Melatonin Enhances Embryo Quality in Japanese Black Cows During the Hot Season

Melatonin is a potent antioxidant hormone that influences bovine reproductive function; however, its practical effects on embryo production under heat stress remain unclear. This study evaluated whether short-term melatonin administration improves embryo production, reduces oxidative stress, and affects pregnancy outcomes in Japanese Black cows using ovum pick-up and in vitro embryo production (OPU-IVP) and multiple ovulation and embryo transfer (MO-ET) systems. In Experiment 1, cows received subcutaneous melatonin (120 mg) or saline on Days 2 and 8 before OPU during the hot season in a crossover design. Melatonin treatment significantly increased the number of freezable embryos obtained by OPU-IVP (rate ratio = 2.85, p < 0.01) and tended to reduce 8-hydroxy-2'-deoxyguanosine (8-OHdG) concentrations in the mixture of follicular fluid and blood surrounding the ovaries. In Experiment 2, the effects of melatonin treatment before embryo recovery (ER) following superovulation were evaluated during the hot and cooler seasons. A single injection had no effect, whereas two injections given 7 and 15 days before ER significantly increased the number of freezable embryos during the hot season (rate ratio = 1.48, p < 0.05). Melatonin treatment significantly decreased serum 8-OHdG concentrations, indicating attenuation of systemic oxidative stress. Moreover, frozen-thawed in vivo embryos derived from melatonin-treated donors showed a tendency towards improved pregnancy outcomes in repeat-breeder (RB) cows. These findings suggest that targeted melatonin administration is a practical strategy for improving embryo quality under heat stress conditions and may be associated with improved pregnancy outcomes following ET in RB cows.