Effects of administration of recombinant bovine somatotropin on the responses of lactating and nonlactating cows to heat stress.

An experiment was performed to determine whether bovine somatotropin (BST) increases the severity of responses to heat stress and whether this effect depends on an increase in milk yield. Lactating and nonlactating cows received daily SC injections of BST (25 mg) or sodium phosphate solution for 12 days. Rectal temperatures and respiration patterns were not affected by BST when cows were maintained in a thermoneutral environment on day 9 of treatment. On day 12, cows were heat stressed by exposure to direct solar radiation. Rectal temperature, respiration rate, and the frequency of open-mouth panting increased as a result of heat stress. For lactating and nonlactating cows, those treated with BST had higher rectal temperatures and rate of open-mouth panting than cows treated with sodium phosphate solution. Administration of BST can increase the severity of responses of cows to heat stress without changing milk yield.