The Roles of Amino Acids in Milk Yield and Components

Unlike the situation in monogastric species (Fuller et al., 1989), it has proven more difficult to predict with accuracy the amino acid (AA) requirements of ruminants for growth and milk production. The current NRC publications for dairy and beef cattle recognise this, and do not explicitly allow for diets to be balanced for essential (EAA) or limiting AA contents, except to recommend a ratio of 3:1 lysine to methionine in metabolizable protein (NRC 2000, 2001). With greater pressures being placed on dairy and beef production to reduce nitrogen wastes and the dairy industries desire to manipulate milk components to meet market demands, the need is there to define not just the limiting AAs, but the general requirement for all AA for production. Current feeding schemes sub-divide EAA requirements into net requirements for maintenance functions, i.e., the first priority, and for growth, lactation and/or reproduction. Therefore, the metabolizable protein requirement reflects only the roles AA serve as precursors for synthesis of proteins, e.g. muscle, milk caseins, conceptus. Conceptually, this approach is satisfactory since a major component of the metabolizable protein prediction will largely be determined from the AA composition of endogenous protein losses, and that needed for net synthesis of muscle, milk and fetal tissue proteins. Where there is less of a conceptual basis and agreement are the conversion or “efficiency” factors used to partition AA in the metabolizable protein to the various fates. It could be argued, therefore, that defining the basis of the “inefficiency” portion of the conversion factors is a goal that holds much greater potential for improving production and predictions than determining the pattern of limiting AA.