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 GnRH-based 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.