Six things to know about how heat stress compromises fertility of the cow (and probably other mammals as well).

Review of the primary literature was used to develop six propositions regarding the nature of actions of heat stress on fertility of the bovine female. The first two propositions are that the reduction in fertility caused by heat stress is due to 1) negative effects on the competence of the oocyte to be fertilized and 2) support embryonic development and death of the embryo when exposed to elevated temperature. The third proposition is that it remains unclear whether damage to the spermatozoa occurs after deposition in the reproductive tract of a heat-stressed female. Fourth, the embryo acquires resistance to heat stress after activation of the embryonic genome. Fifth, heat stress after the first few days of pregnancy usually has only slight effects on embryonic or fetal survival. Sixth, the magnitude of effects of heat stress on fertility is related to the degree of hyperthermia experienced by the female. Thus, physiological and environmental factors that control ability of an animal to regulate its body temperature during heat stress are important determinants of the magnitude of the impact of heat stress on fertility.