Supplementing barley or rapeseed meal to dairy cows fed grass-red clover silage: II. Amino acid profile of microbial fractions

Four ruminally cannulated dairy cows were used to examine the effect of diet on the AA composition of rumen bacteria and protozoa, and the flow of microbial and nonmicrobial AA entering the omasal canal. Cows were offered grass-red clover silage alone, or that supplemented with 5.1 kg DM of barley, 1.9 kg DM of rapeseed meal, or 5.1 kg DM of barley and 1.9 kg DM of rapeseed meal according to a 4 x 4 Latin square design with a 2 x 2 factorial arrangement of treatments. During the first 10 d of each period, cows had free access to silage and, thereafter intake was restricted to 95% of ad libitum intake. Postruminal digesta flow was assessed using the omasal canal sampling technique in combination with a triple marker method. Liquid- (LAB) and particle- (PAB) associated bacteria were isolated from digesta in the reticulorumen and protozoa from digesta entering the omasal canal. Microbial protein flow was determined using 15N as a microbial marker. Flows of AA entering the omasal canal were similar in cows fed silage diets supplemented with barley or rapeseed meal. However, rapeseed meal increased nonmicrobial AA flow while barley increased the flow of AA associated with LAB and protozoa. Diet had negligible effects on the AA profile of microbial fractions. Comparison of AA profiles across diets indicated differences between LAB and PAB for 10 out of 17 AA measured. Rumen bacteria and protozoa were found to be different for 14 out of 15 AA measured. For grass silage-based diets, energy and protein supplementations appear to alter postruminal AA supply through modifications in the proportionate contribution of microbial and nonmicrobial pools to total protein flow rather than as a direct result of changes in the AA profile of microbial protein.