Review on mechanisms of dairy summer infertility and implications for hormonal intervention

In dairy cows and buffaloes, summer heat stress (HS) reduces milk yield and delays return to pregnancy leading to financial loss. Clues for effective interventions against summer infertility (SI) lie in understanding the underlying mechanisms. This article reviews current knowledge on the mechanisms of bovine SI and their implication for hormonal management. Under HS dairy animals encounter anestrous, silent cycles and repeat breeding which extend their open period. These effects are attributed mainly to HS induced disturbances in luteinizing hormone (LH) secretion, follicular dominance and estrogen secretion, ovulation and oocyte competence, luteal development and progesterone secretion, utero-placental function and embryo-fetal development. Hormonal timed artificial insemination protocols and LH support around estrous improved summer pregnancy rates by avoiding need for estrus detection, assisting follicular development and ovulation, enhancing quality oocytes and stimulating luteal function. Progesterone supplementation to enhance embryonic development did not produce significant improvement in summer pregnancy rates. There is need for evaluating integrated approaches combining hormones, metabolic modifier and cyto-protective agents.