Daily oral administration of melatonin from March onwards advances by 4 months the breeding season of ewes maintained under the ambient photoperiod at 57 °N

In three experiments, ewes kept under the ambient photoperiod at 57-degrees-N were given daily at 15:00 h from March onwards an oral dose of 3 mg of melatonin in a 4:1 mixture of water and ethanol or, in the case of controls, the vehicle alone. All ewes were challenged daily with a vasectomized ram for the detection of behavioural oestrus. In Experiment 1, the mean interval (+/- standard error of the mean (s.e.m.)) from the start of melatonin treatment on 5 March to first behavioural oestrus for Border Leicester X Scottish Blackface ewes that had remained barren over the winter period was 87 +/- 5.8 days (n = 8) compared with 222 +/- 10.3 (n= 7) for controls. Corresponding intervals from the start of melatonin treatment on 24 March for the same breed of ewe in Experiment 2 were 80 +/- 3.7 (n = 6) and 211 +/- 9.4 (n = 6) days. In Experiment 3, 22 Scottish Blackface ewes that lambed between 11 January and 22 February were paired by lambing date for weaning at either 6 or 14 weeks. All ewes received melatonin from 22 March until 5 August. Apart from one ewe from the 14 week group that failed to show oestrus, all other ewes responded to melatonin and exhibited first behavioural oestrus at mean intervals from initial dosing of 77 +/- 4.6 days and 75 +/- 3.2 days for the 6 week and 14 week groups, respectively. Melatonin suppressed prolactin in both dry (Experiments 1 and 2) and lactating ewes (Experiments 1, 2 and 3), but in the only experiment in which it was tested (Experiment 1), its continuous administration throughout pregnancy failed to prevent the late-pregnancy rise in prolactin. It had no effect on gestation length or the subsequent growth rate of lambs; its effect on lamb birth weight was equivocal, causing a significant decrease in twin lambs in Experiment 2 and no effect in Experiment 1. Detailed sequential measures of the nature of the episodic release of luteinizing hormone (LH) (Experiment 2) provided no evidence that melatonin induced a progressive increase in the activity of the gonadotrophin releasing hormone (GnRH) pulse generator.

The ovarian response to the daily oral administration of melatonin from March onwards contrasts with literature reports on the failure of continuous-release implants of melatonin to stimulate behavioural oestrus when inserted as early as April or May.