Heat stress has consequences on both the physiology and reproductive performance of
cows, but the most dramatic effect for dairy producers is the decrease produced in fertility.
The effects of heat stress on fertility include an increased number of days open, reduced
conception rate, and larger number of cows suffering different types of anestrus. Once
becomes pregnant, heat stress affects also the reproductive success of the cow through its
direct effects on the ovary, uterus, gametes, embryo, and early fetus. This article reviews
current knowledge of the effects of heat stress on fertility in dairy cows and the hormonal
strategies used to mitigate these effects at the farm level. Administration of GnRH at the
moment of artificial insemination can improve the conception rate. Breeding synchronization
protocols for fixed-time insemination may reduce the calving conception interval
and the number of services per conception. Progesterone-based protocols seem resolve
better the reproductive disorders related to a hot environment (anestrus) than GnRHbased
protocols. The use of combinations of GnRH, eCG, and hCG in progesterone-based
protocols can improve results. Progesterone supplementation during the late embryonic
and/or early fetal period would be useful in curtailing pregnancy losses, mainly in single
pregnancies, whereas a more positive effect of treatment with GnRH than progesterone
has been found in twin pregnancies. Melatonin therapy is emerging as a promising
strategy to improve the natural reproductive performance of cows suffering conditions of
heat stress.