INTERACTIONS OF ENERGY AND BOVINE SOMATOTROPIN WITH HEAT-STRESS

Heat stress, resulting when dairy cows are exposed to hot or to hot an d humid environmental conditions, reduces DMI and milk yield and incre ases maintenance costs of the cow. Decreased DMI may help to maintain homeothermy through reduced metabolic heat production. The high yieldi ng cow has greater metabolic activity and produces more body heat than those with lower yields; thus, greater milk yield may increase heat s tress if the cause of that stress is not mitigated. Use of bST increas es milk yield, and, during hot or hot and humid environmental conditio ns, bST increased milk yield with or without noticeable effects on hea t stress, depending on the study. The use of bST does not change maint enance requirements or partial efficiencies of milk yield. Thus, the g reater heat stress that occurred in some studies probably was related to the increased metabolic activity and heat production associated wit h milk yield and to the difficulties of cows in dissipating the additi onal body heat from environmental conditions. Cows that are administer ed bST are subject to heat stress as are other high yielding cows if s ufficient metabolic heat is not dissipated. Management to minimize eff ects on the cow of environments conducive to heat stress and strategie s that improve DMI are necessary to sustain the high milk yield potent ial offered by technologies, including bST.