Rumen digestion and microbial protein synthesis by growing lambs fed high-concentrate diets: Effects of cereal processing and animal age

Rumen digestion and microbial protein synthesis were studied using 36 growing lambs given either free choice access to separate feeders with whole barley grain and a pelleted soybean meal-based supplement (treatment WB) or a pelleted compound feed (treatment C) made by combining ground barley (0.65) with the same protein supplement (0.35). Free access to barley straw was offered in treatment C but not in WB. While both treatments were imposed before and after weaning (at 42 days), for a third treatment the compound feed was replaced after weaning by whole barley grain and the protein supplement without access to straw (treatment CWB). Six lambs from each treatment were slaughtered at 10 and 30 days post-weaning after ¹⁵N-labelling of microbial N and abomasal digesta flows were estimated using C₃₁ alkane as marker. Processing of barley grain increased (P<0.05) the apparent digestibility of dry matter in the rumen irrespective of age (0.53, 0.48 and 0.43 (S.E. 0.027) for C, CWB and WB). Ruminal pH averaged 5.5 (S.E. 0.06) regardless of cereal processing. The molar ratio of acetate to propionate decreased with age reflecting a higher proportion of grain in the mixed diets (WB and CWB) at 30 than 10 days post-weaning. Even with similar dietary proportions of grain and protein supplement, the acetate to propionate ratio was lower for lambs fed the free-choice (WB and CWB) vs. those fed the compound (C) diet (1.3 vs. 2.2 (S.E.D. 0.37)). Ruminal ammonia concentration increased with age and was lower for treatment C compared with treatments WB and CWB (24 vs. 127 (S.E.D. 22.2) mg/L), reflecting the higher protein intake of the latter two treatments. However, recovery of N intake as abomasal non-ammonia N was low for all treatments due to high protein intake and inefficient microbial growth compared to values reported for mixed diets. The efficiency of microbial N synthesis did not differ between treatments but it was negatively correlated (r = ?0.73) with the organic matter apparently digested in the rumen, resulting in similar microbial yields in spite of the lower digestion in the rumen of whole barley diets. Feeding whole barley is thus a useful strategy to modify the site of digestion in intensive lamb fattening, allowing to reduce ruminal fermentation without depressing microbial N yield.