Association of genetic variants of casein and milk serum proteins with milk, fat, and protein production by dairy cattle.

Polyacrylamide gel electrophoresis methods were used to phenotype caseins for 2045 cows and milk serum proteins for 3870 cows distributed in 63 Quebec dairy herds. Frequencies were: alpha s1-casein A .003, alpha s1-casein B .970, alpha s1-casein C .027; beta-casein A1 .561, beta-casein A2 .421, beta-casein A3 .011, beta-casein B .007; kappa-casein A .744, kappa-casein B .256; beta-lactoglobulin A .387, beta-lactoglobulin B .613; alpha-lactalbumin B 1.00. Overall unadjusted means for 305-day production in first lactation were: 5530 +/- 26.6 kg, 197 +/- 1.0 kg, 172 +/- 1.0 kg, 3.58 +/- .009, 3.12 +/- .009 for milk yield, fat yield, protein yield, fat percentage, and protein percentage for 1687 cows for casein systems. Similar data for 2906 cows phenotyped for milk serum proteins were: 5412 +/- 20.6 kg, 193 +/- .8 kg, 170 +/- .7 kg, 3.57 +/- .007, and 3.13 +/- 007. Least squares analysis of variance showed that herd location, month of calving, age of cow at first calving, and protein variants had significant effects on production traits. alpha s1-Casein B and beta-casein A phenotypes were associated with higher milk, fat, and protein yields than other variants in the two casein systems. Milk from kappa-casein BB and beta-lactoglobulin AA phenotypes contained .13 and .05% more protein than the AA and BB phenotypes with the AB phenotype intermediate.