Abstract:
Animal Production in Australia THE UTILIZATION OF PANGOLA GRASS HAY BY SHEEP AND ANGORA GOATS CAROL WATSON+ and B.W. NORTON* SUMMARY Six Angora goats and 6 Merino crossbred wethers were fed mature (8.2 g N/kg DM) and immature (19.5 g N/kg DM) Pangola grass hay and the intake digestibility and utilisation of these diets compared. Both species consumed, digested and utilised the immature hay with equal efficiency, and voluntary feed intake was depressed to the same extent in sheep and goats given mature hay. Goats digested the organic matter, NDF and ADF fractions of the mature hay to a significantly (P< O.05) greater extent than did sheep. Associated with these higher rates of fibre digestion in goats were significantly (P<O.O5) longer fluid retention times in the rumen, higher ammonia, propionic and isovaleric acid and lower acetic acid concentrations in ruminal fluid. It was concluded that the higher rates of fibre digestion in goats given mature hay was the result of longer feed retention times in the rumen and the maintenance of higher rumen ammonia levels than those found in sheep given the same diet. INTRODUCTION Comparative studies of digestion in goats and other ruminants (sheep and cattle) consuming high quality feeds have shown similar efficiencies of nutrient consumption and utilisation (Schneider 1957; Baumgardt et al. 1964; Doyle and Egan 1980). However where low quality feeds (high fibre and low N content) have been used, goats usually have a higher efficiency of feed utilization (Jones et al. 1972; Wilson et al. 1975; Doyle and Egan 1980), particularly in the digestion of the fibrous fraction of the feed (El Hag 1976; Devendra 1978). In some studies, goats have consumed more low quality feed than other ruminants and digested crude protein to a greater extent (Majumba 1960; Gihad 1976), although few studies have recorded significant differences in N retention between species. Tropical forages have higher fibre and lower N content than do temperate forages (Minson 1982), and are consequently of lower nutritive value. It would seem that, from the above studies, goats may more efficiently use tropical pastures for production than other ruminants and that the extrapolation of dietary recommendations from sheep to goats may not be justified. The purpose of the following experiment was to investigate the digestion and utilization of a tropical grass by sheep and goats and to relate any differences in fibre digestion between the species to observed physiological differences in metabolism. MATERIALS AND METHODS Animals and Diets Six Angora wether goats and 6 wether sheep (Dorset Horn x Border LeicesterMerino) of similar age (3 yr) and fitted with rumen cannulae were held in metabolism cages for the 8 week experimental period. Mean liveweights were 45 and 36 kg for sheep and goats respectively. All animals received trace element supplements and were treated for intestinal parasites (Nilverm, ICI) at regular intervals. Pangola grass (Digitaria decwnbens Stent) was harvested after N fertilizer application (100 kg N/ha) at 3 weeks (Immature) and 12 weeks (Mature), + Department of Agriculture, Wollongbar, N.S.W. 2480. * Department of Agriculture, University of Queensland, St. Lucia,Qld 4067. 467 Animal Production in Australia after slashing, and after drying was hammermilled composition (g/kg dry matter) of the immature (I) was as follows: Nitrogen 19.5 and 8.2; Ash 12.7 (NDF) 798 and 773; Acid Detergent Fibre (ADF) 408 82.6. and and and and stored. The chemical mature (M) hays respectively 19.4; Neutral detergent fibre 388; and Lignin 56.2 and Experimental design Sheep and goats in groups of three were randomly allocated to the two diets and fed ad lib for a 4 week period. In the final 10 d of this period, voluntary feed intake, faecal and urinary excretions were recorded and each animal dosed intra ruminally with 80 ml Cr EDTA (310 mg Cr). Rumen fluid samples were collected at 6 h intervals over 2 d and stored for Cr, VFA and ammonia analysis. Blood was collected from the jugular vein of each animal immediately before and 4 h after feeding, and plasma stored for urea determination. At the end of this period, sheep and goats were again randomly assigned in groups of 3 to each diet, and in a further experimental period of 4 weeks, the same measurements as described above were made. Analytical and statistical methods Feed refusal and faeces were dried (80�C for 36 h) and ground (1 mm screen) samples and urine were analyzed for total N using an autoanalyzer technique (Henzell et al. 1968). Urea in urine and plasma were determined by the method of Marsh et al. (1957). ADF and lignin were estimated by the method of Van Soest (1963) and NDF by the method of Van Soest and Wine (1967) omittingsulphiteaddition. Rumenammoniawas determinedby distillationandtitration, volatile fatty acids (VFA) by gas 1iquidchromatographyandCr EDTAin rumen fluidwasdeterminedby atomic absorption withacrlarrp (425.41~$. Differencesbetweendiets, species andexperimentalperiodswere determinedby analysis of variance using a randomizedblockdesign (Steel tiIbrrie l%O). RESULTS Table 1 shows mean values, with standard errors, for the intake, digestibility and utilization of dietary constituents by sheep and goats. There were no significant differences between sheep and goats in voluntary feed consumption of the same diet, but both species consumed less mature hay. The immature hay contained a relatively low content of crude protein (12.2%) and sheep and goats Table 1 Mean values for the intake, digestibility and nitrogen utilization of Pangola grass hay at two stages of regrowth by sheep and Angora goats * Means within a line with different subscripts differ significantly (P<O.OS>. 468 Animal Production in Australia digested OM, NDF and N to similar extents. Goats digested ADF to a significantly (PcO.05) greater extent than did sheep. However, for the low quality (mature) diet, all dietary constituents with the exception of N were better digested by goats than sheep. Sheep and goats given the immature hay excreted similar amounts of N in urine and faeces, and there were no significant differences in N retention. When given the mature hay, both species were in negative N balance. Table 2 gives mean values, with standard errors, for rumen volume, fluid turnover rate and concentrations of VFA and ammonia in rumen fluid and plasma urea of sheep and goats on the two diets. When rumen volume was expressed as a proportion of liveweight, goats had significantly (PcO.05) larger rumen volumes than sheep, and longer mean retention times of fluid in the rumen. Goats had significantly (PcO.05) higher proportions of propionic and isovaleric acids and lower proportions of acetic acid when compared with sheep. Ruminal ammonia and plasma urea concentrations were similar in sheep and goats given the immature hay, but significantly declined in both species when consuming the mature hay. However, when given this low quality diet goats maintained significantly higher concentrations of both ruminal ammonia and plasma urea than did sheep. TABLE 2 patterns in the rumen * Mean values within a line with dissimilar subscripts differ significantly (P<O. 05). DISCUSSION The results of this experiment confirm previous studies where sheep and goats consumed and digested, with equal efficiency, diets of low fibre high N content, but goats digested high fibre low N diets better than sheep (Devendra 1978). In this study, both diets were of similar fibre content despite differences in maturity, and the major differences between the two diets were their N and lignin content. The lower voluntary feed intake in sheep given the mature hay may be related to the longer retention of feed in the rumen caused by the lower rates of fibre digestion (Thornton and Minson 1973). Although the voluntary feed intakes of goats was depressed to a similar extent when fed the mature hay, fluid retention time was significantly longer than that in sheep and yet the rate of fibre digestion was also significantly greater. Doyle and Egan (198'0) also recorded significant differences between sheep and goats in the mean retention time of 103Ru-Phenanthroline, the greatest differences being found on the diet of lowest N content. Longer feed retention times associated with high rates of digestion may be explained by higher rates of rumination in goats compared with sheep. 469 Animal Production in Australia On both diets, the pattern of fermentation products in the rumen differed between species, suggesting that the higher rates of fibre digestion in goats may be related to the presence of different microbial populations in the rumens of sheep and goats. The higher levels of isovaleric acid and ammonia in goat ruminal fluid on the mature hay suggests that dietary protein was being degraded to a greater extent, either through greater feed particle breakdown from rumination or by higher proteolytic activity of the rumen micro-organisms. Satter and Slyter (1974) found that rumen ammonia levels below 50 mg N/l limited microbial activity. In the present experiment, sheep and goats given the same low quality diet recorded ruminal ammonia concentrations of 43 and 106 mg N/l respectively and clearly microbial activity may be depressed in the sheep but not in the goats on this diet. The higher levels of rumen ammonia in the goats may have been the major reason for higher rates of fibre digestion. Associated with high ruminal ammonia were high plasma urea concentrations in goats, and since there were no species differences in plasma urea concentration when sheep and goats were fed the immature hay, it would appear that the elevated plasma urea levels in goats on the mature hay were a reflection of higher rates of production and absorption of ammonia from the rumen. Goats have also been found to have higher rates of salivary secretion than sheep (Seth et al. 1976), and high rumen ammonia concentrations may have been maintained by a greater urea recycling to the rumen through saliva. It may be concluded that the higher rates of fibre digestion in goats compared with sheep are related to physiological differences between the species, although further studies of rumination behaviour and feed particle degradation are needed to confirm the speculations made above. REFERENCES BAUMGARDT, B-R.+ BYER, W.J., JUMAH, F.H. and KRUEGER, C. (1964). J. Dairy Sci. 47: 160. DEVENDG, C. (1978). World Rev. Anim. Prod. 14: 9. DOYLE, P.T. and EGAN, J.K. (1980). Proc. Aust. Soc. Anim. Prod. 13: 521. Z EL HAG, G.A. (1976). World Rev. Animal Prod. 12: 43. GIHAD, E.A. (1976). J. Anim. Sci. 42: 799. = HENZELL, E.G., WALLIS, I. and LINDQEST, J.E. (1968). Trans. Soil Sci. Congr. 2: 513. JONES,=G.M., LARSEN, R.E., JARED, A.H., DONEFER, E. and GAUDREAU, J.M. (1972). J. Anim. 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