Animal Science1995, 61: 407-417 C 1995 British Society of Animal Science 1357-7298/95/45830407$20.()()

Digestibility and gastro-intestinal transit time of diets containing different proportions of alfalfa and oat straw given to , Shetland , Highland ponies and donkeys

D. Cuddefordl, R. A. Pearson2,R. F. Archibald2 and R. H. Muirheadl

IDepartment of Veterinary Clinical Studies, and Centrefor Tropical Veterinary Medicine, University of Edinburgh, Easter Bush, Roslin, Midlothian EH25 9RG

~bstract

\ series of 4 X 4 Latin-square digestibility trials was carried out to determine whether there were any differences between different types of equid in their ability to digest diets containing different levels of fibre and protein. The equids (Thoroughbreds, Highland ponies, Shetland ponies and donkeys) were offered one of four molassed diets in turn and these contained different proportions of alfalfa and oat straw: 1.0: 0, 0.67: 0.33, 0.33: 0.67, 0: 1, respectively. The apparent digestibility of dry matter (DM), organic matter (OM), gross energy (GE), crude protein (CP), acid-detergent fibre (ADF) and neutral-detergent fibre (NDF) were estimated. The rate of passage of the different diets was measured using Cr-mordanted fibre (Cr-fibre) and Co-ethylenediamine tetra acetic acid. All animals digested the components of the high-fibre diets less well than those of the low-fibre diets (P < 0.001 for , OM, OM, GE, CP and ADF). Donkeys digested fibre more effectively than the other equids (P < 0.01 for ADF and I NDF). The gastro-intestinal ~ransit time 01 the high fibre diets was signif!cantly less than that for the low fibre diets (P < 0.001) and donkeys retaIned food residues longer than the other equlds (P < 0.01).

, tra1!sit time.

significant difference between ( as other herbivores that are hind gut and Quarter mares) and ponies (Shetland are considered to be less efficient than mares); digestibility coefficients for the proximate in plant cell-wall constituents constituents measured in the ponies were higher. 1978). Two groups of This difference was consistent with both diets and Ruudvere, 1955; Loewe and although the magnitude of the difference was 1974) have illustrated the effect that fibre slightly less on the low fibre diet. Similarly, Barth, of the diet cun have on the relative Williams and Brown OY77) found higher .equids. digestibilities of OM and gross energy (GE) in that whilst horses and cattle were Shetland ponies given alfalfa/ orchardgrass hay in digesting low fibre foods, cattle (348g CF per kg OM) than those reported for either able to digest high fibre foods. They alfalfa or orchard grass given to light horses (e.g. that generally, as the fibre content of the diet Fonnesbeck, 1969; Van der Noot and Gilbreath, the digestibility of organic matter (OM) 1970). They suggested that ponies may be more both horses and cattle, but the reduction efficient at digesting roughages compared with was greater. In more recent studies in horses. Hintz 0990), however, following an 53 foods were compared extensive review of the literature, concluded that (Smolders, Steg and Hindle, although the average digestibility of OM and energy were reached. was higher in ponies than in horses, the difference was small enough to justify using ponies in digestion and Hintz (1969) compared the digestion of studies to evalute foodstuffs for horses. (252g crude fibre (CF) per kg dry matter Uden and Van Soest (1982)confirmed that ruminants herbivores.They found no were superior to equids in digesting timothy hay 407

Diet digestibility in equids 409 rationing the groups of animals was the same. The the between-animal stratum (d.f. = 12). Diet effects daily ration was divided into four equal meals and and equid X diet interaction were estimated and given at 08.00h, 12.00 h, 16.00hand 20.00 h, in deep- tested from the animal X period stratum (d.f. = 33). sided troughs in order to reduce spillage. Any food Residual effects of dietary treatment were tested for refusals were collected at 08.00h each day before carry-over using covariance analysis and were not fresh food was offered. found to be significant for any of the measurements.

Measurements and rate of passage markers Each animal was weighed at the start of the Results experiment and twice weekly to the end of the Temperature and relative humidity experimental period. For the first 14 days of each Ambient temperatures and relative humidities were period the animals were allowed to adapt to the new monitored in the climate rooms. The averages of the diet. During the final 7 days, measurements of food daily values achieved for each room for the 9 days at intake and total faecal collections were made. Mean the end of each period {no. = 9) were calculated. retention time (MRT) of two indigestible food Average daily maximum temperatures varied markers (Cr-mordanted hay fibre, a solid phase between 12 and 15°C between rooms, minimum marker and Co-ethylene diamine tetra acetic acid temperatures recorded varied between 9 and 14°C, (EDTA), a liquid phase marker) were measured and average relative humidity at 10.00h ranged using the techniques described by Pearson and between 0.60 and 0.79. There were significant Merritt (1991). differences between rooms only for minimum ambient temperature (P < 0.05). The minimum daily Complete faecal collections were made at regular temperature was associated with washing-out intervals from 23.00h on day 14 (administration of (hosing) of the rooms (once daily). Temperatures markers) for 7 days until the end of the period. For returned to average values within 30 min of the estimation of MRT, faeces from each animal collected completion of room washing. Variation in relative at 9, 11, 13, 15, 17,20,23,31,33,35,37,39,41,46,57, humidity within the day were large (:to.10) possibly 61,65,70,81,85,89,94,105, 113, 129, 137, 153, 161 due to changes in free-standing water. and 177h after marker administration. Individual faecal collections were weighed, thoroughly mixed Period effects on the experimental results and a subsample taken for determination of DM and Experimental observations on each animal lasted for marker concentrations. A further sample 12 weeks, during which time each animal received (proportionately 0.02 by weight) from each collection each dietary treatment for a 3-week period. The was pooled over the 7-day period for each animal for amount of experimental variation that could be subsequent fibre, energy and nitrogen analysis. All accounted for by period effects was not significant. sampleswere dried in a forced draught-oven at 6O°C to constant weight and then ground through a 1 mm Food and water intake screenbefore analysis. Acid-detergent fibre (ADF), The animals were given food to meet maintenance neutral-detergent fibre (NDF), crude protein (CP), energy requirements using the different diets. H;ence GE and OM were determined according to the the daily allowance and the actual intakes of the low methods of the Association of Official Analytical energy, high fibre whole oat straw diet, expressed Chemists(1990). per kg live ,veight, were sig:1ift('~"tly (P ~ 0.001) higher than those of the less fibrous, more energy Calculations and statistical analyses dense whole alfalfa diet. The intakes of the other Apparent digestibilities were calculated from total diets were intermediate between these two values; DM intakes and faecal DM outputs over the 7-day increasing with increasing fibre content (Table 2). collection periods. The MRTs of Cr-fibre and Co- EDTA were calculated as described by Pearson and Most animals consumed all of the food offered each Merritt (1991). day. The exceptions were those given just oat straw. Two of the Shetland ponies intermittently refused up Thedesign of the experiment was a standard change- to one-third of the straw allocation. The over design comprising four Latin squares, one for Thoroughbreds had refusals but proportionately eachtype of equid. The data obtained were subjected they did not exceed 0.2 of their straw allocation. One to of variance using GENSTAT(Lawes of the Highland ponies often left up to one-quarter of Trust, 1990). In the analysis, the total the straw diet, but not on a regular basis. Refusals of of squares was partitioned into three strata the whole straw diet by two other Highland ponies animals, variation proportionately did not exceed 0.1 of the allocation periods, and animal X period interaction. on any day, but again, refusals were irregular. These tested from findings suggest that the amount of oat straw needed

412 Cuddeford,Pearson, Archibald and Muirhead

(P < 0.001). These digestibility coefficients decreased between diets and between individuals when as more straw was included in the diet. The measured in the horses and ponies, particularly differences in the apparent digestibility of fibre when the animals were given the mixed roughage between diets were not so marked as for the other diets. In fact, fibre apparent digestibilities for the nutrients. The greatest difference in AOF apparent single forages (alfalfa or oat straw) tended to be digestibility was seen between the whole alfalfa diet higher than those determined for the mixed forage (0.47)and the other forage mixtures (P < 0.001) (0.39, diets. 0.40 and 0-41, respectively for the oat straw: alfalfa diets 0.33: 0.67, 0.67: 0.33 and 1 : 0). There was no Digestibilities of OM, GE, ADF and particularly consistent difference in NDF apparent digestibility NDF, measured in the donkeys were consistently between diets. The apparent digestibility coefficients higher than those values obtained with the horses or for protein measured in animals given the whole oat ponies (P < 0.01). straw diet are included for completeness, but are of little relevance in view of the very low protein The only significant (P < 0.05) interactions between content of the straw (48 g/kg OM). type of equid and diet occurred when the apparent digestibility of OM, GE and NDF was measured There were significant differences between types of (Table 3). The Thoroughbreds appeared to digest the equid in terms of their ability to digest the diets OM and GE in the diets with the higher levels of (Tabl(' 3). The Shetland ponie& digc3lcd the V~,,{.OM, alfalfa nt-ttt'r than the Shetland parties, but the GE and CP of the different diets significantly less opposite effect was seen when the oat straw diet was well than the other equids (P < 0.01). The effect was given. more noticeable when they were given the more digestible alfalfa diet (mean OM apparent Rate of passageof digesta through the digestive tract digestibility coefficient 0-66)than when given the oat Average recovery rates of Cr-fibre and Co-EDTA straw diet (mean OM apparent digestibility were proportionately 0.88 and 0.93 respectively over coefficient 0-48). The fibre (NOF and AOF) apparent the 7-day collection periods. Differences in recovery digestibility coefficients tended to vary considerably rates between individual animals were not related to

Alfalfa 0.67 Alfalfa: 0.33 Straw 1.()() 1.00

, :.-:;-,.:;-- ~0.80 0.80 ~ ..' .' "","'- .I: ' / ~ 0.60 : , / ~ 0.6() " -; : ' Qj , I / .~ I ..I / " 0.40 :$ 0.40 I :s ..I..I / :3 S 'I oJ e I :s :3 U 0.20 ,.', I u 0.20 I : I J 0 .-,/ J --L~' I I I I I 0 J ---YJI I I I I I

0.33 Alfalfa: 0.67 Straw Straw .00 1.()()

/> .r--' ~-::?:::==::;:;:=;.==.;::.=--.' -., --:;,,";;-;':'-':':_-;':' Qj~0.80 '0.80 ~ t'~.:'1 ~ 0.60 :, I Qj !, oJ 0-60 .~ If I '=~ ,; 0.40 J,'I oS 0.40, .if .: I'.I ..., :3 e .='.,~I e.. I :3 ),, I U=,. 0.20. :' 'I, U 0.20 " / .' .,/ 0 -~ ~./. I I-~/ I I I I 0 0 30 60 90 120 150 180 0 30 60 90 120 150 180 Time (h) Time (h) Figure 1 Mean cumulative proportional recovery rates of Cr-fibre for horses. ponies and donkeys for each dietary treatment (means of four animals): -horse; ---; Highland ; . Diet digestibility in equids 413

Malia 0.67Alfalfa: 0.33 Straw 1.00 .00 I! ~"'==~~-..-:-=.~ :'-:"--- 0.80 -- t- 0.80 ,'/ ~ 0 ; 0.6() ~ / ,1'/ ..0.60 , .~ 0.40 ~ 0.40 ..I:.I,:"'/,.,1 I 1'; , , , , ~ ) e 0.20 u~ 0.20 ..I 0 ." 0

0.33 Alfalfa: 0.67 Straw Straw 1.()() }.()() J ~:;:::::=="'~--oA'"--~ ~ to 0.8() ~0.80 >OJ ,~ 0 ~ 0 I;'~ ~ 0.60 ...~ 0.60 OJ > Qj /' .~ ~ 0-40 ]0-40 '3 =' (t 5 E =' a 0.20 U 0.20

0 0 30 60 90 120 150 180 0 30 60 90 120 ISO 180:~ Time (h) Time (h) Figure 2 Mean cumulative proportional recovery rates of Co-EDTA for horses, ponies and donkeys for each dietary treatment (means of four animals): -horse; ---doni key; Highland pony; .Shetland pony. equid type. The mean cumulative recoveries of Cr- to the donkeys compared with when given to the fibre and Co-EDTA by the different groups of other equids (P < 0.01). Differences in MRTs of the animals and for the different diets are given liquid-phase marker between equids were less respectively in Figures 1 and 2. Proportionately noticeable, although in most cases,the donkeys had about 0.80 of each marker was egested within the longer MRTs, matched only by the Highland ponies first 75 h of the collection period. The cumulative given some of the diets. recovery curves of Co-EDTA show less variation between equid types than those for Cr-fibre. Table 4 There was a significant interaction (P < 0.05) between shows the MRTs of Cr-fibre and Co-EDTA. As might type of equid and diet in the MRT of Cr-fibre. The be expected, the MRT of Cr-fibre (the solid-phase MRT of Cr-fibre in the gastro-intestinal tract of the marker) was longer than that of Co-EDTA (the Thoroughbreds, Highland ponies and donkeys liquid-phase marker) in mc3t ef the .:mmals appeared to be iIilluenced by diet. lIu\'v.:v..!r, in the regardless of diet. The exceptions were the Shetland ponies, diet seemed to have little effect on Thoroughbreds, which showed remarkably similar MRT of Cr-fibre (Table 4). Although no significant MRTs of Cr-fibre and Co-EDT A on each of the diets. interaction between equid type and diet was Diet had a significant effect (P < 0.001) on MRT of observed for Co-EDTA, the Shetland ponies tended Cr-fibre and Co-EDTA. Not surprisingly, MRT was to have the lowest MRTs of Co-EDTA on all of the greatest with the alfalfa diet, the diet with the highest diets, compared with values obtained for the other energy density and therefore given in the smallest equids. amount during the experiment. In contrast, rate of passage of digesta through the tract increased considerably when the animals were given diets that Discussion~, contained straw. Differences in MRT for the three The smallest pornes ,~\Shetlands) ,}",consumed more DM[ diets containing oat straw were small. (g/kg M) compared with the Highland ponies andI Thoroughbr~s. This observation is similar to that Consistently slower rates of passage of the solid- reported by Uden and Van Soest (1982), although in phase marker were measured for all the diets given the present study, the Shetland ponies were not morE'

.., 414 Cuddeford, Pearson,Archibald and Muirhead

Table 4 The mean retention time (h) of Cr-fibre and Co-EDT A in the digestive tract of Thoroughbreds (TB), Highland ponies (H pony) donkeys and Shetland ponies (5 pony) given diets containing different proportions of alfalfa (ALF) and oat straw (OS)

Diet proportions Type of equid Significance ALF OS TB H pony Donkey 5 pony Mean of effects

Mean retention time (h) Cr-fibre 1 0 51.9 63.7 76.7 46.3 59-6 Type .. 0.67 0.33 44.1 504 59.2 44.2 49-5 Diet 0.33 0.67 42-2 51.5 55.3 46.7 49-0 Type X diet 0 1 384 51.3 53.8 46.9 47-6 Mean 44-2 54-2 61.2 46.0 s.e.d.t = 4.58 s.e.d.t = 3.33 s.e.d.§= 1.67 s.e.d.1I= 3.56 Co-EDTA 0 51.1 594 61.8 41.6 53.5 Type 0.67 0.33 44.3 43.0 47.2 40.2 43.7 Diet 0.33 0.67 42.9 45.9 48.6 42.7 45.0 Type X diet 0 1 435 48.3 46.0 44.7 45.9 Mean 454 494 50.9 42.3 s.e.d.t = 5.14 s.e.d.t = 3.70 s.e.d.§= 1.85 s.e.d.1I = 4.02

t s.e.d. for comparison between two means for different types of equid. * s.e.d. for comparison between two means for the same type of equid. § s.e.d. for comparison between overall means for equid types. II s.e.d. for comparison between overall means for diet types.

efficient at digesting food nutrients when compared weight but on the basis of metabolic body size (MO.75 with the Thoroughbred and Highland ponies. The they received significantly less than the heavie apparent digestibility coefficients for DM, OM and animals. GE measured in the Shetland ponies were consistently lower than those for the larger animals fluctuations in live weight could be explained b: (Table 3). This would seemto support the hypothesis changes in gut-fill due to the different diets that wer, that there is a relationship between DM intake offered. Animals consistently showed the heavies (expressed per kg M) and nutrient digestibility, since live weights when given diets with the highest fibr the animals with the lower intakes digested food content and intake allowances (0.67: 0.33 and 1: ( nutrients more effectively. However, donkeys did oat straw: alfalfa). The fact that those animals too not fit this pattern. They consumed almost as much did not consume all of their food ration, but stiJ DM (13.3g/kg M) as the Shetland ponies (14.3g/kg maintained live weight suggests that they had , M), but they consistently digested DM, OM and GE higher efficiency of digestion, a reduced maintenanc, more effectively than the other equids. This superior energy need, or possibly a greater gut-fill. capability to digest fibre compared with ponies agrees with previous results obtained with both The mean apparent digestibility coefficients for OM restricted (Wolter and Velandia, 1970)and ad libitum CP and AOF of dehydrated alfalfa offered to th. fed animals (Pearson and Merritt, 1991; Tisserand, animals in the present study were 0.67,0.72 and 047 Faurie and Toure, 1991; Suhartanto, Julliand, Faurie respectively (Table 3). Previous studies in thi and Tisserand, 1992). This superior ability of laboratory with dehydrated alfalfa (304g AOF per kl donkeys is almost certainly due to the fact that they OM) given to Thoroughbreds showed similar O~ are able to retain food particles for longer in the (0.63) and CP (0.74), but lower AOF (0.35) value digestive tract compared with the other animals. (Cuddeford, Woodhead and Muirhead, 1992). Th That the Shetlands consumed the most DM (g DM coefficients in the current study are similar to thos, per kg M) is not surprising since they were rationed reported by Hintz (1969)for alfalfa hay (310 g CF pe according to an estimate of maintenance energy kg OM) offered to horses, which were 0.59, 0.75 ant requirement that was based on a linear equation that 041 (CF not AOF) respectively. An alfalfa hay give] incorporated a large Y intercept (0.975);this intercept to donkeys was reported (Izraely, Chosniak, Steveru has an effect similar to the use of a power function of Oemment and Shkolnik, 1989) to have an AD body weight. Thus, the Shetland ponies were offered digestibility of 047 which is similar to the valu more food, had the highest intake per kg body (0.50)obtained in the present study. Diet digestibility in equids 415

--I In the current study, energy intake was not limited of oat straw are although there were large differences in protein reported by Hintz (1%9), where intake. Variation in ADF and NDF apparent the CF value was high at 0.51 and the digestibilities obtained between diets and between value was only 0.44. This might have been animals may have partly reflected these differences .for by the fact that the oat straw used in in nitrogen availability and subsequent microbial study contained only 389 g ADF per kg activity. However, the lack of any marked -Hintz (1969)which depression in ADF and NDF apparent digestibility per kg DM-. Digestibility values associated with the decrease in digestible CP intake have not been as the dietary straw content increased (Table 3) other straws are suggests that when the dietary supply of nitrogen is An ADF digestibility has been reported in low, the equid has some mechanism that sustains the fed donkeys as 0.42 for wheat straw nitrogen supply to the hind gut. The donkey appears ., 1989) to be the most successfulequid in terms of digesting --containing 465 g ADF per fibre on low protein diets and this may be because it DM (Suhartanto et al., 1992). These values has the best developed mechanism for nitrogen for oat straw recycling to the hind gut. In support of this, Izraely et j .An ADF digestibility for barley at. (1989)found that when donkeys were given a low ADF per kg DM) offered ad libitum to protein (45 g CP per kg DM) wheat straw compared determined to be 0.52 (Pearson and with when given high protein (360 g/kg DM)alfalfa 1991),with an associated OM digestibility of hay, there was a decrease in the urea filtration rate. 0-48. This was combined with an increase in the fractional urea resorption, which subsequently increased For herbivores, and equids in particular, the fibre retention of urea nitrogen and recycling of nitrogen content of a diet has been found to have a marked into the hind gut. influence on nutrient digestibility (Fonnesbeck, {..ydman, Van der Noot and Symons, 1967); the The situation with horses and ponies is unclear hIgher the fibre the lower the digestibility value. This although Prior, Hintz, Low and Visek (1974) have felationship was confinned in the present study produced evidence to show that urea is recycled to vhen the Thoroughbreds, Highland ponies and the hind gut of ponies. However, nothing is known lonkeys were given the different diets. This of the controlling circumstances under which the ~lationship was less clear in the Shetland ponies, return occurs or of the extent to which it presents a robably because of the low digestibilities measured contribution to, or a penalty on, the nitrogen these animals when given the 0.67 ; 0.33 economy of the animal. HaIfa: oat straw) diet. Generally, the level of fibre , the diet, seemed to have little effect on the The low nutrient apparent digestibilities measured in abilities to digest the fibrous components, the Shetland ponies, compared with those obtair-cd though the combination of 0.07 : O.JJalfalfa and oat in the other animals, were mQre noticeable as the raw depressed both ADF and NDF digestibilities in alfalfa content of the diet increased. The lese animals. It is noteworthy that the other equids measurement of digestibility, particularly of not similarly affected and no explanation can roughage diets, in very small equids (ca. 100 kg) may Jeoffered for this anomaly. not be representative of the situation in larger horses. This may be because their small size may reduce the ~e present study confirmed the observation of effectiveness of digestive processes, particularly Jden and Van Soest (1982)that there is considerable fermentation which is affected by both the size of the ndividual variation between equids in their ability reaction vessel and the out-flow rate (Hume and 0 digest fibre. However, apparent digestibility Sakaguchi, 1991). Hintz (1990)concluded that ponies :>efficientsfor the fibrous components of the single can be used in digestion studies to evaluate )rageswere less variable than those of the mixtures. foodstuffs for horses although the minimum mean of the variation may be explained by the weight of the ponies used in the different ifficulty in achieving an homogenous mix of oat experiments he reviewed was 132 kg and mostly the and alfalfa, subsequent problems of ponies weighed 160kg or more. The immature lequately sampling these mixed forages and ponies used in the current study may have had a )Ssiblefood interactions. smaller large intestine: body weight ratio than the larger equids and could have been disadvantaged. .of energy and nitrogen available to the This relative disadvantage should have been most ~oflora in the hind gut will affect the ability of marked on the high fibre diets (containing oat straw) 1uids to digest fibrous, plant cell-wall components. but in fact the discrepancy was most obvious when 416 Cuddeford, Pearson,Archibald and Muirhead

the dietary alfalfa content increased and intake was an advantage, but, where there are unlimited food reduced. A possible explanation for this apparent supplies, the pony or horse can compensate foJ anomaly could be that the Shetland ponies reduced digestive efficiency by consuming and consumed less digestible CP (g/kg OM) compared processingmore food. with the other animals. Lower water intakes per kg food consumed by donkeys as compared with other equids have been Acknowled~ements reported elsewhere (e.g. Mueller and Houpt, 1991) The authors thank Dengie Crops (Hall Road, Asheldham and may reflect the donkeys' desert origins. Water Southminster, Essex,UK, for generously providing the di~ intake per kg OM increased with increasing straw and I. Campbell, P. Wright, J. Hogg and M. Jordan for can of the animals. The study was carried out with financia inclusion in the diet by up to 0.51. Although, this support from the British Overseas Development increase was not significant, it may reflect greater Administration and the European Community, whOSf water retention in the lumen of the gut which in turn contributions are gratefully acknowledged. may affect the rate of passage of digesta. Hydrophilic polysaccharides, such as hemicellulose, absorb water and hold it in the lumen of the gut. This is reflected in the high water content of the faeces voided by References aniQals given hemicelluI:I~":-lich di~l" (e.g. timothy As~Gciati"n of Official Analytical ('helnis~. l~<)o Offici". hay, straw) compared with those given diets low in methodsof analysis of the Association of Official Analytic". hemicellulose (e.g. alfalfa). Cuddeford et al. (1992) Chemists. 15th ed. Association of Official Analytica have shown that Thoroughbred horses given only Chemists, Virginia. alfalfa (130 g hemicellulose per kg OM) voided four Barth, K. H., Williams, J. V. and Brown, D. G. 1977 times as much water in the urine and produced drier Digestible energy requirements of working and non. faeces than those given timothy hay. However, mean working ponies. Journalof Animal Science44: 585-589. water intakes O/kg OM) were similar, OM intakes Cud deford, D., Woodhead, A. and Muirhead, R. H. 1992 were the same and although rate of passage was A comparison between the nutritive value of short-cutting cycle, high temperature-dried alfalfa and timothy hay fo] quicker for the alfalfa, there were no significant horses. EquineVeterinary Journal 24:84-89. differences. The relationship between water intake and nutrient digestibility in the donkey is as yet Fonnesbeck, P. V. 1%9. Partitioning of the nutrients 01 forage for horses.Journal of Animal Science28: 624-633. untested. Fonnesbeck, P. V., Lydman, R. K., Van der Noot, G. W It has been proposed that compared with the and Symons, L. D. 1%7. Digestibility of the proximat£ nutrients of forage by horses. Journal of Animal Science26 ruminant, the horse is better adapted to deal with 1039-1045. fibrous foods when they are available ad libitum Hintz, H. F. 1969. Review article: (Illius and Gordon, 1990). The adaptation is that the Comparisons of digestion coefficients obtained with cattle horse will eat more low quality roughage than the sheep,rabbits and horses. Veterinarian6: 45-51. ruminant, although the digestibility of the nutrients Hintz, H. F. 1990. Digestion in ponies and horses. Equint will be less in the horse. It is not known if there are Practice12: 5-6. different strategies employed by members of the to digest high fibre diets. For example, is Hintz, H. F., Schryver, H. F. and Stevens, C. E. 1978 Digestion and absorption in the hind gut of non-ruminant there any attempt to regulate intake and rate of herbivores. Journalof Animal Science46: 1803-1807. passage of ingesta thereby affecting exposure time to enzymatic and microbial activity? The results of the Hume, I. D. and Sakaguchi, E. 1991. Patterns of digesta flow and digestion of fore gut and hind gut fermenters. lJ1 current study suggest that donkeys have a different Physiologicalaspects of digestionand metabolismin ruminants. strategy from other equids, because they have a Proceedingsof the seventhinternational symposiumon ruminant higher intake of food and digest nutrients more physiology,pp. 427-451.Academic Press, New York. effectively in contrast to horses and ponies, where IIlius, A. W. and Gordon, I. 1990. Constraints on diet high intakes are associated with lower nutrient selection and foraging behaviour in mammalian herbivores. digestibilities. Previous work (Pearson and Merritt, lJ1Behavioural mechanisms of food selection(ed. R. N. Hughes) 1991)has shown that donkeys given straw ad libitum Springer-Verlag, Berlin. had a slower rate of passage of food residues, and Izraely, J., Chosniak, I., Stevens, C. E., Demment, M. W. higher OM digestibility compared with ponies given and Shkolnik, A. 1989. Factors determining the digestive the same diet. It is proposed that donkeys are more efficiency of the domesticated donkey (Equus asinus) efficient at digesting fibrous foods because they have Quarterly Journalof ExperimentalPhysiology 74: 1-6. a higher mean retention time of food particles in the Janis, C. 1976.The evolutionary strategy of the Equidae and gut regardless of intake. Where the food resource is the origins of rumen and caecal digestion. Evolution 30: limited, the donkey, like the ruminant, would have 757-773. Diet digestibility in equids 417

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