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The effect of melatonin replacement and month of treatment after the winter solstice on the reproductive activity of two Mediterranean breeds of sheep

The present experiment assessed the effect of melatonin implants administered between the winter and the summer solstices on the LH release of Rasa Aragonesa and Assaf ewes kept in the absence of males at latitude 41°. Ovariectomized oestradiol-implanted Rasa Aragonesa and Assaf ewes received a single melatonin implant on either 1 January (M1, n = 6 and 4), 15 February (M2, n = 4 and 4), 1 April (M3, n = 4 and 5) or 15 May (M4, n = 4 Rasa Aragonesa, only). Four ewes of each breed did not receive a melatonin implant and were used as controls (C). Between 17 December and 10 September, blood samples were collected twice weekly and assayed for LH. Data from Assaf ewes C groups revealed a higher degree of sexual seasonality, which exhibited a delay in the onset of the reproductive season compared to Rasa Aragonesa ewes (1 September vs. 15 June; P = 0.06). Five Rasa Aragonesa and three Assaf ewes implanted in January responded to melatonin, either by delaying the end of seasonal anoestrus when compared to controls (10 March vs. 29 December and 21 March vs. 22 January, respectively; P < 0.01) or having a short reproductive season (31 days after implantation). Two Rasa Aragonesa and three Assaf ewes responded to melatonin implants on 15 February. None of four Rasa Aragonesa or four Assaf ewes that received melatonin implants on 1 April were induced to resume reproductive activity (P < 0.05 when compared to the M1 groups). These results confirm the efficacy of using melatonin implants shortly after the winter solstice to influence reproductive activity in Mediterranean ewes.