Effects of bovine somatotropin on milk yield and composition, dry matter intake, and some physiological functions of Holstein cows during heat stress.

Thirteen Holstein cows (46 to 106 d postpartum) were assigned to a partially balanced incomplete block experiment to evaluate effects of bovine somatotropin (20.6 mg monomer/d) and environment (shade and no shade) on milk yield and composition, feed intake, rectal temperature, respiration rate, and concentrations of hormones in plasma. Two treatment periods were 29 d each, preceded by 10-d preliminary periods. Water and cotton-seed hull-based diet were available ad libitum. Mathematical model for statistical analyses included cow, period, treatment, environment, day, and estimable interactions. Black globe temperature and respiration rate and rectal temperature of cows were higher in no shade. Milk and 3.5% FCM yields and feed intake of cows in no shade were approximately 9.5 and 16% less than for cows in shade. Much of the effect of heat stress was associated with reduced DM intake. The 3.5% FCM, but not milk yield, was increased by injections of bovine somatotropin. Dry matter intake was unaffected, but milk, 3.5% FCM, and component yields were increased by bovine somatotropin when adjusted for DM intake. Response of cows to bovine somatotropin was not different in the two environments, except cows injected with bovine somatotropin had slightly higher body temperature and respiration rate. Increased production responses of heat-stressed cows due to bovine somatotropin were less than in cows injected under more temperate conditions.