Small intestinal digestibility of microbial and endogenous amino acids in dairy cows

The small intestinal digestibility of microbial amino acids and the apparen t re-absorption of endogenous amino acids were estimated in three ruminal, duodenal and ileal fistulated lactating dairy cows. The cows were assigned to a 3 x 3 Latin square and fed diets low in amino acids, composed of straw , starch, molasses and urea. The treatments were three straw to concentrate ratios. The digestibilities and re-absorptions were estimated from duodena l and ileal flows of amino acids from feed, microbial and endogenous source s. The duodenal distribution of amino acids between different origins was d etermined by the difference method and by the amino acid profile method. Th e ileal distribution was estimated by a simple method where the feed fracti on was estimated by the mobile bag method, the endogenous fraction as the w ater soluble fraction and the microbial fraction as the rest. In ileal samp les, 8.0 +/- 1.1, 52.2 +/- 2.9 and 39.8 +/- 2.6% of total amino acid nitrog en (AAN) were of feed, microbial and endogenous origin, respectively, compa red with 5.1 +/- 0.9, 61.6 +/- 6.6 and 32.7 +/- 6.9% in duodenal samples. T he small intestinal digestibility of microbial AAN was estimated to be 75.1 +/- 1.8% and the apparent re-absorption of endogenous AAN to be 62.3 +/- 1 1.1%.