AbstractTwo experiments are reported. In the first experiment the amino acid compositions of rumen bacteria and protozoa isolated from sheep given a diet of grass silage were compared with those taken from sheep given a diet of hay and barley; four sheep were assigned to each diet. There were no significant differences (P<0·05) between diets in the amino acid compositions of the microbial fractions and bacterial contents of α‐ɛ‐diaminopimelic acid were also similar for both diets.In the second experiment the digestion of a grass silage and barley diet (65:35; 135 g CP (kg DM)‐1) was studied using four non‐lactating Ayrshire cows fitted with cannulae in the rumen and in the proximal duodenum. The mean N intake was 154 g d‐1and the corresponding duodenal flow was 126 ± 10 g d‐1indicating that 0·21±0·07 of the N intake was absorbed between the mouth and the duodenum. The mean rumen NH3‐N concentration was high, 297 g litre‐1, and the mean rate of microbial protein synthesis in the rumen was low, 17 g microbial N (kg OM apparently digested in the rumen)‐1. There was a low content of bacterial N in the non‐ammonia N at the duodenum (mean proportion 0·52±005) and low concentrations of methionine and lysine in the duodenal digesta protein. The results are discussed in relation to previously published data on the digestion of silage diets in sheep and to the utilization of silage diets for milk production in the cow.It is concluded that with silage diets the supply of methionine and lysine to the duodenum is likely to be low because of the low rates of microbial protein synthesis in the rumen and that in cows those two amino acids may be