EFFECT OF SOMATOTROPIN ON THE CASEIN PROFILE AND KAPPA-CASEIN GLYCOSYLATION IN MILK

Twenty-nine Holstein cows were used to determine the effects of subcut aneous injections of a placebo (control, n = 9), 10.3 mg d of a recomb inant bovine somatotropin (rbST) (n = 10), and 175 mg 14 d-1 rbST in a sustained-release formulation (n = 10) on milk composition. the relat ive proportions of alpha(s1 + s2)-casein, beta-casein and kappa-casein , and the degree of glycosylation Of kappa-Casein. The latter was eval uated by measuring the N-acetylneuraminic acid (NANA) content of casei n. Once a week, morning and evening milk samples were collected and co mposited beginning 4 wk prior to and for 7 wk following the initial in jections (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 inj ections) and 30.9 kg d-1 (biweekly) without affecting percentages of f at, 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 m ug mg-1) was significantly lower (P < 0.05) than in milk from control cows (6.3 mug mg-1). The NANA content Of kappa-casein was, however, no t affected by rbST treatment. Neither the extent of stimulation of mil k production due to rbST treatment nor the level of milk production du ring the treatment significantly affected the NANA content of casein. In conclusion. injection of rbST increased milk yield without affectin g degree Of kappa-casein glycosylation.