Effects of heat stress on ovarian functions and embryonic development: mechanism and potential strategies to alleviate these effects in dairy cows

Reduced reproductive performance of lactating cows during the summer is associated mainly with intensive genetic selection for high milk production, which places a great load on the thermoregulatory mechanism. Today, cooling is the predominant strategy used to alleviate the effects of heat stress. However, it cannot eliminate the decline in reproduction. A better understanding of the mechanism by which heat stress compromises fertility is required to develop new strategies to mitigate its effects. This review summarizes what is known about the multifactorial effect of thermal stress on reproductive functions, which includes alterations in the hypothalamus– pituitary–ovarian axis: gonadotropin secretion, follicular development, steroid production and corpus luteum function. The review provides some new insights into the cellular and molecular responses of oocytes and embryos to elevated temperature, discusses some potential underlying mechanisms, such as nuclear and cytoplasmic maturation, mitochondrial function, apoptotic pathways and oxidative stress, and suggests potential approaches to alleviating these effects. Given the complexity of heat-stress effects on reproduction, comprehensive reproductive management during the summer is suggested, based on: (1) an efficient cooling system as a prerequisite for any other strategies, (2) hormonal treatment targeted to specific subgroups of cows that will benefit from it, rather than the whole herd, and (3) combining two or more strategies in a program, rather than correcting the function of a single aspect of the reproductive system.