High preweaning mortality rates cost the Australian sheep industry an estimated $540 million annually in lost production, with losses significantly greater in twin (≥30%) compared with singleton lambs (≥10%). Previous intensive studies demonstrated that supplementing pregnant ewes with melatonin reduces adverse effects of fetal growth restriction and perinatal hypoxia on the neonatal brain via increased umbilical blood flow, placental efficiency, and antioxidant actions. The current study examined the effects of supplementing ewes with melatonin on the survival of twin Merino lambs under extensive grazing conditions. Pregnant mixed age ewes were implanted with 1 (M1, n = 50) or 2 (M2, n = 53) slow-release melatonin implants (18 mg, Regulin) at gestational days 70 to 90. Control ewes received no supplementation (CTL, n = 54). Ewes were monitored twice daily throughout the lambing period. Lamb survival, weight, and rectal temperature were recorded on the day of birth. Lamb blood samples were taken the following day for serum immunoglobulin G (IgG) analysis. Lamb survival and weight were recorded again at marking (30.6 ± 0.6 d postpartum) and weaning (70.7 ± 0.6 d postpartum). Lamb survival was increased in both melatonin treatments to 3 d postpartum (M1 = 98.0%; M2 = 95.3%; CTL = 83.3%; each P < 0.01), and this improvement was maintained to weaning (M1 = 94.0%; M2 = 92.5%; CTL = 79.6%; each P < 0.01). Melatonin did not affect lamb birthweight, rectal temperature, or growth rate. However, the rates of parturition-related death (dystocia, stillbirth, and birth injury) were greater in CTL lambs than M1 (P = 0.009) and M2 (P = 0.035). This suggests that improved survival is primarily due to melatonin-induced neuroprotection, although further studies are required to clarify the underlying mechanisms. These data provide evidence that supplementing pregnant twin-bearing Merino ewes with melatonin may be a practical strategy to reduce neonatal mortality and improve weaning rates in extensively managed sheep flocks. Although the present data are promising, this study is limited by small sample size and requires further replication.

中文翻译：

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母体褪黑激素植入物可提高双美利奴羊羔的存活率

断奶前死亡率高使澳大利亚绵羊产业每年损失约 5.4 亿美元的生产损失，双胞胎 (≥30%) 的损失明显高于单胎羔羊 (≥10%)。先前的深入研究表明，用褪黑激素补充怀孕的母羊可以通过增加脐血流量、胎盘效率和抗氧化作用来减少胎儿生长受限和围产期缺氧对新生儿大脑的不利影响。目前的研究检查了在广泛放牧条件下为母羊补充褪黑激素对双美利奴羊羔存活率的影响。怀孕的混龄母羊植入 1 (M1, n = 50) 或 2 (M2, n= 53) 在妊娠 70 至 90 天时缓慢释放褪黑激素植入物（18 毫克，Regulin）。对照母羊未接受补充剂（CTL，n = 54）。母羊在整个产羔期间每天监测两次。在出生当天记录羔羊的存活率、体重和直肠温度。第二天采集羔羊血样用于血清免疫球蛋白 G (IgG) 分析。在标记（产后 30.6 ± 0.6 天）和断奶（产后 70.7 ± 0.6 天）时再次记录羔羊的存活率和体重。两种褪黑激素治疗的羔羊存活率均增加至产后 3 天（M1 = 98.0%；M2 = 95.3%；CTL = 83.3%；每个P < 0.01），并且这种改善持续到断奶（M1 = 94.0%；M2 = 92.5 %; CTL = 79.6%; 每个P< 0.01)。褪黑激素不影响羔羊的出生体重、直肠温度或生长速度。然而，CTL 羔羊的分娩相关死亡率（难产、死产和产伤）高于 M1（P = 0.009）和 M2（P = 0.035）。这表明存活率的提高主要是由于褪黑激素诱导的神经保护作用，尽管需要进一步研究来阐明潜在的机制。这些数据提供的证据表明，用褪黑激素补充怀孕的双胎美利奴母羊可能是降低新生儿死亡率和提高广泛管理羊群断奶率的实用策略。虽然目前的数据很有希望，但这项研究受到样本量小的限制，需要进一步复制。