Maternal supplementation with melatonin during mid-pregnancy does not improve the survival of one-year-old ewes and their lambs.

Mating ewes to lamb at 12 months of age can enhance lifetime productivity but is associated with lower reproductive performance and lamb survival compared to mature ewes. Melatonin, an antioxidant, has been shown to improve placental function and lamb health when mature ewes were supplemented during pregnancy. This study investigated the effect of supplementing one-year-old ewes with melatonin during mid-pregnancy on their survival and the live weight and survival of their lambs. Single- (n = 2711) and twin-bearing (n = 2605) ewes were randomly assigned to Melatonin or Control groups at seven farms across southern Australia. The Melatonin group received an 18 mg implant of melatonin at 90-100 days after the introduction of the rams, which releases melatonin for 60-80 days, and the Control group were not supplemented with melatonin. Seven farms recorded paddock level information including fetal number, lambs present 200 days after the introduction of the rams, ewes present pre-lambing and 200 days after the introduction of the rams and ewe pre-lambing live weight. Lamb survival and ewe mortality were analysed with analysis of covariance. Three of the seven farms also collected individual lambs' information (n = 4303 lambs) including weight at birth and weaning of lambs, date of birth and weaning, lamb sex and birth type. Individual lamb live weights and growth rate to weaning were analysed with linear mixed models. Supplementation with melatonin had no effect (P > 0.05) on ewe or lamb survival. Melatonin supplementation did not impact (P > 0.05) individual lamb birthweight, growth rate, or weaning weight, consistent with the observed results in lamb survival. These findings suggest that maternal supplementation with melatonin mid-pregnancy is not an effective strategy to improve the survival or early growth of lambs, or the survival of one-year-old ewes.