Impact of Heat Stress on Production and Fertility of Dairy Cattle

Summer heat stress depresses both milk production and reproductive performance of dairy cows. The use of efficient cooling systems is required because high milk-producing cows are not capable of maintaining normothermia in the summer. Cooling, if efficient enough, is capable of narrowing the gap between winter and summer milk production; however, its positive effect on fertility is limited. Fertility surveys show that, unlike milk yield, the improvement of conception rate by cooling is small. Therefore, additional hormonal treatments are required. Various hormonal approaches have been examined: addition of exogenous progesterone, injection of gonadotrophin releasing hormone (GnRH) at onset of estrus, the timed artificial insemination (AI) (Ovsynch) protocol, embryo transfer, and induction of follicular waves by GnRH and prostaglandin F2á (PGF2á); their potential for improving summer fertility is presented.