Milk Production and Nitrogen Utilization in Response to Postruminal Infusion of Sodium Caseinate in Lactating Cows

Five lactating, rumen-fistulated Holstein cows were used to obtain additional information concerning the effects of postruminal infusion of sodium caseinate on milk production and amino acid utilization. A 7-day continuous abomasal infusion of approximately 450 g/day of sodium caseinate was preceded and followed by 7-day infusions of an isonitrogenous-isocaloric solution of glucose, monosodium glutamate, and urea. Total collections of milk, urine, and feces were obtained during the last 5 days of each infusion period. On the last day of each period, arterial and mammary venous blood samples were obtained for analysis of plasma free amino acids. During infusion of sodium caseinate, milk production, milk protein (NX 6.38) production, and efficiency of nitrogen utilization for milk crude protein production were increased. Arterial plasma concentrations of free histidine, isoleucine, phenylalanine, valine, and total essential amino acids were elevated above control levels during infusion of sodium caseinate, while ornithine and tyrosine were decreased. Calculation of the relative concentration of essential amino acids in arterial plasma and in milk protein indicated that methionine and lysine were least abundant in plasma relative to their requirement for milk protein synthesis. A high percentage extraction from arterial plasma by the mammary gland also suggested that methionine and lysine may have been the essen tial amino acids in most critical supply for milk protein synthesis. Calculation of up take to output ratios of individual plasma amino acids by the mammary gland sug gested that significant quantities of extracted arginine, isoleucine, leucine, threonine, and valine were utilized in pathways other than direct incorporation into milk protein. J. Nutr. 105: 1111-1121, 1975.