The effects of protozoa and of supplementation with nitrogen and sulfur on digestion and microbial metabolism in the rumen of sheep

Abstract

Two experiments were undertaken to study the effects of protozoa on sulfur and nitrogen availability and on fermentation and the composition of bacteria in the rumen of sheep. In Experiment 1, 12 faunated and 12 fauna-free sheep were offered a basal diet of chopped wheaten straw with or without sulfur (S) and urea-nitrogen (N) supplements. Sulfur supplementation increased the rate of straw digestion and the concentration of volatile fatty acids (VFA) in the rumen while reducing methane production. The presence of protozoa did not significantly affected this response, although it increased rumen H2S concentration. In contrast, the response of rumen fermentation to a urea supplement was affected by the presence of protozoa. Unsupplemented faunated sheep had a faster rate of in-sacco straw digestion in the rumen than did fauna free sheep (44 v. 36%DM/day). Supplementary urea increased the rate of in-sacco digestion of the basal ration in fauna free sheep (36 to 42%DM/day) but not in the faunated sheep (44 to 46%DM/day), suggesting that N availability was greater in the rumen of faunated sheep. Ammonia and total VFA concentrations in the rumen were not affected by protozoa, but the molar proportions of butyrate and isoacids in rumen VFA were greater in faunated sheep. Bacteria from the rumen fluid of faunated sheep contained a higher proportion of lipid and a lower proportion of N in their cell DM. In a second experiment, the chemical composition of rumen bacteria of faunated and fauna free sheep was further investigated. In both groups, bacteria associated with the particle-phase of digesta contained a higher proportion of lipid and a lower proportion of N than did fluid-phase bacteria. Fluid-phase bacteria from faunated sheep tended to have more lipid and less N in their DM than did those from fauna-free sheep. It was concluded that the presence of protozoa enables sustained fermentation of diets low in rumen available nitrogen and also increases the lipid content of rumen fluid-phase bacteria.