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Journal of Dairy Science Vol. 59 No. 10 1752-1757
© 1976 by American Dairy Science Association ®
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Effect of Intravenous L-Isoleucine Infusion upon Concentration of Free Isoleucine in Milk

Ovidiu Popescu1, Milton J. Allison and Richard W. Brown

National Animal Disease Center, North Central Region, Agricultural Research Service, P. O. Box 70 Ames, IA 50010

ABSTRACT

The growth of Streptococcus agalactiae in milk is inhibited by elevated free isoleucine. Free amino acid concentrations in both plasma and milk from three lactating dairy cows were studied after intravenous infusions of L-isoleucine at 62, 300, 380, and 840 µmol/kg of body weight. Free isoleucine was lower in milk than in plasma. The concentration of free isoleucine in plasma increased after infusions of isoleucine at the three higher amounts. The highest concentration of free isoleucine in milk measured was 1.5 µmol/100 ml of skim milk. This was observed after infusion of isoleucine for 12 h at 380 µmol/kg of body weight. Free isoleucine in both plasma and milk dropped to the preinfusion concentrations a short time after the isoleucine infusions were stopped. When relatively large quantities of isoleucine were infused into the blood of cows, free isoleucine in blood increased as much as twofold. The source of free isoleucine in milk appeared to be free isoleucine in plasma, but isoleucine in milk only reached concentrations that would give about 50% of the growth inhibition of S. agalactiae obtainable with isoleucine in vitro. It would be difficult to elevate free isoleucine in milk by dietary manipulation sufficiently to inhibit S. agalactiae.

FOOTNOTES

1 Institute of Agronomy, IASI 6600, Romania. Recipient of a Fulbright-Hays grant which was administered through Iowa State University.






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Copyright © 1976 by the American Dairy Science Association ®.