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Centre for Nutrition Modelling, Department of Animal and Poultry Science, University of Guelph, Ontario, N1G 2W1 Canada
2 Corresponding author: jcant{at}uoguelph.ca
Lactating cows were exposed to large amino acid imbalances and deficiencies by i.v. infusion to characterize responses in milk production and plasma concentrations of metabolites and hormones. Six cows in early lactation were fed a basal diet of 9% CP and infused continuously for 6 d with saline (negative control), 1.1 kg/d of a complete amino acid mix (positive control), or the equivalent mix lacking Met, Lys, His, or all 3 branched-chain amino acids. All cows received all treatments in 6 successive periods in a Latin square design. Infusion of the complete amino acid mix resulted in an increase in the plasma concentrations of several essential amino acids, insulin, and glucagon. Milk protein production was stimulated by 19%, which accounted for 10% of the infused amino acid. Plasma urea, acetate, and ß-hydroxybutyrate concentrations were increased. Compared with saline, the amino acid mixtures lacking Met, Lys, or His increased essential amino acids, glucose, insulin, and glucagon concentrations in plasma, and decreased growth hormone. Plasma concentration of the essential amino acid absent from the infusate fell 2-fold but milk protein yield remained within 12% of its basal value. Dry matter intakes were depressed 35% over the first 2 d of infusion of imbalanced mixtures but recovered thereafter. Milk fat yields were increased 258 and 320 g/d by mixtures devoid of Lys and His, respectively. Correction of a Met, Lys, or His deficiency did not affect hormone concentrations in plasma and milk protein yield increased 27% due entirely to increased concentration of the single amino acid in plasma. Although imbalance and deficiency generated similar amino acid profiles in plasma, it was concluded that endocrine responses to total amino acid supply during imbalance can override imperfections in the circulating amino acid profile to maintain milk protein yield at higher levels than expected from deficiency states. Both imbalance and deficiency were characterized by a low protein:fat ratio in milk. Infusion of a mix of amino acids lacking Val, Ile, and Leu, despite a decrease in plasma Leu to 58% of its basal value, increased milk protein and fat yields to the same extent as the complete amino acid mix.
Key Words: amino acid • milk composition • plasma metabolite • hormone
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