Relationships of milk protein types to lifetime performance.
Data from 889 cows at five research stations of Agriculture Canada were used to study the effects of alpha s1-casein, beta-casein, kappa-casein, and beta-lactoglobulin loci on herdlife and total yield over fixed parities (one, two, and three parity) and to a fixed age (36, 48, and 61 mo). Actual yields of all cows were utilized to compute total milk regardless of lactational length. The model consisted of station, breed, year of birth, season of birth, and milk protein types with age at first calving as a covariate. Of the four milk protein types studied, only the kappa-casein locus had significant effects on fixed parity and fixed age total milk and herdlife. Cows with BB kappa-casein type outproduced those with AB or AA kappa-casein types in three parity total milk by 963 and 1657 kg, respectively. Considering total milk accumulated up to 61 mo of age in life, cows with BB kappa-casein type outperformed their counterparts with AB or AA kappa-casein types by 1050 and 1923 kg, respectively. Complete replacement of A by B allele at kappa-casein locus would result in an increase of 1657 kg in three parity total milk and an increase of 1923 kg in 61-mo total milk. The moderate gene frequency of kappa-casein B allele in the current dairy population can be increased to improve lifetime total milk to the benefit of the dairy industry.