Rumen microbial protein synthesis and proportions of microbial and non-microbial nitrogen flowing to the intestines of sheep

Abstract

Microbial protein synthesis in the rumen of mature sheep was measured by a technique dependent upon the incorporation of 35S from radioactive sulphate into microbial sulphur amino acids. In two separate experiments, sheep were fed on four dried forages and three fresh forages. Mean values and standard deviations for microbial protein synthesis per mole of volatile fatty acid produced in the rumen were 16.1 ? 3.4 g and 20.4? 2.3 g for dried and fresh forages respectively. Corresponding values for microbial protein synthesized per 100 g of organic matter digested in the rumen were 15.1 ? 3.6 g and 24.6 ? 4.9 g. Turnover constants for microbial protein were 1.06 ? 0.12 day-1 and 1.42 ? 0.10 day-1 when dried and fresh forages respectively were eaten, and it is suggested that efficiency of microbial protein synthesis was dependent on the rate of digesta flow through the rumen. Of the non-ammonia nitrogen (NAN) reaching the duodenum up to 41, 53 and 68% was non-microbial in origin when perennial ryegrass, Tama ryegrass and white clover respectively were eaten fresh-cut. When either subterranean clover hay, lucerne hay, wheaten hay or a mixture of wheaten hay plus wheat straw was eaten, up to 67, 57, 52 and 57% respectively of NAN at the duodenum was non-microbial in origin. In the latter two cases, the total flow of NAN was relatively low but the apparent digestibility of the non-microbial fraction in the intestines was still appreciable.