Factors Affecting Amino Acids in Blood of Dairy Cows

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Abstract
Concentrations of amino acids in blood plasma were studied in 2 experiments with lactating Holstein cows. In experiment 1, 30 cows were allotted to 5 treatments varying in protein percent and source. Cows were bled during wk 4 and 9 of treatment at 0, 2 and 5 h after feeding. In experiment 2, 68 cows were assigned to 4 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 N). Blood was collected at 42 days postpartum 2 to 3 h after feeding. Seven cows from each treatment 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 (140-220 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. Samples 2 h after feeding had lower branched-chain and aromatic amino acids than samplings at 5 h. 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 2 h 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 suggested limited availability of these amino acids.
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