Effects of melatonin implants on reproductive performance in sheep: a systematic review and meta-analysis

Context Sheep are seasonal breeding animals. Different reproduction systems exist to enhance productivity in non-breeding season. One of the methods used in enhancing reproductive performance in sheep breeding is melatonin administration. Aim This study aimed to evaluate the impact of melatonin application on reproductive performance in sheep by meta-analysis according to different factors. Methods To find eligible studies, PubMed and Web of Science databases were searched. Twenty studies published between 1991 and 2021 were included in the meta-analysis. The effect sizes of pregnancy rate, lambing rate and litter size were calculated according to breeds, production characteristics (dairy-meat breeds) and season (breeding–non-breeding). The study estimated the risk ratio for pregnancy and lambing rates, and standardized mean difference for litter size as effect size. Key results The melatonin hormone increases the pregnancy rate by 1.07 times and the lambing rate by 1.36 times. The effect size (risk ratio) of the Rasa Aragonesa breed (1.15) for pregnancy rate and the Merino breed (1.65) for lambing rate was higher than the Sarda breed. Conclusions The positive effect of melatonin was found to be higher in the non-breeding season than in the breeding season for the three reproductive performance traits examined. Implications This study is a comprehensive research study examining the effect of melatonin application in sheep according to different factors.