Effect of duration of melatonin treatment on the onset and duration of oestrous cyclicity in ewes

removed from their lambs at the end of April and housed under natural daylength at 57\s=deg\N.Treatments (n = 7 ewes per treatment) commenced on 1 May and comprised daily oral dosing at 15:00 h with 3 mg melatonin dissolved in water and ethanol (4:1, v/v) for 30, 60, 90, 120 or 150 days. Control ewes received the vehicle alone. Ovarian activity was assessed by laparoscopy at monthly intervals with an additional interim observation in mid-July. Blood was sampled three times a week by jugular venepuncture and assayed for progesterone, prolactin and follicle-stimulating hormone (FSH). Luteinizing hormone (LH) was determined in blood samples collected at 15 min intervals for 10 h on days 28, 60, 91, 119 and 150. Thirty days of melatonin treatment delayed (P < 0\m=.\01)first ovulation by about 1 month (mean interval \m=+-\sem from 1 May to progesterone> 1 ng ml\m=-\1, 165 \m=+-\4\m=.\5days versus 132 \m=+-\9\m=.\2days for controls). None of the ewes that received melatonin for 60 days ovulated before the end of melatonin treatment, but subsequently six of them did; the mean interval from 1 May to increased progesterone concentration was 75 \m=+-\1\m=.\2 days. All ewes receiving melatonin for 90, 120 and 150 days ovulated with correspond