Exogenous Melatonin Improves the Reproductive Outcomes of Yearling Iberian Red Deer (Cervus elaphus hispanicus) Hinds

SIMPLE SUMMARY: Increasing the reproductive performance of hinds is considered to be a key factor of overall farm deer productivity. In the case of yearling hinds, this aspect becomes more important, as a delay in the pubertal onset will compromise the reproductive performance of the entire herd (decreased fertility), and these yearling hinds will carry this ‘late’ condition throughout their reproductive life. The aim of this study was to explore the use of melatonin implants on yearling Iberian red deer (Cervus elaphus hispanicus) hinds to improve their fertility outcomes, advance the calving date and the calves’ weight, and to prevent the negative impact of yearling hinds’ low liveweight on their reproductive outcomes. Melatonin implants (18 mg), administered three-fold (two implants each time) every 30 days before the breeding season, rendered significantly higher fertility rates (regardless of the yearling hind’s weight) and heavier calves, and advanced the calving date in the yearling hinds by 15 days compared to non-treated hinds. In addition, halving the number of yearling hinds that received melatonin provided a similar benefit to a large-scale treatment of the whole herd, which indicates female-to-female stimulation of the ovarian activity. Taken together, this protocol for melatonin treatment simplifies its administration, reduces its costs, and assures the enhancement of the reproductive productivity of the entire farm. ABSTRACT: The aim of this study was to assess the effect of melatonin implants on the reproductive performance of yearling Iberian red deer (Cervus elaphus hispanicus) hinds. It also explored exogenous melatonin administration as a tool to minimize the negative effect of a low yearling hind’s liveweight on their reproductive efficiency. In addition, the effect of melatonin-treated yearling hinds on non-treated hinds was studied in order to provide a practical and economical protocol to improve farms’ productivity. A total of 4520 Iberian red deer hinds belonging to the same farm were included in this study. Melatonin (108 mg/hind) implants were administered three-fold every 30 days before the breeding season. Fertility rates, calves’ weights and calving dates were registered for each hind. The results showed that exogenous melatonin increased significantly (p < 0.05) the calves’ weight (32.39 ± 1.07 kg vs. 27.65 ± 1.11 kg for Weight 1(calf) (July) and 46.59 ± 1.50 kg vs. 41.79 ± 1.54 kg for Weight 2(calf) (August, at weaning)) and advanced the calving date by 15 days in yearling hinds compared to the non-treated group. In addition, the administration of melatonin implants before the breeding season was able to minimize the negative effect of low yearling hinds’ liveweight (Weight 1(hind)) on their future reproductive outcomes, as the fertility rates increased by 46% and the calves’ weight increased by 7 kg after the melatonin treatment, regardless of the yearlings’ weight. Finally, when both experimental groups (melatonin and non-treated) were kept separate, higher fertility rates (76.73 ± 7.18% vs. 66.94 ± 7.41%) were observed for the melatonin-treated hinds compared to the non-treated hinds. However, when both groups of yearling hinds were maintained together, no significant differences were observed in their fertility outcomes (78.13 ± 21.26% vs. 78.12 ± 23.32%). Therefore, melatonin implants may be used in yearling Iberian red deer hinds as a management tool to improve their reproductive productivity.