Heat stress -HS- is defined as the sum of external forces acting on an animal that causes an increase in body temperature and evokes a physiological response. Summer HS induces immediate and long-lasting deleterious effects on the female reproductive system that lasts until autumn. High milk production is a major cause of sustained hyperthermia during the summer. Heat stress leads to decreased milk production, reduced reproduction rate and growth of dairy cows and seriously affects both welfare and health of dairy cattle. It has been shown that reproduction suffers more from HS than milk production. Heat stress results in profound economic losses that was predicted to cost 879 million dollars in the United States annually. Heat stressed cows show sub-fertility with an increased number of cows suffering from anestrus, low pregnancy/AI, increased number of days open, and higher pregnancy losses. Elevated environmental temperatures also negatively affect the cow’s ability to display estrous behavior, and reduces both the duration and intensity of estrous expression. Different processes of reproduction are impaired in female cattle that lead to low fertility. Heat stress depresses LH secretion and therefore impairs the cascade of events leading to ovulation and formation of a functional corpus luteum and thus reduced steroidogenesis. Heat stress also alters follicular growth dynamics and affects the follicles and their enclosed oocytes in a stage-dependent pattern of resistance and sensitivity. Preimplantation embryos are also sensitive to elevated temperature, in a stage-dependent manner. Two-cell stage embryos are more sensitive to heat stress than those at four- and eight-cell stages. A period of two to three estrous cycles was required for recovery from summer heat damage. Proper cow cooling is still the best way to improve milk production and reproduction during HS and thus any treatment must combined with cooling strategies to overcome the deleterious effects of HS and to improve fertility. The optimal hormone treatment to combat the effects of HS should be suitable for use in cows in fixed-timed AI programs -e.g. Ovsynch-, avoiding the need for heat detection and should resolve anestrus -ovarian cysts or anovulatory follicles-. Progesterone-based protocols, applied either for fixed timed AI or after AI can solve some of the reproductive disorders related to a hot environment -anestrus-. Inclusion of combinations of GnRH, eCG, and hCG in progesterone-based protocols can improve results.

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