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Comparative ingestive mastication in domestic horses and cattle:a pilot investigation

Janis, C M; Constable, E C; Houpt, K A; Streich, W J; Clauss, M

Janis, C M; Constable, E C; Houpt, K A; Streich, W J; Clauss, M (2010). Comparative ingestive mastication indomestic horses and cattle: a pilot investigation. Journal of Animal Physiology and Animal Nutrition,94(6):e402-e409.Postprint available at:http://www.zora.uzh.ch

Posted at the Zurich Open Repository and Archive, University of Zurich.http://www.zora.uzh.ch

Originally published at:Journal of Animal Physiology and Animal Nutrition 2010, 94(6):e402-e409.

Janis, C M; Constable, E C; Houpt, K A; Streich, W J; Clauss, M (2010). Comparative ingestive mastication indomestic horses and cattle: a pilot investigation. Journal of Animal Physiology and Animal Nutrition,94(6):e402-e409.Postprint available at:http://www.zora.uzh.ch

Posted at the Zurich Open Repository and Archive, University of Zurich.http://www.zora.uzh.ch

Originally published at:Journal of Animal Physiology and Animal Nutrition 2010, 94(6):e402-e409.

1

COMPARATIVE INGESTIVE MASTICATION IN DOMESTIC HORSES AND

CATTLE: A PILOT INVESTIGATION

Christine M. Janis1, Emily C. Constable1,2, Katherine A. Houpt3, W. Jürgen Streich4,

Marcus Clauss5*

1Department of Ecology and Evolutionary Biology, Brown University, Providence, RI

02912, USA

2Currently Emily Pershing: 5 Crystal Lane, Cumberland, ME 04021, USA

3Department of Clinical Science, College of Veterinary Medicine, Cornell University,

Ithaca, NY 14853, USA

4Leibniz Institute of Zoo and Wildlife Research (IZW) Berlin, 10315 Berling, Germany

5Clinic of Zoo Animals, Exotic Pets and Wildlife, Vetsuisse Faculty, University of

Zurich, CH-8057 Zurich, Switzerland, mclauss@vetclinics.uzh.ch

*to whom correspondence should be addressed

Running head: Chewing in horses and cattle

2

Summary

It is often assumed that horses chew food more intensively during ingestion than cattle,

which – as ruminants – complete part of the mastication during rumination. This has been

proposed as a reason for more robust mandibles, larger masseter insertion areas and

larger masseter muscles in horses as compared to cattle and other grazing ruminants.

Here, we evaluate results of comparative feeding trials with three horses (338-629 kg)

and three cows (404-786 kg), on four different roughages. Ingestion time (s/g dry matter)

and chewing intensity (chews/g dry matter) differed among animals within a species,

indicating an influence of body mass, and differed significantly between different

forages. However, although numerical differences clearly suggest that horses have longer

ingestion times and higher chewing intensities on high-fiber roughage than do cattle, this

could not be proven in this dataset, most likely due to the small number of individuals

sampled. Further studies are required to corroborate the suspected ingestive behavior

difference between equids and ruminants.

Keywords: Craniodental anatomy, Chewing intensity, Equids, Ingestion, Mastication,

Ruminants

3

Introduction

Equids (horses, donkeys and zebra) and large grazing bovids (such as cattle, other

bovines such as bison, and alcelaphine antelope such as wildebeest) are similar in many

aspects of their biology. In terms of their ecology they occupy similar habitats (e.g., both

zebra and wildebeest are grazers, common on the African savannas), and both have

adaptations for a high degree of ingesta particle size reduction (Fritz et al. 2009).

However, they differ fundamentally in several aspects of their digestive physiology (Janis

1976) and may select different plant species and plant parts (e.g. Hansen et al. 1985).

Horses are hindgut fermenters that achieve ingesta particle size reduction by a

sophisticated dental design, and have adopted a strategy of a high food intake, fast ingesta

passage, and a comparatively low digestive efficiency. In contrast, bovids are foregut

fermenters with a specific sorting mechanism in their forestomach that facilitates not only

extreme particle size reduction, but also a longer ingesta passage with a higher digestive

efficiency. Bovids show a lower overall food intake than equids (of similar body size),

but have an increased food intake as compared to nonruminant foregut fermenters (Foose

1982; Duncan et al. 1990; Jernvall et al. 1996; Stevens and Hume 1998; Clauss et al.

2007; Clauss et al. 2009a; Schwarm et al. 2009). Direct comparisons in foraging patterns

between horses and cattle are rare; however, Arnold (1984) and Duncan et al. (1990)

observed longer daily grazing times in horses as compared to cattle or sheep, and Menard

et al. (2002) observed a higher food intake in free-ranging horses as compared to cattle in

the same habitat.

The concept of a higher food throughput in horses has been implicated in different

studies on the craniodental design of herbivores. A higher food intake should, in theory,

translate into more robust anatomical structures associated with ingestion. Additionally, it

4

has been suggested that the fact that ruminants perform a part of their chewing activity on

‘cud’, i.e., material soaked in rumen fluid and softened by the effect of bacterial digestion,

would mean that there would be an overall reduced load on their masticatory system (as

opposed to these animals doing this same amount of chewing on undigested forage)

(Fortelius 1985). Equids are known to have larger masseter muscles than ruminants, and

also a larger mandibular masseter insertion area (Turnbull 1970), and finite element

analysis shows that hindgut fermenters have more robust jaws than ruminants of similar

size and diet (Fletcher et al. 2009). While grazing ruminants generally tend to have a

shorter premolar tooth row than browsers, no reduction of the premolar tooth row is

observed in perissodactyls (beyond the reduction or loss of the first premolar seen in all

ungulates), and there is even the opposite trend for a more pronounced premolar row

(especially in terms of the molarization of the premolars) in grazing as compared to

browsing perissodactyls (Janis 1990; Mendoza et al. 2002). Consequently, the total

occlusal surface area (and the total occlusal volume) in horses is larger than that of

ruminants of similar diet (Janis 1988). Corresponding to these different findings, higher

bite forces were measured in horses than in cattle (Hongo and Akimoto 2003).

The behavioral correlate of these observations would be an increased ingestive

chewing intensity, and a longer ingestion time per unit of food, in equids as compared to

ruminants. Ruminants initially swallow large particles (Clauss et al. 2009b) and reduce

their size later via rumination, while horses chew their food only once and nevertheless

achieve comparatively small ingesta particles (Fritz et al. 2009). Data on ingestion time

have frequently been recorded for cattle and horses (Table 1). However, these data do not

allow concise conclusions as to differences in the chewing behavior of cattle and horses

because of the large range overlap in the data, and the fact that no single study compares

5

horses and cattle under the same conditions. Compilations of literature data are hampered

by the fact that chewing behavior is influenced by the fiber content of the diet (cattle:

Armentano and Pereira 1997; horses: Meyer et al. 1975; Gallagher and Hintz 1988; Scott

and Potter 1989; Ellis et al. 2005); the physical state of the food (cattle: Shaver et al. 1988;

Woodford and Murphy 1988; Mcleod et al. 1990; other ruminants: Mcsweeney and

Kennedy 1992; Gross et al. 1995); the body size of the animal (cattle: Bae et al. 1983;

horses: Meyer et al. 1975; mammals in general: Druzinsky 1993; Shipley et al. 1994;

Gerstner and Gerstein 2008); variation in intraspecific dental morphology (red deer:

Pérez-Barberìa and Gordon 1998); and the overall intake level (cattle: Bae et al. 1981;

Shaver et al. 1988). Direct comparisons of ingestion and mastication behavior between

equids and ruminants, under identical conditions and with common test diets, are, so far,

unfortunately lacking.

In order to compare the ingestion time, the chewing frequency and the chewing

intensity between horses and cattle, we re-evaluated data generated during a trial that

determined the number of chews during the ingestion of defined amounts of various test

diets in individual horses and cattle of different body weights (previously presented as

conference abstracts, Janis and Constable 1993; Constable et al. 1994). We predicted that

the horses would display a greater number of chews per unit ingested food, and hence a

longer ingestion time per unit food compared to the cattle, and that this difference would

increase with increasing levels of fiber in the food. We also predicted that the horses

generally would have a lower chewing frequency than cattle, as in the data collection of

Gerstner and Gerstein (2008), due to the mechanical constraints of their heavier

mandibles, and that within any one group (horses or cattle) smaller animals would display

a higher rate of chewing than larger ones, due to simple allometric scaling, where chewing

6

frequency and number of chews per unit ingested food are assumed to scale negatively to

body mass (Druzinsky 1993; Shipley et al. 1994; Gerstner and Gerstein 2008).

Methods

This experiment was conducted at the Cornell University College of Veterinary

Medicine, Department of Physiology, in the summer of 1992, and was approved by

Cornell University’s Institutional and Animal Care Use committee.

Animals and Diets

Six animals were used in this study, three horses and three cows (Table 1). Animals

were maintained on a diet of mixed hay (including alfalfa hay, timothy hay, straw, and

clover) fed twice daily together with a supplement of corn, oats, and molasses. Water was

available at all times. Prior to testing, animals were fasted for eight to ten hours by

removing all leftovers from the previous meal and withholding the morning meal, in

order to ensure prompt intake of the test meals in all animals and remove effects of

satiety.

Three experimental diets of hay of differing fiber content were used in the

experiment – alfalfa hay, timothy hay, and a mixed hay similar to the one used for

maintenance of the animals. Alfalfa hay was lowest in total cell wall (neutral detergent

fiber, NDF) and lignocellulose (acid detergent fiber, ADF) content; timothy hay

resembled the mixed hay in total cell wall and alfalfa hay in lignocellulose contents; and

the mixed hay had the highest lignocellulose content (Table 2). Additionally, freshly cut

pasture grass was used that resembled the timothy hay in its fiber content, but had a much

lower dry matter (DM) content than the dried forages (Table 2).

7

Experimental Procedure

Animals were fitted with halters that detected chewing movements as changes in

pressure in a small foam-filled balloon placed underneath the mandible. Pressure changes

were recorded via a pressure transducer on a Grass model 7-D polygraph. In general,

each chewing-induced pressure change was recorded as a deflection on a graph paper

record. Any deflections caused by movements other than chewing were noted by an

observer as they occurred, and were discounted in the analyses.

Each animal received four tests meals of each forage during a period of one day.

Alfalfa hay, the mixed hay, and freshly cut grass were tested on two consecutive days

each, resulting in a total of eight tests per animal and diet; timothy hay was only tested

during one day with a total of four tests per animal. Tests were performed between eleven

am and one pm in all animals.

Exactly a hundred grams of each test diet was offered to the animals in a feed bin

raised 70 cm above ground level. During each test, food spilled by the animals was picked

up by the observer and placed back into the bin. The barn in which the tests were

performed was locked during the procedure to ensure no disturbance for the animals. Each

test run was terminated either when all food was ingested, or when all significant chewing

was complete and animals began to play with left-over stems. The amount of leftovers was

consistently negligible.

Analyses

Results were expressed as the ingestion time for the whole diet or its cell wall

component (s/g DM or s/g NDF, respectively), as the chewing frequency (chews/s), and

8

the chewing intensity (chews/g DM or chews/g NDF, respectively). Results are reported

as means ± standard deviation per species and diet.

ANOVA with species and diet as fixed factors and animal (nested in species) as

random factor, with subsequent post hoc tests (Dunnet T3 test because of inequality of

variances) was used to test for potential differences. Linear contrasts (after sorting the

animals with respect to their body weight) served to test for a correlation between the

body mass and the chewing variables. For further explorative analysis, we additionally

performed an ANOVA with species and diet as fixed factors and body mass as a covariate

using pooled data, ignoring the animals as factor (i.e., pretending all individual

measurement derived from different individual animals). In an explorative analysis, small

p-values have no confirmative power. The significance level was set to α=0.05. The SPSS

16.0 (SPSS Inc., Chicago, IL, USA) statistical software package was used for the

statistical calculations.

Results

When observation time was plotted against the total number of chews, differences

between the species were only evident in those trials in which ingestion time was

prolonged, with a higher chewing frequency in cattle (Fig. 1). Numerical differences in

ingestion times and chewing intensity between the species were as expected (Table 4 &

5). Except for feeding on fresh grass, horses also had a numerically lower chewing

frequency than cattle.

The explorative analysis resulted in highly significant influence of species, diet, and

body mass on ingestion times, with significant diet*species interaction, indicating that the

pattern between the species changed with diets (Fig. 2). That is, for the more fibrous diets

9

the horses spent a longer time chewing than did cattle of similar body mass. For chewing

intensity, highly significant influence of species, diet, and body mass on ingestion times

were also indicated, with significant diet*species interaction (see Fig. 3); species only

tended towards significance (p=0.072) when chewing intensity was based on neutral

detergent fiber, but the diet*species interaction term remained highly significant. Here

again, with the most fibrous diet, the chewing intensity (number of chews per unit of dry

matter) was greater in the horses than in the cows. For chewing frequency, the same

pattern was found, with horses displaying lower frequencies than cattle (see Fig. 1), but

here body mass only tended towards significance (p=0.078), most likely due to the

unusually low chewing frequency in the smallest horse (data not shown).

In the final statistical analyses, differences in parameters between individual

animals within species were always significant (Tables 4 & 5), suggesting an effect of

body mass. In both horse and cattle, linear contrasts indicated correlations of all

parameters with body mass (p always <0.001). In contrast to the explorative analyses,

differences between the species were not statistically significant once the fact that

multiple measurements were performed on the limited number of individuals was taken

into account (Tables 4 & 5). Differences between the diets were often significant (Tables

4 & 5), with fresh grass requiring more time to ingest and more chews per unit of food.

Between the three dried forages, differences between alfalfa hay and the other two hays

were significant when the amount of dry matter was the reference unit, but were not

significant when the amount of cell wall ingested was the reference unit.

Discussion

10

Although the dataset evaluated here shows certain predicted trends, such as an

increased ingestion time and chewing frequency with increasing dietary fiber content, or

an association between ingestion time and chewing intensity and body mass (as suggested

for equines by Mueller et al. 1998), the sample size was too small to allow definite

conclusions about species differences between horses and cattle. However, the numbers

do lend credence to the notion that, especially on more fibrous diets, horses chew their

food more thoroughly on first ingestion than do cattle, as would be expected for a hindgut

fermenter eating more food per day than a ruminant of similar size and diet, and having

only the opportunity at initial ingestion to reduce the particle size of the food. Our results

suggesting that cattle have a higher rate of chewing than horses is also supported by

Putnam’s (1986) study of horses and cattle in the New Forest (England). Here the horses

(New Forest ponies) were observed to take fewer bites per minute than cattle in each

season, and this difference was more pronounced on lower quality grasslands.

However, while the findings supported the expectations numerically, differences

between the species could not be corroborated statistically. In order to decide whether the

numerical trends are really spurious, whether such differences actually do occur,

experiments with a larger number of individuals are required. The data of this study,

which must be considered preliminary, suggests that differences between the species

might be most evident on high-fiber roughages. The fact that differences between the

three forages of similar dry matter content – the three hays – were significant when

expressed on a dry matter basis but not when expressed on a fiber basis (Tables 4 & 5)

indicates that chewing investment is directly linked to the fiber content of the diet. When

comparing the data from this study to that of others, it should be noted that due to the

long fasting regime in the experimental animals used here, chewing rates might be

11

considered “maximum chewing rates”, because animals were probably hungrier than they

would ever be under natural circumstances.

When considering the total chewing activity of horses and ruminants, it is important

to note that ingestive chewing is less important in ruminants and often contributes less to

overall chewing efficiency than rumination (Trudell-Moore and White 1983). Overall,

horses have shorter chewing times per unit ingested food if not only ingestive but also

ruminative mastication is included in the comparison (Mueller et al. 1998), but as

previously noted, the food masticated during rumination is of a different nature (in terms

of likely abrasive quality and toughness) than the food on initial ingestion. Whether the

difference in craniodental morphology between equids and ruminants can truly be related

to differences in feeding behavior depends not only on comparative measurements of

ingestive mastication, but even more on physical characteristics of regurgitated rumen

contents as compared to ingested food. The assumption that regurgitated ingesta is

‘softer’, requires less forceful chewing, and that ruminants can therefore afford less

robust mandibles and smaller masseter insertion areas than equids, should be tested by

applying mechanical tests to different ingesta and digesta fractions, ideally gained from

esophagus-fistulated animals. Well-founded speculations, such as shorter ingestion times

in ruminants, and higher ingestive chewing intensity in horses as compared to cattle on

high-fiber diets, remain to be conclusively corroborated.

Additional studies might also reveal whether the interspecific trend of decreasing

chewing frequencies with increasing body mass (e.g. Druzinsky 1993; Shipley et al.

1994) can also be observed within species; in this pilot investigation, there was no

influence of body mass on this parameter in cattle, and even an inverse relationship in

horses. In contrast, the reported negative interspecific allometry of chewing intensity with

12

body mass (Shipley et al. 1994) was evident in this dataset within species as well (Fig. 3).

The reason for this difference is probably that the negative allometry of chewing

frequency with body mass is rather low at BM-0.13 to BM-0.18 (Druzinsky 1993; Shipley et

al. 1994), whereas that of chewing intensity is much more pronounced at BM-0.85 (Shipley

et al. 1994); thus, chewing frequency may be much more prone to misrepresentation in

low sample size datasets. However, when plotting the results on chewing frequency from

this study and other measurements made in horses fed hays from the literature (Fig. 4), it

appears that available data for horses is rather in line with the general mammalian trend

as reported by Shipley et al. (1994). As for differences between horses and cattle, it is to

be expected that the heavier mandibles of horses, with their more robust bone structure

and higher tooth mass, move at a lower frequency than the more slender mandibles of

cattle.

The craniodental apparatus of equids (and other hindgut fermenters) is more

robustly built than that of ruminants, and we propose that this morphological difference is

functional in origin, rather than merely a phylogenetic artifact. The data presented here

are at least supportive of the hypothesis that horses chew their food more intensively on

initial ingestion than do cattle: this, and the fact that many hindgut fermenters ingest

more food per day than ruminants of equivalent diet and body size, supports the

functional hypothesis that the greater degree of craniodental robustness of hindgut

fermenters reflects a greater amount of stress on the masticatory apparatus.

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Table 1. Ingestion times (seconds per gram dry matter) in domestic cattle, horses and donkeys

Diet Cattle Horse Donkey Straw 2.5 - 3.5 2.9 - 8.0 Grass hay 1.2 - 2.4 1.8 - 4.8 7.2 Haylage 1.7 - 1.8 Grass silage 1.9 - 3.5 Concentrates 0.2 - 0.6 0.5 - 0.9 (data from Balch 1971; Meyer et al. 1975; Mueller et al. 1998; Ellis and Hill 2005; Müller 2009)

Table 2. Animals used in the trials

Species ID Body mass (kg)

Age (a)

Sex

Horse H1 338 7 gelding Horse H2 605 19 female Horse H3 629 12 female Cow C1 404 20 female Cow C2 659 5 female Cow C3 786 6 female

Table 3. Roughages used in the trials and their content of dry matter (DM, in % wet weight),

neutral detergent fiber (NDF, in % DM) and acid detergent fiber (ADF, in % DM)

Food DM NDF ADF Alfalfa hay 92.4 44.5 34.2 Timothy hay 95.1 59.4 34.7 Mixed hay 92.2 61.3 41.4 Fresh grass 24.4 57.8 35.3

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Table 4. Ingestion time (per unit dry matter or neutral detergent fiber) in horses (n=3) and cattle

(n=3)

---------------- Ingestion time ---------------- (s/g dry matter) (s/g NDF) Diet Horses Cattle Horses Cattle Alfalfa 1.57 ± 0.43 1.50 ± 0.53 3.52 ± 0.98 3.37 ± 1.18 Timothy 2.46 ± 0.44 1.69 ± 0.63 4.14 ± 0.75 2.85 ± 1.05 Mixed 2.82 ± 0.69 1.68 ± 0.57 4.60 ± 1.13 2.74 ± 0.93 Fresh grass 3.92 ± 1.23 3.27 ± 0.90 6.78 ± 2.13 5.65 ± 1.55 ANOVA p p Species 0.325 0.354 Diet <0.001 grass>mix/tim>alf <0.001 grass>mix/tim/alf Animal(species) <0.001 <0.001

Table 5. Chewing frequency and intensity (per unit dry matter or neutral detergent fiber) in horses

(n=3) and cattle (n=3)

Diet ------ Chewing frequency ------ ------------------------- Chewing intensity ------------------------- (chews/s) (chews/g dry matter) (chews/g NDF) Horses Cattle Horses Cattle Horses Cattle Alfalfa 1.23 ± 0.14 1.30 ± 0.12 1.88 ± 0.40 1.96 ± 0.72 4.24 ± 0.90 4.41 ± 1.62 Timothy 1.04 ± 0.09 1.25 ± 0.13 2.54 ± 0.40 2.14 ± 0.93 4.27 ± 0.67 3.60 ± 1.56 Mixed 1.13 ± 0.12 1.18 ± 0.13 3.16 ± 0.74 2.01 ± 0.82 5.16 ± 1.21 3.27 ± 1.33 Fresh grass 1.20 ± 0.11 1.12 ± 0.16 4.61 ± 1.17 3.62 ± 1.08 7.97 ± 2.03 6.26 ± 1.87 ANOVA p p p Species 0.589 0.396 0.430 Diet <0.001 mix/tim/grass<alf <0.001 grass>mix≥tim/alf <0.001 grass>mix/tim/alf Animal(species) <0.001 <0.001 <0.001

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Figure 1. Chewing frequency (number of chews vs. unit time) in all individual experiments with horses and cattle from this study.

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a

b

c

Figure 2. Ingestion time (seconds per gram dry matter) in horses and cattle of different body mass fed (with increasing fiber content) a) alfalfa hay, b) timothy hay, c) a mixed hay (repeated measurements in three individuals of each species).

a

b

c

Figure 3. Chewing intensity (chews per gram dry matter) in horses and cattle of different body mass fed (with increasing fiber content) a) alfalfa hay, b) timothy hay, c) a mixed hay (repeated measurements in three individuals of each species).

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Figure 4. Chewing frequency in relation to body mass of cattle and horses of this study, of hay-fed horses from the literature (Meyer et al. 1975; Shingu et al. 2001; Brüssow 2006; Brøkner et al. 2008; Bochnia 2009), and of a variety of mammals (regression line from Shipley et al. 1994).