The effect of frequency of administration of melatonin on the time of onset of the breeding season in anoestrous ewes

Abstract

The results of a study to examine the effect of melatonin administration on the induction of ovarian activity in ewes and ewe lambs are reported. The aims of the experiment were primarily to determine if oral melatonin must be given daily to be effective; secondly, to compare the effects of oral melatonin administration and melatonin implantation on reproductive activity and on circulating levels of melatonin; and thirdly, to establish if melatonin administration affects the pattern of LH secretion. Fifty-four mature ewes (43 Cheviot, 11 Suffolk) were randomised by breed to one of five treatments: (1) Control; (2), (3), (4) dosed daily, three times or once weekly, respectively, with 3 mg melatonin at 16.00 h; (5) implanted with 700 mg sachets of melatonin. Ten ewe lambs were randomised to treatments 1 and 2. Treatments began in July and ended in September. Blood samples were collected for progesterone, LH, and melatonin analysis. Ovaries of ewes were examined by laparoscopy on 28 September. Six ewes from each of treatments 1, 2 and 5 were then slaughtered and carcass and reproductive organ weights recorded. Results of the laparoscopy showed that a significantly higher proportion of animals receiving daily oral melatonin had ovulated ( 12 12 ) by 28 September compared with controls ( 5 12 , P < 0.01 ). A higher proportion of implanted animals had also ovulated compared with controls by this date ( 8 9 , P < 0.05 ). Progesterone analysis showed that daily oral melatonin administration advanced the onset of cyclicity by 21 days (± 1.4 days), compared with controls ( P < 0.01). No significant effect of treatment on LH secretion was found. Melatonin levels were raised to physiological night time levels by both oral melatonin and melatonin implant. Oral melatonin administered daily and melatonin implantation both resulted in significantly heavier uterine weight compared with control animals ( P < 0.01). It was concluded that melatonin treatment can significantly advance the breeding season of the ewe if administered by implant or by daily oral dosing at 16.00 h. However, to be effective melatonin must be administered each day of the treatment period.