Abstract:
Proc. Aust. Soc. Anim. Prod. Vol. 16 COMPARATIVE INTAKES AND DIGESTIBILITY BY SHEEP AND CATTLE FED NaOH-TREATED STRAW K. AMANING-KWARTENG*, R.C. KELLAWAY*, J.C. SPRAGG** and A.C. KIRBY*** SUMMARY Sheep and cattle were fed coarsely-milled, alkali-treated straw, alone and with supplements of cotton-seed meal or barley Organic matter intakes (OMI) expressed as g/kg live weight (W)'*' and g/kg W 4; :75 , cell wall (CW) digestibility and calculated metabolisable energy (ME) intakes relative to maintenance requirements were higher with cattle (P<O.OS) than with sheep. Sheep digested more N and had higher digestible organic matter in dry matter (DOMD) values than cattle (P<O.Ol). The higher intakes by cattle were associated with larger rumen pool sizes (g/kg Woe9 and g/kg Woo7' ) and shorter rumen retention times for solids and liquid. Higher neutral detergent fibre (NDF) digestibility by cattle was associated with a faster rate of NDF digestion when measured 'in sacco'. (Keywords: intake, digestibility, sheep, cattle) INTRODUCTION From a summary of published experiments, Van Soest (1980) concluded that cattle digest low-quality feeds better than sheep, the reverse being true when dry matter digestibility (DMD) is greater than 66%. When components of DM have been considered separately, cattle usually digest fibre more and N less than sheep (Aerts et al. 1984; Prigge et al. 1984). Differences in extent of fibre digestion can be attributable to differences in rate of digestion and retention time in the rumen, the latter being inversely related to intake. In the present experiment we exami ned factors associated with di fferences in intake and d i.gestibi 1.ity between sheep a nd cattle fed high-roughage di ets, MATERIALS AND METHODS Animals, feeds and feeding Six Merino x Border Leicester yearling wethers (35 kg live weight) fitted with rumen and abomasal cannulae, and six Hereford yearling heifers (200 kg live weight) fitted with rumen and abomasal cannulae. were kept in individual metabolism cages under continuous lighting with free access to water. They were fed diets of hammer-milled (30 mm screen) wheat straw (Triticum aestivum, cv. Shortim) (0.48% N), sprayed with NaOH-solution followed by a mineral solution containing urea N, The treated straw (OM, 873; N, PI s, Cu and Co (Amaning-Kwarteng et al. 1984a). 18.3; NDF, 667 and acid detergent lignin, 52.0 g/kg DM) was fed ad libitum either alone (W) or with supplement (150 g/d for sheep and 700 g/d for cattle) of delinted, solvent-extracted cotton-seed meal (WC) or dry-rolled barley (WB), The straw was fed in buckets, automatically, at 2-hour intervals while the supplements were fed once daily and before the straw. Straw refusals were collected and weighed each day and 20% excess straw provided the following day. Digestibility, abomasal diqesta, dilution rate and rumen pool measurements Each 35-day period consisted of 14 days adjus tment to the diet; fol lowed bY 12 days sampling, for which straw was sprayed with Cr-EDTA (200 mg Cr/kg DM) a.nd * Dept of Animal Husbandry, University of Sydney, Camden, M.S.W 2570. ** Fielders Gillespie & Smith Pty Ltd, Tamworth, N.S.W 2340. . *** Dept of Genetics and Biometry, University of Sydney, N.S.W 2006. 131 Proc. Aust. Soc. Anim. Prod. Vol. 16 Yb(N03)3 (150 mg Yb/kg DM) to measure total digesta flow, rumen liquid and particulate matter flows. Individual faecal collections were made over days 1923, subsampled and dried at 50�C. A total of 12 rumen and abomasal samples each were collected at intervals distributed throughout the 24-h cycle over days 20-22. The feeding of marked straw was terminated on day 23 after which seven separate rumen samples were collected at 90-min intervals and assayed for Cr and Yb by atomic absorption spectrtophotometry. Estimates of fractional outflow rates (FOR) of rumen liquid and particulate matter were determined with reference to Cr and Yb dilution rates respectively. Rumen pool size was determined by manually emptying the entire rumen contents on day 26, mixing and subsampling for DM analysis. Betweendays 29 and 35, CW digestion of NaOH-treated straw was estimated by incubating straw, milled through 3 mm screen, in nylon bags (251;1 pores) within the rumen and withdrawing at 0, 3, 6, 9, 12, 24, 36, 48, 72 and 96 h. Bag residues were analysed for NDF. Other details and calculations of rate of CW degradation were as described by Amaning-Kwarteng et al. (1984b). Statistical analysis Animals within species were arranged in two 3 x 3 latin squares which were combined in a single analysis of variance to determine species (Sp), diet (D) and Sp x D effects. Treatment means were compared on the basis of least significant differences (Steel and Torrie 1960). RESULTS Total OMI, expressed as g/kg Woo9 or g/kg Wom7', were greater in cattle than in sheep (P<O.Ol) (Table 1.). Similarly, rumen pools of wet digesta and Table 1 Intakes, rumen pool and fractional outflow rate (FOR) of sheep (S) and cattle (C) fed NaOH-treated straw alone supplements of cotton-seed meal (WC) or barley (Values are means of six animals per treatment within from the rumen (W) or with (WB) species) 132 Proc. Aust. Soc. Anim. Prod. Vol. 16 Table 2 Apparent digestibilities of organic matter (OM), nitrogen (N) and cell wall (NDF), DOMD and calculated ME intakes relative to maintenance requirements (Mm) of sheep (S) and cattle (C) eating NaOH-treated straw diets+ (Values are means of six animals per treatment within species) rumen DM (not presented) were larger in cattle than in sheep and FOR of liquid and particulate matter greater in cattle than in sheep. Digestibility of N and DOMD were 13% and 6% respectively higher in sheep than in cattle, although cattle digested more fibre (5% higher) than sheep which was associated with higher rates of fibre digestion in cattle than in sheep (Table 2). Calculated ME intake relative to maintenance requirements was also higher in cattle (P<O.O5) than in sheep. Mean species differences in NDF digestibility were 5.8, 2.8 and 1.2 on diets W, WC and WB respectively but this interaction between species and diet was not significant. Supplements increased total OM intake, N digestibility and DOMD but had no effect on straw OM intake, wet rumen pool size and FOR. DISCUSSION Graham (1972) demonstrated that live weights expressed as kg Wloo and kg pp.75 were not valid units of metabolic body size in comparisons between sheep and cattle, and recommended the use of kg W 0.9 We observed, however, that voluntar feed intakes by sheep and cattle were similar per kg Wloo but different perkgW 0.5 found intakes to Se similar per and cattle (46 and 405 kg live weight respectively) were fed ryegrass (Lolium perenne) and switchgrass (Panicum vigatum). Such differences between trials may be associated with animal age and feed characteristics, and suggest that no single exponent of live weight may be appropriate for all studies of intake by sheep and cattle. The higher intakes (g/kg were a s iated with larger rumen ) and shorter retention agrees with the observations of (1973) and Prigge et al. (1984). l Woo9 and g/kg W't7 pool sizes (g/kg P71� times of liquid and Thornton and Minson 5 by cattle than sheep o) 0.9 and 8 g/kg f? particulate matter . This (1973), Laredo and Xinson 133 Proc. Aust. Soc. Anim. Prod. Vol. 16 Shorter retention time of solids in the rumen would reduce digestibility of fibre unless the rate of fibre digestion increased. We found that rate of fibre digestion was higher in cattle than in sheep so that overall digestion of fibre was higher in cattle than in sheep, despite shorter solid retention times in the Similar trends for sheep to digest protein more and fibre less than rumen. cattle have been observed by Aerts et al. (1984) and Prigge et al. (1984). When sheep and cattle were fed diets containing 70% ground hay, however, Van Rooyen et al. (1983) observed higher protein digestibility in cattle than in sheep and vice The faster rate of fibre digestion 'in sacco' in versa for fibre digestion. cattle observed by us could have been due to greater numbers of bacteria, different species of bacteria or positive contributions to fibre digestion by protozoa or fungi. The much lower apparent N digestibility in cattle would have contributed This species substantially to DOMD being lower in cattle than in sheep. difference in apparent N digestibility was partly accounted for by greater apparent N digestibility in the intestines of sheep (unpublished),which could be attributable to differences in microbial N contributing 81 and 68% of the total N at the abomasum in sheep and cattle respectively (unpublished).The relative contributions of protozoa and fungi were not determined in this study. Limitations associated with apparent N digestibility values due to metabolic faecal N would also affect N digestibility of cattle more due to their higher DM intake than sheep. The lower proportion of microbial N in abomasal N of cattle suggests less rumen degradation of dietary and endogenous protein in cattle which is likely to be the result of the higher FOR of liquid and particulate matter from the rumen of cattle than sheep. 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