Effects of Insulin and Amino Acids on Milk Protein Concentration and Yield from Dairy Cows

¹ Department of Animal Science, Cornell University, Ithaca, NY 14853-4801

Our study investigated the effect of insulin on the regulation of milk protein synthesis in well-fed cows (n = 4) with or without additional amino acids (AA). The design was a two-way crossed factorial with two 12-d periods involving abomasal infusions of either water or a mixture of casein (500 g/d) plus branched-chain AA (88 g/d). During the last 4 d of each period a hyperinsulinemic-euglycemic clamp was performed; insulin was infused at 1.0 µg·kg of BW^–1·h^–1 to increase circulating levels fourfold, and euglycemia was maintained by infusion of glucose. Cows were fed a diet formulated to exceed requirements for metabolizable energy and protein. During abomasal water infusion, the insulin clamp increased milk protein yields by 15% (+128 g/d); when combined with abomasal infusion of casein plus branched-chain AA, milk protein yield was increased by 25% (+213 g/d). These increases resulted from equivalent increases in milk protein concentration and milk yield. Concentrations of casein and whey proteins in milk were increased by insulin clamp treatments; however, there were no major changes in the relative proportions of individual casein and whey proteins. Plasma concentrations of essential AA were reduced (–33%) during the insulin clamp treatments; effects were most dramatic for the branched-chain AA (–41%) and their keto acids (–45%). Results confirm the important regulatory role of the endocrine system in milk protein synthesis and demonstrate this potential to produce milk protein is not fully expressed.

Key Words: milk • protein • insulin • amino acids

Submitted on December 7, 1998
Accepted on March 19, 1999

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