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The 35S content of ruminal microbial protein following intraruminal infusions of Na235SO4 was used to estimate the relative proportions of microbial and dietary protein in the duodenal digesta of sheep, and hence the proportion of the dietary protein which escaped microbial proteolysis in the rumen. When sheep were given a protein-free diet, the mean ratio of the specific activities of the sulphuramino acids in the duodenal microbial fraction (M) and the unfractionated duodenal digesta (D) was 0.97 �_ 0.08 and 1.00 �_ 0.06 in two separate experiments. When zein was included in the diet the ratio M/D increased to 2.35 �_ 0.14, which indicated that 44�_ 3.4% of nitrogen in the duodenum was microbial. When casein was offered, the ratio M/D was 1.12 �_ 0.05, which indicated that 91 �_ 6.7 % of duodenal nitrogen was microbial. When sheep were offered a partially purified diet containing fish meal, lupin meal, peanut meal or soya bean meal, the ratio MID indicated that 29,65,63 and 39 % of the fish meal, lupin meal, peanut meal and soya bean meal, respectively, was degraded in the rumen; rumen ammonia levels (mg nitrogen/100 ml fluid) were 8.3, 20.9, 16.0 and 6.5, and nitrogen balances were 5.0, 2.4, 2.9 and 5.4 g/day respectively. The 35S-incorporation technique appears to provide realistic estimates of the proportions of dietary proteins which escape degradation in the rumen. |
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