Environmental influences on protein content and composition of bovine Milk.

A total of 24,405 individual milk samples from approximately 2,800 Holstein-Friesian cows in 63 dairy herds enrolled in the Quebec Dairy Herd Analysis Service program were analyzed monthly for protein, serum protein, casein, and somatic cell counts during 17 mo. Unadjusted means for protein, serum protein, and casein were 3.396% +/- .002, .687% +/- .001, and 2.708% +/- .002. Least squares analyses showed significant effects of calendar month of test, stage of lactation, age of cow, and somatic cell count on milk protein content and composition. Total protein, casein, and serum protein contents of milk showed a generally increasing trend from July to December. These components were highest during the first 10 days in lactation when means were 3.81%, 3.05%, and .76% and reached a minimum at 2 mo in lactation to give corresponding means of 3.08%, 2.46%, and .62%. The proportion of casein in milk protein decreased as cows became older. For every unit of increase in log somatic cell count there was an increase in protein content of 0.99%, mainly from change in the serum protein fraction. The same change in somatic cells would decrease ratio of casein to protein by 2.79%.