Effects of pasture allowance on the yield and composition of milk from cows of different beta-lactoglobulin phenotypes.

The objective of this study was to determine whether the differences in the composition of milk from cows of different beta-lactoglobulin beta-LG) phenotypes are affected by the amount of pasture available and, hence, pasture dry matter intake. Twenty-two Friesian cows of each of the AA and BB variants of the beta-LG phenotype were subjected to ad libitum grazing or restricted grazing in crossover experiments during spring (early lactation, approximately 60 d in milk) and summer (mid to late lactation, approximately 180 d in milk). Milk samples were collected from each cow at the end of each 8-d treatment period and analyzed for composition. Cows of the AA variant of the beta-LG phenotype had higher concentrations of whey protein and beta-LG, but lower concentrations of casein (CN), alpha-CN, kappa-CN (summer only), and BSA, than cows of the BB variant. Compared with cows with a restricted allowance, cows grazing ad libitum had higher milk yields and concentrations of protein, casein, whey protein, and all individual proteins except BSA and immunoglobulin. There were no interactions between effects of pasture allowance and phenotype on milk yield or composition. The data show that having adequate pasture for grazing cows is important not only to maximize milk yield, but also to optimize concentrations of protein and casein, and hence the manufacturing potential of milk. Further, the differences in composition of milk from cows of differing beta-LG phenotypes persisted during short-term restrictions in pasture allowance, and between spring and summer.