Effect of β-Lactoglobulin Variant and Environmental Factors on Variation in the Detailed Composition of Bovine Milk Serum Proteins

Between November 1979 and November 1981, 17,086 test-day milk samples were collected from individual Holstein cows in 62 Quebec herds. Samples were analyzed for protein, fat, casein, serum protein, somatic cell count, and the relative percentages of /3-1actoglobulin, ~qactalbumin, bovine serum albumin, and the immunoglobulins. Cows included in the study were phenotyped with respect to ~-lactoglobulin. Unadjusted means -+ standard error for the relative percentages of /3-1actoglobulin, a-lactalbumin, bovine serum albumin, and immunoglobulins were 64.80 .07%, 21.54 -+ .05%, 6.51 -+ .02%, and 7.15 -+ .04%. Least-square analyses showed that month of test, stage of lactation, age of cow, somatic cell count, and phenotype of the cow for/3-1actoglobulin had significant effects on the relative percentages of /3-1actoglobulin, a-lactalbumin, bovine serum albumin, and immunoglobulins. Test-day milk yield, fat percent, protein percent, casein percent, casein number, and serum protein percent, when included in a statistical model as covariates, had a significant effect on the relative percentage of Ig. Relative proportion of /3-1actoglobulin and a-lactalbuminim were not significantly affected by serum protein and total protein contents. For bovine serum albumin, only the covariates fat percent and serum protein percent were not significant.