Effects of exogenous melatonin treatment on out-of-season ram fertility

Abstract

Three field experiments were performed to determine the effects of melatonin implants on the fertility of rams. The experiments were performed on three breeds (Rasa Aragonesa, Assaf and Manchega) in three commercial farms in Spain. Melatonin-treated rams (M) and non-implanted control rams (C) mated either ewes oestrous-induced with FGA intravaginal sponges (Rasa Aragonesa and Assaf farms) or melatonin-treated ewes during the anoestrous period (Manchega farm). Scrotal circumference (SC) was measured at melatonin implantation and at ram introduction (nearly 45 d later). Lambing rate, litter size, and fecundity were recorded at parturition. Melatonin implants in rams increased the number of lambs born per ewe in Rasa Aragonesa (19%), Assaf (9%), and Manchega (7%) because of the higher lambing rates exhibited by ewes mated with melatonin-treated rams (Rasa Aragonesa: 80.2% vs 70.5%, P<0.05; Assaf: 74.2% vs 68.0%, NS; Manchega: 90.0% vs 81.5%, P<0.05; for M and C groups, respectively). As a result, the number of extra lambs born per treated ewe was 0.17, 0.10, and 0.10 in the Rasa Aragonesa, Assaf, and Manchega ewes, respectively. In the M and C groups of Rasa Aragonesa rams, mean scrotal circumference increased significantly between the start of the experiments and the onset of mating, but the two groups did not differ significantly at the implantation or at the onset of mating. In the melatonin-treated and control Assaf rams, scrotal circumference increased over the course of the experiments but, at the onset of mating, the melatonin-treated rams had higher SC than did the control rams. Melatonin-treated Manchega rams had scrotal circumferences that were significantly greater than those of the control rams. In the Rasa Aragonesa and Manchega breeds, treating rams using melatonin implants significantly increased the net lamb production of ewes in field conditions. In the Assaf breed, the number of extra lambs born per ewe mated with melatonin treated rams was lower than in the other breeds, even though rams exhibited a significant increase in scrotal circumference. Further research involving the study of semen quality has been planned.