Strategies to reduce the impact of heat stress on fertility of cows

Heat stress has two major consequences for the physiology of the cow that reduce its probability of becoming pregnant. First, changes in cow behavior (for example, reduced walking time; López-Gatius et al., 2005) and reduced circulating concentrations of estradiol-17β (Gilad et al., 1993) caused by heat stress reduce 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). Secondly, there is 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., 1989). 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).