or

triplet), sire type (Maternal, Merino and Terminal), sex within sire type (wether Merino,

wether Maternal, female Terminal, wether Terminal), dam breed within sire type

(Merino x Merino, Maternal x Merino, Terminal x Merino, Terminal x Border Leicester

x Merino (BLM)) and kill group within site-year. The random terms included sire, and

dam identification by lamb birth year. All appropriate first and second order interactions

between fixed effects and covariates were tested. Non-significant (P > 0.05) terms were

removed in a step-wise manner to derive a base model. Of the total number of carcasses

undergoing CT scanning, 1665 had entries for sex, sire type, birth-type rear-type, dam

breed, and kill group and were included in the base model (Chapter 3, Table 3-4).

The base models described above were also tested with sire ASBVs for PWWT, PEMD

and PFAT in the model. Initially, all 3 ASBVs were included simultaneously as

covariates in the model, as well as their first order interactions with the fixed effects.

This included the interaction between ASBVs and sire type, which was particularly

relevant given that the breeding values were derived from 3 separate data-bases for

Terminal, Maternal, and Merino sired progeny, and thus their magnitudes may not be

directly comparable. Non-significant (P > 0.05) terms were removed in a stepwise

manner. Due to the correlations that exist between these breeding values in this data set

(PWWT vs PEMD = 0.3; PWWT vs PFAT = 0.3; PEMD vs PFAT = 0.1) this process


96

was repeated with the breeding values included one at a time to test the independence of

their effects.

4.4 Results

4.4.1 Impact of production and management on CT fat and bone %

4.4.1.1 Impact on whole carcass

There was considerable variation between site-year combinations in the weight of fat

and bone within the carcass (P < 0.01, Table 4-1). The weight of carcass fat varied by as

much as 15% (Table 4-2) and the weight of carcass bone varied by as much as 19.3%

(Table 4-3).

97

Table 4-1 F-values and numerator and denominator degrees of freedom of factors affecting fat and bone weight of lamb in the whole carcass and the

distribution of fat and bone between the fore, saddle and hind sections of the lamb carcass.

Whole carcass analysis

Fat distribution between sections

Bone distribution between sections

F-values

F-values

F-values

Effect NDF,

DDF Carcass fat Carcass bone

NDF,

DDF

Fore-

section

Saddle-

section Hind-section

NDF,

DDF

Fore-

section

Saddle-

section

Hind-

section

site year 8,177 27.35** 60.79**

7,170 25.19** 94.75** 100.38**

7,169 31.42** 8.96** 43.89**

sex(sire type) 1,177 124.61** 36.89**

1,170 23.91** 55.93** 17.5**

1,169 8.1** 15.08** 2.58

birth-type rear-type NA NA NA

5,170 2.04 5.04** 2.46*

5,169 0.78 2.51* 7.46**

Sire type 2,177 13.58** 13.63**

2,170 10.41** 18.77** 2.71

2,169 11.52** 2.57 3.58*

Kill group (site year) 16,177 7.51** 13.19**

15,170 9.7** 13.78** 8.6**

15,169 7.75** 4.01** 7.68**

Dam breed (sire type) 1,177 11.22** 1.02

NA NA NA NA

1,169 0.71 1.98 6.66*

Site year x sire type 13,177 2.78** 1.27

12,170 1.58 4.23** 4.23**

NA NA NA NA

Site year x Dam breed

(sire type) 5,177 4.48** 1.18

NA NA NA NA

NA NA NA NA

log x 1 1,177 3638.16** 1855.03** 1,170 8357.13** 25459.4** 7738.26** 1,169 7218.74** 3335.74** 4751.55**


NA: term not retained in the final model as was not significant for any component (fat or bone) or section (fore, saddle, hind) 1 In the whole carcass analysis the covariate (x) was carcass weight (kg), however in the distribution analysis the covariate (x) was either whole carcass fat or whole

carcass bone depending on the type of analysis.

* P < 0.05, **P < 0.01


98

Table 4-2 The relative change (% change in weight) for site-year, sex dam breed(sire

type) and birth-type rear-type’s impact on lamb carcass fat weight and the distribution

of fat between the fore, saddle and hind sections of the lamb carcass.

Whole carcass

fat weight Fat weight in each section

(distribution between sections)

Fore-

section

Saddle-

section

Hind-

section

Effect Level % Change in

weight

% Change

in weight

% Change

in weight

% Change

in weight

Site year1

Kirby 2007 0.81a

-0.38

de 5.47

c -9.31

bc

Kirby 2008 12.64

c

-7.48

b 11.80

e -14.76

a


c

-8.08

ab 1.19

ab 8.47

e

Hamilton 2009 5.71

b

-9.82

a 13.0

e -13.32

ab

Struan 2010 14.98

c

-0.62

de 0.63

ab 0.06

d

Turretfield 2009 7.64

b

-1.93

d 1.39

b 0.33

d

Katanning 2007 6.68

b

NA

NA NA

Katanning 2008 8.56

b

-5.34

c 8.19

d -8.68

c

Katanning 2011 0.00

a

0.00

e 0.00

a 0.00

d

Sex-

Dambreed Maternal x Merino M

9.55l

1.97

k -0.69

j -1.62

ijk

(sire type)2 Merino x Merino M 3.23

jk

2.22

k -1.89

i -0.44

kl

Terminal x BLM F

9.88

l

-1.80

i 2.49

k -2.81

i

Terminal x Merino F 5.81

k

-2.33

i 2.75

k -2.32

ij

Terminal x BLM M 1.86

ij

0.34

j 0.03

j -1.05

jkl

Terminal x Merino M 0.00

i

0.00

j 0.00

j 0.00

l

Birth-type Born and raised as single NA

-0.34 -1.35

x 2.72

x

rear-type3

Born twin, raised single NA

-0.26 -0.92

xy 1.89

x

Born and raised as twin NA

-1.48 0.12

z 1.32

wx

Born triplet, raised single NA

0.96 -3.09

w 4.23

y

Born triplet, raised twin NA

-2.11 0.25

z 1.47

x

Born and raised as triplet NA

0.00 0.00

yz 0.00

w

b coefficient (±SE)

log x

4 1.48

±0.025

0.85

±0.010

1.15

±0.007

0.92

±0.011

% change in weight: the difference in loge y values for each fixed effect compared to the fixed

effect with coefficient 0.00, expressed as a percentage.

M = wether; F = ewe; BLM: Border Leicester x Merino

Significant effects are in bold (P<0.05) 1

a–e Within columns for site-year, % change in weight values without a common superscript

differ significantly at P < 0.05. 2 i-l

Within columns for sex-dambreed (sire type), % change in weight values without a common


w-z

Within columns for birth-type rear-type, % change in weight values without a common

superscript differ significantly at P < 0.05. 4 Where carcass fat was analysed, carcass weight was the covariate (x) and when fat distribution

was analysed whole carcass fat was the covariate (x)


99

Table 4-3 The relative change (% change in weight) for site-year, sex dam breed(sire

type) and birth-type rear-type’s impact on carcass bone weight of lamb and the

distribution of bone between the fore, saddle and hind sections of the lamb carcass.

Whole

carcass bone

weight

Bone weight in each section


Fore-

section

Saddle-

section

Hind-

section


weight

% Change

in weight

% Change in

weight

% Change in

weight

Site year1

Kirby 2007 -6.34de

6.30

d 0.40

abc -6.58

a

Kirby 2008 -10.93

bc

1.66

bc 4.69

e -4.31

bc


g

1.75

bc 3.25

de -3.99

bcd

Hamilton 2009 -8.16

cd

0.88

ab 3.47

de -2.58

d

Struan 2010 -15.68

a

2.73

c 3.07

cde -4.80

b


bc

0.11

a -1.02

a 0.89

e

Katanning 2007 -4.73

e

NA NA NA


b

2.17

bc 2.02

bcd -3.20

cd

Katanning 2011 0.00

f

0.00

a 0.00

ab 0.00

e

Sex-

Dambreed

Maternal x Merino M

0.19k

0.95

l -0.24

ij -0.90

i

(sire type)2

Merino x Merino M 4.10l

1.45

l -1.41

i -0.65

i

Terminal x BLM

7 F

-3.19

i

-1.24

i 1.30

jk 0.43

j

Terminal x Merino F -1.73

j

-0.95

ij 2.45

k -0.65

i

Terminal x BLM M 0.47

k

-0.33

jk -0.70

ij 0.79

j

Terminal x Merino M 0.00

k

0.00

k 0.00

ij 0.00

ij

Birth-type

Born and raised as single NA

-1.69 -4.69

w 4.87

y

rear-type3

Born twin, raised single NA

-1.69 -3.86

wx 4.23

xy

Born and raised as twin NA

-1.39 -3.09

wx 3.46

x

Born triplet, raised single NA

-2.14 -3.13

wx 3.87

xy

Born triplet, raised twin NA

-2.03 -2.81

x 3.97

xy

Born and raised as triplet NA

0.00 0.00

y 0.00

w

b coefficient (±SE)

log x

4 0.729

±0.017 1.030

±0.012 1.235 ±0.021

0.816

±0.012

% change in weight: the difference in loge y values for each fixed effect compared to the fixed

effect with coefficient 0.00 expressed as a percentage.

M = wether; F = ewe ; BLM: Border Leicester x Merino

Significant effects are in bold (P<0.05) 1

a–e Within columns for site year, % change in weight values without a common superscript

differ significantly at P < 0.05. 2

i-l Within columns for sexdambreed(sire type), % change in weight values without a common


w-y

Within columns for birth-type rear-type, % change in weight values without a common


Where carcass bone weight was analysed, carcass weight was the covariate (x) and when bone

distribution was analysed whole carcass bone was the covariate (x)


100

Within each site-year there was variation in the fat and bone percentage of the carcass

between kill groups (P < 0.01, Table 4-1). The largest variation in carcass fat weight

was at Kirby 2008 and Katanning 2011, which differed between kill groups by as much

as 12.9% and 12.4% (results of individual kill groups not shown). The younger kill

groups generally had less CT fat % than the older kill groups, except for Katanning in

2008, where the first kill group had the most CT fat%. The largest variation in CT bone

% between kill groups was observed at Katanning in 2008, which differed between kill

groups by as much as 17.8% in the proportion of carcass bone (results of individual kill

groups not shown). In contrast to the impact of kill group on fat, there was no consistent

effect of kill group on CT bone %.

A comparison between sexes was only possible within the Terminal sired lambs where

there were marked differences in the amount of fat and bone within the carcass.

(P<0.01, Table 4-1). On average the ewe lambs had 6.9% more fat (Table 4-2) and

2.8% less bone (Table 4-3) in the carcass than the wether lambs.

4.4.1.2 Impact on fat and bone distribution

The distribution of fat and bone between sections of the carcass varied between sites

(P<0.05, Table 4-1), with Hamilton 2009 showing the greatest variation in carcass fat

weight between sections, with the least fore and one of the least hind section fat but the

most saddle fat. Kirby in 2007 had the most variation in the distribution of bone

between sections, with the most fore-section bone and least hind-section bone (Table

4-3).

The distribution of fat and bone varied between the sexes (P<0.05, Table 4-1).

Carcasses of ewe lambs had less fat and bone (2.2% and 1%) in the fore section, less


101

hind section fat (2.1%), and more fat and bone (2.7% and 2.3%) in the saddle section

when compared to the wether lambs (Table 4-2 and Table 4-3).

The birth-type rear-type did not impact on the proportion of fat or bone in the carcass

but did impact on tissue distribution in the saddle and hind-sections (P<0.05, Table 4-1).

The lambs born and raised as triplets had the most saddle bone and the least hind-

section bone, (Table 4-3) when compared to the other birthing and rear type

combinations. Lambs that were born as triplets and reared as singles had the least saddle

fat and the most hind section fat (Table 4-2).

4.4.2 Impact of genetics on carcass fat and bone

4.4.2.1 Impact of sire, sire type and dam breed

4.4.2.1.1 Impact on whole carcass fat and bone

When evaluating whole carcass fat and bone in the base model there were marked

differences between sires (P<0.01). For fat, 95% of the sire estimates for all three sire

types ranged between ± 8.6% and for bone between ± 5.6% at any given carcass weight.

Sire type comparisons could be made in the wether lambs born to Merino dams. There

were significant differences between sire types for whole carcass fat and bone (P < 0.01,

Table 4-1), with Terminal sired lambs having the least carcass fat. The Maternal and

Merino sired lambs had 9.6% and 6.3% respectively more carcass fat than the Terminal

sired lambs (Table 4-2). Alternatively, the Merino sired lambs had the most bone, with

3.9 and 4.1% more than the Maternal and Terminal sired lambs (Table 4-3).

The impact of dam breed could be assessed in the Terminal sired lambs where BLM

dams produced progeny with more carcass fat than those of the Merino dams (P<0.01,

Table 4-1). This effect was strongest in ewe lambs where progeny from BLM dams


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were 4.1% fatter than lambs from Merino dams (Table 4-2). In contrast this difference

was only 1.9% in wether lambs (Table 4-3).

4.4.2.1.2 Impact on distribution of carcass fat and bone

The distribution of these tissues between carcass sections differed between sires

(P<0.05). The sire estimates for fat within a section varied, with 95% lying between

±3.2%, ±1.8% and ±3.7% for the fore, saddle and hind sections at any given weight of

whole-carcass fat. For bone, 95% of the sire estimates from the saddle and hind lay

between ±2.8% and ±3.7% at any given weight of whole-carcass bone.

There were differences in the distribution of fat and bone between carcass sections (P <

0.01, Table 4-1). The Terminal sired lambs had the least fore section fat, with the

carcasses of lambs sired by Maternal and Merino breeds 2.0% and 2.2% fatter than the

carcasses from Terminal sires (Table 4-2). Additionally, the carcasses of lambs born to

Terminal sires had similar levels of saddle fat to the Maternal sired lambs, although

lambs from Merino sires had 1.9% less saddle fat than the Terminal sired lambs (Table

4-2). In the hind section, the Maternal sired lambs had the most carcass fat, and the

Terminal sired lambs the least (Table 4-2). The carcasses from Terminal sired lambs

had less fore-section bone, with the Merino and Maternal sired lambs having 1.0% and

1.5% more fore section bone (Table 4-3). There was no difference in carcass bone

distribution in the saddle and hind sections between sire types.

There was only a small impact of dam breed on bone distribution with only the hind

section of ewe lambs impacted (P<0.05, Table 4-1). The ewe lambs born to BLM dams

had 1.1% more hind section bone than those from Merino dams (Table 4-3).


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4.4.2.2 Impact of Australian Sheep Breeding Values on carcass fat and

bone

4.4.2.2.1 PFAT ASBV

Decreasing sire PFAT had a marked impact on whole carcass fat and bone weight

(P<0.01, Table 4-4). Carcass fat was reduced by 24.7% across the 5.1 mm PFAT range

(Table 4-5). By contrast carcass bone increased by 6.5% across this same PFAT range

(Table 4-6). Sire PFAT did not impact on the distribution of fat or bone within the

carcass.

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Table 4-4. . F-values and numerator and denominator degrees of freedom in lambs for Australian Sheep Breeding Values affecting whole carcass lamb fat and bone

weight and distribution of fat and bone between the fore, saddle and hind sections of the lamb carcass.

Whole carcass analysis Fat distribution between sections Bone distribution between sections

NDF,

DDF

F-values NDF,

DDF

F-values NDF,

DDF

F-values

Effect Carcass fat Carcass bone

Fore-section Saddle- section Hind-section

Fore-section Saddle- section Hind-section

PWWT 1,156 2.55 0 1,166 0.06 0.95 0.96

1,166 3.06 0.03 4.13*

PWWT x site year 8,156 3.14** 1.24 7,166 1.15 2.2* 2.72*

7,166 0.34 2.35* 3.25**

PWWT x kill group (site year) 16,156 2.16** 1.78* NA NA NA NA

NA NA NA NA

PFAT 1,156 91.81** 11.09** NA NA NA NA

NA NA NA NA

PEMD 1,156 24.58** 1.5 NA NA NA NA

NA NA NA NA

PEMD x sire type 2,156 4.54* 0.4 NA NA NA NA

NA NA NA NA


PWWT: post weaning weight; PFAT: post weaning c-site fat depth; PEMD: post weaning eye muscle depth NA: term not retained in the final model as was not significant for any section

* P<0. 05, **P<0.01


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Table 4-5 Percentage change in weight per unit of Australian Sheep Breeding Value on lamb

carcass fat weight of lamb and fat distribution between the fore, saddle and hind sections of the

lamb carcass.

Whole carcass

fat weight Fat weight in each section


Fore-

section

Saddle-

section

Hind-

section


weight

% Change

in weight

% Change

in weight

% Change

in weight

PWWT

-0.19

-0.02 0.06 -0.09

PWWT x Kirby 2007 -0.85

-0.22 0.00 0.33

site year Kirby 2008 -0.07

0.05 -0.17 0.18

Rutherglen 2010 -0.54

0.16 0.29 -0.68

Hamilton 2009 0.30

0.08 -0.06 -0.21

Struan 2010 -0.05

-0.23 0.12 0.05

Turretfield 2009 0.40

-0.15 0.26 -0.23


NA NA NA


0.21 -0.02 -0.20


-0.08 0.10 0.01

PFAT

4.84

NA NA NA

PEMD

-2.13

NA NA NA

PEMD x Maternal -3.80

NA NA NA

sire type Merino -0.63

NA NA NA

Terminal -1.96

NA NA NA

% Change in weight is per unit of the relevant Australian Sheep Breeding Value


depth


Significant effects are in bold (P<0.05).


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Table 4-6 Percentage change per unit of Australia Sheep Breeding Value on lamb carcass bone

weight and bone distribution between the fore, saddle and hind sections of the lamb carcass.

Whole carcass

bone weight Bone weight in each section


Fore-section Saddle-

section

Hind-

section


weight

% Change

in weight

% Change

in weight

% Change

in weight

PWWT

0.01

-0.10 -0.02 0.12

PWWTx Kirby 2007 0.20

-0.07 -0.40 0.34

site year Kirby 2008 -0.03

-0.06 0.00 0.06


-0.07 -0.38 0.35

Hamilton 2009 0.03

-0.13 0.23 -0.04

Struan 2010 -0.19

-0.11 -0.23 0.23


-0.04 0.38 -0.20

Katanning 2007 0.10

NA NA NA


-0.26 0.24 0.17

Katanning 2011 0.21

-0.06 0.01 0.06

PFAT

-1.27

NA NA NA

PEMD1

-0.39

NA NA NA

% Change in weight: is per unit of the relevant Australian Sheep Breeding Value


depth 1

This term was included in the model as it was significant in the analysis of carcass fat and lean

(Anderson et al. 2015c)(Chapter 3)


Significant effects are in bold (P<0.05).


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4.4.2.2.2 PEMD ASBV

Increasing sire PEMD resulted in a decrease in carcass fat (P<0.01, Table 4-4), but only

in Maternal and Terminal sired lambs and not in Merino lambs. In Maternal sired lambs

carcass fat decreased by 16.3% and in Terminal sired lambs by 15.4% across their

respective ranges of sire PEMD (4.3 and 7.8mm) (Table 4-5). On a per unit PEMD

basis this difference was even more marked, with carcass fat decreasing by 3.8% in

Maternal and 2% in Terminal sired lambs per unit of sire PEMD. The PEMD ASBV did

not impact on whole carcass bone or the distribution of either fat or bone within the

carcass.

4.4.2.2.3 PWWT ASBV

The PWWT ASBV influenced total carcass fat, although the direction and magnitude of

the changes varied between sites and years (P<0.05, Table 4-4), with no main effect.

For those effects that were significant, the greatest decrease in carcass fat was 20.5%

(Table 4-5) and the greatest increase was 9.6% (Table 4-5) across the 24.1 kg PWWT

range.

Sire PWWT ASBV also had an impact on carcass bone weight but this varied between

kill groups at some sites in some years such that on average there was no overall effect

(P<0.05, Table 4-4). The largest variation in bone weight was 13%, with individual kill

group data not shown.

The only breeding value to influence tissue distribution of fat and bone was PWWT

(P<0.05, Table 4-4). When compared at the same weight of carcass fat, an increasing

sire PWWT had at times opposing effects (Table 4-5). At many sites, alterations to


108

proportions of fat in the saddle region were offset by the opposite effect in the hind

section (Table 4-5).

Increasing sire PWWT on average increased hind section bone but this varied between

sites (P<0.05, Table 4-4). The greatest increase in hind section bone across the 24.1 unit

PWWT range was 8.4% (Table 4-6), with this increase in hind section bone largely

offset by a decrease in saddle bone weight.

In both the whole body and tissue distribution analysis, when the three breeding values

were included in the models one at a time, as opposed to all three simultaneously, there

was no change to the effects described above.

4.4.3 Allometric (b) coefficients

There were no significant interactions between fixed effects and the b coefficient.

Compared to the growth rate of the whole carcass, carcass fat was relatively late

maturing and bone early maturing (P<0.01, Table 4-1), with allometric coefficients 1.48

for fat (Table 4-2), and 0.73 for bone (Table 4-3). The rate of maturation of the fat tissue

differed between sections with the fat in the fore section developing earliest, followed

by the hind and saddle sections with allometric coefficients of 0.92 (hind), 0.85 (fore),

and 1.15 (saddle) (Table 4-2). Similarly the rate of bone development differed between

sections, with bone in the hind section developing earliest, followed by the fore and

saddle sections with allometric coefficients of 0.82, 1.03 and 1.23 (Table 4-3).


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4.5 Discussion

4.5.1 Genetic influences on carcass fat and bone

4.5.1.1 PFAT ASBV

In support of our hypothesis, selection for decreasing sire PFAT reduced the weight of

fat uniformly across the three carcass sections for all three sire types. This indicates that

the use of this breeding value, which is based on fat depth measurement at the c-site,

effectively decreases whole carcass fat and that its impact is not solely localised around

its point of measurement – an effect previously identified in pigs (Trezona-Murray

2008, Wood et al. 1983). This associated reduction in CT fat % by decreasing sire

PFAT had previously been reported by Gardner et al. (2010), using a subset of lambs

from this study (those slaughtered in 2007), and was further reflected in the point

measurements of fat depth all located within the saddle region (C5 fat, GR and c-site).

Yet in contrast to these early results, the magnitude of the whole carcass fat effect was

far greater in the current study. Hopkins, Stanley, Martin, Ponnampalam, et al. (2007b)

reported reduced point measures of fat at the c-site, rump and GR, and decreased fat %

as measured by dual energy x-ray absorptiometry, however utilised only Terminal sired

lambs over a limited range of PFAT values.

In partial support of our hypothesis, a reduction in sire PFAT increased bone weight for

all three sire types, however the lack of an effect on bone distribution suggests that this

increase in bone was not proportionally greater in the hind-section as we were

expecting. This redistribution hypothesis was based on an earlier study by Gardner et al.

(2010) in which a small but significant increase in measured hind limb bone weights of

lambs of similar weights and ages was observed.


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The effect of PFAT on fat and bone weights may be due to an increased mature size,

which would also account for the increase in lean weight reported in Anderson et al.

(2015c)(Chapter 3). However analysis of the growth of these lambs indicated that PFAT

only has a small effect on lamb weight (Kelman, pers comm 2015) and has little genetic

or phenotypic correlation with adult weight in Merino lambs (Huisman and Brown

2008). Therefore the alternative explanation is that PFAT is influencing mature

composition. To confirm this theory a longitudinal study of the growth and carcass

composition of lambs from birth through to maturity would be required.

4.5.1.2 PEMD ASBV

Increasing sire PEMD was hypothesised to decrease carcass fat in the saddle only,

however its effect was more far-reaching, uniformly reducing fat in the fore, saddle, and

hind sections. This effect was greatest in the Terminal sired lambs, with a smaller effect

in the Maternal sired lambs, and no effect in Merino lambs. Although erroneous, the

original hypothesis was based on previous work by Gardner et al. (2010) who suggested

a site specific effect of this breeding value after observing a decrease in fat depth at the

c-site, but no change in total weight of short-loin fat or whole carcass fat. There is some

basis for the reduction in whole carcass fatness, with previous work by Martin,

McGilchrist, Thompson, and Gardner (2011) demonstrating that animals with high

PEMD were more responsive to adrenalin within their adipose tissue, and less

responsive within muscle. This catabolic mechanism may explain the reduced adiposity

observed at a whole carcass level. Alternatively, the lack of impact of PEMD on the fat

of Merino sired lambs is difficult to explain.

In support of our hypothesis, sire PEMD did not impact on carcass weight of bone or

bone distribution between sections. This is in contrast, Cake, Boyce, Gardner, Hopkins

and Pethick (2007), who demonstrated that increasing sire PEMD caused a decrease in


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the length of most limb bones and a decrease in carcass length. Increasing sire PEMD

has been shown to increase whole carcass lean and distributed this lean to the saddle

region (Anderson et al. 2015c) (Chapter 3). The current study shows no increase in

bone weight as a result of increasing sire PEMD, which indicates the mechanism for the

increase in muscle appears to be due to muscle hypertrophy rather than an impact on

mature size, lamb maturity or bone weight.

4.5.1.3 PWWT ASBV

Consistent with our hypothesis, increasing sire PWWT decreased whole carcass fat %,

however this effect was small and not consistent across all experimental sites and years.

Lambs of high PWWT have been shown to have a larger mature size (Huisman and

Brown 2008), and therefore when slaughtered at the same age these lambs would be

leaner as they are at an earlier stage of maturity. Furthermore, several studies have

demonstrated that the genetic potential for growth with increasing sire PWWT is limited

when lambs are on low nutrition (Gardner, Pethick, Hopkins et al. 2006, Hegarty et al.

2006a, Hegarty et al. 2006c), which may account for the variation in carcass

composition observed between sites within this study.

Contrary to our hypothesis, increasing sire PWWT had no significant effect on the

percentage of bone in the carcass. There was a small effect of PWWT at some sites, in

some years but this was highly variable and only evident within certain kill groups. Our

result is in contrast to previous work which has shown that an increase in weaning

weight (Thompson et al. 1985a) and post weaning weight (Gardner et al. 2010) resulted

in a proportionately heavier skeleton.

When compared at the same bone weight, lambs of high PWWT sires having increased

proportions of hind section bone. However in contrast, little change in bone distribution


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was observed between high and low growth Merino lambs (Butterfield and Thompson

1983, Perry et al. 1992b). If PWWT increases mature size (Huisman and Brown 2008),

it may account for the increased hind limb bone as this region of bone is early maturing

(b coefficient <1). The impact of PWWT on the other carcass sections was small, which

may explain the lack of impact on whole carcass bone %.

4.5.1.4 The impact of sire type and dam breed on carcass fat and bone

In support of our hypothesis, Maternal sired lambs contained the most carcass fat when

compared to the Merino and Terminal sired lambs at the same carcass weight (wether

lambs born to Merino dams). The dam breed was also consistent with this sire type

effect, with the BLM dams producing lambs with increased fat compared to Merino

dams. These results are similar to previous studies in which lambs from Border

Leicester sires produced lambs with increased point measures of fat (Atkins and

Thompson 1979, Fogarty et al. 2000) or whole carcass fat (Ponnampalam et al. 2007b)

compared to the lambs of other sires, and aligns well with this breeds improved

reproductive capacity (Ferguson et al. 2010). The Terminal sired lambs had the least

carcass fat and the highest portion of lean (Anderson et al. 2015c) (Chapter 3). This

lean/fat profile is consistent with the selection focus for fast-lean-growth within these

breeds, which is likely to impact on mature size. Therefore at the same carcass weight,

the Terminal sired lambs would be less mature and therefore have less carcass fat.

In support of our hypothesis, at any given carcass weight the Merino sired lambs had a

greater proportion of bone than the Maternal and Terminal sired lambs, aligning well

with work by Cake et al. (2007). This is unlikely to be associated with mature size as

Merino’s are smaller at maturity than Maternal or Terminal sired lambs (Hopkins et al.

2007a), hence the differences are more likely to reflect variation in their composition at

maturity.


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The Terminal sired lambs had the least fore section bone compared to the Merino and

Maternal sired lambs. These lambs also had the most fore section lean (Anderson et al.

2015c) (Chapter 3), which supports the notion that lambs selected for muscling, such as

the Terminal sired lambs, undergo bone hypotrophy (Cake et al. 2007). However, if this

were true then we would also expect lambs from sires with high PEMD to have less

bone which was not evident in the current study. The decreased fore section bone in

Terminal sired lambs is not likely due to maturity, with fore section bone maturing at

the same rate as total bone in lambs sired by all sire types.

The Terminal sired lambs had the least fore section fat, with maturity a possible

explanation given fat in this section is relatively early maturing. However the small

differences in fat distribution between sire types in other carcass sections make this

explanation less tenable. The variation in carcass fat and bone distribution in lambs

from different sire types has not been previously reported.

4.5.2 Production factors affecting carcass fat and bone

The differences between carcass composition of ewes and wethers in this study are

similar to previous studies, with reports of ewes having the greatest weight of fat

(Afonso and Thompson 1996, Butterfield 1988) and least bone (Thompson et al. 1979a)

when compared at the same carcass weight. These castrate male and female differences

can be partly attributed to differences in mature size (Hopkins et al. 2007a) and mature

composition. Alternatively, the differences may also relate to fat distribution between

the main fat depots: subcutaneous, intermuscular, abdominal and intramuscular.

Butterfield (1988) showed in Merinos that both sexes are likely to have similar total

body fat weights, with rams having less subcutaneous fat but more intermuscular and

mesenteric fat. Differences in bone may be due to wethers having longer, and thicker


114

bones compared to ewes (Wood et al. 1980). Finally, the rate maturation of fat and bone

may differ in females compared to males but the lack of sex interaction with the b

coefficient makes this scenario less significant.

The carcasses of wether lambs had more bone in the fore section and less bone in the

saddle than ewe lambs which is similar to results found by Thompson, Atkins, and

Gilmour (1979c). This earlier study showed Merino wethers had more forelimb bone

when compared at the same total bone weight, which may be due to increased forelimb

thickness. Our wether lambs also had increased fore section lean (Anderson et al.

2015c) (Chapter 3), perhaps due to the need for more dominant male behaviour as

suggested by Butterfield (1988), explaining the need for an increase in supporting fore

section bone.

Differences in carcass fat and bone between sites, and kill groups, are likely to be

reflective of a range of processing, environmental, nutritional, age effects, or CT

measurement error – all of which this study was not specifically designed to address.

Conflict exists with respect to the impact of a high plane of nutrition on carcass fat, with

some studies showing that animals on a high plane of nutrition have a higher proportion

of fat in the carcass (Berg and Butterfield 1968), while others have shown no change in

carcass fat (Lee et al. 1990). Compared at the same carcass weight there was a trend for

the site-years with the lowest proportion of bone to have the largest proportions of fat,

with the remainder of the weight difference accounted for by lean (Anderson et al.

2015c) (Chapter 3).

Within a site, animals in the younger kill groups generally had less carcass fat, although

carcasses from two sites have results contrary to this trend. Lamb carcasses from earlier


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kill groups represent faster growing animals that reach slaughter weights earlier. These

lambs were likely to be on a growth path to larger mature size and would be at a smaller

portion of their mature weight. Alternative explanations could be associated with

variation in nutrition, i.e. ewe milk output and pasture conditions, or worm burdens

(ewe and lamb impacts). Given there was no consistent trend between carcass bone

weight and kill group (age) there appears to be no obvious association between age and

proportion of carcass bone.

Lambs born as triplets and raised as singles had carcasses with low saddle section fat

and high hind section fat, which may indicate a distributional effect of compensatory

growth. However there were only 11 animals within this category, which is too few to

draw conclusive interpretations. The reasons for lambs born and raised as triplets having

carcasses with altered bone distribution are unclear. These lambs may be late maturing

given the saddle and hind sections are late and early with respect to their rate of

maturation of bone which may be the cause of the high and low weight of bone in these

sections.

4.5.3 Allometric coefficient

Previous studies assessing allometric coefficients used manual dissection rather than

CT, and the allometric coefficients within this study align well with previous

experiments (Afonso and Thompson 1996, Berg and Butterfield 1968, Butterfield et al.

1983a, Butterfield et al. 1984b), demonstrating the well known principles that fat is a

late maturing tissue and bone an early maturing tissue. There were no significant

interactions of the b coefficients with any fixed effects, which is in contrast to studies

by Fourie, Kirton, and Jury (1970), and Thompson, Parks, and Perry (1985b) who

showed the maturation rate of fat to be earlier in ewes compared to rams. Thompson,

Parks, and Perry. (1985b) also showed bone to be earlier maturing in ewes compared to


116

rams. Alternatively, Afonso and Thompson (1996) and Thompson, Atkins, and Gilmour

(1979a) both demonstrated no difference in the rate of fat development between sexes.

The lack of difference in our study may be explained by the fact that our animals were

wethers and not rams or that the age range in this experiment did not provide sufficient

sensitivity to detect differences (24-60 weeks of age) compared to birth to 102 weeks of

age in the study of Thompson, Butterfield, and Perry (1985a).

There are fewer studies that assess the rate of development of fat and bone between the

fore, saddle, and hind sections. The fore section fat was earliest to develop followed by

the hind and saddle section. The early development of the fore section fat negates the

potential to manipulate fatness in this region simply by slaughtering lambs at earlier

ages. The rate of maturation of the bones within the three sections also varied, with the

hind section earliest to mature; the fore section maturing at the same rate as total carcass

bone and saddle bone being late maturing. These results for fat and bone are similar to

those of earlier studies based on manual dissection (Butterfield and Thompson 1983,

Thompson et al. 1979c).


4.5.4.1 Genetic effects on carcass fat and bone

The impact of the ASBVs on carcass fat was far greater than that of the non-genetic

effects. Among these ASBV effects, the impact of PFAT was about 60% greater than

the impact of PEMD on carcass fat, however the latter was not consistent across all sire

types with no effect observed in the Merino sired lambs. The PWWT ASBV also had a

substantial impact on carcass fat, but only at some sites making its use in carcass fat

reduction less important.


117

The magnitude of the ASBV effects on bone was less than that observed for fat. Of the

genetic effects, decreasing sire PFAT had the greatest influence on the proportion of

carcass bone and was approximately 1.6 times greater than that of the sire type effect.

The PWWT effect was variable and only observed in some years at certain sites with

individual kill groups showing no consistent effect. There was no impact of PEMD on

carcass bone weight or its distribution.

Therefore we can conclude that sire PFAT had the most impact on fat composition

within the carcass, having a magnitude of impact at least 50% greater than the effect of

PEMD or PWWT. Similarly, PFAT also had the most marked impact on bone

composition, with neither PEMD nor PWWT having any impact on whole carcass bone.

All three ASBVs had minimal impact on fat and bone distribution, with increased sire

PWWT resulting in only small increases in hind section bone.

Sire type impacted carcass fat and bone composition, however the magnitude of these

effects was less than those of the ASBVs. Maternal sired lambs were fatter and Merino

sired lambs had more carcass bone. In contrast to the whole carcass effects, there was a

greater impact of sire type and dam breed on fat and bone distribution compared to the

ASBVs.

4.5.4.2 Production factors affecting carcass fat and bone

Among the production and management effects, site and kill group had the largest

impact on carcass fat and its distribution between sections. Site had more than double

the impact of the sex and birth-type rear-type effects, demonstrating the potential for

nutrition, and other environmental factors to impact on carcass fat and its distribution.


118

In contrast to fat, the greatest impact on bone was seen with site, year and kill groups,

having up to three times the magnitude of impact compared to the strongest genetic

effects observed with PFAT. Of the other non genetic effects, sex had less impact on

carcass bone and its distribution.. Similarly, birth type-rear type had minor impacts on

bone with its distribution only impacted in lambs born and raised as triplets.

4.6 Conclusions

The effectiveness of using the PFAT and PEMD breeding values to reduce carcass fat is

well demonstrated by this study. Furthermore, the impact of these ASBVs has been

shown to impact not only at the site of measurement in the saddle region, but across all

regions of the carcass. The PFAT breeding value was also shown to increase the

proportion of carcass bone, however this was a comparatively small effect relative to the

decrease in carcsas fat and increase in carcass muscle attributed to this breeding value,

suggesting an overall positive impact on carcass value. The PWWT breeding value does

not appear to have a strong influence on whole carcass fat, with any effects heavily

influenced by lamb nutrition and environmental conditions. The impact of PWWT and

PEMD on carcass bone weight or its distribution throughout the carcass was minimal.

This is an important finding as it implies that selection for improved lean meat yield

does not negatively impacting carcass composition by increasing the proportion of

bone, which is considered undesirable by the consumer.

There are marked differences between sire types with respect to the amount of fat and

bone within the carcass, with Merino sired lambs having the most carcass bone, and

Maternal sired lambs the most carcass fat. Production factors and environmental effects

were also shown to impact both fat and bone in the carcass, with sex having a marked

effect on carcass fat, and environmental effects like site and killgroup having marked


119

effects on bone. Further analysis of the data from this experiment will help determine

the economic implications of these changes.

Chapter 5 – The impact of genetics on retail meat value in Australian lamb

120

Chapter 5. The impact of genetics on retail meat value

in Australian lamb.

The following chapter is the version that was submitted for publication:

Anderson, F., Pethick, D.W., Gardner, G.E. (2015). Meat Science.

5.1 Abstract

Lean (muscle), fat, and bone composition of 1554 lamb carcasses from Terminal,

Merino and Terminal sired lambs was measured using computed tomography scanning.

Lamb sires were diverse in their range of Australian Sheep Breeding Values for post

weaning c-site eye muscle depth (PEMD) and fat depth (PFAT), and post weaning

weight (PWWT). Lean value, representing predicted lean weight multiplied by retail

value, was determined for lambs at the same carcass weight or the same age. At the

same carcass weight, lean value was increased the most by reducing sire PFAT,

followed by increasing PEMD and PWWT. However for lambs of the same age,

increasing sire PWWT increased lean value the most. Terminal sired lambs had greater

lean value irrespective of whether comparisons were made at the same age or weight.

Lean value was greater in Merino compared to Maternal sired lambs at equal carcass

weight, however the reverse was true when comparisons were made at the same age.


121

5.2 Introduction

Hot carcass weight (HCWT) and lean meat yield percentage (LMY%) are important

profit drivers across the entire value chain, but especially for processors. The value of

HCWT is relatively easy to understand as it represents increased volume of product per

fixed cost of slaughter and fabrication of cuts. Alternatively, the financial implications

of LMY% are more complex. For processors, leaner lambs require less fat trimming,

resulting in less wastage and decreased processing costs (Hopkins 1989). Furthermore,

the location of lean in the carcass also influences carcass value, as the price of different

cuts vary at retail. Therefore a carcass with proportionately more lean within the higher

value loin cuts, will be worth more (Pethick, Ball, Banks et al. 2011). To reflect these

profit drivers, Australian processors purchase lambs on the basis of HCWT and GR

tissue depth (tissue depth at the 12th

rib 110mm from the midline) to crudely reflect

LMY% (Pethick et al. 2011).

To indirectly select for LMY% and HCWT, Australian lamb producers make use of

Australian Sheep Breeding Values (ASBVs) for sire post-weaning weight (PWWT), c-

site fat depth (PFAT) and eye muscle depth (PEMD). All three breeding values have

been shown to increase HCWT (Gardner et al. 2015), with PWWT having the biggest

impact producing heavier lambs at the same age, or enabling earlier slaughter at a

targeted weight. Producers also utilise the carcass breeding values to indirectly select

for LMY%, with a breeding value for direct selection for this trait not currently utilised

in Australian lamb breeding programs. A recent study by Anderson, Williams, Pannier,

Pethick and Gardner (2015c) (Chapter 3) used computed tomography (CT) to assess

lamb carcass composition and revealed that when compared at the same carcass weight,

the progeny of sires with increased PEMD and decreased PFAT had a greater

proportion of lean in the carcass. Additionally, it was observed that the increase in lean


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was preferentially distributed to the saddle (mid) section of the carcass (Anderson et al.

2015c), a finding supported by earlier studies (Gardner et al. 2010, Hall et al. 2002,

Hegarty et al. 2006c). Sire PWWT ASBV was shown to increase the proportion of lean

in the saddle region, although the effect on carcass LMY% was smaller and less

consistent than that of sire PFAT and PEMD (Anderson et al. 2015c) (Chapter 3). The

combined effect of these breeding values on LMY% and distribution of lean and

therefore carcass value has not been determined. When comparing lambs at the same

carcass weight we would expect sire PFAT to have the greatest increase on carcass

value, followed by sire PEMD and PWWT. Alternatively, when comparing lambs at the

same age HCWT will be the main profit driver, hence PWWT is expected to have the

biggest impact on carcass value.

Differences between sire types have also been shown to impact carcass composition

(Anderson, Pannier, Pethick et al. 2015a, Anderson et al. 2015c, d, Ponnampalam et al.

2007a) (Chapters 3, 4 and 6). Terminal sired lambs have been shown to grow the fastest

(Gardner et al. 2015), and when compared at the same carcass weight, they also have a

greater proportion of carcass lean than Maternal and Merino sired lambs (Anderson et

al. 2015c, Ponnampalam et al. 2008). Therefore their carcass value should be higher

irrespective of whether compared at the same weight or age.

This experiment analyses data from a large number of lamb carcasses (1554) from the

Information Nucleus Flock (INF) experiment which is run by the Australian

Cooperative Research Centre for Sheep Industry Innovation (Sheep CRC). Previous

analyses of the lamb carcasses from this experiment have examined the impacts of

genetic and non-genetic factors on carcass composition and distribution of fat, lean

(muscle) and bone and are reported in Anderson, Williams et al. (2015c,d) (Chapters 3


123

and 4). Alternatively, this analysis expresses these changes in absolute mass and the

equivalent dollar value, thus focussing on the financial implications that genetic

selection for sires high in PWWT and PEMD and low in PFAT has on the value of lean

in the carcass. We hypothesise that when compared at the same carcass weight,

selection of lambs for increased sire PWWT and PEMD and decreased PFAT will result

in a higher carcass value through an increase in LMY%, with PFAT having the greatest

impact. However, when compared at the same age we hypothesise that PWWT will

have the greatest impact on carcass value. Finally, we hypothesise that the carcass value

of Terminal sired lambs will be greater compared to Merino and Maternal sired lambs

when compared at either the same weight or the same age.

5.3 Materials and Methods

5.3.1 Experimental design and slaughter details.

The design of the Sheep CRC INF is described by Fogarty, Banks, van der Werf, Ball,

and Gibson (2007). In brief, approximately 10,000 lambs were produced by artificial

insemination of Merino or Border Leicester-Merino (BLM) dams over a 5 year period

(year 2007-2011) at eight research stations (Katanning WA, Cowra NSW, Trangie

NSW, Kirby NSW, Struan SA, Turretfield SA, Hamilton VIC, and Rutherglen VIC).

The sire types used in the INF included: Terminal sires (Hampshire Down, Ile De

France, Poll Dorset, Southdown, Suffolk, Texel, White Suffolk), Maternal sires (Bond,

Booroola Leicester, Border Leicester, Coopworth, Corriedale, Dohne Merino, East

Friesian, Prime South African Meat Merino (SAAM), White Dorper), and Merino sires

(Merino, Poll Merino). The combinations of sire and dam crosses included: Merino,

Maternal x Merino, Terminal x Merino and Terminal x BLM. Lambs were weaned at

approximately 100 days of age, grazed under extensive conditions and supplemented

with feed at times when there was limited pasture, with availability of pasture and feed

varying between sites (Ponnampalam et al. 2014). All male lambs were castrated.


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The lambs were divided into groups based on live weights, with each group killed

separately (kill groups) targeting a hot carcass weight at slaughter of between 21 to

24kg, irrespective of condition score. Lambs within kill groups were on average within

5 days of age of each other and within a year there was an attempt to represent all sire

types in each kill group. Within each site, the aim of selection of lambs for CT was to

include at least two progeny from each sire used at the site, selected across a live weight

strata. At all INF sites, lambs were yarded within 48 hours of slaughter, maintained off-

feed for at least 6 hours, and then weighed to determine pre-slaughter live weight.

Lambs were then transported for 0.5-6 hours via truck to one of 5 commercial abattoirs,

held in lairage at the abattoir for between 1 and 12 hours, and then slaughtered. All

carcasses were subjected to medium voltage electrical stimulation (Pearce, Van de Ven,

Mudford et al. 2010) and trimmed according to AUSMEAT standards (Anonymous

2005) and HCWT was then measured within 40 minutes of slaughter. All lambs were

measured and sampled for a wide range of carcass and meat quality traits (Pearce 2009).

Lambs used in the CT study of composition were slaughtered and transported to either

Murdoch University or the University of New England. Lamb carcasses were divided

into three sections (fore, saddle and hind). The fore section was separated from the

saddle by a cut between the fourth and fifth ribs. The hind section was separated from

the saddle by a cut through the mid-length of the sixth lumbar vertebrae. For complete

description of the CT scanning process and calculation of carcass composition see

Anderson et al (2015c)(Chapter 3).

5.3.2 Data used

This experiment utilised the HCWT results obtained from an experiment described by

Gardner et al (2015) for the progeny born from 2007 to 2010. This included 7516


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lambs, representing the progeny of 76 Maternal, 127 Merino and 135 Terminal sires for

which breeding value data was available. The breeding values for PEMD and PFAT are

based upon live ultrasound measurement at the c-site (located at the 12th

rib 45mm from

the midline), and PWWT is based upon live weight, all measured at the post weaning

time point (about 240 days of age). These values are combined into a single index value

(Carcass Plus Index) which is based upon weightings for positive weaning weight

breeding value (0.39), PWWT (0.26), and PEMD (0.30), and negative PFAT (0.05).

CT scanning data was available on 1564 of the lambs from an INF experiment

described by Anderson et al (2015c)(Chapter 3) using 8 site-year combinations where

lean measurement was available within the fore, saddle, and hind-sections of the carcass

(Table 5-1). The mean carcass weight of the slaughtered lambs was 23.3kg, and the

mean, maximum and minimum weights of the fat, lean and bone for each of the carcass

sections is reported in Anderson et al (2015c)(Chapter 3). Of the 81 Maternal, 119

Merino and 144 Terminal sires in the CT experiment, 67, 109 and 143 had ASBV

values for PWWT, PEMD, PFAT and Carcass Plus Index available (Table 5-2).


126

Table 5-1 Average age of lambs at slaughter and number of carcasses scanned using

computed tomography in each lamb kill group at each site.

Site-Year

Kill

group

Average

age (days)

Carcasse

s (n)

Kirby 2007 1 235 72

2 270 63

3 352 96

Kirby 2008 1 269 98

2 345 99

3 408 99

4 420 96

Rutherglen 2010 1 198 57

2 254 59

Hamilton 2009 1 229 56

Struan 2010 1 260 68

2 287 67

3 322 27

Turretfield 2009 1 235 58

2 262 63

3 310 30

Katanning 2008 1 235 20

2 242 29

3 319 29

Katanning 2011 1 168 88

2 238 96

3 280 99

4 355 95

Total - - 1564

Table 5-2 Number of lamb sires and mean (min, max) for the Australian Sheep

Breeding Values for post weaning weight (PWWT), post weaning c-site fat depth

(PFAT) and post weaning eye muscle depth (PEMD) and Carcass Plus Index for each

sire type of lamb carcasses undergoing computed tomography.

Sire type No. of

sires PWWT (kg) PFAT (mm) PEMD (mm) Carcass Plus

Maternal 70 4.7 (-6.1, 12.4) -0.3 (-2.1, 2.6) 0.1 (-2.5, 1.8) 125 (63,184)

Merino 109 1.9 (-5.0, 10.8) -0.2 (-1.9, 1.9) 0.0 (-2.6, 2.6) 112 (68,160)

Terminal 154 12.7 (5.3, 18.6) -0.7 (-2.5, 2.3) 1.2 (-2.8, 5) 179 (133, 209)


127

In both the HCWT analysis and CT analysis, a percentage of sires selected in a year

were used in subsequent years to provide linkage between years. These ASBV values

were sourced from Sheep Genetics, which is Australia’s national genetic evaluation

database for sheep (Brown et al., 2007). The sire breeding values and index estimates

were generated within 3 separate data-bases for Terminal, Maternal, and Merino sired

progeny and were from an analysis completed in April 2013. Some of the youngest sires

used in this experiment lacked industry records and therefore did not have ASBVs

available. For the CT analysis of lean distribution this meant that there were 1501 lambs

with known ASBVs for the analysis.

5.3.3 Data transformation

All data in the lean analysis was converted to natural logarithms in order to utilise

Huxley’s allometric equation (y = axb.) (Huxley and Teissier 1936). Where x is the

independent variable (carcass weight), a is the proportionality coefficient and b the

growth coefficient of y (weight of lean in each section) relative to x. By transforming all

of the values to natural logarithms the equation becomes: loge y = loge a + b.loge x;

which linearises the data and can be solved for y by least squares regression. The b term

was examined with relevant first order interactions with the core terms of sire type, sex


within kill group.

A significant advantage of using the loge form of the equation is that it homogenises the

variance over the entire range of sample data. It also allows for the direct comparison

of the differences in log y values as percent differences (Cole 2000) in lean weight of a

section at a given carcass weight or back-transformation to give the weight (kg) of lean

in each section. The loge transformation introduces a systematic bias into the

calculations and it is well recognised that a correction factor is required to correct for


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this bias when back-transforming predictions (Baskerville 1972, Beauchamp and Olson

1973, Finney 1941, Smith 1993, Sprugel 1983). Hence the correction described by

Sprugel (1983) was used when data was back-transformed from log lean weight to kg

lean weight.

5.3.4 Establishing models for predicting section lean weights.

Linear mixed effects models in SAS (SAS version 9.0, SAS Institute, Cary, NC, USA)

were used to determine the factors affecting the weights of muscle, bone, and fat for

each carcass section. A series of 9 base models were established, where the dependent

variables were the loge weights (kg) of these tissues within each section, including loge

(fore section fat, lean, bone); loge (saddle section fat, lean, bone) and loge (hind section

fat, lean, bone). The covariate (x) was loge whole carcass weight (kg), fixed effects

included: site-year (combined effect of site and year of lamb birth: Katanning (2008,

2011), Kirby (2007, 2008,) Hamilton 2009, Turretfield 2009 and Struan 2010); birth

type and rearing type (combined effect of animals born as single, twin or triplet and

reared as single, twin or triplet); sire type (Maternal, Merino and Terminal); sex within

sire type (wether Merino, wether Maternal, female Terminal, wether Terminal); dam

breed within sire type (Merino x Merino, Maternal x Merino, Terminal x Merino,

Terminal x BLM) and kill group within site-year, and random terms included sire and

dam identification by lamb birth year. The 1554 carcasses with entries for sex, sire type,

birth-type rear-type, dam breed, and kill group were included in the base model (Table

5-3). It was initially established that there were no first order interactions of the b term

with and of the core terms (sire type, sex within sire type, dam breed within sire type,

birth type-rear type, site year, site year within kill group). The 9 base models were

constrained to maintain the same form allowing the relative increases and decreases in

carcass components within each section to offset each other so that the predicted

weights of the components would be additive to equal the weight of the carcass.

Table 5-3 Number of lambs analysed in the base model according to sex, sire type, birthing and rearing type and dam breed.

Sex Birth type-rearing type Dam breed

Single born

and raised

Born as

twin-raised

as single

Born as

twin-raised

as twin

Born as

triplet-

raised as

single

Born as

triplet-

raised as

twin

Born and

raised as

triplet

Female Wether Merino BLM

Maternal 0 332 152 25 141 2 8 4 332 0

Merino 0 251 129 38 79 1 0 4 251 0

Terminal 475 496 379 91 447 8 24 22 457 514

Total 475 1079 660 154 667 11 32 30 1040 514

BLM: Border- Leicester x Merino

129


130

To identify this consistent form we initially used a single multivariate model which was

derived using all of the 9 loge weights (kg) of the tissues within each section as the

dependent variables, and the covariate and fixed effects described above as independent

variables. All relevant interactions between fixed effects and the covariate (loge carcass

weight) were tested within this multivariate model and removed in a stepwise manner if

non-significant (P>0.05).

In a secondary analysis, the above procedure was repeated with the sire ASBVs for

PWWT, PEMD and PFAT included in the base model along with their first order

interactions with the fixed effects. This included the interaction between ASBVs and

sire type, which was particularly relevant given that the breeding values were derived

from 3 separate data-bases for Terminal, Maternal, and Merino sired progeny, and thus

their magnitudes may not be directly comparable. Non-significant (P > 0.05) terms were

removed in a stepwise manner. Although these ASBVs were correlated (PWWT vs

PEMD = 0.3; PWWT vs PFAT = 0.3; PEMD vs PFAT = 0.1) previous analysis has

demonstrated that their effects are still relatively independent when included

simultaneously in a model predicting CT composition (Anderson et al. 2015c, d)

(Chapters 3 and 4).

Finally, to determine the impact of sire Carcass Plus Index on carcass composition and

carcass lean value, the Carcass Plus Index values were included in the base model along

with their first order interactions with the fixed effects, and non-significant (P > 0.05)

terms removed in a stepwise manner. The Carcass Plus Index combines the three

breeding values (PWWT, PFAT and PEMD) into a single index value based on

estimated economic weightings for each trait, and therefore is included in the models

independently to the ASBVs of which it is comprised. Carcass Plus is designed to


131

simplify the selection for improved HCWT and LMY% for producers of Terminal sired

lambs in the Australian Sheep Industry, although the Index values are readily calculated

for the Maternal and Merino sired lambs based on their values for weaning weight,

PWWT, PFAT and PEMD. The Carcass Plus Index is currently based upon weightings

for positive weaning weight breeding value (0.30), PWWT (0.35), and PEMD (0.30),

and negative PFAT (0.05).

5.3.5 Estimating weights of lean tissue within sections

The models described above were used to estimate the lean weight for each carcass

section. These were initially calculated at a constant carcass weight of 23kg, which was

selected because this represents a relevant industry weight and lies well within the

bounds of this dataset. The base model was used to estimate lean weight for the fore,

saddle and hind section for the comparison between ewes versus wethers (within the

progeny of Terminal sires), BLM dams versus Merino dams (within the progeny of

Terminal sires), and Terminal versus Maternal or Merino sire types (within the wether

progeny of Merino dams). The model with the 3 ASBVs was used to estimate loge lean

weights in each section for Maternal, Merino and Terminal sires at the extremes of their

breeding value ranges for PFAT, PWWT, and PEMD (Table 5-2). In all cases, the lean

weights estimated were back transformed to kilogram weight and the correction

described by Sprugel (1983) applied to account for log-transformation error. The

predicted weight of lean (kg) was subsequently used to determine the value of lean

across the range of breeding values (see 5.3.6. calculating lean value). Similarly, the

model containing the Carcass Plus index was used to estimate lean weights in each

section for Maternal, Merino and Terminal sires at the extremes of their Carcass Plus

Index values (Table 5-2).


132

This procedure was then repeated to make these same comparisons at a constant

slaughter age of 280 days. This age represents the raw mean age of lambs at the time of

slaughter. In this instance, carcass weight was not held constant, but instead allowed to

vary to reflect the true carcass weight for the comparisons being made. For example, the

section lean weight of ewes was predicted at 23.5kg carcass weight, and wethers at

24.6kg carcass weight. The carcass weights used to predict lean weights for each sire

type are summarised in Table 5-4 and were derived from the models described by

Gardner et al. (2015). These weights were converted to loge values prior to analysis.

5.3.6 Calculating lean value

To determine carcass lean value the estimates of lean weight for each carcass section

(kg), as described above, were multiplied by the average retail value of lean in each

section and summed to arrive at a whole carcass lean value. The average retail values

used were $17.33, $27.02 and $20.25 per kilogram for lean from the fore, saddle and

hind sections. These values were determined using average retail prices of two major

Australian lamb retailers (April 2015) for a set of commercial cuts (AUS-MEAT

description codes (AUS-Meat Limited, 2015) in parenthesis) which included: 5 cuts

from the fore section, eye of shoulder (5151), boneless shoulder (5047), fore-shank trim

(5030), breast trim (5000), neck meat (5020), and fore-section lean trim (5290); 4 cuts

from the saddle section, eye of shortloin (5150), tenderloin butt off (5082), eye of rack

(5153), trimmed boneless flap (5173), and saddle section lean trim (5270); and 6 cuts

from the hind section, topside (5073), round (5076), silverside (5071), rump (5130), butt

tenderloin (5081), hind shank butt off (5031), and hind section lean trim (5270). Within

each carcass section the average of these prices was calculated, after being weighted by

the proportion that each cut represented of the total mass of lean for that section. The

commercial cuts described above contain a small amount of subcutaneous and inter-

Table 5-4 Predicted hot carcass weights (kg) ± SE of the Maternal, Merino and Terminal sired wether lambs born to Merino dams at an average age of

280 days.

Carcass

weight (kg)

Carcass weight (kg)

Carcass weight (kg)

Carcass weight (kg)

Carcass weight (kg)

Sire type

Base model

(no ASBVs)

PWWT

range

(high, low)1

PWWT

low

PWWT

high

PEMD

range

(high, low)

PEMD

low

PEMD

high

PFAT

range

(high, low)

PFAT

low

PFAT

high

Carcass

plus range

(high, low)

Carcass

plus low

Carcass

plus high

Maternal

21.9

± 0.18

(-6.1, 12.4)

20.0

± 0.53

23.3

± 0.36 (-2.5, 1.8)

21.5

± 0.23

22.1

± 0.21 (-2.1, 2.6)

22.2

± 0.24

21.3

± 0.26 (64,184)

19.0

± 0.60

24.6

± 0.52

Merino

19.11

± 0.22

(-5.0, 10.8)

16.3

± 0.32

21.9

± 0.33 (-2.6, 2.6)

18.3

± 0.25

19.1

± 0.25 (-1.9, 1.9)

19.1

± 0.25

18.3

± 0.26 (69,160)

17.2

± 0.42

21.2

± 0.32

Terminal

23.38

± 0.19

(5.3, 18.6)

22.2

± 0.29

24.6

± 0.28 (-2.8, 5)

22.9

± 0.26

24.1

± 0.26 (-2.5, 2.3)

23.9

± 0.21

22.9

± 0.27 (134, 209)

22.4

± 0.31

24.5

± 0.30 PWWT: post weaning weight; PFAT: post weaning c-site fat depth; PEMD: post weaning eye muscle depth 1 The HCWT for each sire type has been predicted at the maximum and minimum range for that particular sire type as described in Table 5-2 using models and data from Gardner et

al. (2015).

133


134

muscular fat, however this would have little effect on the proportion that each cut

represents of the total lean in each section, therefore having little impact on the overall

estimate of lean value for that section.

5.4 Results

5.4.1 Impact of sex on carcass composition and lean value

A comparison between sexes was only able to be made within the Terminal sired lambs.

When compared at the same carcass weight, the carcass composition varied between

sexes (P<0.01, Table 5-5), with wethers generally having more lean than ewes (Table

5-6). This was evident in the fore, saddle and hind sections where the wether progeny

from BLM dams had 4.83%, 2.04% and 1.69% more lean than ewe lambs (Table 5-6).

Likewise in lambs born to Merino dams, the wethers had 4.26%, 1.85% and 1.08%

more lean than the ewe lambs in the fore, saddle and hind sections (Table 6). The

decreased lean in the ewe lambs was largely offset by an increase in fat across all

sections of the carcass (Table 5-6).

In a 23kg carcass, these compositional differences equated to an additional 0.38 kg and

0.31 kg of lean in wether lambs born to BLM and Merino dams, which was worth $7.55

and $6.27 per carcass more than ewe lambs (Table 5-7). The carcasses of wether lambs

were also 1.07 kg heavier than ewe lambs at the same age (ie 280 days) (see Gardner et

al. 2015). When this carcass weight difference was factored into the calculation the

additional carcass lean was worth $19.22 and $16.81 for the wether lambs born to BLM

and Merino dams (Table 5-7).

Table 5-5 F-values and degrees of freedom for the numerator (NDF) and denominator (DDF) for factors affecting the proportions of fat, lean and bone

in the fore, saddle and hind sections of the lamb carcass.

Fore-section Saddle section Hind section

Effect NDF,DDF

Fat Lean Bone

Fat Lean Bone

Fat Lean Bone

Core model

Site year 7,171

13.24** 82.05** 43.13**

49.31** 38.24** 29.33**

54.11** 35.82** 85.21**

sex(sire type) 1,171

35.86** 156.21** 43.30**

151.57** 19.84** 3.37

41.65** 21.52** 31.89**

Sire type 2,171

21.31** 5.13** 17.46**

8.56** 26.13** 1.31

13.67** 30.86** 14.8**

Kill group(site year) 15,171

3.21** 9.15** 11.86**

9.88** 11.12** 6.95**

9.53** 4.89** 11.46**

Dam breed(sire type) 1,171

5.83* 8.33** 2.24

11.07** 1.78 0.19

2.76 9.04** 0.09

Flock year x sire type 12,171

2.82** 1.24 0.76

2.57** 3.11** 1.16

4.01** 3.00** 1.90*

Flock year x dam breed (sire type) 5,171

2.08 5.88** 2.29*

4.42** 1.89 0.32

3.32** 2.50* 1.96

log (CT whole carcass weight (kg)) 1,171

2308.07** 3729.09** 1109.13**

2446.08** 2383.67** 615.23**

2332.49** 4582.8** 1525.07**

Australian Sheep Breeding Values

PWWT 1,153

3.41 0.01 0.09

0.35 8.67** 0.48

3.69 0.6 0.06

PWWT*site year 7,153

2.84** 1.67 1.27

3.88** 3.43** 1.14

3.84** 4** 3.48**

PFAT 1,153

44.4** 29.17** 9.2**

70.12** 49.65** 6.06*

58.53** 48.56** 8.77**

PFAT *Kill group(Site year) 22,153

1.34 1.28 1.33

1.33 1.75* 1.78*

1.93* 1.75* 1.79*

PEMD 1,153

14.75** 3.24 1.96

24.33** 53.23** 0.12

42.16** 31.64** 0.09

PEMD*sire type 2,153

2.07 1.47 0.46

8.13** 2.84 1.12

4.48* 6.14** 1.60

Carcass plus

Carcass plus 1,167

12.14** 1.31 0.17

5.05* 22.53** 0.16

14.2** 12.64** 0.01

Carcass Plus*sire type 2,167 1.65 3.61* 1.03

2.72 2.24 0.44

1.15 1.60 1.48


* P<0.05, **P<0.01

135

Table 5-6 The relative change (% change in weight) for fat, lean and bone in the fore, saddle and hind sections of the carcass due to sex, sire types,

dam breeds and Australian Sheep Breeding Value effects for lambs slaughtered at 23kg.

Fore section (% change in

weight) Saddle section (% change

in weight) Hind section (% change in

weight)

Effect Level

Fat Lean Bone

Fat Lean Bone

Fat Lean Bone

Sexdambreed (sire type)1

Maternal x Merino M

10.92d

-2.42c

1.36c

10.05

b -6.78

v -0.58

ab

8.17

d -5.26

a 0.23

c

Merino x Merino M

6.31

c -0.35

d 5.63

d

1.08

a -4.79

w 1.80

b

4.00

bc -4.13

ab 4.37

d

Terminal x BLM F

6.61

c -5.64

a -4.32

a

14.38

c -3.01

wx -1.9

a

6.19

cd -2.93

b -2.76

a

Terminal x Merino F

3.30

b -4.26

b -2.90

ab

9.48

b -1.85

xy -0.10

ab

3.63

b -1.08

c -1.89

ab

Terminal x BLM M

1.74

ab -0.81

d 0.25

c

2.92

a -0.97

yz 0.96

ab

0.19

a -1.24

c 0.32

c

Terminal x Merino M

0.00

a 0.00

d 0.00

bc

0.00

a 0.00

z 0.00

ab

0.00

a 0.00

d 0.00

bc

Australian Sheep Breeding Values2

PWWT

-0.28 -0.01 -0.04

-0.10 0.28 0.13

-0.32 0.05 0.02

site year Kirby 2007

-0.97 0.13 0.28

-0.89 0.37 0.03

-0.39 0.31 0.47

Kirby 2008

-0.11 0.06 0.05

-0.45 -0.05 0.15

0.18 0.07 -0.01

Rutherglen 2010

-0.47 0.12 -0.12

-0.33 0.41 -0.19

-1.42 0.38 0.16

Hamilton 2009

0.28 -0.09 0.09

0.42 -0.14 0.28

0.14 -0.18 -0.17

Struan 2010

-0.40 -0.22 -0.31

0.22 0.25 -0.38

0.02 -0.06 -0.06

Turretfield 2009

0.23 -0.21 -0.22

0.83 0.21 0.05

0.34 -0.29 -0.50

Katanning 2008

-0.49 0.02 -0.28

-0.65 1.06 0.79

-1.31 0.15 0.22

Katanning 2011

-0.31 0.13 0.15

0.00 0.11 0.32

-0.14 0.01 0.08

PFAT

3.87 -1.72 -1.56

6.25 -2.74 -1.75

5.15 -1.84 -1.07

PEMD

-1.96 0.62 -0.40

-2.73 1.88 -0.19

-2.77 1.16 -0.19

PEMD* sexdambreed Maternal x Merino M

-3.17 0.61 -0.63

-6.27 2.89 1.16

-5.30 2.24 0.74

(sire type) Merino x Merino M

-0.69 -0.07 -0.98

-0.39 2.39 0.01

-3.25 0.40 0.10

Terminal x BLM F

-2.29 1.00 0.24

-2.71 1.35 -1.61

-1.46 1.28 -0.49

Terminal x Merino F

-1.77 0.40 -0.29

-2.37 1.46 -0.10

-1.83 1.07 -0.31

136

Table 5-6 continued…

Fore section (% change)

Saddle section (%

change) Hind section (% change)

Effect Level Fat Lean Bone Fat Lean Bone Fat Lean Bone

PEMD* sexdambreed Terminal x BLM M -2.31 0.87 -0.32 -2.88 1.51 0.09 -2.64 1.23 -0.28

(sire type) Terminal x Merino M -1.51 0.91 -0.42 -1.75 1.71 -0.71 -2.12 0.75 -0.93

Carcass plus3

Carcass Plus

-0.96 0.25 -0.09

-0.80 0.76 -0.03

-0.88 0.40 -0.15

Carcass Plus* sexdambreed Maternal x Merino M

-0.90 0.35 -0.43

-1.39 0.88 0.03

-1.43 0.56 -0.10

(sire type) Merino x Merino M

-0.23 -0.22 0.21

0.08 0.14 0.28

-0.34 0.12 0.30

Terminal x BLM F

-1.54 0.47 0.13

-0.67 0.93 -0.81

-0.58 0.63 -0.42

Terminal x Merino F

-1.00 -0.04 0.17

-0.80 0.85 0.59

-0.71 0.36 -0.18

Terminal x BLM M

-1.31 0.43 -0.54

-1.12 1.07 0.07

-1.43 0.53 -0.32

Terminal x Merino M

-0.79 0.49 -0.05

-0.93 0.71 -0.35

-0.80 0.20 -0.20

M = wether; F = ewe; BLM: Border Leicester- Merino

PWWT: post weaning weight; PEMD: post weaning eye muscle depth; PFAT: post weaning c-site fat depth 1

a–e

Within columns for sexdambreed(sire type), % change values without a common superscript differ significantly at P < 0.05 and represents the difference in loge y values for

each fixed effect compared to the fixed effect with coefficient 0.00 expressed as a percentage. 2 % change in weight: the % increase or decrease in each tissue type per unit of Australian Sheep Breeding Value (PWWT, PFAT, PEMD)

3 % change in weight: the % increase or decrease in each tissue type per 10 units of the Carcass Plus breeding value

Effects in bold are significant P<0.05

137

Table 5-7 Weight (kg) and value ($) of lean in the fore, saddle and hind sections of the lamb carcass at the same weight (23 kg) and the same age

(280 days).

Fore-section

Saddle-section

Hind-section Whole carcass

Effect Level Lean weight

(kg)

Lean value

($)

Lean weight

(kg)

Lean value

($)

Lean weight

(kg)

Lean value

($)

Lean weight

(kg)

Lean value

($)

Lambs compared at the same carcass weight (23kg)

Sexdambreed Maternal x Merino M 4.28 74.13

3.45 93.29

5.10 103.36

12.83 270.79

(sire type) Merino x Merino M 4.37 75.71

3.53 95.28

5.16 104.59

13.06 275.58

Terminal x BLM F 4.14 71.69

3.59 97.07

5.23 105.90

12.96 274.66

Terminal x Merino F 4.20 72.73

3.64 98.23

5.33 107.92

13.17 278.88

Terminal x BLM M 4.35 75.36

3.67 99.11

5.32 107.75

13.34 282.21

Terminal x Merino M 4.38 75.97

3.70 100.08

5.39 109.10

13.47 285.15

Lambs compared at the same age (280 days)

Sexdambreed Maternal x Merino M 4.10 71.04 3.19 86.19 4.91 99.45 12.20 256.68

(sire type) Merino x Merino M 3.72 64.46 3.00 81.06 4.44 89.93 11.16 235.44

Terminal x BLM F 4.40 76.24 3.81 102.94 5.54 112.21 13.75 291.39

Terminal x Merino F 4.10 71.04 3.55 95.92 5.21 105.53 12.86 272.48

Terminal x BLM M 4.80 83.17 4.04 109.16 5.84 118.29 14.68 310.61

Terminal x Merino M 4.45 77.10 3.76 101.59 5.46 110.59 13.67 289.29

M = wether; F = ewe; BLM: Border Leicester x Merino

138


139

5.4.2 Impact of genetics on carcass composition and lean value in the

carcass

5.4.2.1 Sire type

Sire type comparisons were possible in wether lambs born to Merino dams. When

compared at the same carcass weight, the amount of lean in each section varied between

the Maternal, Merino and Terminal sired lambs (P<0.01, Table 5-5). For lean, 95% of

the sire estimates lay between ±3.7%, ±5.4% and ±3.9% for the fore, saddle and hind

sections respectively and any given carcass weight.

Compared to the Terminal sired lambs, the Maternal and Merino sired lambs 6.78% and

4.79% less lean in the saddle and 5.26% and 4.13% less lean in the hind section (Table

5-6). The Maternal sired lambs had the least fore section lean with 2.08% and 2.42%

less lean than the Merino and Terminal sired lambs (Table 5-6).

When compared at the same carcass weight (23kg) these compositional differences

equated to an additional 0.42kg and 0.62kg of lean in Terminal sired lambs compared to

Merino and Maternal sired lambs, which was worth an extra $9.57 and $14.36 per

carcass (Table 5-7).

Carcasses of Terminal sired lambs were 4.27kg and 1.48kg heavier than Merino and

Maternal sired lambs at the same age (ie 280 days) (Gardner et al. 2015). When this

carcass weight difference was factored into the calculation the additional carcass lean

was worth $53.85 and $32.61 (Table 5-7).

5.4.2.2 Dam breed

A comparison of dam breeds was possible in the Terminal sired lambs. In the fore

section, differences only existed within ewe lambs (P<0.01, Table 5-5), where the


140

progeny of Merino dams had 1.46% more lean than those from BLM dams (Table 5-6).

In the hind section, differences were evident within both sexes (P<0.01, Table 5), where

lambs from Merino dams had 1.26% (wethers) and 1.90% (ewes) more lean than lambs

from BLM dams (Table 5-6). In the ewe lambs the increased lean in Merino lambs was

offset by reductions in fat within the fore, saddle and hind sections (3.1%, 4.28% and

2.41%, Table 5-6) compared to lambs from BLM dams.

When compared at the same carcass weight (23kg), these compositional differences

equated to an additional 0.17kg more lean in the progeny of Merino dams (average of

wethers and females) compared to the progeny of BLM dams (Table 5-7). This resulted

in Merino dams producing lambs with a predicted carcass lean value of $3.58 more,

when compared to those born to BLM dams (Table 5-7). When compared at the same

age, (280 days) the BLM dams produced lambs that on average had more lean (0.95kg,

Table 5-7) and a carcass lean value that was $20.12 greater than the lambs from Merino

dams.

141

Table 5-8 Predicted carcass value for the fore, saddle and hind sections of carcass for the Maternal, Merino and Terminal sired lambs for the range of

post weaning weight (PWWT), c-site fat depth (PFAT), c-site eye muscle depth (PEMD) and Carcass Plus Index values – wether progeny of Merino

dams at 23kg carcass weight.

Sire type Breeding value Fore section lean

value ($)

Saddle section

lean value ($)

Hind section

lean value ($)

Carcass value

($)

$ difference

(high-low)

$ per unit

breeding value

PWWT

Maternal Low (-6.1) 74.65 92.11 103.52 270.28

Maternal High (12.4) 74.52 96.88 104.48 275.87 5.59 0.30

Merino Low (-5) 76.70 96.31 106.53 279.54

Merino High (10.8) 76.58 100.57 107.38 284.52 4.98 0.32

Terminal Low (5.3) 75.46 98.52 106.30 280.28

Terminal High (18.6) 75.36 102.22 107.01 284.58 4.31 0.32

PFAT

Maternal PFAT low (-2.1) 76.93 99.67 107.55 284.15

Maternal PFAT high (2.6) 70.71 86.83 98.25 255.79 -28.36 -6.03

Merino PFAT low (-1.9) 76.64 98.17 106.89 281.69

Merino PFAT high (1.9) 71.63 87.94 99.41 258.99 -22.70 -5.97

Terminal PFAT low (-2.5) 77.78 105.64 110.26 293.68

Terminal PFAT high (2.3) 71.36 91.75 100.52 263.63 -30.05 -6.26 PEMD

Maternal PEMD low (-2.5) 73.72 89.45 96.69 259.85

Maternal PEMD high (1.8) 75.68 96.68 106.00 278.36 18.51 4.30 Merino PEMD low (-2.6) 75.76 92.56 106.40 274.71

Merino PEMD high (2.6) 78.20 101.61 108.61 288.42 13.70 2.64

Terminal PEMD low (-2.8) 74.07 91.86 105.40 271.33

Terminal PEMD high (5.0) 77.65 105.33 111.57 294.54 23.22 2.98

142

Table 5-8 continued….

Sire type Breeding value Fore section

lean value ($)

Saddle section

lean value ($)

Hind section

lean value ($)

Carcass value

($)

$ difference

(high-low)

$ per unit

breeding value

Carcass plus

Maternal Carcass Plus low (64) 72.29 90.51 102.04 264.84

Maternal Carcass Plus high (184) 75.32 98.76 106.94 281.03 16.19 0.13

Merino Carcass Plus low (69) 76.30 90.79 102.23 269.32

Merino Carcass Plus high (160) 74.77 97.07 105.96 277.80 8.47 0.09

Terminal Carcass Plus low (134) 74.36 94.64 104.70 273.69

Terminal Carcass Plus high (209) 77.09 100.03 107.84 284.96 11.27 0.15

PWWT: post weaning weight; PFAT: Post weaning c-site fat depth; PEMD: Post weaning c-site eye muscle depth


143

5.4.2.3 Australian Sheep Breeding Values

5.4.2.3.1 Impact of sire PWWT on carcass composition and lean value

Sire PWWT had a significant effect on lean weight of the saddle section (P<0.01, Table

5-5). The magnitude of this effect varied between site-years, however on average saddle

section lean weight increased at 0.28% per unit increase of PWWT ASBV (Table 6).

When lambs were compared at the same carcass weight (23kg), the lean value increase

due to sire PWWT was similar for all three sire types (Table 5-8). However, due to their

different breeding value ranges this meant that the impact of PWWT on lean value was

greatest in Maternal sired lambs and worth an additional $5.59, compared to only $4.98

and $4.31 for the Terminal and Merino sired lambs (Table 5-8) across their respective

ranges of sire PWWT.

Sire PWWT breeding value also had a marked effect on carcass weight which differed

between sire types (Gardner et al. 2015). Thus when these differences between carcass

weights (Table 5-4) were factored into the calculation at a constant age (280 days) the

additional carcass lean was worth $3.95 per unit sire PWWT for the Merino sired

lambs, compared to only $2.07, and $2.10 per unit sire PWWT for the Maternal, and

Terminal sired lambs (Table 5-9). Across the range of sire PWWT breeding value for

each sire type this equated to a total of $62.46, $38.39, and $28.20 for the Merino,

Maternal, and Terminal sired lambs (Table 5-9).

5.4.2.3.2 Impact of sire PFAT on carcass composition and lean value

When compared at the same carcass weight, decreasing sire PFAT increased lean

(P<0.01, Table 5-5) in all carcass sections. The greatest magnitude of effect was in the

saddle region where at any given carcass weight a decrease in sire PFAT by one unit


144

increased saddle lean by 2.74%, compared to increases of only 1.72% and 1.84% for

lean in the fore and hind sections (Table 5-6). The increases in lean were accompanied

by changes in the proportion of fat and bone in all carcass sections (P<0.05, Table 5)

with the proportion of fat decreasing and bone increasing (Table 5-6). The impact of

PFAT for all sections of the carcass was consistent across sites and years, although

varied between kill groups (P<0.01, Table 5-5) (data not shown).

When compared at the same carcass weight (23kg), the increase in carcass lean value

for each unit decrease in sire PFAT was similar for all three sire types (Table 5-6). Due

to the different ranges of breeding values between sire types, this meant that the

Terminal sired lambs had the greatest increase in carcass value ($30.05, Table 8),

followed by the Maternal and Merino sired lambs ($28.36 and $22.70, Table 5-8).

Sire PFAT had a small impact on carcass weight that was consistent across sire types

(Table 5-4)(Gardner et al. 2015). Therefore when lambs were compared at the same

age, these increases in carcass weight resulted in a per unit value of lean of $8.42, $7.60

and $7.11 per unit decrease in PFAT for the Terminal, Maternal and Merino sire types

(Table 5-9). Across the sire range of PFAT values for each sire type this equated to an

increased lean value of $40.42, $35.71 and $27.03 for the Terminal, Maternal and

Merino sired lambs (Table 5-9).

5.4.2.3.3 Impact of sire PEMD on carcass composition and lean value

When compared at the same carcass weight, increasing sire PEMD increased lean in the

saddle and hind sections (P<0.01, Table 5-5). In the saddle each unit increase in sire

PEMD increased saddle lean by 1.99% (Table 5-6). In the hind section the effect varied

between sire types. PEMD had minimal impact on the proportion of lean in the Merino


145

sired lambs, however in the Terminal and Maternal sired lambs there was a 0.75 and

2.24% increase in hind section lean per unit of PEMD (Table 5-6). This increase in lean

was largely off-set by carcass fat (P<0.01, Table 5-5) which when averaged across sire

types was equivalent to a reduction in fat of 1.96%, 2.73% and 2.77% in the fore, saddle

and hind sections per unit of increasing sire PEMD (Table 5-6).

When compared at the same carcass weight, Maternal sired lambs had the greatest

increase in lean value per unit increase in PEMD ($4.30, Table 8), which was greater

than both of the Terminal ($2.98) and Merino ($2.64) sired lambs (Table 5-8). Across

their respective ranges of PEMD breeding values this equated to an increase in carcass

lean value of $18.51, $23.22 and $13.70 for the Maternal, Terminal and Merino sired

lambs (Table 5-8).

Sire PEMD had only a small impact on carcass weight which was consistent across sire

types (Gardner et al. 2015). Thus when this was factored into the calculation and

comparisons made at the same age, the Maternal sired lambs still had the greatest per

unit increase in carcass lean value ($5.65, Table 5-9) compared to either the Terminal or

Merino sired lambs. However, when calculated across the range of sire PEMD values

within each sire type the Terminal sired lambs had the greatest increase in carcass lean

value at $36.06 (Table 5-9), compared to $24.30 and $20.06 in the Maternal and Merino

sired lambs.

5.4.2.3.4 The impact of the Carcass Plus Index on carcass composition and lean value

The sire Carcass Plus Index impacted on the saddle and hind section lean, with the

impact on fore section dependent on sire type (P<0.01, Table 5-5). For every 10 units of

increase in Carcass Plus there was an increase of 0.35% and 0.49% in fore section lean


146

in the Maternal and Terminal sired lambs which amounted to a 4.2% and 3.7% increase

in lean across their respective ranges of the Carcass Plus Index values (Table 5-6). The

increase in lean for all sire types in the saddle and hind sections was 0.76 and 0.40% per

10 units of the Carcass Plus Index value (Table 5-6).

When lambs were compared at the same carcass weight (23kg), the Terminal and

Maternal sired lambs had similar increase in lean value per unit of Carcass Plus ($0.15

and $0.13, Table 5-8), with a smaller impact on the lean value of Merino sired lambs

($0.09/ unit Carcass Plus). Across the range of Carcass Plus this equates to an increase

of $11.27, $8.47 and $16.19 for the Terminal, Merino and Maternal sired lambs (Table

5-8).

Sire Carcass Plus Index also impacted on lamb carcass weight, although this varied

across the three sire types (Table 5-4) with the Merino sired lambs having the greatest

increase in HCWT per unit Carcass Plus (Table 5-4). When lambs were compared at the

same age, the Merino sired lambs had the greatest increase in lean value of $0.54 per

unit Carcass Plus (Table 5-9), compared to $0.48 and $0.37 for the Maternal and

Terminal sired lambs (Table 5-9). When compared across their respective ranges of

Carcass Plus this resulted in the greatest increase in lean value of $57.59 in the Maternal

sired lambs (Table 5-9).

.

Table 5-9 Predicted carcass value for the fore, saddle and hind sections of carcass for the Maternal, Merino and Terminal sired lambs for the range of

post weaning weight (PWWT), c-site fat depth (PFAT), c-site eye muscle depth (PEMD) and Carcass Plus Index values – wether progeny of Merino

dams at 280 days of age.


lean value ($)

Saddle section

lean value ($)

Hind section

lean value ($)

Carcass value

($)

$ difference

(high-low)

$ per unit

breeding value

PWWT

Maternal Low (-6.1) 66.10 81.70 92.26 240.06

Maternal High (12.4) 75.23 97.79 105.42 278.44 38.39 2.07

Merino Low (-5) 57.33 72.30 80.87 210.50

Merino High (10.8) 73.38 96.44 103.13 272.96 62.46 3.95

Terminal Low (5.3) 73.30 95.75 103.42 272.47

Terminal High (18.6) 79.73 108.06 112.88 300.67 28.20 2.10

PFAT

Maternal PFAT low (-2.1) 74.74 96.88 104.65 276.28

Maternal PFAT high (2.6) 66.15 81.32 93.09 240.57 -35.71 -7.60

Merino PFAT low (-1.9) 65.27 83.79 91.81 240.87

Merino PFAT high (1.9) 58.85 72.46 82.52 213.84 -27.03 -7.11

Terminal PFAT low (-2.5) 80.39 109.13 113.76 303.28

Terminal PFAT high (2.3) 71.14 91.48 100.23 262.86 -40.42 -8.42

PEMD

Maternal PEMD low (-2.5) 69.47 84.38 91.41 245.27

Maternal PEMD high (1.8) 73.23 93.60 102.74 269.57 24.30 5.65 Merino PEMD low (-2.6) 62.32 76.37 88.44 227.14

Merino PEMD high (2.6) 66.76 86.94 93.50 247.20 20.06 3.86 Terminal PEMD low (-2.8) 73.82 91.56 105.07 270.45

Terminal PEMD high (5.0) 80.89 109.65 115.97 306.51 36.06 4.62

147

Table 5-9 continued…..


lean value ($)

Saddle section

lean value ($)

Hind section

lean value ($)

Carcass value

($)

$ difference

(high-low) $ per unit

breeding value

Carcass plus

Maternal Carcass Plus low (64) 66.10 81.70 92.26 240.06

Maternal Carcass Plus high (184) 79.89 104.67 113.08 297.64 57.59 0.48 Merino Carcass Plus low (69) 57.33 72.30 80.87 210.50

Merino Carcass Plus high (160) 69.69 90.55 99.12 259.36 48.86 0.54

Terminal Carcass Plus low (134) 73.30 95.75 103.42 272.47

Terminal Carcass Plus high (209) 81.33 105.48 113.45 300.26 27.79 0.37

PWWT: post weaning weight; PFAT: Post weaning c-site fat depth; PEMD: Post weaning c-site eye muscle depth

148


149

5.5 Discussion

5.5.1 The impact of genetics on the value of the lamb carcass

5.5.1.1 The impact of sire type and dam breed on the value of the lamb

carcass

In support of our hypothesis, Terminal sired lambs had the greatest lean value when

compared at the same carcass weight. This was a reflection of their greater lean weight

across all three carcass sections compared to both Merino and Maternal sired lambs,

consistent with their larger mature size and therefore reduced maturity when compared

at the same weight (Huisman and Brown 2008). On the basis of these differences the

carcasses of Terminal sired lambs were worth $9.57 and $14.36 more than the Merino

and Maternal sired lambs.

When compared at the same age, the difference between sire types was much greater

due to their marked difference in carcass weights (Gardner et al. 2015). Thus the lean

from the carcasses of Terminal sired lambs were worth $53.84 and $32.60 more than

the Merino and Maternal sired lambs. These values also highlight the re-ranking of

Merino and Maternal sired lambs, with Merino’s more valuable when compared at the

same carcass weight, but less valuable when carcass weight (and therefore growth rate)

is factored in to the calculation. This partly contradicts the conclusions of

Ponnampalam, Hopkins, Dunshea, et al. (2007b) who stated that purebred Merino

lambs will always be less productive in terms of carcass weight and muscle related

productivity traits than Maternal sired lambs.

Compared at the same carcass weight, Merino dams produced lambs that had more fore

and hind lean compared to the BLM dams. This difference in lean weight was worth

$3.58, and aligns well with the sire type effect discussed above where the lean from


150

Merino lambs was worth $4.80 more than lambs from Maternal dams. Although not

previously compared on a dollar basis, the mass differences align well with previous

studies where, after correcting for carcass weight, lambs from BLM dams had less

carcass lean (Anderson et al. 2015c, Ponnampalam et al. 2008) and a higher proportion

of fat (Anderson et al. 2015d) than lambs from Merino dams. The increased fatness of

lambs from BLM genetics may be linked to their selection for improved reproduction

capacity (Ferguson et al. 2010).

When compared at the same age, the opposite trend was evident, with lean value of the

lambs from the BLM dams worth $20.12 more than that of the Merinos. Again, this

aligns well with the sire type comparison between Merino and Maternal sires, reflecting

the marked difference in carcass weights of lambs from these two dam breeds (Gardner

et al. 2015).

5.5.1.2 The impact of Australian Sheep Breeding Values on carcass

value

5.5.1.2.1 Post weaning c-site fat depth

In support of our hypothesis, when comparing lambs at the same carcass weight,

decreasing sire PFAT increased lean weight, corresponding with a substantial increase

in the value of carcass lean. Furthermore, this increase in the value of lean across the

range of PFAT ASBVs in the sired used, was substantially larger than the

corresponding effects of PEMD or PWWT. This aligns well with previous work which

showed that PFAT not only caused substantial increases in the mass of lean across the

entire carcass, but also a favourable distribution of lean to the more highly valued

saddle region (Anderson et al. 2015c) (Chapter 3). This is in contrast to PEMD where

only the weight/value of the saddle and hind sections were increased.


151

When lambs were compared at the same age, the difference in carcass lean value was

still quite similar across the PFAT range. This reflects the relatively small effect of the

PFAT breeding value on carcass weight, thus the main impact of PFAT on carcass value

is via its effect on composition. The potential impact of PFAT on the retail lamb value

through its impacts on eating quality (Pannier et al. 2014a) and intramuscular fat (IMF)

% (Pannier, Pethick, Geesink et al. 2014d) in addition to the value of the lean meat are

yet to be quantified.

5.5.1.2.2 Post weaning weight

The impact of PWWT on lean was predominantly focused on the saddle region, with no

effects evident in the fore or hind sections, contrary to our initial hypothesis. Thus lean

weight within the saddle increased across the PWWT range when compared at the same

carcass weight, resulting in a small increase in carcass lean value. Previous analyses

have demonstrated no overall increase in carcass lean weight due to the PWWT

breeding value (Anderson et al. 2015c) (Chapter 3), most likely because the small

increases in saddle lean were masked by the lack of effect elsewhere.

In contrast to these small effects, when compared at the same age, sire PWWT had a

marked impact on the lean value within the carcass, largely reflecting the substantial

impact of this breeding value on carcass weight. Furthermore, lean value of the Merino

sired lambs was almost twice the per unit increase evident within the Maternal and

Terminal sired lambs. This is due to the greater impact of PWWT on carcass weight

within the Merino sired lambs compared to the Terminal and Maternal sired lambs

(Gardner et al. 2015).


152

5.5.1.2.3 Post weaning eye muscle depth

At any given carcass weight, increasing sire PEMD increased the weight of lean in the

saddle and hind sections, with no significant increase in the fore section. These

combined effects lead to a substantial increase in lean value, aligning well with our

initial hypothesis. Earlier studies by Anderson, Williams et al. (2015c) (Chapter 3)

showed that PEMD preferentially increased lean weight in the saddle at the expense of

the fore section. Therefore it would be expected that the combined effects of the overall

increase in lean weight and the lean redistribution away from the fore section would

result in no net change to fore section lean weight. Likewise the overall increase in

carcass lean weight, and the redistribution of lean to the saddle region would amplify

the increase in lean weight in the more expensive saddle region cuts – as was evidenced

in this study. The Maternal sired lambs had the greatest increase in carcass lean value

per unit PEMD which was at least 1.5 times that of the Terminal and Merino sired

lambs. The Merino sired lambs had only a small increase in hind section lean weight

and therefore the smallest increase in lean value.

The increase in carcass weight caused by PEMD was relatively small, and consistent

across all three sire types (Gardner et al. 2015). Therefore, when lambs were compared

at the same age the increase in lean value resulting from increasing sire PEMD for all

sire types was greater than when lambs were compared at the same carcass weight.

5.5.1.2.4 Carcass plus

This breeding value index represents a way for lamb producers to select their Terminal

sires on the basis of a weighted combination of PEMD, PWWT, and PFAT. This

optimises the production of lambs that reach slaughter weights early, while maximising

lean meat and maintaining a lean carcass. It is not used by Merino breeders as it does

not incorporate selection for wool traits, with index selection in Maternal sires having


153

more emphasis on traits such as number of lambs weaned and ewe resilience, including

maintaining fat to optimise fertility. Despite these different selection pressures this

analysis provides important information that quantify the effect of the Carcass Plus

index on lean value in all sire types.

When lambs were compared at the same carcass weight, the Maternal and Terminal

sired lambs had a similar increase in lean value per unit of Carcass Plus. The Merino

sired lambs had less increase due to the negligible impact of Carcass Plus on the fore

section of the carcass.

However, when lambs were compared at the same age, the Merino sired lambs had the

greatest per unit increase in lean value compared to the other sire types. This is due to

the heavy weighting of PWWT in the Index and the increased impact of PWWT on the

carcass weight of the Merino sired lambs (Gardner et al. 2015).

This analysis provides important information regarding the impact of the Carcass Plus

index on the value of lean in the carcass. In the future, similar analyses may provide

some basis for restructuring the Carcass Plus Index to maximise lamb profitability. This

was well demonstrated in one of our previous analyses (Anderson et al. 2013a) which

calculated the loss in retail carcass value when the emphasis on PFAT within the index

was reduced from 30% to 5%, which was performed in order to maintain intramuscular

fat at a level that did not negatively impact eating quality (Pannier et al. 2014a, Pannier

et al. 2014d).

5.5.2 The impact of sex on the value of the lamb carcass

At the same carcass weight the wether lambs had more lean in all carcass sections

resulting in greater carcass value equivalent to $6.46 within a 23kg carcass. The largest


154

differences in lean weight were seen in the fore section, with the hind section showing

the least differences in lean weight between the wethers and ewe lambs. However, given

the high value of the saddle lean the difference within this tissue was amplified when

expressed in dollar terms. The increase in lean in wethers when lambs are compared at

the same weight (Anderson et al. 2015c, Lee et al. 1990, Ponnampalam et al. 2008)

(Chapter 3) and age (Ponnampalam et al. 2008) has been reported previously, although

not quantified holistically across carcass regions or expressed in dollar terms. These

effects on composition are a reflection of the impact of sex on mature composition, with

ewes being fatter than wethers (Anderson et al. 2015d, Ponnampalam et al. 2008)

(Chapter 4) and a reflection of mature size, with wethers being less mature and therefore

leaner when compared at the same weight (Butterfield 1988).

When comparisons were made at the same age, the mass difference between wethers

and ewes was amplified, due to the more rapid growth rate and therefore heavier

slaughter weights of wether lambs compared to ewes at the same age (Gardner et al.

2015). This leads to an even greater financial difference with lean value of wethers

being $18.01 greater than ewes. This difference represents an important advantage for

self-replacing ewe flocks where all wether lambs are slaughtered while the bulk of ewes

are retained for breeding.


When compared at the same carcass weight, the biggest influence on carcass lean value

was through the two breeding values, PFAT and PEMD. Reducing PFAT and

increasing PEMD both increased carcass lean value, although the impact of both was

greatest in Terminal sired lambs. This effect when assessed across the breeding value

range was at least double the next largest factor – in this case the sire type difference

between Terminal and Maternal sired lambs. It is possible that reducing sire PFAT and


155

increasing PEMD has an even greater impact beyond its impact on lean weight and

carcass weight due to the substantial decrease in carcass fat (Anderson et al. 2015d,

Gardner et al. 2010) (Chapter 4). These carcasses are less likely to attract penalties at

processing plants for being over fat. The impact of gender was the next largest effect,

with the difference between ewes and wethers only half the difference between

Terminal and Maternal sired lambs. The PWWT breeding value had a relatively small

effect, particularly when compared to the other two breeding values. The magnitude of

effect of PWWT across the breeding value range was just over half the magnitude of the

difference between genders. Lastly dam breed had a relatively small effect which was

similar in magnitude to the impact of PWWT.

When lambs were compared at the same age this allowed for the differences in lamb

size and therefore carcass weight at a given age to be included in the calculation of the

weight and value of lean. There was marked re-ranking of magnitudes between the

factors affecting value, predominantly driven by those which had the greatest impact on

carcass weight. As such, the difference between Terminal and Merino sire types and the

effect of the PWWT across its breeding value range, particularly within the Merino

sires, had the largest impact on carcass lean value. The differences between genders and

dam breeds also increased when differences in carcass weight at a given age were

accounted for, yet these differences were still less than the effect of the PFAT and

PEMD across their respective breeding value ranges.

5.5.4 Limitations and future work

This analysis enables the relative effects of gender and genetics on carcass lean to be

evaluated without relying solely on a selected few cuts like the topside, and loin to

indicate this value. However, allocating prices to this lean based on market prices as of

April 2015 immediately exposes this analysis to currency issues due to price


156

fluctuations. However, the ranking of prices for each cut within the lamb carcass has

remained relatively constant over time (Pethick et al. 2010), hence the results can still

be interpreted with respect to the ranking of fore, saddle, and hind quarter lean within

the carcass. Likewise, the effects identified in this paper are also likely to maintain their

relative proportional differences.

Other limitations to this work are the additional factors that are impacted upon by the

traits examined – particularly the carcass breeding values. For example the magnitude

of the changes observed in bone and fat have not been incorporated into this financial

evaluation which was instead restricted to the value of lean within the carcass. Indeed,

this value differential could also be extended to include the costs of trimming of

excessive fat from carcasses in the retail market. Likewise, this analysis does not

capture the impact of genetic selection (carcass breeding values, dam breed, sire type)

on the number of lambs weaned, feed efficiency, wool growth or ewe longevity. This

more detailed economic analysis is beyond the immediate scope of this study, yet would

certainly be of value to the industry when attempts are made to justify the introduction

of new technologies to capture this value.

Lastly, the introduction of eating quality assessment through Meat Standards Australia

(Meat Standards Australia, 2013), creates an opportunity for the associated effect on

eating quality to be incorporated into the economic values. Work by Pannier, Gardner,

et al. (2014a) showed that selection for improved LMY% using increased sire PEMD

and reduced sire PFAT had a negative impact on the eating quality of longissimus and

semimembranosus samples, partially through their impact on intramuscular fat

percentage. Therefore future multi-trait indexes are required that balance eating quality

(reflected through intramuscular percentage) with the existing growth and carcass traits


157

(Mortimer et al. 2014, Swan, Pleasants and Pethick 2015). More widespread

introduction of technology that accurately assesses LMY%, IMF% and eating quality

commercially and in breeding programs, alongside payment schemes that reward

producers, would encourage the use of these multi index selection tools.

5.6 Conclusion

The results of this study enable the direct comparison of carcass lean value of different

sire types, sexes and breeding values at the same carcass weight and at the same age.

When compared at the same carcass weight sire PFAT had the greatest impact on

carcass lean value, its magnitude of effect across the PFAT range being 1.5 times

greater than the next most influential breeding value, PEMD. However when compared

at the same age increasing sire PWWT had the greatest impact on carcass lean value,

due to largely to its effect on carcass weight. Compared to lambs from other sire types,

Terminal sired lambs had the greatest lean value irrespective of whether compared at

the same weight or age, closely followed by Merinos when compared at the same

weight, and Maternal sired lambs when compared at the same age.

Chapter 6 – Intramuscular fat and the impact of selection for lean meat yield

158

Chapter 6. Intramuscular fat in lamb muscle and the

impact of selection for improved carcass lean meat

yield.

The following chapter is the version accepted for publication:

Anderson, F., Pannier, L., Pethick, D.W., Gardner, G.E. (2015) Animal 9(6): 1081-

1090.

6.1 Abstract

Intramuscular fat percentage (IMF%) has been shown to have a positive influence on

the eating quality of red meat. Selection of Australian lambs for increased lean tissue

and reduced carcass fatness using Australian Sheep Breeding Values has been shown to

decrease IMF% of the M. longissimus lumborum. The impact this selection has on the

IMF% of other muscle depots is unknown. This study examined IMF% in 5 different

muscles from 400 lambs (M. longissimus lumborum, M. semimembranosus, M.

semitendinosus, M. supraspinatus, M. infraspinatus). The sires of these lambs had a

broad range in carcass breeding values for post weaning weight, eye muscle depth, and

fat depth over the 12th

rib (c-site fat depth). Results showed IMF% to be highest in the

M. supraspinatus (4.87±0.1, P<0.01) and lowest in the M. semimembranosus (3.58±0.1,

P<0.01). Hot carcass weight was positively associated with IMF% of all muscles.

Selection for decreasing c-site fat depth reduced IMF% in the M. longissimus

lumborum, M. semimembranosus and M. semitendinosus. Higher breeding values for

post weaning weight and eye muscle depth increased and decreased IMF% respectively,

but only in the lambs born as multiples and raised as singles. For each percent increase

in lean meat yield % there was a reduction in IMF% of 0.16 in all 5 muscles examined.


159

Given the drive within the lamb industry to improve lean meat yield %, our results

indicate the importance of continued monitoring of IMF% throughout the different

carcass regions, given its importance for eating quality.


160

6.2 Introduction

Consumers in both domestic and international markets have an increasing desire for

lamb which produces retail cuts of meat that are well muscled and low in salvage fat,

representing value for money and healthy meal options (Pethick et al. 2006a). The

Australian lamb industry has responded to these market drivers to select for larger,

leaner lambs (Banks 2002, Hall et al. 2000, Laville et al. 2004). Health characteristics

such as low levels of fat and the nutritional content will remain important (Harper and

Pethick 2004b, Pannier et al. 2014b), however eating quality has also been shown to be

important to consumers (Harper and Pethick 2004b). Intramuscular fat percentage

(IMF%) is a key determinant of eating quality in red meat and it is well accepted that

IMF% has a positive impact on flavour, juiciness and tenderness (Hopkins et al. 2006a,

Pannier et al. 2014a, Thompson 2004b). In lamb, it has been suggested that a minimum

of 4-5 IMF% is required for Australian consumer satisfaction with regard to palatability

(Hopkins et al. 2006a). Accordingly the IMF% of the Muscularis longissimus

lumborum (short loin) has been identified as a key factor for maintaining premium

eating quality of lamb (Pannier et al. 2014a, Pannier et al. 2014c).

Given that the loin musculature contains the most valuable cuts in the carcass, previous

research has focused on the eating quality and IMF% of this muscle (McPhee et al.

2008, Pannier et al. 2014a, Pannier et al. 2014c). The IMF% of muscles other than the

M. longissimus lumborum has not been well described in lamb. These levels are likely

to vary between muscles, in part as a consequence of functional variation due to muscle

fibre type (Hocquette et al. 2010). Muscles composed of more oxidative fibres contain

more triglycerides and so IMF (Hocquette et al. 2010). Muscles responsible for

maintenance of posture tend to be more oxidative and are predominantly comprised of

Type 1 fibres with a propensity for higher IMF% (Picard et al. 2002).


161

A key factor driving reduced IMF% in the M. longissimus lumborum is selection for

lean growth. Pannier et al. (2014c) demonstrated an association between various carcass

indicators of fatness and IMF% in the M. longissimus lumborum, whilst Gardner et al.

(2010) reported a negative phenotypic (-0.24) and genetic correlation (-0.46) between

IMF% and lean meat yield percentage (LMY%). Selection for improved LMY% in

Australian sheep is performed indirectly through the use of Australian Sheep Breeding

Values (ASBVs) to increase post weaning weight (PWWT) and c-site eye muscle depth

(PEMD), and reduced c-site fat depth (PFAT; c-site defined as a measure 45 mm from

the midline over the 12th

rib). Pannier et al.(2014c) demonstrated that decreasing PFAT

and increasing PEMD both reduced IMF% in the M. longissimus lumborum. A

decreased PFAT breeding value reduces total carcass fatness which would include IMF.

Moreover, sires with high PEMD and/or low PFAT breeding values had increased

weight of the M. longissimus lumborum, located in the saddle section (Anderson et al.

2013b, Anderson et al. 2015c, Gardner et al. 2010)(Chapter 3) and had proportionately

more lean weight in the saddle (loin) section than in other regions of the carcass.

Therefore it seems likely that the impact of these breeding values on IMF%, delivered

through their effects on muscle hypertrophy and so dilution of IMF%, will be greater in

the saddle musculature.

Lambs from sires selected for high PWWT may also have reduced IMF%. These lambs

are faster growing due to their larger mature size (Huisman and Brown 2008), thus

when compared at the same carcass weight, they will be leaner and less mature and

subsequently have reduced IMF%. However, in the study of Pannier et al. (2014c) there

was no association of PWWT with IMF% in the M. longissimus lumborum, suggesting


162

that any maturity linked effect may be too subtle to impact. This is likely to extend to

other muscles of the carcass.

Therefore, given the negative phenotypic correlation of LMY% and IMF% we

hypothesised that as LMY% increases, measured in this study by computed tomography

(CT) lean percentage (lean%), IMF% will decrease. Based on the impact of ASBVs

used to improve LMY% on carcass composition, we hypothesised that lambs from high

PEMD sires or from low PFAT sires will have reduced IMF% in the short loin, but to a

lesser extent in the hind and fore sections, and that increasing sire PWWT will have no

impact on IMF% in the carcass. Lastly, we hypothesised that the IMF% of muscles in

postural regions of the carcass will have greater IMF% than muscles in locomotive

regions.



The Australian Cooperative Research Centre for Sheep Industry Innovation established

an Information Nucleus Flock commencing in 2007 (Fogarty et al. 2007). This paper

examines data from 400 lambs born at Katanning, Western Australia in 2011. The

lambs (Merino, Maternal x Merino, Terminal x Merino and Terminal x Border

Leicester-Merino) were the progeny of 97 industry sires, representing the major sheep

breeds used in the Australian industry. The sires types included 46 Terminal sires

(Hampshire Down, Poll Dorset, Suffolk, Texel, White Suffolk), 16 Maternal sires

(Border Leicester, Coopworth, Corriedale, Dohne Merino, Prime South African Meat

Merino), and 35 Merino sires (Merino, Poll Merino). These sires had ASBVs for

PWWT, PEMD, and PFAT which were all sourced from Sheep Genetics, which is

Australia’s national genetic evaluation database for sheep (Brown et al., 2007). The sire

breeding values were generated within 3 separate data-bases for Terminal, Maternal,


163

and Merino sired progeny. This was from an analysis completed in August 2013, and

excluded progeny from the Information Nucleus Flock. Some of the youngest sires used

in this experiment lacked industry records and therefore did not have ASBVs available.

The ranges for these ASBVs varied within sire types as shown in Table 6-1.

Table 6-1 Number of lamb sires and mean (min, max) of Australian Sheep Breeding

Values for each sire type.

Sire type No. of

sires PWWT (kg) PFAT (mm) PEMD (mm)

Maternal 16 5.9 (-3.1, 12.4) -0.8 (-2.1, 0.6) -0.1 (-1.6, 1.8)

Merino 35 2.7 (-3.6, 10.8) -0.1 (-1.4, 1.9) 0.1 (-2.6, 2.0)

Terminal 46 13.3 (7.3, 18.6) -0.44 (-1.7, 1.3) 1.5 (-0.7, 3.8)

PWWT= Post weaning weight; PFAT= Post weaning c-site fat depth; PEMD= Post

weaning eye muscle depth.

Pregnant ewes were maintained on grass and supplementary fed grain only if pasture

was limited. Ewes were scanned for multiple pregnancies and managed in groups with

stocking density appropriate for paddock size, with no difference in the pasture

available between different sire groups or birthing and rearing types.

Lambs were yarded the day prior to slaughter and transported to a commercial abattoir

in Katanning, held in lairage overnight and slaughtered the following day at a target

average carcass weight of 21.4 kg. Carcasses were subjected to medium voltage

electrical stimulation (Pearce et al. 2010), and then sampled the day after slaughter for a

wide range of carcass and meat quality traits after being chilled overnight (4°C).

6.3.2 Sample collection and measurements

Hot carcass weight (HCWT) was measured immediately after dressing and carcasses

were transported to Murdoch University to undergo computed tomography (CT)

scanning within 72 hours of slaughter (For details of computed tomography scanning,


164

see Anderson et al. (2015b)(Chapter 7). CT scanning of the carcasses enabled the lean

percentage to be determined, which was used as a covariate in some of the statistical

models.

Following CT scanning, the individual muscles were dissected from each carcass,

weighed and samples collected for IMF% measurement: from the fore section, the

Muscularis supraspinatus and Muscularis infraspinatus; from the saddle section, the M.

longissimus lumborum; and from the hind section, Muscularis semimembranosus and

Muscularis semitendinosus. Due to carcass imperfections all five muscle could not

always be obtained from each carcass. Intramuscular fat was determined on all of these

muscles using the method described by Anderson et al. (2015b) (Chapter 7).


The data analysed in the base model has been summarised in Table 6-2. The IMF%

were analysed using linear mixed effect models (SAS Version 9.1, SAS Institute, Cary,

NC, USA). The base model included fixed effects for muscle (M. semimembranosus, M.

semitendinosus, M. longissimus lumborum, M. supraspinatus, M. infraspinatus), sex

within sire type (Merino wether, Maternal wether, Terminal female and Terminal

wether), birthing and rearing type (term representing if lamb was born and reared as a

single, born as multiple and raised as single or born and raised as a multiple), sire type

(Merino, Maternal, Terminal), kill group and dam breed within sire type (Merino x

Merino, Maternal x Merino, Terminal x Merino and Terminal x Border Leicester-

Merino). Sire identification, dam identification and animal identification were included

as random terms. All relevant first order interactions between fixed effects were tested

and non-significant (P>0.1) terms were removed in a stepwise manner.

165

Table 6-2 Number of lambs analysed in the base model according to sire type, sex, birthing and rearing type, dam breed and kill group.

Sex Birth-rearing type Dam breed Kill group1

Female Male

Single born

and raised

Born as multiple-

raised as single

Born and raised

as multiple Merino BLM

2 167K11 238K11 280K11 355K11

Maternal 0 92

34 6 52

92 0

6 16 32 38

Merino 0 70

32 10 28

70 0

0 1 13 56

Terminal 111 127

96 24 117

140 95

95 83 55 5

Total 111 289 162 40 197 302 95 101 100 100 99 1Kill group= average age of lambs at slaughter followed by location and birth year (2011); K= Katanning.

2BLM: Border Leicester x Merino


166

The association between IMF% and sire ASBVs for PWWT, PEMD and PFAT were

tested in the base model. These ASBVs were initially included concurrently as

covariates along with linear and quadratic interactions with the fixed effects. Non-

significant (P>0.1) terms were removed in a stepwise manner. Correlations exist

between the three ASBVs, therefore this process was repeated with ASBVs included

one at a time to determine the independence of their effects.

The base and ASBV models described above were additionally tested with HCWT

included in the model as a covariate to determine the effect of HCWT on IMF%, and

whether the fixed and ASBV effects were associated with HCWT. Similarly this

process was repeated using CT lean% and carcass weight at the time of CT scanning to

determine the impact of carcass composition on IMF%. The relevant linear and

quadratic interactions with these covariates were also included. The mean and

distribution of the HCWT and CT lean% data is shown in Table 6-3.


167

Table 6-3 Lamb hot carcass weight (kg) and carcass lean percentage as measured by

computed tomography displaying raw mean ± SD (min, max).

Hot carcass weight (kg) CT Lean %

Birth type-rear type

Born and raised single

20.9 ± 2.78

(15.3, 29.0)

59.2 ± 2.62

(52.1, 66.2)

Born multiple-raised single

21.4 ± 2.50

(17.0, 27.3)

58.8 ± 2.57

(54.1, 65.0)

Born and raised as multiple

21.9 ± 2.87

(13.5, 27.8)

58.1 ± 2.82

(50.9, 64.7)

Sire type x Dam breed Sex

Maternal x Merino Wether 21.1 ± 2.51

(13.9, 26.4)

57.6 ± 2.83

(50.9, 65.9)

Merino x Merino Wether 19.6 ± 2.28

(13.5, 24.6)

58.9 ± 2.20

(53.9, 63.5)

Terminal x Merino Wether 22.5 ± 2.78

(17.0, 29.0)

59.5 ± 2.42

(55.0, 66.2)

Terminal x Merino Female 21.8 ± 2.57

(17.0, 27.1)

58.3 ± 2.52

(53.5, 63.3)

Terminal x BLM

Wether 22.0 ± 3.06

(17.5, 27.7)

59.6 ± 2.80

(54.0, 64.7)

Terminal x BLM Female 22.2 ± 2.90

(16.5, 28.4)

58.3 ± 2.90

(51.9, 65.0)

Kill group

167 K11

22.5 ± 2.78

(17.0, 29.0)

59.5 ± 2.42

(55.0, 66.2)

238 K11

19.6 ± 2.16

(16.5, 26.6)

61.2 ± 2.10

(56.0, 66.2)

280 K11

21.1 ± 1.97

(16.2, 25.8)

57.2 ± 2.37

(50.9, 62.9)

355 K11

20.7 ± 2.79

(13.5, 27.4)

58.3 ± 2.46

(51.9, 63.5)

CT Lean % = percentage of lean in the carcass as measured by computed tomography

BLM: Border Leicester x Merino

Kill group: average age of lambs at slaughter, followed by location (where K=

Katanning) and birth year (2011).


168

6.4 Results

6.4.1 Effect of non-genetic factors

The base model used 1900 of the 1908 observations available, after excluding animals

with missing data and described 52% of the total variance in IMF%. The IMF% varied

between all muscles examined (P<0.01, Table 6-4). The mean for IMF% from highest

to lowest was M. supraspinatus (4.87±0.1), M. semitendinosus (4.54±0.1), M.

longissimus lumborum (4.21±0.1), M. infraspinatus (3.86±0.1) and M.

semimembranosus (3.58±0.1). Within the Terminal sired lambs, females (4.59±0.08)

had on average 0.2 IMF% more (P<0.01, Table 6-4) than wether lambs (4.38±0.08).

Birth type-rear type impacted on IMF% (P<0.05, Table 6-4). However this was only

evident in the Merino sired lambs, with the multiple born and raised lambs (4.23±0.14)

being 0.5 IMF% higher than that of the single born and raised or multiple born and

single raised lambs. The IMF% increased with each successive kill group (P<0.01,

Table 6-4), and on average there was an increase of 1.3 IMF% between the first and last

kill group (Table 6-5). This increase varied between muscles (P<0.01, Table 6-4), with

the greatest increase of 1.8 IMF% seen in the M. supraspinatus and the smallest

increase of 0.9 IMF% seen in the M. semimembranosus (Table 6-5).

Table 6-4 F values, and numerator and denominator degrees of freedom, for the effects of the base linear mixed effects model, corrected for hot carcass

weight (HCWT), computed tomography lean % and Australian Sheep Breeding Values on intramuscular fat % of lamb muscles (Muscularis

semimembranosus, Muscularis semitendinosus, Muscularis supraspinatus, Muscularis infraspinatus and Muscularis longissimus lumborum).

Effect

Model not corrected for hot

carcass weight (kg)

Model corrected for hot

carcass weight (kg)

Model corrected for

computed tomography lean

%

Model corrected for

Australian Sheep Breeding

Values NDF, DDF F-value NDF, DDF F-value NDF, DDF F-value NDF, DDF F-value

Muscle

4, 1476 153.31*** 4, 1423 0.47

4, 1446 150.98*** 4, 1407 104.14***

Sex(sire type)

1, 1476 7.58***

1, 1423 5.8**

NS NS

1, 1407 7.43***

Birth-type rear-type

2, 1476 3.45**

2, 1423 3.93**

2, 1446 5.83***

2, 1407 5.12***

Sire type

2, 1476 7.8***

2, 1423 3.07**

2, 1446 3.24**

2, 1407 3.12**

Kill group

3, 1476 29.77***

3, 1423 23.85***

3, 1446 25.06***

3, 1407 25.87***

Dam breed(sire type)

1, 1476 7.46***

1, 1423 4.51**

1, 1446 2.37

1, 1407 6.06**

Muscle x sire type

8, 1476 3.52***

8, 1423 1.88*

8, 1446 3.68***

8, 1407 4.1***

Muscle x kill group

12, 1476 5.69***

12, 1423 4.43***

12, 1446 5.35***

12, 1407 5.19***

Muscle x dam breed(sire type)

4, 1476 6.54***

4, 1423 5.39***

4, 1446 7.11***

4, 1407 6.72***

Birth-type rear-type x sire type

4, 1476 3.21**

4, 1423 3.01**

4, 1446 2.92**

4, 1407 2.69**

HCWT2

- -

1, 1423 13.25***

- -

- -

HCWT x muscle

- -

4, 1423 2.57**

- -

- -

HCWT x sire type

- -

2, 1423 2.22

- -

- -

HCWT x muscle x sire type

- -

8, 1423 2.14**

- -

- -

CT3 Weight (kg)

- -

- -

1, 1446 7.55***

- -

CT Weight (kg) x CT Weight

(kg) - -

- -

1, 1446 6.73***

- -

CT Lean %

- -

- -

1, 1446 44.77***

- -

169

Effect

Model not corrected for hot

carcass weight (kg)

Model corrected for hot

carcass weight (kg)

Model corrected for

computed tomography lean

%

Model corrected for

Australian Sheep Breeding

Values NDF, DDF F-value NDF, DDF F-value NDF, DDF F-value NDF, DDF F-value

PWWT4

- -

- -

- -

1, 1407 0.64

PWWT x birth-type-rear-type

- -

- -

- -

2, 1407 4.63***

PFAT5

- -

- -

- -

1, 1407 0.65

PFAT x muscle

- -

- -

- -

4, 1407 2.31*

PFAT x PFAT

- -

- -

- -

1, 1407 0.06

PFAT x PFAT x muscle

- -

- -

- -

4, 1407 2.13*

PEMD6

- -

- -

- -

1, 1407 2.5

PEMD x birth-type-rear-type

- -

- -

- -

2, 1407 3.05**

PEMD x PEMD

- -

- -

- -

1, 1407 0.38

PEMD x PEMD x birth-type-

rear-type - - - - - - 2, 1407 3.42**


HCWT: hot carcass weight. CT: computed tomography. PWWT: post weaning weight; PFAT: Post weaning c-site fat depth; PEMD: Post weaning c-site eye muscle depth;

NS: not significant

* P < 0.1; ** P < 0.05; ** P < 0.01

170

Table 6-4 continued……

Table 6-5 Lamb intramuscular fat percentage for sex, dam breed, sire type and kill groups within the Muscularis semimembranosus, Muscularis

semitendinosus, Muscularis supraspinatus, Muscularis infraspinatus and Muscularis longissimus lumborum (not corrected for hot carcass weight).

M. infraspinatus M. longissimus

lumborum

M. semi-

membranosus

M.

supraspinatus

M.

semitendinosus

Sex Dam

breed Sire type

Least Squared Means ± SE

Wether Merino Maternal

3.78± 0.13 ab

4.33± 0.13ab

3.63± 0.13a 4.83± 0.13

ab 4.60± 0.13

b

Wether Merino Merino

3.71± 0.15 a 3.89± 0.15

c 3.36± 0.15

a 4.66± 0.15

a 3.95± 0.15

a

Female BLM Terminal

4.24± 0.23 b 4.54± 0.23

b 3.74± 0.23

a 5.29± 0.23

bc 5.54± 0.23

d

Female Merino Terminal

4.02± 0.15 ab

4.45± 0.15b 4.00± 0.15

b 5.05± 0.15

abc 4.98± 0.15

bc

Wether BLM Terminal

4.18± 0.16 ab

4.45± 0.15b 3.62± 0.15

a 5.31± 0.16

c 5.12± 0.15

cd

Wether Merino Terminal

3.88± 0.14 ab

4.15± 0.13abc

3.69± 0.13a 4.80± 0.14

a 4.66± 0.14

bc

Kill group

167K11

3.18± 0.15

w 3.59± 0.14

w 3.39± 0.14

wx 4.04± 0.14

w 3.97± 0.14

w

238K11

3.51± 0.12

x 4.09± 0.12

x 3.30± 0.12

w 4.60± 0.12

x 4.41± 0.12

wx

280K11

4.08± 0.10

y 4.39± 0.10

x 3.37± 0.10

w 4.98± 0.10

y 4.62± 0.10

x

355K11 4.67± 0.11z 4.75± 0.11

z 4.28± 0.11

x 5.86± 0.11

z 5.17± 0.11

y

BLM: Border Leicester x Merino

Kill group: average age of lambs at slaughter followed by location and birth year (2011); K= Katanning. a-d

Values within a column with different superscripts differ significantly at P<0.05. w-z

Values within a column with different superscripts differ significantly at P<0.05

171


172

When the model was corrected for HCWT, the impact of kill group was reduced with

IMF% differing by only 1 IMF% on average across the four kill groups. When both CT

Lean % and carcass weight at scanning were included in the model sex(sire type) was

no longer significant.

6.4.2 Effect of genetic factors

6.4.2.1 Effect of sire type and dam breed

In the base model, sire was significant at P=0.08 with sire estimates varying between

3.66 and 4.81 IMF% within Maternal, 3.34 and 4.49 IMF% within Merino and 3.91 and

5.06 IMF% within Terminal sired lambs.

Comparison of IMF% between sire types was possible only in the male progeny of

Merino dams. The Merino sired lambs had on average 0.15 and 0.32 less IMF% than

the Maternal and Terminal sired lambs (P<0.01, Table 6-4 and Table 6-5). The greatest

difference in IMF% was seen in the M. semitendinosus where the Merino sired lambs

(3.95± 0.15) had 0.71 and 0.65 IMF% less than both the Terminal and Maternal sired

animals. In the M. longissimus lumborum, the Merino sired lambs had 0.44 IMF% less

than the Maternal sired lambs, but were not different compared to Terminal sired lambs.

There were no differences between sire types in the other muscles.

The impact of dam breed on IMF% was assessed in the Terminal sired lambs.

Differences were evident (P<0.01, Table 6-4) within the M. infraspinatus, M.

supraspinatus and M. semitendinosus, where the lambs from Border Leicester-Merino

dams had IMF% of 4.21±0.11, 5.29±0.11, 5.34±0.11, compared to lambs from Merino

dams with IMF% of 3.96±0.11, 4.92±0.11, 4.81±0.11 (values represent mean of male

and female progeny in Table 6-5). A similar trend (P<0.1) was seen in the M.


173

longissimus lumborum where lambs from Border Leicester-Merino dams had IMF% of

4.50±0.10 compared to 4.30±0.10 for lambs from Merino dams.

Correcting the model for HCWT accounted for the difference between sire types in the

M. longissimus lumborum, and partly accounted for the difference in the M.

semitendinosus, with the Merino sired lambs having only 0.5 IMF% less in the M.

semitendinosus than the Maternal and Terminal sired lambs. The difference between

dam breeds was relatively unchanged. Including CT Lean % and carcass weight at

scanning accounted for the differences between dam breeds.

6.4.2.2 Effect of Australian Sheep Breeding Values

When the sire ASBVs for PWWT, PFAT and PEMD were included at the same time in

the base linear mixed effects model, all three demonstrated a significant effect (P<0.1,

Table 4).The impact of sire PWWT on IMF% only impacted in lambs born as a

multiples and raised as a singles (P<0.01, Table 6-4). In this group of lambs a 1 unit

increase in sire PWWT was associated with a 0.08 IMF% increase (Figure 6-1). The

PWWT effect was independent of its impact on HCWT.


174

Figure 6-1 The relationship between intramuscular fat % in lamb and sire post weaning

weight (PWWT) Australian Sheep Breeding Value, for lambs born as multiples and

raised as singles. Symbols (●) represent residuals for each lamb as deviations from the

predicted means for intramuscular fat%. Line represents least square means (±SE as

dashed lines) across the PWWT range.

The impact of sire PFAT ASBV differed between muscles (P<0.05, Table 6-4). This

effect was non-linear and across a 2.75 unit range (1.0 to -1.75 mm) of decreasing

PFAT values, IMF% in the M. semitendinosus and M. semimembranosus decreased by

0.4 IMF% (Figure 6-2) and the M. longissimus lumborum decreased by 0.3 IMF%.

There was only one lamb with a sire PFAT value greater than 1.5 mm, however when

this animal was removed from the analysis there was no change to the magnitude of the

responses. The fore section muscles showed no change in IMF% in response to

decreasing sire PFAT.

2

2.5

3

3.5

4

4.5

5

5.5

-4 1 6 11 16

Intr

am

uscula

r fa

t %

Sire post weaning weight (kg) Australian Sheep Breeding Value


175

Figure 6-2 The relationship between intramuscular fat % in lamb and sire post weaning

c-site fat depth (PFAT) Australian Sheep Breeding Value, for the M.

semimembranosus. Symbols (●) represent residuals as deviations from the predicted

means for intramuscular fat%. Line represents least square means (±SE as dashed lines)

across the PFAT range.

An impact of increased sire PEMD was observed only in the lambs born as multiples

and raised as singles (P<0.05, Table 6-4). This effect was non-linear and across a 3 unit

range of sire PEMD (-1.1 to 2.25 mm) IMF% decreased in single raised lambs by 0.9

IMF% (Figure 6-3).

2

2.5

3

3.5

4

4.5

5

5.5

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

Intr

am

uscula

r fa

t %

Sire post weaning c-site fat depth (mm) Australian Sheep Breeding Value


176

Figure 6-3 The relationship between intramuscular fat% in lamb and sire post weaning

eye muscle depth (PEMD) Australian Sheep Breeding Value, for lambs born as

multiples and raised as singles. Symbols (●) represent residuals as deviations from the

predicted means for intramuscular fat%. Line represents least square means (±SE as

dashed lines) across the PEMD range (±SE).

The association of the ASBVs with IMF% did not alter when the ASBVs were included

individually in the base model. When HCWT was included in the ASBV model, the

effects did not change except for the magnitude of the PWWT effect which was reduced

by a quarter.

Including CT Lean % and carcass weight in the ASBV statistical model reduced the

magnitude of the PEMD and PWWT effects in the lambs born as multiples and raised as

singles by 0.2 IMF%. Within the same model, the impact of PFAT was altered, with a

decrease in IMF% observed only in the M. semimembranosus (0.1 IMF% across the

decreasing 2.75 unit PFAT range). In this model the M. supraspinatus and M.

infraspinatus increased in IMF% by 0.4 and 0.2 as PFAT reduced from 1 to -1.75.

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

-1.25 -0.75 -0.25 0.25 0.75 1.25 1.75 2.25 2.75 3.25

Intr

am

uscula

r fa

t %

Sire post weaning eye muscle depth (mm) Australian Sheep Breeding Value


177

6.4.3 Effect of hot carcass weight and lean meat yield percentage

When HCWT was included in the model as a covariate, heavier carcasses had more

IMF%, although this varied between the different muscles and sire types (P<0.01, Table

6-4). On average, IMF% increased by 0.82 units over a 13 kg HCWT range (15 to 28kg)

across all muscles and sire types which is equivalent to 0.06 IMF%/kg of HCWT. Most

muscles followed this trend, with the outlier to this being the M. supraspinatus in the

Merino sired lambs where IMF went up by 0.27 IMF%/kg of HCWT (Table 6-6).

Excluding the M. supraspinatus, the Merinos increased at 0.07 IMF% per kg HCWT

compared to the Maternal and Terminal sired lambs which increased on average by 0.04

IMF% per kg HCWT. When comparing muscles, the M. semitendinosus increased at

the greatest rate per unit HCWT (0.07 IMF%/kg HCWT), followed by the M.

infraspinatus (0.05 IMF%/kg HCWT), M. longissimus lumborum (0.04 IMF%/kg

HCWT), M. semimembranosus (0.04 IMF%/kg HCWT).

Table 6-6 Coefficients ± standard error (SE) for the Muscularis semimembranosus,

Muscularis semitendinosus, Muscularis supraspinatus, Muscularis infraspinatus and

Muscularis longissimus lumborum in the model corrected for hot carcass weight.

Coefficient ± SE

Muscle Maternal Merino Terminal

M. infraspinatus 0.03±0.04 0.10±0.05 0.01±0.03

M. longissimus lumborum 0.03±0.04 0.06±0.05 0.04±0.03

M. semimembranosus 0.04±0.04 0.04±0.05 0.04±0.03

M. supraspinatus 0.06±0.04 0.27±0.05 0.03±0.03

M. semitendinosus 0.07±0.04 0.09±0.05 0.04±0.03

When CT Lean % was included in the base model along with carcass weight at scanning

it was associated with a decrease in IMF% (P<0.01, Figure 4). On average IMF%

decreased by 1.3 units as whole carcass CT Lean % increased from 52% to 66% (Figure

6-4).


178

Figure 6-4 The relationship between intramuscular fat % in lamb and the percentage of

lean in the carcass, as measured by computed tomography (CT). Symbols (o) represent

residuals as deviations from the predicted means for intramuscular fat%. Line

represents least square means (±SE as dotted lines) across the range of CT lean %.

6.5 Discussion

Recent work by Pannier et al. (2014c) shows IMF% of the M. longissimus lumborum in

Australian lamb to be 4.2 IMF%, aligning well with the results in this study (4.2±0.1).

Other Australian work by Warner et al. (2010b), showed a mean IMF% of the M.

semitendinosus of 3.11±0.3, which is slightly lower than the IMF% in this study

(3.6±0.1) but similarly to our study shows IMF% to be lower than the M. longissimus

lumborum. Comparisons of IMF% in studies from other countries are difficult, as lambs

are from vastly different genotypes, environments, and are slaughtered at greatly

varying ages, all of which have been shown to impact IMF% (Pannier et al. 2014c). The

variation in the IMF% of the five muscles examined was not as large as had been

observed in other species such as beef (Brackebrush et al. 1991), with the ranking of

muscles also being vastly different. Brackebrush et al. (1991) found the highest IMF%

1.5

2.5

3.5

4.5

5.5

6.5

7.5

50 52 54 56 58 60 62 64 66

Intr

am

uscula

r fa

t %

Carcass % of lean tissue


179

in the M. infraspinatus, followed by the M. longissimus lumborum, M. supraspinatus,

M. semimembranosus and M. semitendinosus. The reasons for the differences in the

Brackebrush et al. (1991) study compared to ours is difficult to explain, although the

comparison is imperfect given that the animals were older in the study of Brackebrush

et al. (1991), and this may be influenced by variation in the maturation patterns of the

individual muscles and therefore IMF%.

In contrast to our hypothesis, the variation in IMF% between muscles was not strictly

related to postural versus locomotive muscle function. Variation in IMF% has been

shown to relate to species, breed type, gender and muscle fibre type (Hocquette et al.

2010, Pannier et al. 2014c). Fibre type is related to the function of the muscle, therefore

muscle location and function is likely to impact on IMF%. Muscles responsible for

maintenance of posture are more oxidative and are predominantly comprised of Type 1

fibres with a propensity for higher IMF% (Picard et al. 2002). Within a muscle, more

oxidative fibres contain more phospholipids and triglycerides, and conversely muscles

with high glycolytic activity have lower IMF% (Hocquette et al. 2010). On this basis

we expected that the M. longissimus lumborum would have the highest IMF% of the

five muscles examined, being a stabiliser muscle, however this was not observed in our

results. Alternatively, the M. supraspinatus could be considered a postural muscle as it

helps to bear body weight and therefore would be expected to have high IMF%. It had

greater IMF% than the M. infraspinatus which can be considered a locomotive muscle

as it is used for extension and flexion of the shoulder joint in sheep (Suzuki 1995).

A study examining the metabolic characteristics in sheep classified both the M. supra-

and M. infraspinatus muscles as being more oxidative relative to the M.

semimembranosus, M. semitendinosus and M. longissimus lumborum (Briand, Talmant,


180

Briand et al. 1981). Based on fibre type and metabolic activity, it is unclear why the M.

infraspinatus has less IMF% than the M. semitendinosus and M. longissimus lumborum.

The M. semitendinosus is considered a fast glycolytic muscle (Briand et al. 1981,

Gardner, Hopkins, Greenwood et al. 2007, Hocquette et al. 2012) especially in

comparison to the M. semimembranosus and M. longissimus lumborum. Therefore it

would be expected for the M. semitendinosus to have less IMF% than the latter two

muscles, which was contrary to our findings. Our findings would support the latter

findings indicating that muscle function/fibre type alone cannot be used to predict

IMF% of a muscle.

6.5.1 Genetic influence on intramuscular fat

6.5.1.1 The impact of Australian Sheep Breeding Values on

intramuscular fat

In support of our hypothesis, decreasing sire PFAT ASBV decreased IMF%, with the

magnitude of this effect varying between muscles. As expected there was a marked

impact in the M. longissimus lumborum, with a 0.3 unit decrease in IMF% over a 2.75

unit range of PFAT. However, there was a marginally greater effect in the M.

semimembranosus and M. semitendinosus, and no effect in the muscles of the fore

section. Assuming that this impact is associated with increased muscle hypertrophy

(Hocquette et al. 2010), this aligns well with the hypertrophy effects induced by PFAT

which Anderson et al. (2013b) demonstrated were focused on the saddle section of the

carcass, with least impact in the fore section. The reduced IMF% in the M. longissimus

lumborum was slightly less than that observed by Pannier et al. (2014c) where IMF%

decreased by 0.17% per mm reduction in PFAT, however the results of Pannier et al.

(2014c), in a larger data set, were only evident for the Terminal sired animals.


181

Contrary to our hypothesis, increasing sire PEMD ASBV had no effect on the IMF% of

muscle tissue in the majority of lambs studied in this experiment. The only effect was

seen in multiple born lambs raised as singles, where IMF% was reduced in all of the

five muscles examined. The basis for our PEMD hypothesis was associated with muscle

hypertrophy in the saddle section, with previous studies demonstrating that PEMD

increased the weight of the loin muscle (Gardner et al. 2010), and increased the

proportion of lean in the saddle section (Anderson et al. 2013b). However, a recent

analysis of the composition data derived from CT scanning of the 400 animals in this

study showed that PEMD did not increase lean saddle weight, so the lack of PEMD

impact is not unexpected. These results contrast with work by Pannier et al. (2014c)

who demonstrated that increasing PEMD reduced IMF% in the M. longissimus

lumborum. However, this effect was only evident in the Terminal sired lambs, and

appeared to be driven by a small number of sires that were extremes for this ASBV.

Indeed, the lack of impact of PEMD in this analysis may be due to the absence of these

extreme sires which had PEMD values between 4 – 5 mm in the study of Pannier et al.

(2014c), contrasting with a maximum of only 3.8 in this study. The marked effect of

PEMD on IMF% in the multiple born and single raised lambs is difficult to explain, and

has not been previously documented.

As expected, increasing sire PWWT ASBV did not impact on IMF% in the majority of

lambs used in this experiment. Previously analyses of this trait have shown no PWWT

maturity-linked impact on IMF% (Pannier et al. 2014c). Alternatively, there was a

substantial effect of PWWT on IMF% in the multiple born and single raised lambs,

increasing it across all muscles. This is the same sub-group of lambs where the PEMD

effect was identified, representing only 10% (40 lambs) of the population used in this

study, and like the PEMD effect has not been documented previously. Furthermore, in


182

an analysis of a much larger data (n=5642) set Pannier et al. (2014c) found no such

interaction of PWWT with birth or rear type, and therefore more work is required before

attributing confidence to this effect.

The results of this study demonstrate the need to carefully manage the potentially

negative impact that selecting for reduced sire PFAT has IMF% of the lamb carcass.

Alternatively, they also highlight that some monitoring of the hind section muscles may

be required, given that the impact of PFAT was greater in this region of the carcass.

PEMD and PWWT largely had no effect, although the unusual result found in the

multiple born and single raised lambs may require future investigation.

6.5.1.2 Intramuscular fat percentage differences between sire types

and dam breeds

In contrast to our hypothesis the Maternal sired lambs did not have more IMF% than the

Terminal sired lambs. The only differences were for the Merino sired lambs which had

less IMF% than both the Maternal and Terminal sired lambs in the M. semitendinosus,

and less IMF% compared to the Terminal sired lambs in the M. longissimus lumborum.

These differences in the M. semitendinosus were still present (although slightly

diminished) after correcting the model for HCWT, inferring that size/maturity only

partly accounts for the sire type differences. These results are in contrast to those of

Pannier et al. (2014c) who found no differences in IMF% between sire types in the M.

longissimus lumborum, although when compared at the same HCWT the Merinos had

the highest IMF%.

The Border Leicester-Merino dams produced lambs with more IMF% in four muscles,

although the effect was not as large as had been previously reported in the M.

longissimus lumborum (Hopkins et al. 2007c, McPhee et al. 2008, Pannier et al. 2014c).


183

This affect is likely to align with greater whole body adiposity in the Border Leicester-

Merino dams, an assertion supported by the inclusion of CT lean% which accounted for

the difference between dam breeds.

In conclusion, the generally lower IMF% levels of Merino sired lambs can only partly

be attributed to differences in weight. More importantly, this highlights that IMF% is

unlikely to account for the superior eating quality of Merino lambs as demonstrated by

Pannier et al. (2014a).

6.5.2 The impact of lean meat yield percentage and hot carcass weight

on intramuscular fat

In support of our hypothesis increasing CT Lean % led to a decrease in IMF%, with this

effect being consistent across all sire types and muscles. Increasing muscularity is

thought to dilute the final fat content in muscle (Hocquette et al. 2010) and therefore

reduce IMF%. When CT Lean % was included in the ASBV model the impact of the

breeding values on IMF% was reduced however still significant which indicates that CT

Lean % does not account for all the variation in IMF%. As with previous studies,

increasing HCWT was associated with an increase in IMF% of the M. longissimus

lumborum, (McPhee et al. 2008, Pannier et al. 2014c), however the variation in the

association across muscles has not been previously reported in Australian sheep. In

particular, the large increase in IMF% in the M. supraspinatus of the Merino sired

lambs is a unique finding, the reason for which is not currently known. These results,

particularly those in response to phenotypic increase in CT Lean %, highlight the

importance of maintaining IMF% as the lamb industry continues to select for lean

growth. Furthermore this demonstrates that this impact is not restricted to the loin,

affecting muscles in both the fore section and hind section of the carcass.


184

6.5.3 Production and management effects on intramuscular fat

The increase in IMF% with each successive kill group is likely to be a reflection of age

and weight. The average age in days of these kill groups was 167, 238, 280 and 355,

therefore the linear increase in IMF% with age aligns well with the impact of maturity

on adiposity as has previously been observed in the M. longissimus lumborum (McPhee

et al. 2008, Pannier et al. 2014c). None-the-less other factors that may also have

impacted, such as changing nutrition/pasture quality across this period cannot be

completely discounted. When corrected for HCWT, the magnitude of the kill group

effect was reduced, though still significant, indicating that increasing animal size

contributes to IMF% but that there are likely to be effects of age or maturity that impact

beyond their simple correlation with weight. The increase in IMF% present across all

muscle types varied in magnitude between muscles, with the M. supraspinatus showing

the greatest increase in IMF% and the M. semimembranosus showing the least. These

differences are likely to reflect development towards differing IMF% at maturity (i.e.

higher in the M. supraspinatus, and lower in the M. semimembranosus), although the

possibility of differential maturation rates to the same IMF% at maturity cannot be

ignored.

The impact of birthing and rearing types was only evident within the Merino sired

lambs. The IMF% of the multiple born and raised lambs was higher than that of the

multiple born-single raised and singleton born and raised lambs. This does not appear to

be the result of an impact of HCWT as its inclusion in the base model did not alter the

magnitude of this effect. One explanation may be associated with maternal nutritional

restriction. During the early gestation period in sheep restriction has been shown to

increase the IMF% of the M. longissimus lumborum of offspring at eight months of age

(Zhu et al. 2006). If the IMF% is related to gestational nutritional restriction it is unclear


185

why the lambs born as multiples and then raised as singletons have low IMF%, though

it is possible that the postnatal growth of musculature in the single raised lambs results

in a dilution effect on IMF%. Another explanation may be associated with fibre type as

the muscle of multiple raised lambs has been shown to be metabolically more oxidative

and less glycolytic than those reared as singletons (Greenwood, Harden and Hopkins

2007). Given that oxidative muscle types have been shown to associate positively with

IMF% (Hocquette 2010), this may account for the higher IMF% in this group. It is

unclear why the birth-rear type effect was only observed in the Merino sired lambs.

Female lambs had higher IMF% than the wethers, which was consistent across all

muscle types. This effect aligns well with the greater whole body fatness of females

(Butterfield 1988) and is further supported in this study through the correction of the

model for body composition (CT lean% corrected for carcass weight) which accounted

for the difference between sexes. The sex differences in IMF% between females and

wethers has previously been demonstrated, however only in the M. longissimus

lumborum (Craigie et al. 2012, Pannier et al. 2014c).

6.6 Conclusion

There are no previously published comparisons of IMF% over a range of carcass

regions in Australian lamb, making this study unique. The use of sires with reduced

PFAT breeding values results in a decrease in IMF%, however this effect was greater in

the muscles of the hind section indicating that additional monitoring may be required in

these muscles for managing the broader impact of PFAT. The PEMD and PWWT

breeding values largely had no impact on IMF%, although their effect identified in the

multiple born–single raised lambs may require further investigation. Lastly, the marked

effect of phenotypic CT Lean % across all muscles of the carcass further emphasises the


186

need to manage the potential impact of this selection goal in Australia to maintain

optimum eating quality.

Chapter 7 – Correlation of intramuscular fat (IMF) between muscles and the computed tomography prediction of IMF

187

Chapter 7. The correlation of intramuscular fat

content between muscles of the lamb carcass and the

use of computed tomography to predict intramuscular

fat percentage in lambs.

The following chapter is the version that was accepted for publication:

Anderson, F., Pannier, L., Pethick, D.W., Gardner, G.E. (2015). Animal, 9(7), 1239-

1249

7.1 Abstract

Intramuscular fat (IMF) % contributes positively to the juiciness and flavour of lamb

and is therefore a useful indicator of eating quality. A rapid, non-destructive method of

IMF determination like computed tomography (CT) would enable pre-sorting of

carcasses based on IMF% and potential eating quality. Given the loin muscle

(longissimus lumborum) is easy to sample, a single measurement at this site would be

useful, providing it correlates well to other muscles. To determine the ability of CT to

predict IMF%, this study used 400 animals and examined 5 muscles from three sections

of the carcass: from the fore-section the m. supraspinatus and m. infraspinatus, from the

saddle-section the m. longissimus lumborum and from the hind-section the m.

semimembranosus and m. semitendinosus. The average CT pixel density of muscle was

negatively associated with IMF% and can be used to predict IMF% although precision

in this study was poor. The ability of CT to predict IMF% was greatest in the m.

longissimus lumborum (slope -0.07) and smallest in the m. infraspinatus (slope -0.02).

The correlation coefficients of IMF% between the five muscles were variable, with the

highest correlation coefficients evident between muscles of the fore section (0.67


188

between the m. supraspinatus and the m. infraspinatus) and the weakest correlations

were between the muscle of the fore and hind section. The correlation between the m.

longissimus lumborum to the other muscles was fairly consistent with values ranging

between 0.34 to 0.40 (partial correlation coefficient). The correlation between the

proportion of carcass fat and the IMF% of the 5 muscles varied and was greatest in the

m. longissimus lumborum (0.41).


189

7.2 Introduction

Intramuscular fat (IMF) has been identified as an important factor influencing the eating

quality of red meat though its influence on juiciness (Shorthose and Harris 1991,

Thompson 2004a) and flavour (Thompson 2004a). In beef it accounts for up to 15 % of

the variation in palatability (Dikeman 1987) and in lamb it is thought that a minimum of

4-5% IMF is required for consumer satisfaction with regard to palatability (Hopkins et

al. 2006a). Loin IMF% is a useful predictor of eating quality and its measurement can

help maintain premium eating quality of lamb (Pannier et al. 2014a, c). To improve

targeted marketing, value adding to cuts and feedback to suppliers, a rapid, non-

destructive tool to determine IMF % of muscle prior to sale would be advantageous.

There is variation in IMF% between muscles throughout the carcass in lamb (Anderson

et al. 2015a, Warner et al. 2010b)/(Chapter 6) and beef (Brackebrush et al. 1991). The

study in beef by Brackebrush et al. (1991) demonstrated a strong linear relationship

between IMF% of the muscle longissimus lumborum and other muscle depots.

Therefore the potential exists to use the IMF % of the m. longissimus lumborum to

predict other muscles. In sheep, loin musculature (m. longissimus lumborum) is

considered of premium quality with research focused on the eating quality and

intramuscular fat levels of this muscle (Pannier et al., 2014a, c). In lamb, the IMF% of

muscles other than the m. longissimus lumborum has not been well described. A

companion study in lamb by Anderson et al. (2015a)/(Chapter 6) describes the IMF% of

an additional 4 muscles (m. supraspinatus, m. infraspinatus, m. semimembranosus and

m. semitendinosus) in three regions of the carcass. If a high correlation exists between

these muscles then one measurement may be used to predict IMF% of other muscle

depots, enabling a single site measurement for grading for eating quality purposes and


190

development of an IMF% breeding value. Conversely, poor correlation may indicate a

need for IMF% to be measured in multiple muscle depots for these purposes.

X-ray computed tomography (CT) has been used to accurately determine carcass

composition of fat, muscle and bone in sheep and pigs (Gardner et al. 2010, Kolstad,

Jopson and Vangen 1996, Simm, Lewis, Collins et al. 2001). Karamichou et al (2006)

and demonstrated muscle CT density was correlated to meat quality traits such as

IMF%, lamb juiciness, flavour and overall liking in the loin and m. semimembranosus.

The use of in vivo CT to predict eating quality through assessment of IMF% and shear-

force of the m. longissimus lumborum has been investigated (Lambe et al. 2009).

Clelland et al. (2014) has investigated the use of in vivo CT scanning of lamb loin to

predict IMF%, with predicted carcass fat accounting for much of the variation in IMF%.

The prediction of IMF% in dissected loins has also been investigated in lambs (Lambe

et al. 2010), beef (Prieto et al. 2010) and pork (Font-i-Furnols et al. 2013). There are

currently no studies in lamb that investigate the use of CT to predict IMF% in muscles

other than the loin. The ability of CT to accurately predict IMF% in other muscles

would enable better assessment of carcass IMF% and therefore indicate quality prior to

sorting, boning and sale of other cuts of meat.

The Australian Cooperative Research Centre (CRC) for Sheep Industry Innovation

established an Information Nucleus Flock (INF) experiment in 2007. In addition to the

standard carcass measurements, 400 lamb carcasses underwent computed tomography

(CT) scanning to determine proportion of fat, lean and bone. The IMF% of 5 muscles

(m. longissimus lumborum, m. supraspinatus, m. infraspinatus, m. semimembranosus

and m. semitendinosus) from 3 sections of the carcass, with the impact of selection for

improved lean meat yield on IMF% content is reported in Anderson et al. (2015a)/


191

(Chapter 6). Additional analysis is reported in this paper, where the correlation of

IMF% between these muscles and also with the percentage of fat within the carcass has

been determined. Additionally, the ability of CT to predict IMF% in dissected muscles

from 5 locations has been investigated. We hypothesise that the IMF% of the m.

longissimus lumborum muscle will be correlated with the IMF% of other muscles

examined. Additionally, we hypothesise that CT pixel density will adequately predict

the IMF% of CT scanned muscles, allowing non-destructive rapid determination of

IMF% throughout the lamb carcass.



Details of the design of the Sheep CRC’s Information Nucleus Flock (INF) were

presented by Fogarty et al. (2007). The 400 lambs used in this experiment were the

progeny of sires representative of a wide range of traits. All lambs were born and raised

at Katanning, Western Australia in 2011, with information recorded about the lambs

including: sire type (whether the sire was a Maternal, Merino or Terminal sire); birth

type and rearing type (combined effect of animals born as single or multiple and reared

as single or multiple); sex (wether or ewe); dam breed (Merino or Border Leicester-

Merino). Lambs were slaughtered at a target carcass weight of 21kg and therefore

divided into 4 different kill groups at average ages of 167d, 238d, 280d, and 355d. The

number of lambs in each category for sire type, sex, dam breed, birthing and rearing

type and kill group are shown in Table 7-1. The breeding values used to select for

improved lean meat yield were also available for the three sire types with results for the

impact of selection for improved carcass lean meat yield presented in Anderson et al.

(2015a)/ (Chapter 6).

Table 7-1 Number of lambs used according to sire type, sex, birthing and rearing type, dam breed and kill group.

Sex Birth-rearing type Dam breed Kill group1

Female Male

Single born

and raised

Born as multiple-

raised as single

Born and raised as

multiple Merino BLM

1 167K11 238K11 280K11 355K11

Maternal 0 92

34 6 52

92 0

6 16 32 38

Merino 0 70

32 10 28

70 0

0 1 13 56

Terminal 111 127

96 24 117

140 95

95 83 55 5

Total 111 289 162 40 197 302 95 101 100 100 99

Kill group= average age of lambs at slaughter followed by location and birth year (2011); K= Katanning.

1 BLM: Border Leicester-Merino

192


193

Lambs were yarded the day prior to slaughter and transported to a commercial abattoir

in Katanning, held in lairage overnight and slaughtered the following day at an average

carcass weight of 21 kg. Carcasses were subjected to a medium voltage electrical

stimulation (Pearce et al. 2010).

7.3.2 Sample collection and measurements

Hot carcass weight (HCWT) was determined immediately following dressing.

Carcasses were stored at 4 ºC within 1 hour of slaughter, with measurements taken at 24

hours for: GR tissue depth (total tissue depth above the surface of the 12th

rib 110mm

from the midline); c-site fat depth (mm) and eye muscle depth (mm) (subcutaneous fat

depth and eye muscle depth (mm), taken at the 12th

rib, 45 mm from midline).

Chilled carcasses were transported to Murdoch University to undergo CT scanning

within 72 hours of slaughter. Prior to CT scanning each carcass was split into three

primal components, the fore, saddle, and hind sections. The fore-section was separated

from the saddle by a cut between the fourth and fifth rib, the hind-section was separated

from the saddle by a cut through the mid-length of the sixth lumbar vertebrae. These

sections were then CT scanned as detailed in the section on Computed tomography

scanning below). Following scanning, individual muscles were dissected from each

carcass section, trimmed to remove all external and inter-muscular fat, weighed, CT

scanned separately, and then stored at -20 ºC until processing for IMF determination

using a near infrared procedure (NIR). These muscles included: from the fore section,

the m. supraspinatus and m. infraspinatus; from the saddle section, the m. longissimus

lumborum; and from the hind section, m. semimembranosus and m. semitendinosus.

Due to carcass imperfections and muscle trimming (e.g. faecal contamination, abscess

or excessive bruising), all five muscles could not always be obtained from each carcass,

with numbers of available muscles shown in Table 7-2.


194

Table 7-2 Showing for each muscle (m. semimembranosus, m. semitendinosus, m.

supraspinatus m. infraspinatus and m. longissimus lumborum) the number available for

analysis, intramuscular fat % as measured by near infrared spectroscopy and average

pixel density (Hu).

Number

of

muscles

Intramuscular

fat%1 ± SD

(min, max)

Average CT pixel

density2 ± SD

(min, max)

All muscles 1908 4.4 ± 1.1

(2.2, 9.9)

46 ± 11.4

(5, 71)

m. semitendinosus 390 4.8 ± 1.2

(2.6, 9.1)

41 ± 6.4

(15, 63)

m. semimembranosus 391 3.7 ± 0.8

(2.2, 6.1)

55 ± 3.8

(40, 64)

m. supraspinatus 374 5.0 ± 1.1

(2.9, 9.9)

39 ± 6.2

(21, 60)

m. infraspinatus 374 4.0 ± 0.9

(2.2, 7.9)

36 ± 8.3

(5, 62)

m. longissimus

lumborum 379

4.3 ± 0.8

(2.5, 8.1)

61 ± 3.7

(44, 71) 1 Intramuscular fat % as determined by near infrared spectroscopy

2 Average CT pixel density = the average of the Hounsfield units from within the muscle

The IMF % of each muscle was determined using a near infrared procedure (NIR).

Samples were commercially freeze-dried using a Cuddon FD 1015 freeze dryer

(Cuddon Freeze Dry, Blenheim, New Zealand). NIR measurements were taken using a

Spectro Star 2400 calibrated against chloroform solvent extraction as detailed by (Perry

et al. 2001). Calibration samples (n=160) were taken from all muscles represented in

this study from lambs aged between 155 to 355 days of age with intramuscular fat levels

ranging between 2.2-9.9%. There was an R2 of 0.94 between IMF% from the NIR and

soxhlet derived results. Additionally, samples were cross validated with the laboratory

used to analyse IMF in lamb for samples in Pannier et al’s (2014c) analysis of

intramuscular fat in the longissimus with R2 of 0.91.


195


CT scanning was undertaken with a Picker PQ 5000 spiral CT scanner (Cleveland,

Ohio, United States). The spiral abdomen protocol was selected with settings: pilot scan

length of 512mm, field of view set at 480, KV 100, Index 20, mA 150, revs 40, pitch

1.5 and standard algorithm. The carcass and individual muscles were scanned in 10mm

slice widths, with each slice taken 10 mm apart. The muscles were analysed by taking

multiple images throughout the muscle so that more pixel data was available, rather

than relying on one cross sectional image for each muscle. Image J was used to edit the

images so that only the pixels obtained from the internal structure of the muscle were

recorded, therefore excluding any fat external to the muscle. The average of all the

pixels obtained in the scanning of each muscle was used in the analysis of CT density’s

ability to predict IMF%.

The method described by Gardner et al. (2010) was used to determine the % of lean and

fat in the carcass. The images produced from the CT scan were edited to remove non-

carcass image artefacts and were partitioned into bone, muscle and fat components

using the software program Image J (Image J version 1.37v, National Institutes of

Health, Bethesda, MD, USA) in conjunction with Microsoft Excel. The discrimination

point to identify the Hounsfield barriers for associating pixels with bone, muscle or fat

were –235 to 2.3 for fat, 2.4 to 164.3 for lean and >164.3 for bone (Alston et al. 2005).

An estimate of volume using Cavalieri’s method (Gundersen and Jensen 1987,

Gundersen et al. 1988) was calculated as follows:

m

VolumeCav = d × Σ areag - t × areamax

g=1


196

in which m is the number of CT scans taken and d is the distance between cross-

sectional CT scans, in this case 10 mm. The value of t is the thickness of each slice (g),

in this example 10 mm, and area max is the maximum area of any of the m scans.

The average of the Hounsfield units of the pixels of each component was then

determined and converted into density (kg/L) using a linear transformation (Mull 1984).

This was then used along with the volume of each component to determine the weight

of fat, lean and bone, which was then expressed as a percentage of total carcass weight

at the time of scanning. Given the density of the marrow tissue, it is classified as either

fat or lean using the boundary discrimination method described above. Additional

editing within Image J enabled the isolation of the marrow component of bone within

all images. Thus the above procedures could be repeated on the ‘marrow only’ images.

This enabled back correction for these pixels, reallocating them as bone and removing

their associated volumes from the lean and fat components of the first iteration of image

analysis. Thus using the CT scans it is possible to determine the percentage of fat, lean

and bone within each carcass.

7.3.4 Raw Data Description

The raw average IMF% ± standard deviation, as determined by NIR (NIR-IMF%) for

each of the muscles tested was 4.8 ± 1.2, 3.7 ± 0.8, 5.0 ± 1.1, 4.0 ± 0.9 and 4.3 ± 0.8, for

the m. semitendinosus, m. semimembranosus, m. supraspinatus, m. infraspinatus, m.

longissimus lumborum (Table 7-2). Likewise, the average pixel density for these

muscles was 41 ± 6.4, 55 ± 3.8, 39 ± 6.2, 36 ± 8.3, 61 ± 3.7 (Table 7-2). The

distribution of this data is shown in Figure 7-1. The raw average (± standard deviation)

for HCWT, CT lean %, CT fat %, GR tissue depth (mm), eye muscle depth (mm) and c-

site fat depth (mm) for the lambs in this experiment was 21.4 ± 2.85, 58.6 ± 2.76, 23.4 ±


197

3.39, 10.8 ± 4.65, 32.6 ± 4.62, 4.9 ± 2.47. (Details of the mean and distribution of the

above measurements are shown in Table 7-3).

Table 7-3 Lamb age (days), hot carcass weight (kg), carcass lean and fat percentage as measured by computed tomography, eye muscle depth

(mm), c-site fat depth (mm) and GR tissue depth (mm) displaying raw mean ± SD (min, max).

Age (days) Hot carcass

weight (kg) CT Lean % CT Fat%

Eye muscle

depth (mm)

C-site fat

depth (mm)

GR tissue

depth (mm)

All animals

259.5 ± 1.52

(162, 364)

21.4 ± 2.85

(13.5, 29.0)

58.6 ± 2.76

(50.9, 66.2)

23.4 ± 3.39

(15.4, 32.6)

32.6 ± 4.62 (20.0,

46.0)

4.9 ± 2.47

(1.0, 12.0)

10.8 ± 4.65

(3.0, 25.0)

Birth type-rear type

Born and raised single

233.7 ± 71.59

(162.0, 364.0)

20.9 ± 2.78

(15.3, 29.0)

59.2 ± 2.62

(52.1, 66.2)

22.6 ± 3.36

(15.4, 31.4)

32.3 ± 4.65

(20.0, 43.0)

4.3 ± 2.36

(1.0, 11.0)

9.5 ± 4.51

(3.0, 25.0)

Born multiple-raised single

261.0 ± 60.67

(166.0, 361.0)

21.4 ± 2.50

(17.0, 27.3)

58.8 ± 2.57

(54.1, 65.0)

23.3 ± 3.10

(18.1, 29.2)

33.1 ± 4.97

(21.0, 46.0)

5.2 ± 2.58

(1.0, 12.0)

11.2 ± 4.43

(3.0, 22.0)

Born and raised as multiple

280.9 ± 58.47

(165.0, 362.0)

21.9 ± 2.87

(13.5, 27.8)

58.1 ± 2.82

(50.9, 64.7)

24.1 ± 3.34

(16.4, 32.6)

32.7 ± 4.53

(21.0, 43.0)

5.2 ± 2.43

(1.0, 12.0)

11.8 ± 4.57

(3.0, 25.0)

Sire type x dam breed Sex

Maternal x Merino

Wether

295.6 ± 57.13

(164.0, 364.0)

21.1 ± 2.51

(13.9, 26.4)

57.6 ± 2.83

(50.9, 65.9)

24.5 ± 3.40

(15.4, 30.9)

31.2 ± 5.11

(20.0, 43.0)

5.1 ± 2.46

(1.0, 12.0)

10.8 ± 3.89

(3.0, 21.0)

Merino x Merino Wether

338.7 ± 33.18

(232.0, 361.0)

19.6 ± 2.28

(13.5, 24.6)

58.9 ± 2.20

(53.9, 63.5)

22.5 ± 2.87

(16.1, 28.9)

33.0 ± 4.77

(21.0, 42.0)

3.0 ± 1.39

(1.0, 9.0)

7.5 ± 3.04

(3.0, 16.0)

Terminal x Merino Wether

237.1 ± 41.25

(162.0, 352.0)

22.5 ± 2.78

(17.0, 29.0)

59.5 ± 2.42

(55.0, 66.2)

22.8 ± 2.83

(16.6, 28.1)

33.2 ± 4.03

(25.0, 42.0)

5.3 ± 2.31

(1.0, 12.0)

12.1 ± 4.67

(4.0, 25.0)

Terminal x Merino Female

246.1 ± 47.70

(168.0, 352.0)

21.8 ± 2.57

(17.0, 27.1)

58.3 ± 2.52

(53.5, 63.3)

24.0 ± 3.16

(18.9, 30.8)

32.6 ± 4.48

(21.0, 46.0)

5.7 ± 1.97

(2.0, 11.0)

12.3 ± 4.34

(5.0, 23.0)

Terminal x Border Leicester-

Merino Wether

207.2 ± 47.09

(162.0, 287.0)

22.0 ± 3.06

(17.5, 27.7)

59.6 ± 2.80

(54.0, 64.7)

22.3 ± 3.60

(16.4, 30.5)

32.7 ± 3.91

(23.0, 42.0)

5.0 ± 2.81

(1.0, 12.0)

10.6 ± 4.86

(4.0, 22.0)

Terminal x Border Leicester-

Merino Female

212.1 ± 51.80

(162.0, 362.0)

22.2 ± 2.90

(16.5, 28.4)

58.3 ± 2.90

(51.9, 65.0)

24.2 ± 3.61

(18.0, 32.6)

33.1 ± 4.64

(24.0, 43.0)

5.4 ± 2.52

(1.0, 11.0)

12.4 ± 5.19

(5.0, 25.0)

198

Table 7-3 continued

Age (days) Hot carcass

weight (kg) CT Lean % CT Fat%

Eye muscle

depth (mm)

C-site fat

depth (mm)

GR tissue

depth (mm)

Kill group

Kill group 1

(average age 167 days)

167.4 ± 3.12

(162.0, 175.0)

19.6 ± 2.16

(16.5, 26.6)

61.2 ± 2.10

(56.0, 66.2)

20.5 ± 2.54

(15.4, 28.2)

32.2 ± 4.06

(25.0, 43.0)

3.6 ± 2.02

(1.0, 10.0)

7.8 ± 2.83

(3.0, 16.0)

Kill group 2


237.9 ± 3.37

(230.0, 244.0)

24.6 ± 1.69

(20.8, 29.0)

58.0 ± 2.39

(52.5, 63.0)

25.2 ± 2.86

(18.9, 31.4)

34.4 ± 4.52

(22.0, 46.0)

6.5 ± 2.34

(2.0, 12.0)

15.4 ± 3.83

(6.5, 25.0)

Kill group 3


279.6 ± 4.04

(271.0, 287.0)

21.1 ± 1.97

(16.2, 25.8)

57.2 ± 2.37

(50.9, 62.9)

23.8 ± 2.97

(17.7, 30.2)

29.3 ± 4.10

(20.0, 38.0)

5.7 ± 2.12

(2.0, 11.0)

10.1 ± 3.65

(3.0, 21.0)

Kill group 4


355.1 ± 3.60

(346.0, 364.0)

20.7 ± 2.79

(13.5, 27.4)

58.3 ± 2.46

(51.9, 63.5)

24.0 ± 3.31

(16.1, 32.6)

34.1 ± 3.99

(23.0, 42.0)

3.6 ± 1.94

(1.0, 10.0)

9.8 ± 4.38

(3.0, 25.0) CT Lean % = percentage of lean in the carcass as measured by computed tomography

CT Fat % = percentage of fat in the carcass as measured by computed tomography

.

199

Figure 7-1 Raw data of the intramuscular fat % in lamb of the m. semimembranosus, m. semitendinosus, m. supraspinatus, m. infraspinatus and m.

longissimus lumborum as it relates to average computed tomography pixel density (Hu) of the fat and muscle pixels.

0

2

4

6

8

10

0 10 20 30 40 50 60 70

Intr

am

uscula

r fa

t %

Average computed tomography pixel density (Hu)

M. semitendinosus

M. semimembranosus

M. supraspinatus

M. infraspinatus

M. longissimus lumborum

200


201


Simple correlations of NIR-IMF% between each of the five muscles (m. longissimus

lumborum, m. semimembranosus, m. semitendinosus, m. supraspinatus, m.

infraspinatus) was determined using PROC CORR in SAS (Version 9.1, SAS Institute,

Cary, NC, USA). Partial correlations of NIR-IMF% between the 5 muscles were also

determined using a multivariate analysis (Version 9.1, SAS Institute, Cary, NC, USA)

that included the fixed effects: birth type and rearing type; sex within sire type (wether

Merino, wether Maternal, female Terminal, wether Terminal); dam breed within sire

type (Merino x Merino, Maternal x Merino, Terminal x Merino, Terminal x Border

Leicester-Merino) and kill group, plus their first order interactions. Thus the robustness

of the simple correlations can be assessed by comparison with the partial correlation

coefficients (Table 7-4). Additionally, the simple and partial correlations of the IMF in

each of the 5 muscles and the percentage of fat in the carcass (CT fat%) was determined

using the same method as described above.

The information obtained from the CT scanning of muscles was used in a general linear

model to predict IMF% (SAS Version 9.1, SAS Institute, Cary, NC, USA). A number of

models were tested to reflect scenarios where varying amounts of information was

available for predicting NIR-IMF%. These models were constrained to reflect the

availability of information within Australian abattoirs in the present and future context.

The standard deviation of the pixel density was included in the models, however was

not significant and therefore not retained in the final models described below. The

models included varying combinations of the following terms:

1. Average CT pixel density and muscle type (see models 1-3; Table 7-5)

2. Average CT pixel density, muscle type and basic carcass measurements

(HCWT, GR tissue depth) (see models 4 and 5; Table 7-5)


202

3. Average CT pixel density and muscle type plus more detailed carcass

measurements including HCWT, eye muscle depth (mm) and c-site fat depth

(mm). (see models 6 and 7; Table 7-5)

4. Average CT pixel density and muscle type plus percentage of fat or lean tissue

determined using CT scanning. (see models 8-11; Table 7-5)

5. Average CT pixel density, percentage of fat or lean tissue determined using CT

scanning, carcass measurements, muscle type plus the inclusion of known

production factors including sex, sire type, birth type-rear type, kill group, and

dam breed. Sex and dam breed were both fitted within sire type, and in this

experiment kill group in part described age, given that the average age for kill

groups 1 to 4 was 167, 238, 280 and 355 days. (see models 12-17; Table 7-6)

6. Prediction of NIR-IMF% for each of the five muscles individually (m.

longissimus lumborum, m. semimembranosus, m. semitendinosus, m.

supraspinatus, m. infraspinatus) using average CT pixel density, percentage of

carcass fat determined using CT scanning and CT density of the m. longissimus

lumborum (Table 7-7).

7.4 Results

The correlation coefficients of IMF% between the five muscles were variable with the

highest correlation coefficients evident between the m. supraspinatus and the m.

infraspinatus (Table 7-4) in the fore section of the carcass. The correlation between the

m. longissimus lumborum to the other muscles was fairly consistent with values ranging

between 0.34 to 0.40 (partial correlation coefficient), and the weakest correlations were

those between the forequarter and hindquarter muscles. Simple correlations

demonstrated similar trends to the partial correlation coefficients, and were only mildly

inflated above the partial correlation coefficient values.


203

The correlation of IMF% and carcass fat % (CT fat %) varied between muscles from

0.24 to 0.41 (partial correlation coefficient, Table 7-4). The strongest correlation of CT

fat % was with the IMF of the m. longissimus lumborum ( 0.41), which was 25% greater

than the correlation between CT fat% and the IMF% of the other muscles.

Table 7-4 Partial correlation coefficients (above the diagonal) and simple correlation

coefficients (below the diagonal) of the IMF% and computed tomography derived % of

fat (CT fat%) in lamb between the m. semimembranosus (SM), m. semitendinosus (ST),

m. supraspinatus (SS) m. infraspinatus (IS) and m. longissimus lumborum(LL).

SM ST SS IS LL

CT Fat

%

SM 1.00 0.43 0.30 0.38 0.40 0.30

ST 0.42 1.00 0.25 0.29 0.40 0.24

SS 0.41 0.30 1.00 0.68 0.34 0.29

IS 0.48 0.34 0.75 1.00 0.34 0.25

LL 0.45 0.47 0.45 0.45 1.00 0.41

CT Fat

% 0.24 0.32 0.36 0.31 0.48 1.00

All correlations are significantly different from zero (P<0.05)

CT Fat % = percentage of fat in the carcass as measured by computed tomography

7.4.1 Prediction of IMF% in lamb using average CT pixel density and

muscle type

There was a negative linear relationship (P<0.01) between IMF% and average CT pixel

density which varied between muscles. (Figure 7-2). This relationship was greatest in

the m. longissimus lumborum with a linear regression coefficient of -0.07±0.013,

followed by the m. supraspinatus with a linear regression coefficient of -0.06±0.008.

This relationship was smallest in the m. infraspinatus with a linear regression

coefficient of -0.02±0.006. As such, for the model using a combination of average CT

pixel density and muscle, there was adequate precision for predicting IMF%. This

model described 25% of the variation (coefficient of determination (R2) = 0.25) in

IMF% with 2/3 of the data falling within 0.94 IMF% units (Root mean square error


204

(RMSE) = 0.93) of the predicted value (see Model 3, Table 7-5). However most of the

variation is explained by muscle alone (see Model 1, Table 7-5), therefore when the

muscle is unknown the precision falls (R2 = 0.07, RMSE 1.05; Model 2, Table 7-5).

Figure 7-2 The relationship between intramuscular fat % in lamb and computed

tomography pixel density (Hu) (model 3) for the m. infraspinatus (slope = -0.02±0.006,

intercept=4.60), m. longissimus lumborum (slope = -0.07±0.013, intercept=8.50), m.

semimembranosus (slope = -0.04±0.013, intercept=5.92), m. supraspinatus (slope =

0.06±0.008, intercept=7.29) and m. semitendinosus (slope = -0.03±0.007,

intercept=5.89). Lines represent least square means (±SE as dotted lines) across the

range of average computed tomography pixels (Hu).

3

3.5

4

4.5

5

5.5

6

6.5

0 10 20 30 40 50 60 70

Intr

am

uscula

r fa

t %

Average computed tomography pixel density (Hu)

M. infraspinatus

M. longissmus lumborum

M. semimembranosus

M. supraspinatus

M. semitendinosus

Table 7-5 F-values, coefficient of determination (R-square), and root mean square error (RMSE) for models predicting intramuscular fat % in lamb

using muscle type, average computed tomography pixel density of fat and muscle (CT density), hot carcass weight, GR Tissue depth (mm), eye

muscle depth (mm), c-site fat depth (mm) and computed tomography derived % of fat and/or lean tissue (CT Lean % or Fat%).

Model

1

Model

2

Model

3

Model

4

Model

5

Model

6

Model

7

Model

8

Model

9

Model

10

Model

11

Muscle 125** - 131** - 14.8** - 14.37** - - - 11.88**

CT Density - 131** 82** - 87.2** - 86.01** - - - 43.18**

CT Density*muscle - - 6.59** - 6.1** - 5.91** - - - 4.39**

Hot carcass weight - - - 5.59* 3.81 5.24* 4.81* 0.94 1.03 1.19 0.73

GR tissue depth1 (mm) - - - 32.3** 38.65** 22.02** 24.98** 1.35 5.57 6.34* 4.46*

Eye muscle depth (mm) - - - - - 1.19 1.75 - - - -

c-site fat depth2 (mm) - - - - - 0.31 0.99 - - - -

CT Lean % - - - - - - - 97.97** - 0.88 0.42

CT Fat % - - - - - - - - 148.83** 49.13** 46.28**

R-Square 0.21 0.07 0.25 0.03 0.28 0.025 0.28 0.08 0.1 0.1 0.34

RMSE3

0.97 1.05 0.94 1.08 0.93 1.08 0.929 1.05 1.04 1.04 0.89 1GR tissue depth: tissue depth at the 12

th rib, 110mm from the midline

2C-site fat depth: depth of the fat at the 45mm from the midline at the 12

th rib

3RMSE: Root means square error

*P<0.05, **P<0.01

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206

Table 7-6 F-values, coefficient of determination (R-square), and root mean square error

(RMSE) for models predicting intramuscular fat % in lamb using muscle type, average

computed tomography pixel density (CT density) of fat and lean, % computed

tomography fat and lean (CT Lean% or CT Fat%), on-farm information and carcass

measurements.

Model

12

Model

13

Model

14

Model

15

Model

16

Model

17

Muscle 140.0** - - 38.1** 36.7** 37.4**

CT Density

- 98.9** - 16.6** 17.3** 18.1**

CT Density*muscle - - - 13.8** 13.0** 13.9**

Sex(sire type) - - - - 1.4 1.5

Sire type - - - - 7.8** 4.6*

Kill group 76.8** 141.3** 59.4** 138.4** 109.0** 102.8**

Birth-type rear-type - - - - 8.8** 10.6*

Dam breed(sire type) - - - - 2.2* 1.4

Hot carcass weight - - - - - 0.9

GR tissue depth1 (mm) - - - - - 0.04

CT Lean %

- 1.1 - 0.9 0.01 0.01

CT Fat %

- 49.0** - 47.2** 24.8** 14.1**

R-Square 0.29 0.21 0.09 0.39 0.40 0.41

RMSE2 0.92 0.97 1 0.85 0.85 0.84

1GR tissue depth: tissue depth at the 12

th rib, 110mm from the midline

2RMSE: Root mean square error

**P<0.01

7.4.2 Prediction of IMF% in lamb using average CT pixel density,

muscle type, and other carcass measures

Including hot carcass weight (HCWT), and GR tissue depth (mm) with average CT

pixel density and muscle, only marginally improved the precision of prediction (Model

5, Table 7-5) compared to CT pixel density and muscle type alone. Additional carcass

tissue depth measures (Eye muscle depth (mm), and c-site fat depth (mm)) did not

deliver any further improvements in precision (Model 7, Table 7-5). The inclusion of

whole body estimates of fat% and lean% derived from CT scans (i.e. CT fat% and CT

lean%) in addition to CT density, muscle and carcass measurements (HCWT and GR

tissue depth (mm)) provided a further improvement in precision for predicting IMF%

(R2 = 0.34 and RMSE = 0.89, Model 11, Table 5). However these terms alone only

described similar variance as CT density (see Models 8, 9, and 10, Table 7-5).


207

7.4.3 Prediction of IMF% in lamb using average CT pixel density, CT

fat%, CT lean%, muscle type, and on-farm information

The inclusion of on-farm information such as sex, sire type, kill group (age), birth-type

rear-type, and dam breed also improved the prediction of IMF% (see Models 12-16,

Table 7-6). When all available on farm information was incorporated as well as hot

carcass weight and GR tissue depth the prediction of IMF% only slightly exceeded that

demonstrated by models 15 and 16 containing CT fat% (R2 = 0.41 and RMSE = 0.84,

Model 17, Table 7-6).

Of the production factors tested, kill group (age) described the largest portion of

variance (Model 17, Table 7-6), however used alone was a poor prediction of IMF%

(Model 14, Table 7-6).

7.4.4 Prediction of IMF% within each muscle (m longissimus

lumborum, m semimembranosus, m. semitendinosus, m.

supraspinatus, m. infraspinatus) using CT density, CT Fat% and the

CT density of the m. longissimus lumborum

When each muscle was treated separately, the precision of prediction (R2

/ RMSE) of

IMF using CT density varied and was highest in the m. longissimus lumborum

(0.09/0.8), and m. supraspinatus (0.1/1.1) (Table 7-7). In the remaining muscles the

precision of prediction was less than half this (Table 7-7).

The precision of prediction of IMF% using CT fat% alone varied between muscles

(Table 7-5) and was highest in the m. longissimus lumborum (R2 0.23/ RMSE 0.73,

Table 7-7). In the other muscles it was approximately half that of the loin (Table 7-7)

and lowest in the m. semimembranosus (R2 =

0.06, RMSE = 0.75, Table 7-7).


208

When the CT density of the m. longissimus lumborum was used to predict the NIR-

IMF% of the other muscles the precision was similar to when the CT density of each

individual muscle was used (Table 7-7). The exception to this was with the m.

supraspinatus where the precision was halved.

Table 7-7 F-values, coefficient of determination (R2), and root mean square error

(RMSE) for models predicting intramuscular fat % in the m. semimembranosus, m.

semitendinosus, m. supraspinatus m. infraspinatus and m. longissimus lumborum in

lamb using average computed tomography pixel density (CT density) of fat and lean,

computed tomography derived % of fat (CT Fat%), and CT density of the m.

longissimus lumborum.

CT Density CT Fat %

CT density in

m. longissimus

lumborum

R-

square r.m.s.e

1

CT Density

M. longissimus lumborum 37.19** - - 0.09 0.80

M. semimembranosus 16.01** - - 0.04 0.76

M.semitendinosus 8.05** - - 0.02 1.14

M. supraspinatus 41.37** - - 0.10 1.06

M. infraspinatus 8.11** - - 0.02 0.89

CT Fat%

M. longissimus lumborum - 112** - 0.23 0.73

M. semimembranosus - 23.06** - 0.06 0.75

M.semitendinosus - 41.12** - 0.1 1.1

M. supraspinatus - 54.65** - 0.13 1.04

M. infraspinatus - 36.72** - 0.09 0.87

CT Density of the M. longissimus

lumborum

M. longissimus lumborum - - 37.19** 0.09 0.80

M. semimembranosus - - 25.14** 0.06 0.75

M.semitendinosus - - 9.38** 0.02 1.15

M. supraspinatus - - 13.27** 0.04 1.1

M. infraspinatus - - 14.1** 0.04 0.9 1

r.m.s.e: Root means square error *P<0.05, **P<0.01


209

7.5 Discussion

7.5.1 Correlation of IMF% between the muscles

Aligning with our initial hypothesis, IMF% within lamb muscles from the fore and hind

sections are consistently correlated with IMF% in the M. longissimus lumborum.

Furthermore, the relatively similar values of the partial and simple correlation

coefficients suggests that production factors such as sex, sire type, dam breed, birthing

and rearing type, and kill group do not bias the simple correlations. From an industry

perspective this is important given that industry measurement of IMF% is likely to be

focused on the m. longissimus lumborum. This measurement can thus be extrapolated to

other muscles without concern over production factors biasing this correlation.

Correlations between the other muscles of the carcass appear more dependent upon co-

location rather than an absolute amount of IMF. This is evidenced by the fact that the

highest correlations were seen between the muscles of the fore-section (m.

supraspinatus and m. infraspinatus) and the hind section (m. semimembranosus, and m.

semitendinosus). These high correlations are in spite of the fact that these muscles have

different functions, for example the m. supraspinatus is considered a stabilising muscle

and the m. infraspinatus is used for extension and flexion of the shoulder joint (Suzuki

1995). Also aligning with the theme of correlations between co-localised muscles, the

poorest correlation of IMF% was between muscles of the fore and hind sections and the

m. longissimus lumborum had a moderate and similar correlation for IMF% with all

other muscles. In beef, the correlations that exist between muscles is higher, where the

R2 between the m. longissimus lumborum and the m. supraspinatus, infraspinatus, m.

semimembranosus, and m. semitendinosus are 0.63, 0.69, 0.83 and 0.77 (Brackebrush et

al. 1991). However this may be a reflection of a greater range in IMF% across which

these R2 were estimated. A single measurement of IMF% taken from the m. longissimus


210

lumborum in lamb carcass is still likely to be an adequate predictor of IMF% within the

rest of the carcass.

The correlation of whole carcass fat (CT fat %) and the NIR-IMF% varied between the

muscles examined, which is a finding unique to this study. The highest correlation was

between carcass fat % and the NIR-IMF% of the m. longissimus lumborum (partial

correlation 0.41). The correlation of carcass fat % and the NIR-IMF% of the other

muscles was 25% lower than with the m. longissimus lumborum. Knowledge of carcass

fat % does however offer a useful contribution to prediction of IMF% in all muscles

examined, in addition to knowledge of average pixel density (Hu) of the muscle.

7.5.2 Prediction of IMF% based on CT density and muscle

The average CT pixel density based on the Hounsfield units within each image was

negatively associated with increasing IMF%. As such and in support of our hypothesis,

average CT pixel density could be used to predict IMF% within a muscle type albeit

with relatively poor precision across muscles. Similar to Clelland et al. (2014) but in

contrast to Lambe et al. (2010), the use of standard deviation of CT density did not

improve the prediction of IMF%.

The ability to predict IMF% using average CT pixel density alone within this study is

less than that of previous studies. Lambe et al. (2009) showed that using one CT

scanner they were able to predict loin IMF% with similar precision in both live animals

and dissected loins on the basis of muscle density alone (R2 0.36 and 0.33). However,

their precision of prediction diminished when data was obtained from the CT scanning

of loins using other CT scanners, with R2

similar to those obtained in our study.


211

The density change correlating with greatest increase in IMF% was in the m.

longissimus lumborum and m. supraspinatus. In contrast, the m. infraspinatus

demonstrated little change in IMF % but a greater magnitude of density change. This

also highlights that the CT is likely to be a more sensitive predictor of IMF within the

m. longissimus lumborum. Thus when CT was used to predict IMF in each individual

muscle, the R2 was greatest for the m. longissimus lumborum and m. supraspinatus and

lower in the m. infraspinatus, m. semitendinosus and m. semimembranosus, in part

reflecting this greater magnitude of change in IMF% per unit change in CT pixel

density.

The precision of IMF% prediction in lamb was markedly improved if the muscle is

known and used in the prediction model. This indicates there are factors more

influential than IMF% that elicit differences in density between muscles. This is further

evidenced by the poor association of IMF% and average CT pixel density between

muscles, an example being the m. longissimus lumborum which despite having the

highest average CT pixel density did not have the lowest IMF%. The reason for these

discrepancies is likely attributed to the fact that muscles like the m. longissimus

lumborum appear to be quite homogenous on a CT image, with the majority of the

pixels classified as being muscle, with fibres predominantly running parallel to each

other. In contrast the m. supraspinatus, m. infraspinatus and m. semimembranosus have

more multidirectional fibres when examined on cut surface. This is related to

differences in structure and function, which may influence how they appear as a CT

image. The amount of elastin in the m. semitendinosus for example, may influence the

average CT pixel density of the muscles and therefore interfere with the prediction of

IMF between muscles. Additionally the size and shape of the intramuscular fat within

muscles may impact on the calculated CT density of the muscle. Furthermore, post-


212

mortem changes to muscles may impact the CT density of the muscle, with this impact

potentially varying between muscle locations.

Given the moderate and similar correlation of NIR-IMF% from the m. longissimus

lumborum to other muscles examined, it was thought the knowledge of the CT density

of the m. longissimus lumborum may be sufficient to predict IMF% in other locations.

Similar precision of prediction was achieved when the CT density of the m. longissimus

lumborum was used to predict IMF% of the other muscles as when using the CT density

of the muscle itself. The exception to this was the m. supraspinatus where use of CT

density in the m. longissimus lumborum to predict NIR-IMF% halved the precision,

compared to using the CT density of the muscle itself. The reason for this is difficult to

explain given there is similar correlation between the loin and the other muscles

examined. It does however emphasise the difficulty in relying solely on correlations of

IMF between regions of the carcass to predict IMF using CT scanning of the loin.

Furthermore, reliance on these correlation should be used with caution if selection for

IMF% in breeding programs is based solely on the measurements in the loin, as it

remains to be determined if correlations between muscles will remain constant with

such selection over time.

7.5.3 Incorporating additional information for predicting IMF%

Given that industry routinely measures HCWT and fat score at the GR site we tested the

potential for CT to predict IMF% in the presence of these terms. This led to only a

marginal improvement in the prediction of IMF%, with the R2 increasing from 0.25 to

0.28. None-the-less, this aligns well with the work of Pannier et al. (2014c), who

demonstrated an association between IMF% and HCWT with an increase in IMF% of

2.08% across a 28 kg range in HCWT. Additionally an increase in GR tissue depth from

0.5 to 25mm increased IMF% in the longissimus by 1.57%. Further work is currently


213

underway in Australia to develop methods to rapidly and accurately measure carcass

eye muscle depth (mm) and c-site fat depth (mm) to improve the precision of predicting

lean meat yield. However, these additional measurements provided no further

improvement in prediction precision for IMF% and would therefore not be of specific

benefit for inclusion in a prediction equation.

Similar to previous studies (Clelland et al. 2014) inclusion of a measure of carcass fat

(CT fat %) did provide improvement in yield prediction. In the study of Clelland et al.

(2014), the inclusion of predicted carcass fat from a reference CT scan described the

majority of the variation in IMF of the m. longissimus lumborum (R2= 0.51). In our

study, across all muscles, the inclusion of CT fat % accounted for some of the variation

in IMF (R2 = 0.1), however this varied between muscles and is a reflection of the

differences in correlation between CT fat% and the IMF% of the different muscles. In

the current study, the precision of IMF% prediction using CT fat% was highest in the

loin (R2 0.23), reflecting the high correlation between CT fat% and IMF% in this

muscle. The reason for the difference in the precision of IMF% prediction in the loin in

our study using CT fat % and that of Clelland et al. (2014) is unclear. However, the

lambs in the study of Clelland et al. (2014) were from one breed type (Texel), were

younger (mean slaughter age 149 days) and had lower IMF% (mean1.48%) than the

lambs in this study which may account for some of the differences.

Importantly, if CT measurements were available, then IMF% could be predicted from

CT fat%, with relatively little further information provided by CT pixel density.

However, Pannier et al. (2014c) has shown that focusing only on genetic selection for

reduced carcass fat in the Australian lamb industry has concurrently decreased IMF% in

the longissimus. Therefore the concern with predominantly utilising CT fat% to predict


214

IMF% is that it does not enable uncoupling of selection for low carcass fat and

maintenance of IMF%.

The Australian industry is moving towards improved individual animal tracking.

Therefore information regarding individual animal production factors such as sex, birth

type-rear type etc. may become readily available to abattoirs. As such we also tested the

addition of pre-slaughter information into the prediction equation and saw improved

accuracy of prediction of IMF%. In particular there was a marked improvement in the

prediction of IMF% when kill group was included in the model. Although potentially

confounded by specific day effects, kill group is likely to largely reflect the impact of

age (Anderson et al. 2015a, Pannier et al. 2014c)(Chapter 6) and maturity, both of

which have been previously demonstrated to increase IMF% (Pannier et al. 2014c). As

such, knowledge of the animals’ age offers the most potential for improving the

prediction of IMF%. This improvement in the prediction of IMF% using pre-slaughter

production information is in contrast to work by Prieto et al. (2010) who used the CT

pixel density only, as additional information did not further improve the precision of

IMF% prediction.

In a study in cattle Prieto et al. (2010) predicted IMF% in two breeds of cattle with R2

values of 0.76 and 0.71 based on CT pixel density only. One possibility to explain this

discrepancy with our study may be associated with larger cell size in the muscle of

cattle (approximately 4000µm2) compared to lamb (approximately 2300 µm

2)

(Greenwood et al., 2006, 2009) for the m. longissimus lumborum. Given the pixel

resolution of the CT images (1mmx1mm), the increased cell size in cattle may result in

better tissue/density differentiation between pixels, potentially amplifying the density

differences between high and low IMF samples. However in contrast to this, the IMF%


215

of pork (Font-i-Furnols et al. 2013) and lamb (Clelland et al. 2014) has been predicted

using CT, indicating cell size alone does not account for the relatively poor prediction

of IMF% in our study.

7.6 Conclusion

The intramuscular fat % (IMF%) of lamb is linked to eating quality. Currently in

Australia, IMF% is determined in lamb for experimental purposes only, mainly in the

longissimus. The use of technologies such as CT to predict IMF% in muscle would

allow for the rapid and non-destructive determination of IMF% prior to carcass sorting.

CT has the potential for non-invasive point measurement in the loin for rapid prediction

of intramuscular fat, and therefore eating quality, of multiple muscles in the carcass.

The scanning of lamb carcasses using a CT scanner has the ability to predict IMF%

within individual muscles of the carcass, however in this experiment the precision was

relatively poor. The majority of the variation in IMF% between muscles was described

by knowledge of the muscle type alone. However, if used in conjunction with pre-

slaughter information and carcass measurements, particularly CT fat%, the prediction of

IMF% was greatly improved (R2 = 0.41). The precision of CT prediction of IMF%

using only CT density of the muscle in the lamb carcass needs to be improved before

this will become a viable method of IMF% prediction across the carcass.

The IMF% of the m. longissimus lumborum correlated with each of the other muscles in

this study, with the strength of this correlation similar for all muscles. However, the

strongest correlations in IMF% existed between muscles located within the same region

of the carcass. Knowledge of the correlation between the m. longissimus lumborum and

other muscles within the lamb carcass may allow CT measurement of IMF% in this

muscle to predict IMF% elsewhere, however if selection for IMF% remains focussed

only on the loin, then the correlation of IMF% between muscles should be monitored.


216

With inclusion of technologies such as CT in sheep abattoirs there is the potential to

better predict IMF% prior to carcass sorting and boning, allowing better utilisation of

carcasses based on estimates of their eating quality.

Chapter 8 – General discussion

217

Chapter 8. General discussion

8.1 Selection for lean meat yield and implications for the

Australian Sheep Industry

Improvements to LMY% in the lamb carcass are important to the Australian lamb

industry. More muscle results in increased saleable meat and therefore improved

profitability for processors per lamb carcass. Additionally, excessive fat and bone is

undesirable as there are associated costs with its trimming from primal portions for both

processors and consumers (Hopkins 1989, Pethick et al. 2011). An increasing

proportion of lambs are sold directly to the abattoirs (Australian Bureau of Agricultural

and Resource Economics 2014) with farmers currently paid on the basis of an estimate

of saleable meat through HCWT and GR tissue depth. The feed costs associated with

growing older and fatter lambs are considerable, therefore it is also of benefit to produce

lambs that reach slaughter weights earlier or that reach heavier weights without

becoming excessively fat. If LMY% is rewarded by abattoirs, there is significant

incentive to select for increased lean yield. The widespread introduction of technologies

to accurately and affordably predict lean yield at the processing plant will accelerate the

adoption of grid based payment schemes for lamb that reward LMY% (Goers and Craig

2008). Therefore the benefits of improving LMY% are likely to become as important to

farmers as they are to processors and this thesis demonstrates these benefits.

The results of this thesis offer important information regarding the changes that occur to

the composition of the lamb carcass due to selection for LMY% using Australian Sheep

Breeding Values (PWWT, PEMD and reduced PFAT). An allometric approach was

used to examine the impact on carcass composition of the three carcass breeding values


218

(Chapter 3 and 4). Additionally the impact these factors have on the distribution of the

three carcass tissues (fat, lean and bone) between the fore, saddle and hind sections was

explored. Previous investigations have focussed on the use of indicator muscles

(Gardner et al. 2010), site measurements (Gardner et al. 2010), or whole carcass

composition with no differentiation between carcass regions (Gardner et al. 2010,

Ponnampalam et al. 2007b) to determine the impact of genetic and non-genetic factors

on carcass composition.

The carcass breeding values were assessed to determine whether they deliver the

changes in carcass composition they have been designed for – are breeding values

delivering what they promise? (Chapters 3 and 4) The answer is yes - and with good

financial gains (Chapter 5), with this information offering a new and more

comprehensive understanding of the financial implications of using genetics to select

for LMY%. Previous studies have looked at the relationship between genetic

improvements in carcass weight and gross value of production in the lamb industry

(Banks 2003, Thatcher and Couchman 1983). Pethick et al (2006a) describes the growth

of the lamb industry that occurred in response to genetic improvements in lamb growth

and carcass leanness. This thesis analysed data obtained from CT of lamb carcasses and

provides information about the combined effects of changes in whole carcass

composition and distribution of the tissues between sections, which has not been

previously reported (Chapter 5).

The differences in carcass composition due to other non-genetic factors (birthing and

rearing types, sex, location of production system (site) and age of slaughter (kill group)

were also investigated (Chapters 3, 4). The differences in LMY% and financial impact

these factors have on the lamb carcass also have to be reported (Chapter 5).


219

Eating quality is important to the Australian consumer (Pethick et al. 2011) and IMF%

positively influences eating quality (Pannier et al. 2014a, Pannier et al. 2014b). This

thesis show that selection using carcass breeding values impacts lean in the fore, saddle

and hind sections of the carcass (Chapter 3). Given work by Pannier et al (2014c) which

showed that selection for leanness using PEMD and PFAT reduced IMF% in the

longissimus, it was important to investigate the impact in other carcass regions. The

impact of LMY%, and the carcass breeding values on IMF% across the lamb carcass

was explored in Chapter 6. The use of CT to determine IMF% and the correlation of

IMF% in the longissimus with other carcass muscles were also examined (Chapter 7).

8.2 Use of the allometric analysis

An allometric analysis was used to examine carcass composition (Chapters 3, 4 and 5)

which was originally described by Huxley (Huxley and Teissier 1936, Huxley 1932).

The equation y = axb describes the relative growth rate of one component of an

organism relative to the growth of another component. In many instances the equation is

used in the log-log form and appears as log y = log a + blog x. This equation can then

be used in least squares regression to solve for the coefficients a (intercept) and b

(slope). This equation has been used previously in lamb (Cake et al. 2006a, Notter,

Ferrell and Field 1983, Thompson et al. 1979a, Thompson et al. 1979c) and beef (Berg

and Butterfield 1968, Tulloh 1963) to assess the growth rate of the three carcass

components (fat, lean and bone).

The reasons for undertaking our analysis using an allometric form are:

1. Growth/composition data of this type is subject to heteroskedasticity within the

residuals (ie residuals become larger as weights increase). The log

transformation compensates for this problem, normalising the variance and

reducing the influence of outliers (Huxley 1932, Niklas 1994, Peters 1986)


220

2. Perhaps more importantly, this enables the presentation of results in a concise

form as the use of the natural logarithm allows for the differences in log a to be

directly interpreted as percentage differences in tissue weight between

comparative groups (Cole 2000). This makes describing the results far simpler,

with the effects (ie percentage differences in tissue weight) able to be applied to

any weight of animal within the range specified in our data set. The alternative

use of a general linear model would require the need to output estimates at a

given weight, however these absolute differences (in kg) could not be easily

applied to animals of heavier or lighter weight as the absolute value of the

differences would change.

3. The allometic model enables output of the relative rate of growth of one tissue

type as it relates to the growth of another region. The b coefficient can be used

to determine the relative rates of growth of a component (x) as it relates to the

growth of the whole organism (y) (Notter et al. 1983, Seebeck 1968). This study

has analysed the relative growth of the three carcass tissues as they relate to the

growth of the carcass as a whole. The approach also allows the relative rate of

lean deposition within one carcase region to be compared to the total carcase

lean. Thus relative rates of maturation can be compared.

For the most part this analysis reports percentage differences between production

factors that affect lamb carcass composition or percent increases/decreases per unit of

breeding values. The introduction of bias in the back-transformation of logarithmic

analysis has been well documented (Finney 1941, Smith 1993, Sprugel 1983). To

minimise this error, we have chosen to use the error correction described by Sprugel

(1983) when results have been back-transformed in Chapter 5.


221

8.3 Changes in carcass composition due to genetic and non-

genetic factors.

8.3.1 Impact of genetics on carcass composition and value

8.3.1.1 Sire type and dam breed

The strength of the analysis in this thesis has been that the sire types and dam breed

effects on their slaughtered progeny were compared at the same carcass weight (Chapter

3). Previous studies have compared lambs from different sire types at the same age

(Ponnampalam et al. 2008) and weight (Gardner et al. 2010, Ponnampalam et al. 2008),

generally reporting changes in whole carcass lean weight, and the weights of indicator

muscles, however the change in lean weight and value within different regions of the

carcass has not previously been reported.

Compared at the same carcass weight, the Terminal sired lambs had more lean weight

than the Merino and Maternal sired lambs (Chapter 3). When compared at the same lean

weight, the Terminal sired lambs had the most lean in the saddle and hind sections, but

the least fore section lean (Chapter 3). As hypothesised in Chapter 5, this has led to the

Terminal sired lambs having the greatest carcass lean value when compared to the

Maternal and Merino sired lambs at equal carcass weight and the same age. This is the

first time that the financial advantage of the Terminal sired lambs has been

comprehensively reported.

8.3.1.1.1 Border Leicester and other Maternal breeds.

When lambs are compared at the same carcass weight, Maternal sired lambs had the

most carcass fat (Chapter 4), with this result thought likely to reflect their superior

breeding efficiency (Ferguson et al. 2010). The increased fat of these lambs may reduce

their carcass value if they attract penalties at the processing plant due to increased fat

scores. The dam breed differences supported the differences seen between sire types,


222

with the progeny of Border Leicester x Merino dams producing more carcass fat than

those of the Merino dams. The effect was strongest in ewe lambs, and given that many

of the females are retained for breeding, this small difference in the fat of the wethers is

a favourable result.

8.3.1.1.2 Merinos

The Merino sired lambs have been criticised for having increased bone and a ‘leggy’

appearance and are considered inferior for meat production compared to the other sire

types, partially due to their slow growth (Hopkins et al. 2007a, Ponnampalam et al.

2007b). In support of our hypothesis Merino sired lambs had more bone than the

Maternal and Terminal sired lambs (Chapters 4 and 5). Further assessment of sub-

primal cuts of meat between sire types is needed to determine the retail implications for

the increased bone.

When compared at the same carcass weight, Merino sired lambs have greater carcass

lean value (assuming the same $ value per kg of lean) than the Maternal sired lambs

which was unexpected, with previous research indicating that Merino lambs will always

be less productive in terms of carcass weight and muscle related productivity traits than

Maternal sired lambs (Ponnampalam et al. 2007b). The Merino sired lambs had greater

lean weight in both the saddle and hind sections (Chapter 5) and the Maternal sired

lambs had the least fore section lean weight of all three sire types. Therefore, despite

Merinos having increased bone (Chapter 4), they have potential for favourable financial

returns on their lean weight when lambs are compared at the same carcass weight. The

slow growth rate of Merino sired lambs (Hopkins et al. 2007a) mean they will reach

slaughter weights later than other sire types and this will impact on the cost of

production and this should be taken into account.


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Despite being bred for their wool, Merinos respond favourably to selection for LMY%

with carcass ASBVs. Merino sired lambs showed an increase in carcass lean in response

to increasing sire PEMD and decreasing PFAT (Chapter 3) which is similar to that of

the Maternal and Terminal sired lambs. However, the decrease in carcass fat in response

to selection for increased sire PEMD was more limited than the other sire types

(Chapter 4). The large increase in HCWT in response to selection for increased PWWT

ASBV (Gardner et al. 2015) means when Merino lambs of the same age are compared,

lambs Merinos born to sires with increased PWWT have increased lean value (Chapter

5). Additionally, on a per unit of PWWT comparison the increase in lean value is

greater than that of the Maternal and Terminal sired lambs. Therefore, similar to other

sire types there are benefits to the using carcass breeding values where it does not

overly compromise wool production/value.

Merino sired lambs have been shown to have higher sensory scores for tenderness,

overall liking, juiciness and flavour compared to the Terminal sired lambs (Pannier et

al. 2014a). This result was not solely linked to differences in IMF%. Therefore,

although Merino sired lambs are clearly bred for wool production, they offer reasonable

returns for lean yield, and coupled with their often superior eating quality they could

theoretically be used for high quality meat production – however there are other issues,

especially meat colour and high ultimate pH (Warner, Dunshea, Ponnampalam et al.

2006).

The effects of dam breed were compared within the Terminal sired lambs as Border

Leicester X Merino dams were not mated to Maternal or Merino sires. It was the

Merino dams that produced lambs with greater carcass lean and at a carcass weight of

23kg, with these compositional differences equated to an additional 0.17kg lean in the


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progeny of Merino dams (average of wethers and females) compared to the progeny of

Border Leicester X Merino dams (Chapter 5). This resulted in Merino dams producing

lambs with a predicted carcass lean value of $3.58 more, when compared to those born

to Border Leicester X Merino dams. These results indicate that the use of a Terminal

sire over Merino dams can be financially rewarding. It is likely this would be most

beneficial in circumstances where feed was abundant as the merino lambs take longer to

reach slaughter weights and in the advent of supplementary feeding the financial

benefits may be lost. When compared at the same age, it is the Border Leicester X

Merino dams that have more lean (0.95kg) and a carcass lean value that was $20.12

greater than the lambs from Merino dams.

8.3.1.2 Australian Sheep Breeding Values

8.3.1.2.1 Post weaning weight Australian Sheep Breeding Value

Although we had expected that increasing sire PWWT would increase the proportion of

lean in the carcass, results from this thesis (Chapter 3) reveal an increase in lean weight

only at some sites in some years. This finding may be attributed to the variation in

expression of this breeding value varying under different nutritional conditions (Hegarty

et al. 2006c). Maternal genetics cannot be discounted for this variation as ewes at each

site are of differing genetics. But in general the increase in LMY% is small, especially

compared to the other carcass breeding values, with the benefits of PWWT

predominantly delivered through its impact on HCWT (Chapter 5).

A new and important finding was that increasing PWWT resulted in redistribution of

lean to the saddle region when lambs are compared at the same lean weight (Chapter 3).

The mechanism for this redistribution effect is difficult to explain. Given the b

coefficients in this study were all similar there appears to be no difference in rate of

development of lean in carcass section. However, these maturation differences can not


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be completely discounted as early work by Butterfield et al (1984a) did show the

longissimus to be early maturing so this theory has credence. It is possible that PWWT

is affecting lamb conformation, where the carcass is longer, therefore further

investigation into carcass dimensions and sire PWWT may be useful.

In contrast to our hypothesis, increasing sire PWWT did not increase carcass bone, with

the effect very inconsistent, with increases only evident within some kill groups within

certain site-years (Chapter 4). This is in contrast to earlier work by Hegarty et al

(2006a) who showed that sires with increased sire PWWT had increased bone trim,

thought likely due to them being at an earlier stage of maturity due to their larger mature

size. The distribution of bone was impacted by sire PWWT, with increased hind limb

bone observed.

Similar to the impact on bone, and in contrast to our hypothesis there was little

consistent impact of sire PWWT on fat within the carcass, with it decreasing at some

sites in some years (Chapter 4).

The combined effects of sire PWWT on whole carcass composition and distribution of

lean, reveal a net increase in saddle lean weight (Chapter 5). In the Terminal sired

lambs, where selection for increased PWWT is most relevant, there is the potential to

increase carcass lean value by $6.26, which is $0.32 per unit of PWWT. However

compared to the other breeding values (PFAT and PEMD), the increase in sire PWWT

has a much smaller impact on carcass lean value. The full benefit of PWWT is realised

when its impact on HCWT was accounted for (Gardner et al. 2015) and therefore had a

far greater impact on carcass value when lambs are compared at the same age, due to

the increase in HCWT. Due to the variable impact of PWWT on HCWT between sire


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types (Gardner et al. 2015), Chapter 5 shows a corresponding variation in the per unit

rate of increase in carcass lean value when lambs are compared at the same age. The

greatest response was observed in the Merino sired lambs, but given the range of

PWWT values in this sire type is generally limited the maximum lean value is still

lower than that of the Terminal and Maternal sired lambs.

8.3.1.2.2 Post weaning fat depth Australian Sheep Breeding Value

The PFAT ASBV has a substantial impact on carcass composition, impacting weights

of lean, fat and bone (Chapter 3, 4 and 5). Of the three carcass breeding values, reducing

sire PFAT has the greatest impact on carcass lean (Chapter 3) and substantially

increases lean value in the carcass (Chapter 5), which until now has not been as

extensively quantified. The magnitude of this effect was greatest in the Terminal sired

lambs that were compared at the same weight (23kg) and age (269 days), with

decreasing sire PFAT worth $6.26/ unit PFAT and $8.42/ unit PFAT respectively. The

CT study of Gardner et al (2010) reported a small increase in lean which was very

localised to the saddle section with increases in loin weight and eye muscle area. The

redistribution of lean from the fore section to the saddle (Chapter 4) has not previously

been identified, making the findings of this thesis unique.

The greatest magnitude of PFAT’s effect on carcass composition is in the reduction of

carcass fat (Chapter 4). As hypothesised, carcass fat was uniformly decreased in all

sections and this effect was found to be consistent across all sire types. Gardner et al

(2010) has shown similar results, however the magnitude of carcass fat reductions was

much smaller than the a 24.7% reduction in the weight of carcass fat shown in this

thesis. There have been concerns that point measures such as the c-site may over time

result in a decrease in fat only at this point of measurement as occurred in pigs

(Trezona-Murray 2008, Wood et al. 1983). Previous studies in lamb did show sire


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PFAT to have a more localised impact in the loin region (Hegarty et al. 2006a).

Although current genetic selection appears to have a whole carcass impact on fat, it

would be advisable to continue to monitor carcass composition to ensure this single site

selection for leanness (reduced sire PFAT) remains appropriate.

Similar to work by Gardner et al (2010) and in partially supporting the hypothesis, a

decreasing sire PFAT increased carcass bone weight (Chapter 4), but did not impact

bone distribution in the carcass. The magnitude of the increase in bone shown in the

work of this thesis is much greater than previously identified by Gardner et al (2010).

This result is unlikely due to an increase in mature size as there is no evidence that

PFAT has a substantial increase in growth rate (Kelman et al. 2014a), and there is only

a small increase in carcass weight reported (Gardner et al. 2015). It will be important to

monitor the impact of the increase in bone, as changes in the conformation of the

carcass may influence the appeal of certain cuts of meat to consumers.

The selection of sires of reduced PFAT may decrease carcass fat by altering the

expression of lipogenic enzymes in the liver and adipose tissue. Work in pigs showed

that selection for reduced back fat decreased hepatic expression of fatty acid synthase

and Δ6-desaturase (Muñoz, Estany, Tor et al. 2013). Similar experiments in lamb may

reveal a similar process in the adipose tissue and account for the fat reduction observed.

Adipose tissue from obese humans has reduced adrenaline responsiveness resulting in

reduced lipolysis (Jocken and Blaak 2008). Martin et al (2011) hypothesised that if the

reverse were true then lambs selected for reduced fat may have increased adrenaline

responsiveness and therefore increased lipolysis.


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Administration of growth hormone has been shown to increase growth rate, reduce fat

deposition and induce muscle hypertrophy (Beermann, Fishell, Roneker et al. 1990).

Pigs selected for low back fat depth were shown to have increased mRNA levels of

growth hormone and its transcription factors ultimately leading to increase growth

hormone (te Pas, Freriksen, van Bijnen et al. 2001). In pigs, the magnitude of the

response to growth hormone differed between muscles (Ono, Solomon, Evock-Clover et

al. 1995), with early maturing muscles respond better to exogenous porcine

somatotropin. Therefore the regional influence of growth hormone on muscle

hypertrophy may account for regional differences in lean response in the lamb.

Previous studies in lamb have focussed on levels of IGF-I as representative regulator of

the somatotrophic axis (Hegarty, McFarlane, Banks et al. 2006b). In this study there

was little ability of IGF-I to predict carcass composition however further investigation

into the relationships between selection for reduced c-site fat depth and the somatrophin

axis may elicit the mechanism by which this selection reduced carcass fat and increases

regional muscularity. Furthermore, the local production of IGF-I may be influenced by

sire PFAT, where it is more difficult to measure and quantify changes.

A study by Pogoda et al (Pogoda, Egermann, Schnell et al. 2006) shows that high levels

of leptin in ewes resulted in decreased osteoblastic activity and trabecular bone

formation. The highest level of leptin mRNA is located in the subcutaneous back fat of

sheep (Kumar, Francis, Suttie et al. 1998). Therefore if lambs selected for low levels of

subcutaneous fat depths (low sire PFAT) also have low leptin, then it is possible this

may contribute to the increased bone of their progeny.


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8.3.1.2.3 Post weaning eye muscle depth Australian Sheep Breeding Value

One of the most significant findings of this thesis was that sire PEMD causes a

substantial increase in carcass lean weight, contrary to what was initially hypothesised

(Chapter 3). Previous work had identified only a localised effect of this breeding value

focused around its site of measurement in the loin (Gardner et al. 2010, Hegarty et al.

2006a). None-the-less, and aligning with this previous work, the effect in this study was

still of greatest magnitude in the saddle region with redistribution of lean from the fore

section to the saddle occurring. In Terminal sired lambs this was worth as much as

$30.05 more in retail lean value (Chapter 5).

Countering the lean response, PEMD had a much greater impact on fat which decreased

in all carcass sections to a similar magnitude with no redistribution effects evident. This

effect varied substantially between sire types, being most prominent for Maternal sired

lambs and non-existent for Merino’s (Chapter 4). There was also no impact of PEMD

on carcase bone weight within any of the sire types, contrast with Cake et al (2007) who

concluded that there was higher muscle:bone ratios in lambs selected for muscling due

to a decrease in bone rather than an increase in carcass lean.

The increase in whole carcass lean due to PEMD discussed in Chapter 3 is not likely

due to an effect on mature size. If this were the case then we would also expect to see

proportionately more bone. Additionally there has been no evidence of a phenotypic or

genetic correlation between PEMD and mature weight (Huisman and Brown 2008). The

leaner and more muscular composition appears to be independent of maturity.

Increased muscle mass can be due to hypertrophy or hyperplasia. Postnatal hyperplasia

in lamb is uncommon, with increases in muscle due predominantly to hypertrophy


230

(Greenwood et al. 2000, McDonagh, Ferguson, Bacic et al. 2006, Zhu et al. 2004, Zhu

et al. 2006). Hypertrophy can occur due to increased protein synthesis and decreases

protein degradation (Koohmaraie, Shackelford, Wheeler et al. 1995, McDonagh et al.

2006). Greenwood et al (2006b) showed increased amounts of RNA with increased

PEMD in the longissimus, semimembranosus and semitendinosus, which may indicate

increased translational capacity due to this genetic selection. In this same study of

Greenwood et al (2006b) the longissimus also had increased protein/DNA ratio which

supports a theory of increased protein accretion, though reduced protein degradation

cannot be discounted. Greenwood et al (2006d) suggested hyperplasia of the

longissimus may be the cause of increased muscle mass in high PEMD sires as there

was no significant increase in myofibre cross sectional area.

In lamb, type II (glycolytic) muscle fibres are larger in diameter than type I (oxidative)

fibres (Greenwood et al. 2007), therefore increasing the proportion of Type II muscle

fibres may increase the size of the muscle. Increased sire PEMD led to more glycolytic

fibres compared to oxidative fibres in the longissimus (Greenwood et al. 2006b) and an

increase the proportions of type IIX fibres (Greenwood et al. 2006d). Kelman et al

(2014b) supported a theory of increased glycolytic fibres, with a reduction in oxidative

indicators like myoglobin and ICDH in lambs of high PEMD. A switch in fibre type to

more type IIX fibres in certain regions/muscles of the lamb carcass (i.e. the saddle and

hind sections) may account for the localised effect of PEMD. Given the study of

Greenwood et al (2006b, 2006d) only used a small number of Dorset Horn sires and it

would be useful to include other sire types and a range of muscles to investigate this

theorem further. Type II fibres are more responsive to β adrenergic agonists (McDonagh

et al. 2006). In pigs, glycolytic fibres also more likely to respond to administration of

porcine somatotropin (Ono et al. 1995). Ono et al (1995) also showed that location of


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the muscle and fibre type/metabolism will have different effects on muscle growth

which may account for the regional differences in muscle growth in lambs of increased

sire PEMD.

Insulin like growth factors can be synthesised locally to cause hypertrophy (Musarò,

McCullagh, Naya et al. 1999). Therefore studies that measure serum insulin like growth

factors may be too simplistic to identify a mechanism for hypertrophy of this nature

(Nattrass, Quigley, Gardner et al. 2006).

Pathways involving adrenergic receptors on protein and fat metabolism are complex

(Johnson, Smith and Chung 2014, Mersmann 1998). It is possible the density of β

adrenergic receptors varies depending on the relative maturity of an animal (Johnson et

al. 2014). Greife et al (1989) showed that older rats were more responsive to

clenbuterol than younger rats which may support this notion. PEMD does not impact

mature size (Huisman and Brown 2008) and is therefore unlikely to impact on lamb

maturity and adrenergic receptor density through this mechanism although if the

maturity of individual muscles are altered by such selection then this theory is more

plausible. Alternatively in rats, it was shown that oxidative muscle types have an

increased density of β adrenergic receptors (Martin, Murphree and Saffitz 1989). It was

shown by Greenwood et al (2006d) that lambs selected for muscling had reduced

oxidative and increased glycolytic fibres, therefore may have decreased density of β

adrenergic receptors in muscle. This would result in a decrease in muscle

responsiveness to adrenaline which has been observed in both sheep (Martin et al.

2011) and cattle (Gardner, Martin, McGilchrist et al. 2005). Future studies could

investigate the relative distribution of -adrenergic receptors on muscle and adipose

tissue to see if the above effects caused by PEMD are a result of receptor distribution.


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The reduction in fat caused by an increase in sire PEMD, (Chapter 4) may be due to

adrenaline responsiveness. Martin et al (2011) showed that lambs selected for muscling

had increased adipose responsiveness to adrenaline at 4 months of age. They proposed

that this response maybe due to increased vascularisation of adipose tissue from lean

animals, which may increase the adrenaline responsiveness of fat and increase lipolysis

(Gregory, Christopherson and Lister 1986).

8.3.2 Non genetic effects

The purpose of this experiment was not to specifically test for the impact of nutrition

and environmental conditions on carcass composition. However these impacts were

taken into account by the inclusion of site or location in the models.

Non-genetic effects had less impact than ASBVs on fat and bone when adjusted to a

constant carcass weight. Of the production factors, site and kill group had the largest

impact which demonstrates the potential for nutrition and other environmental factors to

impact carcass fat and its distribution.

The impact of sex on carcass composition of fat, lean and bone was similar to those of

previous studies. An important aspect of this experiment was to quantify those

differences (Chapters 3 and 4) and to describe the financial advantages of wethers when

compared to ewe lambs at both the same carcass weight and age. At the same carcass

weight, the wethers were $9.91 greater in value than the ewes, with this difference in

value doubling when lambs were compared at the same age.


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8.4 Investigations into intramuscular fat % in muscles across

the lamb carcass.

8.4.1 IMF% of muscles from different regions across the Australian

lamb carcass and correlations with the m. longissimus lumborum

The impact of selection for leanness on IMF% has only been documented in the

longissimus (Pannier et al. 2014c), with the impact for this selection not previously

reported for other carcass regions (Chapter 6). The average IMF% within each of the

muscles tested was 4.3%, 5.0%, 4.0%, 3.7% and 4.8% for the m. longissimus, m.

supraspinatus, m. infraspinatus, m. semimembranosus, and m. semitendinosus (Chapters

6, 7). The average IMF% of the m. longissimus is similar to that reported by Pannier et

al (2014c) in a much larger, but related study of Australian lamb. This gives confidence

in the reporting of IMF% from the other muscles in this experiment.

The correlation of IMF% of the m. longissimus lumborum with the 4 other muscles (m.

supraspinatus, m. infraspinatus, m, semitendinosus and m. semitendinosus) was fairly

consistent, ranging from 0.34 to 0.40 (Chapter 5), which has not previously been

reported. Knowledge of this moderate and consistent correlation between muscles from

different regions of the carcass means that selection for IMF% in the m. longissimus

lumborum will provide significant estimation of the changes in IMF% in the other

regions of the carcass. The correlation of carcass fat % (as measured by CT) was highest

with the m. longissimus lumborum (0.41, partial correlation coefficient) and at least

25% higher than the correlation between CT fat % and the IMF of the other muscles.

This indicates that although carcass fat % is reasonably well correlated with IMF % of

the m. longissimus lumborum, it is not a good predictor of IMF% in other regions of the

carcass.


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The IMF content of muscles was not strictly related to the function of the muscle

(postural v locomotive) as the m. longissimus lumborum did not have the highest IMF as

expected, being solely a stabilising muscle. However, the m. supraspinatus, a stabilising

muscle of the shoulder (Suzuki 1995), did have higher IMF% than the m. infraspinatus

(locomotive) which does conform to the notion that stabiliser muscles are more

oxidative and have higher IMF%.

The metabolic function of a muscle has also been shown to influence the IMF% content,

with more oxidative muscles often having higher IMF% (Hocquette et al. 2010). It

appears that IMF% is reduced in the m. longissimus lumborum in response to selection

for leanness (Pannier et al. 2014c) and that selection for LMY% has resulted in less

oxidative metabolism (Kelman et al. 2014b). Further work to characterise the muscle

metabolic type in other regions of the carcass would help to determine if selection for

LMY%, IMF% and muscle metabolism are linked in other carcass regions.

8.4.2 Prediction using CT

Management of carcass fat and IMF% are important drivers of carcass value with

carcasses more desirable if they have low levels of carcass fat to maximise lean yield,

however IMF% must be maintained at a level for acceptable consumer satisfaction and

eating quality (Hopkins et al. 2006a). Unfortunately the selection for improved LMY%

has a negative relationship with IMF% (Pannier et al. 2014c) which makes the balance

of these two traits difficult. The Australian lamb industry can effectively modify carcass

fat using a point measure of fat at the c-site (reduced sire PFAT) (Chapter 4). If IMF%

is to be managed independently of carcass fat then there needs to be an accurate, rapid

method of measuring carcass IMF%.


235

This thesis investigated the relationship between CT and IMF% in Australian lamb

across a wide range of ages and sire types with moderate variation in IMF% (Chapter

7). There was a negative correlation between pixel density and IMF% similar to other

studies (Lambe et al. 2010), however the IMF in the loin could only be predicted with

quite low precision. Prediction of IMF% using CT in the other regions of the carcass

was inferior to the prediction in the loin, possible due to fat cell size and shape and

muscle structure in the different muscles (Chapter 7).

The relationship of IMF% with eating quality poses an interesting question for the lamb

industry as in order to select for a trait such as IMF%, it is necessary to have knowledge

of the heritability, variation within the lamb population and an accurate measure of this

trait. Although there is knowledge of heritability (Mortimer et al. 2014) and the

population range (Pannier et al. 2014c) of IMF% in the Australian lamb population, the

degree of change of IMF% which consumers can detect and therefore the degree of

accuracy required in a measurement system is unknown. Given that there is currently no

commercial incentive for producers to select for IMF% in their breeding programs, the

realisation of in plant measurement of IMF% seems distant. Future studies that

investigate the relationships between the magnitude of change in IMF% required to

elucidate a consumer preference in lamb will help to determine the degree of accuracy

of future in plant measurement systems such as CT.

The prediction of IMF using CT has been used with varying success in lamb, beef and

pigs in predict IMF% (Clelland et al. 2014, Font-i-Furnols et al. 2013, Karamichou et

al. 2006, Prieto et al. 2010). The reason for the differences between studies may be due

to different scanners and possibly due to the different populations studied (species,

breeds of sheep, age and IMF% content). A further reason could be due to the different


236

statistical techniques used. Partial least squares regression was used by some of the

studies (Font-i-Furnols et al. 2013). We did not use this method because as our study

size was large and there is no evidence to suggest that using partial least squares

regression is any better than linear regression in this instance. None-the-less it may be

beneficial to investigate alternative methods of IMF prediction.

8.4.3 The impact of lean meat yield on intramuscular fat throughout

the lamb carcass.

Previous experiments in lamb have focussed on the IMF% and eating quality of the m.

longissimus lumborum (Pannier et al a, c), which is located in the highly valued saddle

region. Carcass breeding values used to select for leanness (reduced sire PFAT and

increasing sire PEMD) have been shown to reduce IMF% in the m. longissimus

(Pannier et al. 2014c), however the impact of carcass breeding values on other regions

has not been reported. In support of the hypothesis in this thesis, selection for reduced

sire PFAT resulted in decreased IMF% in the m. longissimus, m. semimembranosus and

m. semitendinosus (Chapter 6), with the magnitude of the effect varying between these

muscles and greatest in the hind section muscles. Increasing PEMD had an impact only

on the IMF% of the lambs born as multiples and raised as singles (Chapter 6), which is

in contrast to findings by Pannier et al (2014c) where sire PEMD had an impact on the

Terminal sired lambs. An unexpected result was for the same subset of lambs (born

multiple, raised single) to have decreased IMF% in response to an increase in sire

PWWT. Previous investigations by Pannier et al (2014c) did not reveal any influence of

sire PWWT on IMF%. The reason for the response of PEMD and PWWT on the lambs

born as multiples and raised as singles is difficult to describe, however given these

lambs represent 10% of lambs in the experiment, further investigations are

recommended before attributing confidence to this result.


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This thesis further demonstrates the impact of selection for LMY% using ASBVs on

IMF%. An increase in CT lean% led to a decrease in IMF% that was consistent across

all muscles investigated (Chapter 6). As HCWT increased, IMF% also increased, which

is similar to previous investigations (McPhee et al. 2008, Pannier et al. 2014c).

However it was a new finding that this decrease varied between sire types and muscles,

with the greatest effect in the supraspinatus of the Merino sired lambs (Chapter 6).

Future investigations into muscle metabolism may elicit a reason for the variation

between muscles and sire types. The variation in the reduction of IMF% as LMY%

increases indicates that despite the good correlation of IMF% from the longissimus with

other muscles (Chapter 7), a breeding value for IMF% may be better if incorporating

information about IMF from more than one site. This further iterates the need for a

rapid, non-destructive method to evaluate IMF% such as CT.

8.5 Alterations to experimental design and future studies

8.5.1 Alterations to experimental design

The lambs for this experiment were selected from the INF which is a large experiment

with a comprehensive genetic design to assess production and biological parameters

(Fogarty et al. 2007). Despite careful attention to the experimental design, there are

some areas that could be addressed differently if given the opportunity and additional

resources. For example, sire type comparisons could only be made in the wether

progeny of the Merino dams as this was the only category where all three sire types

were equally represented. This was due to the ewe lambs from the Maternal and Merino

progeny being unavailable for slaughter and thus this information could not be recorded.

Similarity the dam breed effects could only be compared within the Terminal sired

lambs as Border Leicester X Merino dams were not used in the Merino and Maternal

sire type groups.


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To investigate the impact of nutrition on carcass composition it would be necessary to

quantify feed intake. This was not possible in this study which used INF lambs that

were raised on pasture in different environments/geographical locations spread around

Australia. Thus pasture availability and environment (worm burdens, weather

conditions), differencing between locations and years all contribute to the variation in

the data. Whilst these differences were modelled through the inclusion of site and year

in our analyses, the confounded nature of these factors mean that nutritional

interpretations are difficult to elucidate. Key experimental factors such as sire type and

breeding value impacts were all carefully balanced by ensuring representation of sires

across each of the sites and therefore nutritional environments.

The financial analysis (Chapter 5) uses the average retail value to describe the value of

the lamb carcass. The most accurate way to describe carcass value would be to dissect

the carcass into retail cuts and collect each of their individual weights. Due to the

carcasses being sampled for a range of other traits as part of the INF experiment it was

not possible to dissect and weigh retail cuts. The estimation of the financial value of

each section by multiplying the weight of lean (determined by CT) in each section by

the average lean value is still a good estimation of the retail lean value of each carcass.

It allows for differential effects between breeding values and fixed effects to be

compared.

In addition, the financial cost of removing bone and fat trim was not accounted for in

the calculation of retail value. Further experiments may consider incorporating a

commercial bone out so that CT lean can be directly converted to retail saleable yield.

Indeed the recommended way forward is to have CT based composition as the gold

standard for carcass composition and have this connected to a number of commercial


239

bone-out scenarios. However to obtain such data would be expensive, with the CT value

of lean perhaps remaining the best and most consistent estimate of carcass value.

8.5.2 Future studies

The slaughter age of lambs in these experiments ranges from 168 to 420 days and

carcass weights of 13.3 to 40.0 kg. It is well accepted that the carcass proportions of fat,

lean and bone alter from birth to maturity (Berg and Butterfield 1968, Butterfield 1988,

Butterfield et al. 1983a) and that comparisons made at the same age will be different to

comparisons made at the same stage of maturity. It would be ideal to document the

composition of lambs from birth through to maturity, rather than making an assessment

only at slaughter. Information from serial DXA scanning of lambs has been performed

but reports on whole carcass composition (Ponnampalam et al. 2007b). Serial CT

scanning would provide more detailed information regarding the distribution of the

tissues between carcass sections, rather than only whole carcass composition of fat, lean

and bone, and would also be superior to serial slaughter and bone out procedures as

individual animals can be assessed through to maturity, rather than at a single slaughter

time-point.

The use of CT on live animals would also allow a more comprehensive evaluation of

the impact of selection for reduced c-site fat on fat distribution, including abdominal fat.

In the current experiment the amount and distribution of fat in the carcass was

examined, which is important from a commercial and retail point of view, however CT

of live animals would allow the abdominal depots of fat to be examined. This would

determine if whole body fat is being affected by selection for LMY% or only the

subcutaneous fat. These results could have important implications for Maternal lambs as

phenotypic fat has been identified as being important for reproduction, with extreme

selection for reduced PFAT potentially impacting reproductive efficiency.


240

Although the main reason to examine the composition of a carcass is to look at the

economic implications at slaughter age/weight, the scanning of lambs from birth to

maturity would provide valuable information about the impact that selection for LMY%

has on mature size and composition. This information will resolve some questions

posed by the current research such as whether differences are due to stage of maturity,

or mature composition. It would be important to note differences in mature composition

between lambs of different genetics (breeding values, sire types, dam breeds),

production and management effects. Such an experiment would need to incorporate a

representative sample on lambs of known genetics, ideally from a number of regions

across Australia so that the information obtained has commercial relevance to the

Australian Sheep industry. It would be ideal if within sites the nutritional inputs of the

lambs could be controlled to provide information regarding composition under

conditions of high and low nutrition.

8.5.2.1 Mechanistic experiments

Future work could examine the protein, RNA and DNA contents of muscles throughout

the carcass, including muscles from the fore section and may elucidate the mechanism

for increased muscle mass. Additionally, characterisation of the muscle metabolic type

by measurement of enzymes such as ICHD, LDH and oxidative indicators such as

myoglobin would be useful to try and determine how selection for muscling, which has

been shown to impact on all carcass sections (Chapter 3), affects muscle metabolism

throughout the carcass. It would be essential to include a range of sire types and

muscles in such a study to expand on previous work which has focussed on Terminal

sired lambs and muscles of the saddle and hind sections only.


241

Although the motivation for assessing carcass composition is primarily to estimate the

value of the carcass lean, eating quality has become more important with the

introduction of Meat Standards Australia into the lamb industry (Watson et al. 2008).

Fibre type has been shown to impact eating quality (Karlsson, Klont and Fernandez

1999) and selection for muscling and leanness has been shown to affect fibre type

(Greenwood, Gardner and Hegarty 2006c, Greenwood et al. 2006d, Kelman et al.

2014b, Warner et al. 2010a). Given that increased sire PEMD and reduced sire PFAT

was shown to impact the lean (Chapter 3), it would be useful to document associated

changes in fibre type and metabolic activity in all three carcass sections through

collection of muscle biopsies at post mortem.

Martin et al (2011) showed that lambs selected for muscling had increased adipose

responsiveness to adrenaline in lambs at 4 months of age. To determine the impact that

selection for decreased sire PFAT and increased sire PEMD has on fat and muscle it

would be useful to determine the number and types of adrenergic receptors in muscle

and fat depots. Additionally, the lipogenic processes in adipose tissue and the liver may

reveal the mechanism for fat reduction.

Given the possible implication of growth hormone and IGF-I in the reduced fat of lambs

from low sire PFAT, further studies could incorporate measurements of hormones from

the somatotrophin axis. The cost of such an experiment on a large scale may however be

prohibitive.

The detection of genes that are specific for a desired trait, such as muscling, can be

incorporated into a pedigree to assist development of testing for such traits, with these

genes known as quantitative trait loci (QTL). The mechanisms that underpin the


242

changes in carcass composition are complex. Genetic causes of muscle hypertrophy are

the callipyge gene, identified on chromosome 18 of sheep; the Texel muscling QTL,

also on chromosome 18, which increased weight of the m. longissimus lumborum

(Macfarlane, Lambe, Bishop et al. 2009) and the Carwell gene, which has been mapped

to a similar region of the sheep genome and has been shown to increase muscling of the

longissimus dorsi (McLaren, Broad, McEwan et al. 2001). These QTLs can be used in

genetic improvement programmes using linked markers called marker assisted selection

(Dodds, McEwan and Davis 2007). Marker assisted selection is likely to be beneficial in

improving carcass composition as the carcass measurements cannot be taken from the

breeding animal (Van der Werf 2007). There are a number of genes identified in

relation to muscling in sheep (Broad, Glass, Greer et al. 2000, Freking, Murphy, Wylie

et al. 2002, Johnson, McEwan, Dodds et al. 2005, Laville et al. 2004, McRae, Bishop,

Walling et al. 2005, Nicoll, Burkin, Broad et al. 1998). Future research into carcass

composition could include genomic marker research to identify genetic markers that

identify regions of the genome that cause the increased muscling and redistribution

observed in the current study. These markers may be able to be incorporated into future

genomic selection programs.

8.6 Summary

This thesis reveals some important impacts that the selection for LMY% through the use

of ASBVs has on carcass composition. The proportion of lean in the carcass is increased

by the use of sires with increased PEMD and decreased PFAT, with both these breeding

values increase the amount of lean in the saddle section. Both breeding values lead to an

increase in carcass value, with sire PFAT having the greatest influence on carcass lean

value. Sire PWWT has less impact on lean weight in the carcass, with its value best

realised through its impact on carcass weight and therefore carcass lean weight, when

lambs are compared at the same age.


243

Use of sires with decreased PFAT ASBVs produces lambs with less carcass fat. This

effect is seen uniformly throughout all carcass sections and is not localised to its point

of measurement at the c-site in the in the saddle section. Combined with its impact on

lean, decreasing sire PFAT has the greatest contribution to carcass leanness and lean

value. This reinforces the notion that an independent breeding value is required to

maintain adequate IMF% in the carcass as a decrease emphasis on PFAT to manage

falling IMF% will negative impact carcass lean value considerably.

This thesis provides the most comprehensive reporting of the IMF% of muscles

throughout the Australian lamb carcass. There is continued concern on the impact of

leanness on IMF% levels in the lamb carcass. This thesis shows that sire PEMD and

PFAT decrease IMF% in muscles other than the longissimus, however larger studies are

required to investigate this further.

The use of computed tomography to determine IMF% in the lamb carcass lacks

precision, with further work required for this technique to be useful as a research tool to

determine IMF% levels in the carcass. CT prediction of IMF% was most accurate in the

longissimus, with this muscle showing good correlation of IMF% of the other muscles

examined. This indicates that the current research practice of measuring IMF% in the

longissimus is appropriate, however it would be advised to continue to monitor the

correlations of IMF% between muscles in the future.

References

244

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