Effect of methionine and fishmeal added to the diet of cows with different beta-lactoglobulin genotypes on the composition and physical properties of milk

The effect of protected methionine (20 mg/head/day) and fishmeal (300 g of crude protein/head/day) on milk yield, composition and physical properties, as well as the proportion of individual casein fractions, was analysed for 41 Black-and-White cows with different milk beta-lactoglobulin genotypes (AA, AB and BB). A significant increase in the content and production of beta-casein in milk was observed in all groups of cows examined. The addition of protected methionine to the diet of cows with the BB beta-lactoglobulin genotype resulted in a significant (P ≤ 0.01) increase in the content of crude protein and production of beta-casein, and a significant (P ≤ 0.05) increase in the content of casein and beta-casein in milk. The addition of fishmeal to the diet of AB beta-lactoglobulin genotype cows resulted in a significant decrease in the content of urea together with an increase in the content of crude protein and casein, including alpha- and beta-caseins, in milk.