System of equations for fulfilling net energy and absorbed protein requirements for milk component production.

Objectives were to develop a system of equations for formulating rations to meet absorbed protein requirements, to define NEL requirements for individual milk components, and to evaluate the effects of changes in milk composition on requirements for absorbed protein and NEL recommended by NRC or by a proposed system based on milk components. By combining parameters in the NRC degradable protein system, a set of two equations was derived that can be solved to meet absorbed protein requirements when either ruminally available protein or energy limits microbial protein synthesis. Heats of combustion were used to estimate NEL requirements for milk components. Maintenance requirements for absorbed protein and NEL were obtained from NRC. To eliminate the dependence of DMI prediction on milk fat concentration, as assumed by NRC, DMI was estimated using the NDF concentration of the feed and the NEL requirement of the cow. The proposed system of requirements, based on milk components and the matrix of equations for meeting absorbed protein requirements, not only accounts for differences in the degradability and microbial yield of feeds but also more logically matches differences in nutrient requirements for milk components associated with changes in milk composition.