Partitioning of amino acids flowing to the abomasum into feed, bacterial, protozoal, and endogenous fractions

We partitioned the flow of amino acids (AA) to the abomasum among rumen und egradable protein (RUP) and bacterial, protozoal, and endogenous fractions using four Holstein cows in midlactation that were equipped with ruminal an d abomasal cannulas. A 2 x 2 factorial design with four diets, combinations of high or low ruminally degradable organic matter, and rumen degradable p rotein, was employed. Crude protein (CP) and AA contents of ruminal bacteri a and protozoa and abomasal digesta were determined. Equations for the sour ce compositions and in vivo flows of CP and 16 AA were then solved simultan eously with a linear program to estimate the contribution of RUP, bacterial , protozoal, and endogenous CP to AA flows. The flows of RUP and bacterial AA were not affected by diet. Low dietary RDP increased the flow of protozo al AA to the abomasum, but the ruminally degradable organic matter content of the diet did not affect protozoal AA flow. Across diets, RUP, bacterial, protozoal, and endogenous fractions provided 55, 33, 11, and <1% of the CP , and 62, 26, 12, and <1% of the AA that reached the abomasum. The linear p rogram was a useful tool for partitioning AA that flows to the abomasum. Th e technique may also allow dietary effects on ruminal microbes and the AA p rofile of protein flowing to the duodenum to be better understood and perha ps manipulated.