The effects on reproductive performance in the short and medium term of the combined use of exogenous melatonin and progestagen pessaries in ewes with a short seasonal anoestrous period

Three hundred and sixty-one adult Rasa Aragonesa ewes were used to evaluate the effect on their reproductive parameters of melatonin implants used in association with the classical synchronization treatment (progestagen plus pregnant mare serum gonadotrophin (PMSG)) in an accelerated breeding system (3 lambings in 2 years). The animals were divided into four groups on the basis of body condition score (H, greater than or equal to 2.75; L, less than or equal to 2.5) and treatment (M) or no treatment (C) with a subcutaneous implant containing 18 mg of melatonin on 17 February 1997. Intravaginal pessaries containing 30 mg of fluorogestone acetate were inserted into all the ewes between 3 and 13 March and 500 IU PMSG was injected intramuscularly when the pessaries were withdrawn 14 days later. Fertile rams were introduced on 17 March. The pregnancy rate and litter size were unaffected by either melatonin treatment or body condition. The overall fertility at first plus second oestrus was between 76.4% (HC) and 82.8% (HM), and the litter size was between 1.58 (LM) and 1.82 (HC) lambs born per lambing. However, melatonin did seem to have a positive effect on the reproductive parameters of non-pregnant ewes at the second mating period 2 months later. Finally, no detrimental effect of melatonin implants on fertility and litter size was recorded in the medium term.