Effect of somatotropin on the casein profile and κ-casein glycosylation in milk

Abstract

Twenty-nine Holstein cows were used to determine the effects of subcutaneous injections of a placebo (control, n = 9), 10.3 mg d−1 of a recombinant bovine somatotropin (rbST) (n = 10), and 175 mg 14 d−1 rbST in a sustained-release formulation (n = 10) on milk composition, the relative proportions of αs1+s2-casein, β-casein and κ-casein, and the degree of glycosylation of κ-casein. The latter was evaluated by measuring the N-acetylneuraminic acid (NANA) content of casein. Once a week, morning and evening milk samples were collected and composited beginning 4 wk prior to and for 7 wk following the initial injections (105 ± 7 d postpartum). Treatment with rbST increased milk yields (P < 0.06) from 28.4 kg d−1 (control) to 30.6 kg d−1 (daily injections) and 30.9 kg d−1 (biweekly) without affecting percentages of fat, protein and non-fat solids in milk or the relative proportions of the caseins. The NANA content of casein in milk of treated cows (5.6 μg mg−1) was significantly lower (P < 0.05) than in milk from control cows (6.3 μg mg−1). The NANA content of κ-casein was, however, not affected by rbST treatment. Neither the extent of stimulation of milk production due to rbST treatment nor the level of milk production during the treatment significantly affected the NANA content of casein. In conclusion, injection of rbST increased milk yield without affecting degree of κ-casein glycosylation. Key words: Somatotropin, casein, κ-casein glycosylation, lactation