Amino acids truly digestible in the small intestine : the AADI system for the dairy cow

Abstract

This new AADI system is integrated into the PDI system by estimating the composition in amino acids of the PDI. The feedstuff values are estimated from 1/ the amount of duodenal microbial and undegraded feed protein, 2/ their amino acids (AA) composition and 3/their intestinal digestibility. This estimate was validated by comparison to the AA profile of digestive contents from 133 diets. As a consequence, some adjustments should be made to improve the initial estimates; relevant equations are given. This work permitted to draw up a table of nearly 700 feedstuffs giving contents of intestinally digestible amino acids for eachof 9 essential AA. These values were used to establish the dose-responsecurves of milk protein production and content to different levels of several AADI. The recommended allowances for any AA are given as a percentage of PDIE supply. In dairy cows, methionine and lysine are generally most limiting AA; recommended allowances could be established with some confidence at 7.3 % LysDI %PDIE and 2.5 MetDI % PDIE. However practical and economical considerations led to the idea that onecould pay attention to LysDI and MetDI short ages mainly when supplies are below critical levels set at 6.8% LysDI and 2.1 % MetDI. The values for other AADIare not so well established; they could be at least 8.8% for LeuDI and within the range 2.5-3.2 % for HisDI, that are possibly limiting in some situations.