Factors Affecting Amino Acids in Blood of Dairy Cows

Abstract

Concentrations of amino acids in blood plasma were studied in two experiments with lactating Holstein cows. In Experiment 1, 30 cows were allotted to five treatments varying in protein percent and source. Cows were bled during wk 4 and 9 of treatment at 0, 2, and 5h after feeding. In Experiment 2, 68 cows were assigned to four rations. One contained 12.5% crude protein from natural sources and another 16.0%. Two other rations contained 16.0% crude protein (12.2% natural and 3.8% from nonprotein nitrogen). Blood was collected at 42 days postpartum 2 to 3h after feeding. Seven cows from each treatment also were sampled 14 days prepartum and 126 days postpartum.Cows fed 9.3% crude protein had lower branched-chain amino acids and higher glycine in plasma than those fed 13 to 14%. High yielding cows in mid-lactation (140 to 220 days) showed decreased branched-chain amino acids and increased glycine in plasma compared to low yielders. Production did not alter amino acids in early lactation (42 days). Several significant changes in amino acids of plasma were related to stage of lactation, but groups of acids did not follow consistent patterns. Samplings 2h after feeding had lower branched-chain and aromatic amino acids than samplings at 5h. Ratios of essential to nonessential amino acids were depressed by rations of low natural protein, by high milk production in early lactation, and in samples taken 2h postfeeding. Estimates of minimum blood flow to the mammary gland needed for output of amino acids in milk identified methionine, phenylalanine, threonine, and lysine (in that order) as those in lowest supply. Ratios of essential amino acids of plasma to milk also suggested limited availability of these amino acids.