Variation in milk protein concentrations associated with genetic polymorphism and environmental factors.

Concentrations of alpha s-casein, beta-casein, kappa-casein, beta-lactoglobulin, alpha-lactalbumin, serum albumin, and immunoglobulin in milk from 1888 Holstein cows were determined monthly over the lactation period. Cows were phenotyped for genetic variants of alpha s1-casein, beta-casein, kappa-casein, and beta-lactoglobulin. Least squares analyses showed variations in individual proteins due to parity number, month of test, stage of lactation, somatic cell count, fat content, milk yield, and phenotypes of cows for milk proteins. beta-Casein declined and serum proteins increased with advancing age of cows. Concentration of individual proteins decreased during the first 2 to 3 mo in lactation and then increased as lactation progressed. alpha s1-Casein variants significantly affected concentrations of alpha s-casein (BC greater than BB greater than AB) and beta-lactoglobulin (AB greater than BB greater than BC). Variant B for beta-casein is associated with lower alpha s-casein, beta-lactoglobulin, immunoglobulins, and higher beta-casein and alpha-lactalbumin concentrations than variant A1, A2, or A3. Milk from BB kappa-casein, and BB beta-lactoglobulin cows contained more alpha s-casein, kappa-casein, and less beta-lactoglobulin than milk from AA cows for the two proteins. Concentrations of all proteins were negatively correlated with milk production. Increased somatic cell counts were associated with lower beta-casein and higher concentrations of other proteins. Fat content of milk was positively correlated with the three casein fractions and beta-lactoglobulin.