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Use of Plasma Amino Acid Concentration to Identify Limiting Amino Acids for Milk Production¹

Departments of Dairy Science and Biochemistry, University of Wisconsin, Madison 53706

ABSTRACT

The hypothesis that an essential amino acid will accumulate in blood plasma only when supplied in excess of requirement was the basis of an attempt to identify essential amino acids most likely limiting lactation. In a 5 X 5 Latin-square experiment, five lactating cows were fed a corn-based basal ration containing 9.0% crude protein. Casein treated with .8% wt/wt formaldehyde to prevent protein breakdown by ruminal microorganisms was fed in five quantities to give diets containing 9.0 (basal), 11.2, 13.5, 15.7, and 18.0% crude protein (dry matter basis). Milk production and milk crude protein content for each of the five treatments were: 20.4 kg and 2.98%; 22.1 and 3.04; 24.7 and 3.14; 26.8 and 3.17; and 25.7 and 3.15. This indicated that amino acid supply was inadequate at the two lowest but adequate at the two highest percents of dietary protein. Comparison of milk samples obtained at the lowest and highest protein feeding indicated that 82% of the increase in milk nitrogen content was accounted for as true protein (trichloroacetic acid-precipitable nitrogen). There was little evidence of change in the amino acid composition of milk protein with increased protein intake. Essential amino acid concentrations in jugular and mammary vein plasma for cows at all intakes of formaldehyde-treated casein indicated that methionine, valine, and lysine were the most likely limiting amino acids for milk production.

¹ Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison, and by Hatch Project 1891.

² Present address: Department of Animal Science, Texas A and M University, College Station, TX 77843.

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