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GENETICS

Selection Strategies for Body Weight and ReducedAscites Susceptibility in Broilers

A. Pakdel, P. Bijma, B. J. Ducro, and H. Bovenhuis1

Animal Breeding and Genetics Group, Wageningen Institute of Animal Sciences,PO Box 338, 6700 AH Wageningen, The Netherlands

ABSTRACT Ascites syndrome is a metabolic disorderin broilers. Mortality due to ascites results in significanteconomic losses and has a negative impact on animalwelfare. It has been shown that genetic factors play aconsiderable role in susceptibility of birds to ascites,which offers perspectives for selection against this syn-drome. The aim of the present study was to evaluate theconsequences of alternative selection strategies for BWand resistance to ascites syndrome using deterministicsimulation. In addition to the consequences of current

selection (i.e., selection for increased BW only) alternativeselection strategies including information on different as-cites-related traits measured under normal or cold condi-tions and the consequences of having information on theunderlying genes (i.e., MAS) were quantified. Five differ-ent breeding schemes were compared based on the selec-

(Key words: broiler, ascites, breeding program, deterministic simulation, quantitative trait loci)

2005 Poultry Science 84:528535

INTRODUCTION

Ascites syndrome is a metabolic disorder of fast-grow-ing meat-type chickens. It has been suggested that thissyndrome is the consequence of allomorphic differencesin heart and lung growth relative to the size of the animal(Siegel and Dunnington, 1997). Ascites results in signifi-cant economic losses to the broiler industry due to highmortality, especially at later ages, and has a negativeimpact on animal welfare. The incidence of ascites is influ-enced by environmental and genetic factors. The mostimportant environmental factors causing the develop-ment of ascites in broilers are high altitudes and coldtemperatures (Smith et al., 1954; Siller and Hemsley, 1966;Bendheim et al., 1992; Shlosberg et al., 1992). Several stud-ies have shown that traits related to ascites syndromehave a relatively high heritability (Lubritz et al., 1995;Maxwell and Robertson, 1997; De Greef et al., 2001; Pakdel

2005 Poultry Science Association, Inc.Received for publication July 14, 2004.Accepted for publication December 3, 2004.1To whom correspondence should be addressed: henk.bovenhuis

@wur.nl.

528

tion response for BW, ascites susceptibility, and the rateof inbreeding. Traits investigated in the index as indica-tors for ascites were hematocrit value (HCT) and ratio ofright ventricle to the total ventricular weight of the heart(RV:TV). The results indicated that by ignoring ascitessusceptibility in the breeding goal, the gain for BW is 130g and the birds will become more susceptible to ascites.Testing 50% of the birds under cold temperature condi-tions and including information of ascites related traits(HCT and RV:TV) measured under normal and cold con-

ditions makes it possible to achieve a relatively high gainfor BW (111.4 g) while controlling the genetic level forascites susceptibility (selection response was 0). The re-sults of scenarios including QTL information of ascitessusceptibility showed that QTL information could beused very effectively in controlling ascites susceptibility.

et al., 2002). This indicates that genetic factors play aconsiderable role in susceptibility of birds to ascites,

which offers perspectives for selection against thissyndrome.

In practice, selection will be for a combination of pro-duction traits, such as BW and ascites susceptibility (AS).A few studies have reported genetic correlations betweenBW and ascites-related traits (De Greef et al., 2001; Mogh-adam et al., 2001; Pakdel et al., 2005a). Under normalclimatic conditions, Moghadam et al. (2001) found a posi-tive genetic correlation between ascites and BW. Undercold conditions, however, De Greef et al. (2001) and Pak-del et al. (2005a) reported a negative genetic correlation

between traits related to ascites and BW. In addition Pak-del et al. (2005b) estimated a weak but positive geneticcorrelation among ascites-related traits measured undercold conditions and BW measured under normal condi-tions. These results indicate that there is considerablescope for simultaneous selection of birds for increased

Abbreviation Key: AS = ascites susceptibility; BLUP = best linearunbiased prediction; EBV = estimated breeding value; F = rate ofinbreeding; HCT = hematocrit value; RV:TV = ratio of right ventricularweight to the total ventricular weight.

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BREEDING STRATEGIES TO CONTROL ASCITES 529

BW while controlling susceptibility to ascites. However,it is not obvious which selection strategy is optimal toachieve this goal.

Designing effective selection strategies for ascites resis-tance not only has to deal with the identification of goodindicator traits but also the choice of the selection environ-ment. Indicator traits, such as hematocrit value (HCT) orratio of right ventricle to the total ventricular weight of

the heart (RV:TV), differ not only with respect to theircorrelations to AS but also differ in that HCT can bemeasured on live birds, whereas RV:TV cannot. Further,measurement of birds under cold-stressed conditions isexpected to reveal to a larger extent the genetic differencesthat exist in AS between birds as compared with thetesting of birds under normal temperature conditions.New opportunities to reduce AS by means of selectionwill become available if genetic markers linked to ASare identified.

The aim of the present study was to evaluate the conse-quences of alternative selection strategies for BW andresistance to ascites syndrome using deterministic simula-

tion. In addition to the consequences of current selection(i.e., selection for increased BW only) alternative selectionstrategies including information on different ascites-re-lated traits measured under normal or cold conditions,and QTL information for AS will be quantified. Schemeswill be compared based on the selection response for BWand AS as well as the rate of inbreeding.

MATERIALS AND METHODS

Population Structure

A population with discrete generations was simulated

in which 50 males were randomly mated to 350 femaleswith a mating ratio of 1 male to 7 females. Each femaleproduced 32 progeny, 16 males and 16 females, and eachmale had 112 progeny of each sex. The total number ofprogeny of each sex was 350 16 = 5,600. Among theprogeny, the best males and females were selected asparents of the next generation. In the starting situation50 male parents out of 5,600 available males were selected(selected proportion = 0.009), and 350 females were se-lected out of the available 5,600 (selected proportion =0.063). Base generation individuals were assumed to orig-inate from a large population in Hardy-Weinberg andgametic phase equilibriums.

Breeding Goal and Index

The breeding goal was a combination of 2 traits: BWand AS,

H = VBW ABW + VAS AAS

where H is a weighted sum of the true breeding values,VBW is the economic value of BW, ABW is the true breedingvalue for BW, VAS is the economic value of AS, and AASis the true breeding value for AS. In practice an index I

is used to predict value for the breeding goal H of eachselection candidate. The index I is the estimated breedingvalue (EBV) of the breeding goal:

I = VBW EBVBW + VAS EBVAS .

The breeding goal applied to normal husbandry con-ditions.

Two different breeding goals were used: 1) base situa-tion in which selection was only for BW and with noemphasis on AS (VAS = 0) and 2) the economic value forBW was fixed at 1, and the economic value of AS in the

breeding goal (VAS) was changed to obtain zero responsein AS. In the base situation information on BW measuredunder normal temperature conditions was available forall birds. Selection for increased BW in this situation re-flected selection based on the EBV from an animal model

best linear unbiased prediction (BLUP) procedure. Alter-native selection strategies considered differed with re-spect to the type of ascites indicator trait and theenvironmental conditions the traits were measured in.

With respect to the type of ascites indicator trait, traitscan be distinguished that can be measured on the live

bird, such as HCT, and indicator traits for which the birdhas to be slaughtered, such as RV:TV. It was assumedthat the birds for which RV:TV was recorded were notavailable as selection candidates. Four different scenarioswere considered:

A-1) Measurement of BW and HCT on all birds andunder normal conditions, which is indicated as [100%(BWN + HCTN)].

A-2) Measurement of BW and HCT on all birds andmeasurement of RV:TV on 50% of the birds, which

is indicated as [100% (BWN +HCTN) + 50% (RV:TVN)].All birds were kept under normal temperature condi-tions, and birds for which RV:TV was measured werenot available for selection.

A-3) 50% of the birds are tested under cold-stress condi-tions. In this scenario the traits BW and HCT weremeasured under normal and cold conditions,whereas the trait RV:TV was measured only undercold conditions. Birds for which RV:TV was mea-sured were not available for selection [50% (BWN +HCTN) + 50% (BWC + HCTC + RV:TVC)].

A-4) All birds were tested under cold-stress conditions.In this scenario the traits BW and HCTwere measuredon all birds. Fifty percent of the birds were killedto measure the trait RV:TV [100% (BWC + HCTC) +50% (RV:TVC)].

Table 1 summarizes the traits recorded and the numberof half-sibs and full-sibs in the different breedingschemes. For BW and HCT, in addition to the averagephenotypic performance of full-sibs and half-sibs, the

birds own performance was available. The RV:TV traitwas not available for the selection candidate.

In addition to the above indicated alternative scenarios,breeding schemes with QTL information were investi-

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TABLE 1. The selected proportion per sex and the number of half-sib and full-sib birds for whichphenotypic information was available under different selection strategies

Normal conditions Cold conditions

Selected BWN HCTN1 RV:TVN BWC HCTC RV:TVC

proportionScheme per sex FS2 HS FS HS FS HS FS HS FS HS FS HS

Base 5,600 31 192 A-1 5,600 31 192 31 192 A-2 2,800 31 192 31 192 16 96

A-3 2,800 15 96 15 96 16 96 16 96 16 96A-4 2,800 31 192 31 192 16 96

1HCT = hematocrit value;RV:TV= ratio of right ventricular weight to total ventricular weight, where subscriptC indicates that the trait was measured under cold conditions, and subscript N indicates that the trait wasmeasured under normal temperature conditions.

2FS = number of full-sib birds; HS = number of half-sib birds.

gated. The QTL that were considered explained 5, 10, 20,or 50% of the genetic variance of the breeding goal traitAS. These situations will be referred to as Q05, Q10, Q20,and Q50, respectively. This fraction of the variance can

be explained by one or a number of genes. The remaininggenetic variation for AS results from polygenes (i.e., notmarked). Details on the calculations are given in appen-dix 1.

Genetic Parameters

The genetic parameters used in the simulation werebased on estimates from Pakdel et al. (2002, 2005a,b).Results of Pakdel et al. (2002) indicated that there is asignificant maternal genetic effect for traits BW andRV:TV. Therefore, Pakdel et al. (2002) analyzed these

traits using a model that accounts for a maternal geneticeffect. In the current study, the estimated heritabilities fordirect genetic effects were used. In the model calculationsonly direct genetic effects were considered, and maternalgenetic effects were not included in the analyses.

For the breeding goal trait AS, no genetic parameterswere available. The most apparent sign of ascites is theaccumulation of fluid in the abdominal cavity of the bird.Fluid accumulation in the abdominal cavity appears inthe latest stages of ascites, that is, before death occurs. Inthe current study the genetic parameters for AS werethose that were obtained for the trait fluid accumulationin the abdominal cavity, which was scored as 0, 1, or 2.

The variation for susceptibility to ascites is only partlyexpressed under normal conditions. Under cold condi-tions, however, the trait fluid accumulation in the abdo-men reflects more accurately the variation in AS.Therefore, parameters for AS were based on fluid accu-mulation in the abdomen under cold conditions.

The genetic correlation matrix was singular, and, there-fore, a bending routine was used to make the geneticcorrelation matrix nonsingular. This resulted in smallchanges as compared with the original parameters esti-mated by Pakdel et al. (2002, 2005a,b). Table 2 shows thegenetic parameters that were used in the simulations.

Analytical Comparisonof Breeding Strategies

Predictions of the rates of genetic change and inbreed-

ing were performed by deterministic simulation of single-stage selection schemes with discrete generation, usingthe program SelAction (Rutten et al., 2002). This programpredicts the rate of genetic gain using multitrait pseudo-BLUP (Villanueva et al, 1993). The program accounts forreduction in variance due to selection (Bulmer, 1971) andcorrects selection intensities for finite population size andfor the correlation between index values of family mem-

bers (Meuwissen, 1991). The program is an accurate ap-proximation of stochastic simulation with selection onanimal model BLUP, because full pedigree information isaccounted for (Wray and Hill, 1989). Pedigree informationconsists of the EBV of the sire and dam and the mean EBV

of the dams of each half-sib group. The mating structure isassumed to be hierarchical and random, and selectionresponse is predicted for the Bulmer equilibrium situation(Rutten et al., 2002). Prediction of the rate of inbreedingwas based on the long-term genetic contribution theory(Wray and Thompson, 1990).

RESULTS

Base Situation

Table 3 gives the response for BW, AS, and the rate ofinbreeding for the base and alternative scenarios. In the

base situation, in which AS is ignored in the breedinggoal and selection is for BW only, a selection responsefor BW of 130 g was observed. The correlated selectionresponse for AS was +0.025 units. The AS is a score traitthat is categorized as 0, 1, or 2. The mean of this scoretrait was 0.08 with a phenotypic standard deviation of0.38. A value of 0.025 is, therefore, equal to 0.2 additivegenetic standard deviation. When assuming a normal dis-tribution for the underlying continuous variable, the per-centage of birds under cold conditions with a score of 1or 2 in the base situation was equal to 4.2%. A selectionresponse of 0.025 corresponded to an increase of the num-

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TABLE 2. The genetic parameters used in the simulation study1

Genetic parameters2

Traits 2p BWN AS HCTN RV:TVN BWC HCTC RV:TVC

BWN 60,000 0.20 0.18 0.55 0.46 0.46 0.27 0.37AS 0.14 0.10 0.46 0.55 0.01 0.64 0.73HCTN 5 0.10 0.20 0.64 0.14 0.82 0.46RV:TVN 20 0.11 0.10 0.10 0.06 0.73 0.82BWC 60,000 0.30 0.20 0.21 0.25HCTC 15 0.30 0.40 0.40 0.55

RV:TVC 60 0.50 0.30 0.50 0.25

1AS = ascites susceptibility trait; HCT = hematocrit value; RV:TV = ratio of right ventricular weight to totalventricular weight; subscript C indicates that the trait was measured under cold conditions, subscript N indicates

that the trait was measured under normal temperature conditions; 2p = phenotypic variance.2Genetic correlations are above, phenotypic correlations are below, and heritabilities are on the diagonal.

ber of birds with score 1 or 2 to 4.8%. Therefore, underthis scenario the fraction of ascites-susceptible birds willincrease gradually.

Use of Ascites-Indicator Traits Measured

Under Normal Temperature ConditionsWhen including information for a nondestructive indi-

cator trait (HCT) for ascites measured on all the birdsand these birds were kept under normal conditions(scheme A-1), the response for BW decreased 11.4% ascompared with the base situation. The rate of inbreedingincreased from 3% in the base situation to 4.3% per gener-ation in the scheme A-1 (Table 3). By including informa-tion on RV:TV measured on 50% of the birds kept undernormal conditions (scheme A-2), the number of selectioncandidates was reduced to 2,800 birds per sex. The birdson which RV:TV is measured were no longer available

as selection candidates. In this scheme, selection responsefor BW was reduced by 17.3% as compared with the basesituation. The predicted rate of inbreeding for this schemewas 2.3% per generation.

Use of Ascites-Indicator Traits MeasuredUnder Cold Temperature Conditions

In scenarios A-3 and A-4, information on traits mea-sured under cold conditions was used in the index. In

TABLE 3. The effect of alternative scenarios on BW, ascites susceptibility (AS), and rate of inbreeding

Number of Economic Selection Selectionselection value for response response F4

Scheme Traits in the index candidates AS1 for BW2 for AS3 (%)

Base 100% (BWN) 5,600 0 130 0.025 3.0A-1 100% (BWN + HCTN) 5,600 1,050 115.2 0 4.3A-2 100% (BWN + HCTN) + 50%(RV:TVN) 2,800 775 107.5 0 2.3A-3 50% (BWN + HCTN) + 50% (BWC + HCTC + RV:TVC) 2,800 400 111.4 0 1.7A-4 100% (BWC + HCTC) + 50% (RV:TVC) 2,800 580 54.1 0 2.1

1The economic value of AS in the breeding goal. The economic value of BW in each scheme was equal to 1.2Response on BW, the unit of BW is grams. The additive genetic standard deviation (a ) for BW is equal to 109.5 g. Therefore 130 g = 1.2 a.3The accumulation of fluid in the abdomen is a score trait as 0, 1, or 2. The additive genetic standard deviation for AS is equal to 0.12 unit.

Therefore 0.025 = 0.21 a.4F = rate of inbreeding per generation.

scheme A-3, half of the birds were kept under normaltemperature conditions, and the other half were kept un-der cold conditions. In this scheme, information wasavailable for BW and HCT under cold and normal condi-tions, and the trait RV:TV was measured only under cold

conditions. The selection response achieved for BW inthis scheme was higher (111.4 g) than that of scheme A-2 (Table 3). In addition, the rate of inbreeding in thisscheme was reduced to 1.7% per generation, which wasthe lowest inbreeding rate among the different scenariosthat were considered. The economic value that was re-quired to keep AS at the same level was lower than thatrequired for the other scenarios.

The last scenario considered was one in which all thebirds were kept under cold conditions (scheme A-4). Inthis scheme traits BW and HCT were measured on allthe birds. To measure the trait RV:TV, half of the birdswere euthanized. The number of selection candidates in

this scheme was equal to the number of selection candi-dates in the schemes A-2 and A-3. The selection responsefor BW in this scheme was 54.1 g (Table 3). This is clearlylower than the response under the other schemes.

Effects of Proportion of Birds Euthanized

In some of the schemes considered so far, 50% of thebirds were euthanized to measure the trait RV:TV. Thisproportion might not be optimal, and in order to investi-

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TABLE 4. The effects of the proportion of birds that is sacrificed on scenarios A-2, A-3, and A-4 1

25% 50% 75%

Economic Selection Economic Selection Economic Selectionvalue for response F4 value for response F value for response F

Scheme AS3 for BW (%) AS for BW (F) AS for BW (%)

A-2 825 114.6 3.2 775 107.5 2.3 750 92.3 1.4A-3 425 120.9 2.4 400 111.4 1.7 410 92.6 1.0A-4 575 58.4 2.8 580 54.1 2.1 580 46.5 1.2

1For each alternative, the response for ascites susceptibility was fixed at zero.2Scheme A-2 including traits BWN, hematocrit value (HCTN), and ratio of right ventricular weight to the total

ventricular weight (RV:TVN), scheme A-3 including traits BWN, HCTN , BWC, HCTC , and RV:TVC and schemeA-4 including traits BWC, HCTC , and RV:TVC. Subscripts N or C indicate that traits were measured undernormal or cold temperature conditions, respectively.

3The economic value of ascites susceptibility (AS) in the breeding goal. The economic value of BW in eachscheme was equal to 1.

4F = rate of inbreeding.

gate the effect of the proportion of birds euthanized, thisproportion was changed from 50 to 25 or 75%. In eachscheme, response for AS was kept constant, and responsesfor BW and inbreeding rate are presented in Table 4. The

results indicated that by increasing the number of birdseuthanized, selection response for BW decreased as wellas the rate of inbreeding. In scheme A-4, in which all

birds kept under cold conditions, the proportion of birdseuthanized had a relatively small effect on the results.

Comparison of Schemes at EqualRates of Inbreeding

The scheme A-1 showed higher response for BW thanscheme A-3 when the proportion of euthanized birds was50 or 75%, but the inbreeding rate in scheme A-1 wasrelatively high. Ideally, schemes should be compared atequal rates of inbreeding. To make scheme A-1 compara-

ble to other schemes, the rate of inbreeding was reducedby increasing the number of selected sires (Table 5). Whenthe number of sires increased from 50 to 90 or 140, theinbreeding rate in scheme A-1 was reduced to 2.4 and1.7%, respectively, which was approximately equal to therate of inbreeding in scheme A-2 (2.3%) and scheme A-3 (1.7%). At equal rates of inbreeding, the gain achievedfor BW in scheme A-1 was higher than that in scheme A-2 (110.1 vs. 107.5 g) but was lower than that in scheme A-3 (105.8 vs. 111.4 g). Therefore, when comparing selectionresponse at the same rate of inbreeding, scheme A-3 (i.e.,

TABLE 5. The effect of changing the number of sires used in scheme A-1

SelectionEconomic response

Number of sires value for AS1 for BW2 F (%)3

50 1,050 115.2 4.390 1,075 110.1 2.4140 1,090 105.8 1.7

1The economic value of ascites susceptibility (AS) in the breeding goal. The economic value of BW in eachscheme was equal to 1.

2Response for BW in grams. Response for AS was fixed at zero level in each scheme.3 = rate of inbreeding.

including information of BWN, HCTN, BWC, HCTC, andRV:TVC in the index) was the best among alternative

breeding schemes considered. In this scheme the geneticprogress of BW was reduced by around 18.4 g as com-

pared with the base situation.

Genetic Progress in BreedingSchemes Using Genetic Markers

Results of schemes with different fractions of varianceexplained by the QTL were compared with the basescheme and scheme A-3. The results are summarized inTable 6. By using information on QTL that describe only5% of the genetic variance (scheme Q05) ascites could becontrolled while selection response for BW was 122.3 g.This was only 6% lower than the achieved gain for BW

in the base scheme (130 g) and 10% higher than the gainobtained for BW in scheme A-3. Inbreeding for the MASschemes was at a similar level as that of the base scheme.It is good to keep in mind that in the MAS schemes,similar as in the base scheme, all the birds were availableas selection candidates and they are kept under normaltemperature circumstances. However, in scheme A-3 aproportion of the birds was kept under cold conditions,and they had to be euthanized to measure ascites-relatedtraits. If the proportion of the genetic variance that wasexplained by the markers increased, (scheme Q10, Q20,and Q50), the gain for BW improved further. For Q50 the

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TABLE 6. The effect of amount of variance explained by the QTL on the breeding goal traits

Number of Selection Selectionselection Economic response response

Scheme candidates value for AS1 for BW for AS F (%)

Base2 5,600 130.0 0.025 3.0A-33 2,800 400 111.4 0 1.7Q054 5,600 550 122.3 0 3.0Q10 5,600 450 123.8 0 2.9Q20 5,600 320 125.9 0 2.9Q50 5,600 165 128.0 0 2.9

1AS = ascites susceptibility. The economic value of BW in the breeding goal was equal to 1 in all schemes.2Base scheme including information on BW only.3Scheme A-3 including information of BW and hematocrit value (HCT) measured on 50% of birds under

normal temperature conditions and BW, HCT, and ratio of right ventricular weight to the total ventricularweight (RV:TV) measured on the rest of birds under cold temperature conditions.

4Q refers to the amount of variance explained by the QTL. This value is 5, 10, 20, or 50%.

gain for BW was 128 g, which was close to the 130 g ofthe base scheme.

DISCUSSION

To combine selection for increased BW and resistanceto AS in broilers, different schemes were compared bydeterministic simulation. In the alternative schemes thetraits HCT and RV:TV measured under normal and coldconditions were used as index information. It has beenaccepted that as an adaptation to hypoxia, HCT increases,and so direct measurement of HCT is useful in accessingascites progression (Maxwell, 1991, Mirsalimi and Julian,1991, Lubritz and McPherson, 1994, Shlosberg et al., 1996).Moreover, RV:TV is a pathological trait that has value asa predictive indicator of ascites (Maxwell, 1991; Enkvet-chakul et al.,1995; Wideman et al., 1998). However,Shlosberg et al. (1996) showed a high nonascites mortality

in experimental groups with a low hematocrit value, and,therefore, selection against HCT needs more attentionand care than selection against RV:TV.

The results of the current study indicated that by usinginformation of ascites-related traits measured under nor-mal conditions AS can be controlled. However, controlof AScomes at a cost (i.e., a reduction inselection responsefor BW). This reduction in BW was higher in scheme A-2, in which RV:TV was measured on a proportion of the

birds kept under normal temperature conditions. How-ever, when ascites-related traits were measured on a pro-portion of the birds kept under cold conditions (schemeA-3), the gain achieved for BW was higher. This findingalso holds when results are compared at the same levelof inbreeding. If all information was collected for birdskept under cold conditions (scheme A-4), the selectionresponse for BW was reduced considerably. This findingcan be ascribed to the low genetic correlation betweenBW measured under cold and normal conditions (0.49).It has to be realized that the breeding goal is definedat normal temperature conditions. Therefore, in case nomarker information on AS is available, the best schemefor improving BW while controlling AS is scheme A-3 inwhich ascites-related traits are measured under cold andnormal conditions.

Inbreeding

It has been generally recommended that the rate ofinbreeding in broiler lines should be kept at a level lowerthan 1% (Morris and Pollott, 1997). In all the simulations

in the current study the rate of inbreeding (F) wasgreater than this critical level. SelAction calculates therate of inbreeding for a Bulmer equilibrium situation ac-counting for the effect of selection (Rutten et al., 2002).Prediction of the rate of inbreeding is based on the long-term genetic contribution theory (Wray and Thompson,1990; Woolliams and Bijma, 2000). In the current simula-tion it was assumed that selection is for BW and ASonly. However, in a practical poultry breeding programselection will be for several other traits. Therefore, inpractice the number of selection candidates will be lowerthan what has been assumed in the current study. If thenumber of available selection candidates was reduced by

50%, to allow for selection on other traits not correlatedto the breeding goal traits, on average F was reduced

by about 40% in each scheme (e.g., in the base schemeF reduced from 3 to 1.9% or in scheme A-3 F reducedfrom 1.7 to 1.1%).

In addition to the number of parents, there are othermechanisms that affect F. For instance, more sib infor-mation in the index increases the probability of co-selec-tion of relatives, which in turn increases F. By changingthe number of sires used in scheme A-1, the F of thatscheme was made equal to that of schemes A-2 and A-3. The results indicated that at equal rate of inbreedingthe gain achieved for BW in this scheme was lower thanthe gain achieved for BW in scheme A-3.

There are, however, more efficient ways of maximizinggain while restricting inbreeding (i.e., by using stochasticsimulation rather than deterministic simulation). Meu-wissen (1997) introduced a dynamic selection tool to max-imize genetic gain while restricting the rate of inbreeding.In this method the number of parents and the number ofoffspring per parent may vary, depending on the candi-dates available in a particular generation, which resultsin a dynamic breeding program. Dynamic selection toolsare likely to reduce the variance of the rate of inbreedingas well (Meuwissen, 1997).

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MAS

The use of MAS can be especially useful for traits thathave a lowheritability or are difficultto measure (Dekkersand Hospital, 2002). Both of these characteristics applyto AS, and, therefore, it is expected that MAS can make animportant contribution. Rabie (2004) performed a wholegenome scan aimed at the mapping of QTL for ascites.

Three significant QTL affecting ascites-related traits weredetected, whereas 2 QTL reached the genome-wide sug-gestive threshold.

The results of present study showed that QTL informa-tion could be used very effectively in controlling AS. Evenif the QTL explained only 5% of the genetic variance, thereduction in gain for BW was limited as compared witha scheme in which selection was for BW only (base). Byusing QTL, information no longer requires that some ofthe birds are euthanized (e.g., in scheme A-3 a proportionof birds have to be euthanized to measure the traitRV:TV). This illustrates that MAS can make an importantcontribution in selection of birds that are resistant to asci-

tes. Whether implementation of MAS in a breedingscheme is economically beneficial will depend upon thecost and the benefits from increased genetic progress.

Literature

The results of current study showed that selection forincreased BW and resistance to ascites syndrome waspossible in broilers. This result was in agreement withthe previous results obtained by Wideman and French(2000), Anthony et al. (2001), Balog et al. (2001), andMcMillan and Quinton (2002). In a theoretical study,McMillan and Quinton (2002) investigated the effects of

selecting for a production trait, such as growth rate, ona fitness trait, such as ascites, using stochastic simulation.They reported that the use of sib information and anindicator trait for ascites reduces the genetic level for AS.

In experimental studies several researchers have cho-sen a strategy to test an animals ability to withstandsevere stress and not succumb to ascites (Wideman andFrench, 2000; Anthony et al., 2001; Balog et al., 2001).Wideman and French, (2000) demonstrated improvementin ascites resistance of progeny from broiler breeders that,for 2 consecutive generations, had survived the rigorousselection pressure imposed by unilateral pulmonary ar-tery occlusion. Chronic unilateral pulmonary artery oc-clusion triggers a pathophysiological progression thatterminates with a high incidence of mortality diagnosedas ascites. Wideman and French (2000) concluded that byusing this technique, genetic selection for ascites resis-tance could be accomplished. However the final BW innonascitic broilers in one experiment was less than fornonascitic base population birds, but in the second experi-ment there was no difference for final BW among nonas-citic birds. Anthony et al. (2001) made sire-familyselections from siblings in broilers based on mortalitydata obtained in a hypobaric chamber. They reportedsuccess at selecting an ascites-resistant and an ascites-

susceptible population of broilers. Balog et al. (2001) re-ported that the selection for ascites resistance or suscepti-

bility did not affect weight gain.In conclusion, alternative selection schemes were con-

sidered for simultaneous selection on BW as well as re-duction of AS in the present study. The alternativeselection strategies indicated that by selection for in-creased BW only and ignoring AS in the breeding goal,

the AS of the birds increased. Inclusion of information onascites-related traits (HCT and RV:TV) measured undernormal and cold conditions made it possible to achieverelatively high gain for BW (111.4 g) while keeping theAS level constant (no genetic change). The results of usingQTL information indicated that MAS could be used veryeffectively in the breeding scheme for ascites resistanceeven if the QTL explains only 5% of the genetic varianceof the AS.

ACKNOWLEDGMENTS

The authors thank Johan Van Arendonk and Addie

Vereiken for valuable discussions. This work was finan-cially supported by the Islamic Republic of Iran, Ministryof Science, Research, and Technology (MSRT).

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APPENDIX 1Heritability of BW was 0.2, and the phenotypic variance

was 60,000. The heritability of AS was 0.10, and pheno-typic variance was 0.14. The economic value of BW wasset equal to 1, and the economic value of AS was changedto obtain zero response for AS. The information sourcefor the genetic evaluation of BW was the animals record,pedigree information, average phenotypic performanceof full-sibs, and average phenotypic performance of half-sibs. The information sources for the genetic evaluationof AS were the QTL information. All birds were keptunder normal conditions, and no birds were killed. The

QTL considered explained 5, 10, 20, or 50% of the geneticvariance of AS. These situations are referred to as Q05,Q10, Q20, and Q50, respectively. Variance could be ex-plained by one or number of genes. The remaining geneticvariation for AS resulted from polygenes (i.e., notmarked). The QTL information was modeled as a traitthat was correlated to AS in the breeding goal and hada heritability of 1. It was assumed that the QTL had nopleiotropic effects on BW, and, therefore, the genetic cor-relation between the QTL and BW was zero. Further, itwas assumed that the correlation between the QTL andthe polygenic component was 0 in the base generation.(i.e., prior to selection). The correlation between the QTL

and the breeding goal trait AS depended on the amountof variation that was explained by the QTL.

The genetic correlations of AS with the QTL and the

polygenic component areq and1 q, respectively, and

the phenotypic correlation of AS with QTL and polygenic

component are q h2AS and (1 q) h2AS, respectively,

where q is the fraction of the genetic variance in AS traitexplained by the QTL. The genetic correlation between

polygenic component and BW is equal torgAS,BW

1 qso that

for any q, the total genetic correlation of BW and AS isequal to 0.18. The heritability of the polygenic component

is1 q

1

h2AS q

. The phenotypic variance of QTL is equal to

additive genetic variance of QTL, which is equal to q

2AAS

, and the phenotypic variance of polygenic compo-

nent is equal to 2PAS 2QTL.

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