SummaryEffects of genetic variants of the milk proteins, αs1-casein, β-casein, κ-casein and β-lactoglobulin (βlg), on milk yield and composition, particularly the protein composition, were investigated in milk samples from 289 Jersey and 249 Friesian cows in eight commercial herds.Milk protein genotypes had no significant effect on yields over a complete lactation of milk and fat, but significant differences in fat content were detected for β-casein (B, A1B, A2 > A1A2) and βlg (B, AB > A) variants. Significant differences between β-lg variants were also found with total solids (B, AB > A), casein (B, AB > A), whey protein (A > AB > B) and βlg (A > AB, AC > B > BC) concentrations. Casein genotypes were not significantly different in total protein and casein concentrations but many differences were found in casein composition. αs1-Casein variants significantly affected αs1-casein (BC > B) and κ-casein (B > BC) concentrations. β-Casein variants affected concentration and proportion of β-casein (A1B, A2B > A1, A1A2, A2, B), αs1-casein (A1, A2 > B) and κ-casein (B > A2) and concentration of whey protein (A1 > most other β-casein variants). κ-Casein variants affected concentration and proportion of κ-casein (B > AB > A), proportion of αs1-casein (A > AB > B) and concentration of βlg (A > AB, B) and α-lactalbumin (A, AB > B). Differences in milk composition were found between breeds, herds and ages, and with stage of lactation. The potential use of milk protein genotypes as an aid in dairy cattle breeding is discussed.
