Manipulation of amino acid supply to the growing ruminant

Quality of protein (indicated by amino acid [AA] composition) that enters the small intestine (SI) of growing ruminants is dictated largely by the AA composition of microbial protein. The AA supply is well-balanced and, although individual AA can be experimentally determined to be first- or second-limiting, it seems that several AA (sulfur AA, lysine, histidine, and possibly threonine, valine, and isoleucine) may be colimiting in many circumstances. Quality of the postruminal AA supply can be altered by increasing (maximizing) net microbial protein synthesis, manipulating supplemental protein source, or feeding ruminally protected AA. Defaunating the rumen increases postruminal AA supply by increasing flow of both bacterial and nonbacterial AA. Defaunation has little effect on proportions of individual AA entering the SI. Different feed proteins vary greatly in the quantity of individual AA that they supply for absorption from the SI. Most proteins are a poor source of at least one essential AA; feeding combinations of proteins may be the most practical approach to supplying AA in optimal proportions. Feeding individual ruminally protected AA can alter the profile of AA reaching the SI, but work is needed to identify dietary conditions under which use of such products will be most beneficial.