METABOLIZABLE ENERGY AND ABSORBED PROTEIN-REQUIREMENTS FOR MILK COMPONENT PRODUCTION

Metabolic pathways of milk component synthesis were used to estimate m etabolizable energy and absorbed protein requirements for lactation. A mounts of ATP and AA used for synthesis of each component from absorbe d substrates were determined. Coefficients were adjusted to account fo r additional inefficiencies and to define requirements in terms of die tary supply based on NRC energy requirements and N balance data. Assum ing that 10% of glucose required was derived from AA, metabolizable en ergy and absorbed protein requirements were 6.02 Mcal and .1 36 kg/kg of lactose, 13.43 Mcal and .127 kg/kg of fat, and 7.57 Mcal and 1.069 kg/kg of protein, respectively; an additional .144 Mcal/kg of milk was required for milk volume. For production of milk containing 4.8% lact ose, 3.5% fat, and 3.3% protein, absorbed protein required for lactose and fat may account for 14.1 and 9.6%, respectively, of total absorbe d protein required for milk production. Efficiency of protein utilizat ion for milk protein synthesis may be as high as 90% when 10% of gluco se requirements must be supplied by AA. Expressing nutrient requiremen ts for lactation on a component basis enables calculation of requireme nts for milk production of any composition and does not rely on correl ations between major milk components.