Impact of heat stress on the reproduction of farm animals and strategies to ameliorate it

Animals are susceptible to heat stress under tropical environmental conditions. Exposure of growing male and female animals to severe heat stress during summer adversely affects their growth and reproductive traits, reduces the resistance to diseases and induces infertility in farm animals and represents a major source of economic loss to the livestock sector. During the summer season, animals usually suffer from summer sterility due to prevailing hot and humid conditions. In female animal's age at puberty, estrous signs, ovulation time, ova quality, conception rate, embryonic development, embryo size, embryo weight and hormonal balance are affected by exposure to heat stress. The most sensitive to heat stress are growing ovarian follicles. Heat stress-induced changes in growing follicles can be expressed later as compromised maturation and developmental capacity of the ovulating oocyte. In males, sex hormone levels, spermatogenesis, temporary sterility, libido, ejaculate volume, macroscopic as well as microscopic semen characteristics in an ejaculate are affected, and sperm abnormalities and dead sperm increase by exposure to the heat stress. Various heat stress amelioration technologies have evolved to circumvent the fertility reduction in dairy cattle and other livestock animals. New breakthroughs such as estrus synchronization, embryo transfer technique, cryopreservation of gametes and genetic manipulation, nutraceuticals (feed additives, pre and probiotics) and managerial changes may exist to improve summer fertility. This review summarizes the most up-to-date information on the effects of heat stress on the male and female reproduction especially under tropical conditions.