Interactions of energy and bovine somatotropin with heat stress.

Heat stress, resulting when dairy cows are exposed to hot or to hot and humid environmental conditions, reduces DMI and milk yield and increases maintenance costs of the cow. Decreased DMI may help to maintain homeothermy through reduced metabolic heat production. The high yielding cow has greater metabolic activity and produces more body heat than those with lower yields; thus, greater milk yield may increase heat stress if the cause of that stress is not mitigated. Use of bST increases milk yield, and, during hot or hot and humid environmental conditions, bST increased milk yield with or without noticeable effects on heat stress, depending on the study. The use of bST does not change maintenance requirements or partial efficiencies of milk yield. Thus, the greater heat stress that occurred in some studies probably was related to the increased metabolic activity and heat production associated with milk yield and to the difficulties of cows in dissipating the additional body heat from environmental conditions. Cows that are administered bST are subject to heat stress as are other high yielding cows if sufficient metabolic heat is not dissipated. Management to minimize effects on the cow of environments conducive to heat stress and strategies that improve DMI are necessary to sustain the high milk yield potential offered by technologies, including bST.