THE MODIFYING EFFECTS OF MELATONIN, RAM EXPOSURE AND PLANE OF NUTRITION ON THE ONSET OF OVARIAN ACTIVITY, OVULATION RATE AND THE ENDOCRINE STATUS OF EWES

In four experiments, ewes were kept in natural daylength conditions at 57-degrees-N and given daily at 15:00 h an oral dose of 3 mg of melatonin. Irrespective of feeding level, this method of administering melatonin sustained plasma concentrations of the hormone at night-time values from 1 h after dosing to the onset of natural darkness (7 h or less), but by the following morning concentrations were the same as those for control ewes. For Experiment 1, which started on 6 July and involved 24 Border Leicester x Scottish Blackface ewes, the mean intervals and (ranges) in days to the onset of ovarian activity (plasma progesterone above 1 ng ml-1) were 50 (40-61) and 70 (35-96) for melatonin-treated and control ewes kept in isolation from the male. Corresponding values for those exposed once daily to a vasectomized ram were 29 (5-44) and 34 (21-61). In Experiment 2, the intervals from the start of treatment on 6 June were 67 (57-75) days for ten Scottish Blackface ewes receiving melatonin compared with 107 (75-141 ) for nine of ten controls that showed ovarian activity before the experiment ended on 29 October. For a further ten ewes exposed continuously to an oestrous ewe and vasectomized ram, the interval was 72 (36-124) days. Experiment 3 started on 10 June and involved 32 Scottish Blackface ewes which were exposed once daily to a vasectomized ram and received two levels of feeding (5.8 or 11.6 MJ of metabolizable energy daily), either with or without melatonin in a 2 x 2 factorial design. Here, the intervals to behavioural oestrus were 79 (70-91) days and 92 (71-111) days for treated and control ewes, respectively, given the low feeding level and 74 (57-86) days and 85 (70-102) days for those given the high feeding level. Melatonin increased the ovulation rate and litter size, with the increases being greater for the low- than high-plane ewes. In Experiment 4, the mean interval from the start of melatonin treatment on 7 July to the onset of ovarian activity for low-plane Scottish Blackface ewes was 61 +/- 12.5 days versus 78 +/- 11.4 days for controls (n = 10 per group). The ovulation rate, at first behavioural oestrus was not affected by melatonin, but at the subsequent three oestrous cycles was always higher for those receiving melatonin. Melatonin decreased circulating prolactin concentrations and obliterated their circadian rhythm (Experiment 1). Luteinizing hormone concentrations in plasma samples taken at frequent intervals for up to 24 h at 1, 3, 6 and 9 weeks after the start of melatonin treatment (Experiments 1 and 3), favoured the hypothesis that it initiates ovarian activity in ewes via a sudden increase in the activity of the hypothalamic gonadotrophin-releasing hormone pulse generator just prior to oestrus, rather than by a gradual and progressive increase throughout the treatment period.