EFFECTS OF TREATMENT WITH MELATONIN ON THE RESPONSE OF SEASONALLY ANOVULAR EWES TO THE INTRODUCTION OF RAMS

Sixty-four mature ewes (equal numbers of Bluefaced Leicester male x Swaledale female and Suffolk male x (Bluefaced Leicester male x Swaledale female) which had lambed in March were separated from their lambs on 1 June. They were randomly allocated from breed and live-weight blocks to one of four treatments arranged in a 2 x 2 factorial design. All received vaginal implants, with or without melatonin, on 1 June. The implants were removed, all ewes given 20 mg progesterone in oil intramuscularly and equal numbers isolated or introduced to rams on 12 July. Ovarian activity was monitored by progesterone concentrations in blood taken three times weekly from the jugular vein. Four ewes receiving melatonin and four control ewes were bled for 24 h on 6 July to determine plasma melatonin concentrations. Melatonin treatment increased plasma melatonin concentrations during the day. All ewes were anovular prior to 12 July. There were no differences between the two ewe types in their response to treatment. The number of ewes ovulating before 27 August was: control 1; melatonin 6; ram 16; melatonin + ram 16. The mean date of ovulation for both the ram and melatonin + ram groups was 21 July. The number of ewes showing oestrous behaviour was 16 and 15 for the ram and melatonin + ram treatments respectively. The proportion of ewes responding to ram introduction was judged as unusually high, possibly as a result of the favourable male:female ratio. Pregnancy rate was in excess of 80% for both treatments, with resulting litter sizes of 1.4 for the ram and 1.8 for the melatonin + ram treatments respectively (P > 0.05).