Rumen-protected amino acids for dairy cattle: Progress towards determining lysine and methionine requirements

Progress has been made in determining lysine (Lys) and methionine (Met) requirements of the lactating dairy cow, Dose-response relationships for both amino acids (AA) have been obtained by increasing postruminal supplies of Lys and Met in graded fashion via abomasal infusion while production responses and AA flows to the duodenum are measured and by an indirect dose-response approach. From the established dose-response relationships, it is concluded that for cows fed conventional diets, Lys must contribute 14.7-16.7% and Met 5.3-5.6% of total essential AA in duodenal digesta (7.0-8.0% and 2.5-2.7% of total AA, respectively) for maximum content and yield of milk protein, However, further research must be directed toward the refinement of these requirements, particularly for the early lactation cow, The attention given to Lys and Met thus far also must be given to other potentially limiting AA and rumen undegradable intake protein sources appropriately balanced for them if the maximum benefits of optimizing Lys and Met nutrition are to be realized, Because the production response to graded levels of an AA is one of diminishing returns, the ''practical'' requirements to which one formulates will be governed by economic considerations. Production studies that are designed to determine the value of optimizing intestinal AA balance must be initiated at or before parturition. Only in this way can the full effects on herd health and lactational performance be realized.