Abstract:
Proc. Aust. Soc. Anim. Prod. Vol. 19 COMPARATIVE DIGESTIVE EFFICIENCY OF CATTLE AND SHEEP CONSUMING TROPICAL FORAGES AND ITS SIGNIFICANCE IN PREDICTING DIGESTIBILITY FROM IN VITRO TECHNIQUES A. J. ASHA and A. C. SCHLINK AB ACSIRO Division of Tropical Crops and Pastures, Davies Laboratory, Private Mail Bag, Post Office, Aitkenvale, Qld. 48 14. -Present address: CSIRO, Division of Animal Production, P.O. Wembley, W.A. 6014. SUMMARY The comparative digestive efficiency of cattle and sheep fed 10 tropical forage diets was evaluated in a pen experiment. The significance of this relationship in predicting digestibility from in vitro techniques was also examined. Efficiency of forage digestion was 2 units higher in cattle than in sheep for low quality diets whereas sheep digested high quality forages better than cattle. The difference between this relationship and other published sheep-cattle comparisons may be explained by variation in level of feeding between studies. Differences in the digestive efficiency of cattle and sheep were not, however, apparent when compared in a regression between in vitro and in vivo digestibility. This indicates that errors in estimating digestibility by in vitro techniques can be larger than differences in digestive efficiency between sheep and cattle. In studies where in vivo digestibility of cattle is being estimated from in vitro methods standardised with sheep greater emphasis should be placed on minimising errors associated with the in vitro technique. Keywords: digestibility, tropical forages, cattle, sheep. INTRODUCTION Efficiency of forage digestion is generally greater in cattle than in sheep (Cipolloni et al. 1951) because of the longer time forage is retained in the reticula-rumen by cattle (Poppi et al. 1981). The differences in digestibility (DMD) of forages between sheep and cattle have been quantified in a number of studies (Cipolloni et al. 1951; Playne 1978; Reid et al. 1988; Minson 1990). In 2 of these studies (Cipolloni et aZ. 195 1; Minson 1990) the relative differences between cattle and sheep were constant across a range of forage qualities while the results of Playne (1978) and Reid et al. (1988) showed that the differences in digestibility decrease as the quality of the forage improves. The nature of this relationship between cattle and sheep can have important implications where accurate estimates of in vivo digestibility are required for the prediction of such variables as feed intake (Playne 1978) and the nutritive value of forages. Direct measurements of digestibility with cattle in vivo are both laborious and expensive and frequently require impracticably large amounts of feed. These problems are reduced by use of sheep instead of cattle or avoiding the use of animals altogether by applying in vitro fermentative techniques such as the pepsin cellulase technique (McLeod and Minson 1980). Such techniques are standardised using samples of known in vivo digestibility, usually determined with sheep. This necessitates a further conversion, and hence a further source of error, where estimates of cattle DMD are required. At present there are insufficient data comparing the digestive efficiency of sheep and cattle consuming tropical grasses and legumes of differing qualities. Further, it is not known what significance this relationship has for the prediction of digestibility using in vitro techniques. This paper describes an experiment designed to examine these relationships. MATERIALS AND METHODS Experimental detaiZs Twenty Droughtmaster steers about 30 months old and with a mean weight of 434 kg were used in the cattle study while the same number of mature Merino wethers (mean weight 38 kg) were used in the sheep component of the experiment. Cattle were kept in individual pens with concrete floors while sheep were housed in metabolism cages. Ten diets were compared in a series of digestion trials. Animals were assigned at random to each diet within each digestion trial with each diet fed to 4 cattle and 5 sheep. The diets used were a range of 8 tropical hays, luceme (Medicago sativa) and a 50% maize grain/50% Verano hay mix. The tropical hays were spear grass, Heteropogon contortus; sabi grass, Urochloa mosambicensis; rhodes grass, ChZoris gayana (mid-cut and mature late-cut); forage sorghum, Sorghum bicolor; Verano, StyZosanthes hamata; lablab (dolichos), LubZab puvureus. The chemical 331 Proc. Aust. Sot. Anim. Prod. Vol. 19 composition of the diets is shown in Table 1. Hays were either made from forage grown at the CSIRO Lansdown Research Station, situated 50 km south of Townsville, or obtained from commercial sources in the Townsville region. All hays were chaffed into approximately 2 cm lengths prior to feeding. Maize grain was purchased from the Burdekin region and crushed before feeding. All animals were fed once a day at approximately 0800 hours. Table 1. Chemical composition (g/kg dry matter) of the diets fed to cattle and sheep in vivo and in vitro digestibility determination Each 21-day digestion trial consisted of a 14-day preliminary period followed by which forage intake and faeces output were measured. During the adaptation period adjusted to about 90% ad Zibitum to minimise refusals. Refusals were less than 1% of faecal output was recorded and samples of feed, refusals and faeces were collected measurement period. a 7-day period in feed intakes were feed offered. Total during the 7-day Analytical procedures Feed, refusals and faeces were dried at 70�C in a forced draught oven and ground to pass through a 1 mm screen. Organic matter (OM) was determined after ashing dried samples in a muffle furnace for 3 h at 55OOC. Analysis for acid detergent fibre (ADF) and lignin was performed as described by Goering and Van Soest (1970) while total nitrogen (N) and phosphorus (P) concentrations in samples were measured calorimetrically following Kjeldahl digestion (AOAC 1975). In vitro digestibility (DMD) of the forages was determined by the pepsin-cellulase technique (McLeod and Minson 1980). The samples were incubated in 50 mL Gooch crucibles which were sealed with caps both top and bottom. Linear regressions and differences between the slopes and intercepts of the regressions were tested using the procedures of Steel and Torrie (1960). RESULTS AND DISCUSSION Dry matter intake and apparent DMD of the diets fed to sheep and cattle are shown in Table 2. The mean DMD and intake of the 10 diets by cattle and sheep were similar at 50.1 and 49.6%, and 24.5 and 28.7 g/kg W'e9, respectively. The regression of cattle DMD on sheep DMD (Fig. 1) was linear and significant (P < 0.01) with the slope of the line being significantly different from unity (P < 0.01). Efficiency of forage digestion was 2 units higher in cattle than in sheep for low quality diets whereas sheep digested high quality forages, particularly the legumes, better than cattle. This relationship is not consistent with other studies (Cipolloni et al. 1951; Playne 1978; Reid et al. 1988; Minson 1990) which show cattle to be more efficient at digestion of forages of all qualities. This inconsistency between our study and other published relationships may be explained by differences in level of feeding. Prigge et al. (1984) showed that at restricted levels of feeding sheep digested a high quality forage with a greater efficiency than cattle but with ad Zibitum feeding cattle digested the forage better than sheep. In contrast, cattle digested a low quality forage better than sheep at both levels of feeding. Presumably differences between sheep and cattle in retention time of forage in the rumen, a contributing factor in the greater digestive efficiency of cattle (Poppi et aZ. 1981), are likely to be less under a restricted feeding regime, particularly with high quality forages. Animals in our study were fed at about 90% ad Zibitum compared with other studies where feed was offered at about 115% ad Zibitum 332 Proc. Aust. Sot. Anim. Prod. Vol. 19 Fig. 1. Regression of cattle DMD on sheep DMD (-,A) for the 10 forages fed in the experiment (y = 0.84~ + 8.44, r = 0.97, s.e. y = 2.01). The line y = x shown as (---). (Poppi et aZ. 1981; Reid et al. 1988). These findings suggest that relationships comparing the digestibility of sheep and cattle need to be interpreted with some caution, and the level of feeding needs to be clearly stated. The relationships between in vitro and in vivo digestibility for sheep and cattle are shown in Fig. 2. The lines of best fit were: where y = DMD in vivo and x = DMD in vitro. These relationships are comparable with other studies which have employed the pepsin+ellulase method and covered a similar range of digestibilities (McLeod and Minson 1978). Approximately 27% of the error in both regression relationships could be accounted for by the very low in vivo digestibility values recorded for lablab compared with its laboratory measure of digestibility. This is consistent with the results of McLeod and Minson (1978) where in vivo DMD of lablab was considerably lower than other species which had the same in vitro DMD. The low digestibility of lablab may be associated with its pattern of fragmentation and passage through the rumen (McLeod et aZ. 1990). Table 2. Dry matter intake (g/kg W'm9) and in vivo DMD (%) of tropical forage diets fed to sheep and cattle in pens 333 Proc. Aust. Sot. Anim. Prod. Vol. I9 Fig. 2. Regressions relating in vivo DMD to in vitro (pepsincellulase) DMD for sheep (---,A) and cattle (-,A ) fed a range of tropical forage diets. There was no significant difference (P > 0.05) in these regression lines between cattle and sheep for either the slope or the intercepts. This result is in contrast to the direct comparison of in viva digestibilities where the slope of the relationship between sheep and cattle diverged significantly from unity. It would appear then that the errors in estimating digestibility of tropical forages by the in vitro technique can be larger than the differences in digestive efficiency between sheep and cattle. This result highlights the problems associated with using the pepsin-cellulase in vitro technique, or any other indirect method with similar errors, particularly in quantitative studies where dry matter intake is being estimated The use of standards similar to the test species is necessary to reduce the variability in all indirect techniques for estimating digestibility (Minson 1990). Only after such variability is minimised will further improvements be possible in estimating in vivo digestibility from a suitable regression equation relating the digestive efficiency of cattle to sheep. ACKNOWLEDGMENTS The authors wish to thank Jeff Corfield and Clem King for technical assistance in the collection of digestibility data from pens. REFERENCES AOAC (1975). ` Official Methods of Analysis.' 12th ed. (Association of Official Agricultural Chemists: Washington, DC). CIPOLLONI, M. A., SCHNEIDER, B. H., LUCAS, H. L. and PAVLECH, H. M. (1951). J. Anim. Sci. 10: 337-43. GOERING, H. K. and VAN SOEST, P. J. (1970). ` Forage Fibre Analysis.' (United States Department of Agriculture: Bethesda, MD). McLEOD, M. N. and MINSON, D. J. (1978). Anim. Feed Sci. Technol. 3: 277-87. McLEOD, M. N. and MINSON, D. J. (1980). Anim. Feed Sci. Technol. 5: 347-50. McLEOD, M. N., KENNEDY, P. M. and MINSON, D. J. (1990). Br. .I. Nutr. 63: 105-19. MINSON, D. J. (1990). ` Forage in Ruminant Nutrition.' (Academic Press: San Diego). PLAYNE, M. J. (1978). Anim. Feed Sci. Technol. 3: 5 l-5. POPPI, D. P., MINSON, D. J. and TERNOUTH, J. H. (198 1). Awt. J. Agric. Res. 32: 109-2 1. PRIGGE, E. C., BAKER, M .J. and VARGA, G. A. (1984). J. Anim. Sci. 59: 23745. REID, R. L., JUNG, G. A. and THAYNE, W. V. (1988). J. Anim. Sci. 66: 1275-91. STEEL, R. G. D. and TORRIE, J. H. (1960). ` Principles and Procedures of Statistics.' (McGraw-Hill: New York). 334