Factors affecting response of cows to recombinant methionyl bovine somatotropin

Eighty lactating Holstein cows were randomly assigned to one of two treatments: Placebo(C) or 500 mg recombinant methionyl bovine somatotropin (BST). Cows were intramuscularly injected with C or BST every 14 d for 36 wk beginning 60 d postpartum. Factors analyzed for differences in milk yield response to BST included: production level, genetic potential, stage of lactation, and heat stress. Response of low producing cows to BST was numerically greater than that of medium or high producers (averaging 27%, 13%, and 20%, respectively, above controls). However, there was not a significant interaction between BST treatment and production group. Cows of a lower genetic potential exhibited a greater response to BST than cows of a higher potential, but the treatment x genetic potential interaction was not significant. Response of cows to BST in mid to late lactation was slightly greater than in cows in early lactation (13.7% and 15.1% vs. 9.6%, respectively) but no significant interaction was noted. No difference in response was noticed during periods of high ambient temperature