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

Forty-two Scottish Blackface ewes that lambed outdoors in March were removed from their lambs at the end of April and housed under natural daylength at 57 degrees 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 < 0.01) first ovulation by about 1 month (mean interval +/- SEM from 1 May to progesterone > 1 ng ml-1, 165 +/- 4.5 days versus 132 +/- 9.2 days 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 +/- 1.2 days. All ewes receiving melatonin for 90, 120 and 150 days ovulated with corresponding mean intervals of 83 +/- 2.7, 85 +/- 1.3 and 87 +/- 2.2 days, respectively (P < 0.001 compared with controls).(ABSTRACT TRUNCATED AT 250 WORDS)