Effects of bovine somatotropin on physiologic responses of lactating Holstein and Jersey cows during hot, humid weather.

Thirty-one lactating Holstein and Jersey cows were used to determine the effects of daily injections of 0 or 20 mg of recombinant bST on physiologic responses during hot, humid weather. Body temperature was determined by measuring milk temperature at each milking. Jugular blood was sampled for serum analysis of selected hormones, blood metabolites, and fatty acids, and arterial blood was sampled for blood pH and blood gas analysis. Milk was characterized for fatty acid composition. Blood pH was unchanged, but partial pressure of blood CO2, blood bicarbonate, base excess, and total CO2 declined with administration of bST. Serum triglycerides increased 89% in cows receiving bST. Blood urea nitrogen tended to decline in cows receiving bST. Serum cortisol, triiodothyronine, and thyroxine did not change, but insulin-like growth factor-1 increased 128% with bST use. Reduced milk short-chain fatty acids, increased milk long-chain fatty acids, and increased blood serum C18:1 fatty acid content occurred in cows administered bST and probably reflected tissue mobilization. Cows administered bST in hot weather had higher milk temperatures. Alterations in physiologic and metabolic measures in association with higher milk temperature suggest an interaction of bST use with hot, humid weather and reflect the need to minimize the effects of heat stress.