Effects of energy and protein supply on milk protein yield responses in dairy cows

Consumers demand production of animal protein with minimal environmental impact, which requires improving the efficiency of conversion of feed nitrogen into milk and meat protein. Accurate determination of animal protein requirements and evaluation of dietary protein supply are critically important for optimizing production with minimum nitrogen input in dairy production systems. As milk nitrogen efficiency is inversely related to dietary nitrogen input, diets with greater protein concentration predictably result in lower milk protein conversion efficiency. Most current feeding systems, however, assume a constant efficiency of conversion of metabolizable protein, above maintenance requirements, into milk protein. Thus, milk protein yield is overestimated at high and underestimated at low protein intakes. This analysis discusses milk protein responses to dietary energy and protein supply in dairy cows and suggests improvements to current milk protein yieldpredicting models. A meta-analysis of studies with post-ruminally infused casein showed transfer efficiency of casein into milk protein of 0.29, which is clearly below default values of conversion of metabolizable protein into milk protein in protein evaluation systems. This decreased efficiency can be partially explained by enhanced liver metabolism and use of absorbed amino acids as an internal energy source by the mammary gland. Previous meta-analysis showed that milk protein yield can be predicted better from energy than from metabolizable protein supply, which along with substantial and consistent production responses to post-ruminal casein infusions suggests that inaccurate estimation of metabolizable protein supply, particularly from feed, contributes to variable efficiency of metabolizable protein utilization. Data from milk production and nutrient infusion studies demonstrate an important role of energy supply in transferring metabolizable protein into milk protein. An empirical model based on energy intake and feed metabolizable protein predicted milk protein yield responses better than models based on the supplies of metabolizable protein or metabolizable energy, demonstrating the significant role of energy in protein utilization. Increased energy supply provides other nutrients, glucose and acetate, that are needed to support increased milk production in addition to amino acids. Future models should integrate the concepts of nitrogen metabolism in the digestive tract and tissue metabolism of amino acids and translate this into mechanistic models that are capable of predicting milk protein yield and efficiency of nitrogen utilization for wide ranges of dietary inputs and genetic potential of the cows. To develop such models that are better than current empirical models, not only understanding of whole body amino acid metabolism must be improved, but also more accurate estimates of the supply of absorbed, especially feed, amino acids are needed.