Abstract
The first experiment was performed in order to determine the effect of exogenous melatonin followed by a fluorogestone acetate (FGA) and pregnant mare's serum gonadotrophin (PMSG) treatment on terminal follicular dynamics, ovulation rate and luteinizing hormone (LH) pulsatility in Suffolk ewes at the beginning of the breeding season. Therefore, 10 ewes (MEL/FGA/PMSG) were injected melatonin 22 days before receiving sponges containing 40 mg of FGA for 14 days and 800 IU of PMSG at the removal of pessaries. Ten other ewes (FGA/PMSG) were synchronised with the same FGA/PMSG treatment without melatonin. The second experiment was undertaken to determine the effects of exogenous melatonin followed by a FGA-PMSG synchronisation on the plasma FSH, E 2 and P 4 concentrations. The investigations on terminal follicular growth and LH pulsatility were repeated. Six ewes were submitted to a MEL/FGA/PMSG treatment and six other to a FGA/PMSG control following the same design as in the first experiment. Melatonin treatment was effective in increasing ( p < 0.05) the circulating melatonin during a period of more than 14 days but was ineffective in lowering plasma PRL concentrations. LH pulsatility during and after the hormonal treatment and the concentrations of plasma FSH and P 4 were comparable among MEL/FGA/PMSG and FGA/PMSG groups. Concentrations of plasma E 2 during the luteal phase but not during the follicular phase of the synchronised estrous cycle were lower ( p < 0.05) in ewes treated with melatonin when compared to controls. The two groups differed only at the time of selection : the percentage of medium follicles undergoing atresia was lower ( p < 0.05) in the MEL/FGA/PMSG than in the FGA/PMSG (38% and 35.5% versus 65% and 61.5%); this resulted in a increased ( p < 0.05) mean number of large follicles (4.7 ± 0.5 versus 3.2 ± 0.5) and ovulation rate (2.6 ± 0.4 versus 1.45 ± 0.35).
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