Effects of repeated high dose administration of recombinant bovine somatotropin in lactating dairy cows.

Four multiparous Holstein cows were utilized to determine the effects of high doses of recombinant bovine somatotropin (rbSt) on milk production, feed intake, somatotropin concentration in milk, blood and urine constituents and the presence or absence of treatment-related lesions. Treatments were 0 (control) or 430 mg rbSt daily for 21 days as im injections beginning approximately 150 days into lactation. Cows were fed concentrate according to level of milk production and corn silage ad libitum. Milk yield of rbSt-treated cows increased 34% while dry matter intake and net energy intake decreased. Serum and milk bSt concentrations were elevated following treatment. Red blood cell count, hemoglobin, and hematocrit decreased in rbSt-treated cows; however, values remained within the normal range. Blood urea nitrogen concentration was decreased following treatment. Serum alkaline phosphatase activity, insulin and glucose concentrations were increased in 1 treated cow. No changes were observed in serum thyroxine, cortisol or bSt antibody concentrations. Evidence of irritation at the injection site was examined microscopically and was consistent with irritant swelling. Fetuses of rbSt-treated cows were normal, and cows did not abort. No serious adverse effects were observed following daily injections of 430 mg rbSt.