Physiological and nutritional factors affecting oestrous activity and pregnancy in the ewe, with emphasis on the role of melatonin
In seven experiments carried out under natural-daylength conditions at 57oN, Scottish Blackface and Border Leicester x Scottish Blackface ewes of varying physiological states and nutritional regimes received daily at 1500 h an oral dose of either melatonin dissolved in water and alcohol or the vehicle alone. Throughout the experiments blood samples were collected thrice weekly for progesterone, prolactin and follicle stimulating hormone concentrations (FSH) and at 10 to 15 minute intervals for periods of up to 24 h for luteinizing hormone (LH). Ovulation rates were determined by laparoscopy. When given from the beginning of June and mid-March respectively melatonin advanced and reversed the breeding season, the reversal being as effective in ewes with an extended lactation as in those weaned early or those that had not bred in the previous year. Short-term treatment (30 days) with melatonin during mid-anoestrus not only failed to advance the breeding season but delayed it. Longer term treatment (60 to 90 days) produced transient oestrous cyclicity with ewes returning to anoestrus within one month after the end of treatment. For ewes on a low plane of nutrition melatonin increased ovulation rate at first behavioural oestrus and appeared to reduce embryo mortality. In all experiments melatonin suppressed plasma prolactin 10 to 15 days after the start of treatment with levels remaining low throughout the period of treatment except during late pregnancy. There was no evidence that melatonin promoted a progressive increase in the frequency of the pulsatile release of LH or had any stimulatory effect on FSH secretion in the first 6 weeks of treatment. It did however increase plasma progesterone concentrations in the post-oestrus period.