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.

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Seven Friesian cows in each of four treatment groups consumed hay and a rolled barley concentrate in the ratios of 100:0, 80:20, 60:40 and 37:63 respectively. Cows fed on only hay consumed less dry matter than cows fed on barley and hay and produced less milk, fat, solids not fat, protein and casein. Solids not fat, protein, casein, β-lactoglobulin and α-lactalbumin percentages were generally higher when the ration contained a higher proportion of concentrate, although milk fat percentages were similar for all treatments. Casein, β-lactoglobulin and α-lactalbumin concentrations relative to the concentration of total protein were similar for all treatments. Milk fat from cows fed on hay only was generally lower in C10, C12, C18:2 fatty acids and higher in C18:1 and C18:3 fatty acids than milk fat from cows fed on hay and concentrate. The rennet curd firmness of the milk increased as the proportion of concentrate in the diet increased. The mean maximum heat stability of the skimmed milk from individual cows generally decreased as the proportion of concentrate in the diet increased.