Effect of concentrates in a hay diet on the contribution of anaerobic fungi, protozoa and bacteria to nitrogen in rumen and duodenal digesta in sheep

Abstract

Two sheep, cannulated at the rumen and the duodenum, were given diets of chopped orchard grass hay (H) and 3 parts grass hay to 2 parts concentrates (M). The the pools and flows of anaerobic fungi, protozoa, liquid-associated bacteria (LAB) and solids-associated bacteria (SAB) were measured. The anaerobic fungi contributed only 11–35 g N/kg of the microbial-N in the rumen and 7–27 g N/kg of the microbial-N flowing to the duodenum. Inclusion of concentrates in the diet increased fungal zoospore counts and biomass and, in sheep 1, resulted in a more than four-fold increase in rumen protozoal counts: the protozoal contribution to its microbial N increased from 41 to 66% in the rumen and from 4 to 15% in duodenal flow. For sheep 2, there was an almost two-fold increase in protozoal counts and their contributions to its rumen and duodenal microbial N increased from, respectively, 35 to 57% and 7 to 9%. These changes were associated with a decrease in the yield of rumen microbial-N from 31 to 25 g/kg OM digested in the stomach for sheep 1 but an increase from 37 to 42 g/kg OM digested in the stomach for sheep 2. In addition, there were decreases in non-ammonia-N (NAN) flow to and digestion in the intestines of sheep 1 but increases in sheep 2. Protozoa obtained 40–58% of their N from bacteria. When diaminopimelic acid was used as the marker, bacterial-N was overestimated in rumen digesta and appeared to be underestimated in duodenal digesta. The ratio of duodenal NA 15N to either rumen or duodenal LAB- 15N underestimated the contribution of microbial-N to duodenal NAN. The mean retention times (MRT) of protozoa were much longer than those of digesta particles, reflecting protozoal sequestration in the rumen. On the hay diet, the apparent MRTs of LAB were the same as those for SAB and particles whereas, when concentrates were included in the diet, the MRTs of LAB were similar to those of solutes in sheep 2 and between those of solutes and particles in sheep 1; apparent MRTs of SAB were always similar to particle MRT. These results and the calculated generation times of LAB and SAB were consistent with the conclusion that, on the hay diet, LAB were probably SAB “in transit” between particles but, with the inclusion of concentrates, they were predominantly starch-utilizing organisms whose proliferation in the rumen of sheep 1 was limited by predation by its large protozoal population. Ammonia was produced in and absorbed from the omasum and the 15N-incorporation data were consistent with the growth of LAB in the omasum.