Impact of Heat Stress on Female Fertility

The Nature of the Problem Heat stress has two major actions on physiology of the female that reduce the probability of a cow becoming pregnant. First, heat stress reduces ability to detect estrus. On one dairy in Florida, only about 18-24% of estruses in hot months were detected by herdsmen while 45-56% of estrus periods were detected in cool months (Thatcher and Collier, 1986). The reduction in estrus detection is the result of effects of heat stress on cow behavior (for example, reduced walking time; López-Gatius et al., 2005) and on reduced circulating concentrations of the hormone estradiol-17β that causes estrous behavior (Gilad et al., 1993). Secondly, heat stress causes a large reduction in fertility. In lactating dairy cows, pregnancy rates per insemination in the summer can be as low as 10-20% (Hansen and Aréchiga, 1999). Fertility is reduced because heat stress can damage both the oocyte and early embryo (Hansen, 2013). The oocyte can be compromised by heat stress as early as 105 days before ovulation (Torres-Júnior et al., 2008) and as late as the peri-ovulatory period (Putney et al., 1989b). The early embryo is also initially sensitive to heat stress but quickly becomes resistant so that heat stress on day 1 after estrus reduced embryonic development whereas heat stress at day 3 had no effect (Ealy et al., 1993). The times in the reproductive cycle in which the cow is sensitive to heat stress are illustrated in Figure 1.