Relative intakes, digestibility and bacterial protein synthesis by sheep and cattle fed high-roughage diets

Abstract

Six each of yearling Merino and Border Leicester crossbred wethers (35.9 ± 0.67 kg liveweight) and yearling Hereford heifers (226.7 ± 5.90 kg liveweight), all fitted with rumen and abomasal cannulae were offered coarsely-milled, NaOH-treated wheat straw ad libitum, either alone (W) or with supplements of cotton-seed meal (WC) or barley (WB). Animals within species were arranged in two 3 × 3 latin squares which were combined in a single analysis of variance to determine species (Sp), diet (D) and Sp× D effects. Voluntary dry matter intakes by sheep and cattle were similar when expressed as g/kg liveweight (W) 1.0 (30.4 and 28.7, respectively), but were higher in cattle when expressed as g/kg W 0.9 ( P < 0.05) and g/kg W 0.75 ( P < 0.01). Cell wall (CW) digestibility and calculated metabolizable energy intakes (MEI) relative to maintenance requirements (Mm) were higher in cattle than in sheep ( P < 0.05). The relationship between relative metabolizable energy intakes by cattle and sheep was: y = 0.79 (± 0.195) + 0.64 (± 0.120) x, r = 0.798 ( P < 0.05), where y = MEI Mm for cattle and x = MEI Mm for sheep, Sy· x = ± 0.0164. Sheep digested more nitrogen (N) ( P < 0.01) and had 6.7% higher digestible organic matter in dry matter (DOMD) values than cattle. The higher intakes by cattle (g/Kg W 0.9 and g/kg W 0.75) were associated with larger rumen pool sizes (g/kg W 0.9 and g/kg W 0.75) and higher liquid fractional outflow rates from the rumen of cattle. Higher CW digestibility by cattle was associated with a faster CW digestion when measured by incubating straw in nylon bags within the rumen. The higher apparent N digestibility in the whole tract of sheep was associated with a higher proportion of bacterial N in abomasal N of sheep (81 compared to 68% in cattle), and was accounted for by greater apparent N digestibility in the intestines of sheep. The much lower N digestibility in cattle would have contributed substantially to DOMD being lower in cattle than in sheep. It is apparent that measurements of nutritional value of alkali-treated straw diets should be made with the animal species for which the diets are intended.