Abstract
Seasonally or postpartum anestrous Suffolk ewes were divided into the following 4 groups: Groups 1 (barren, n=5), 2 (postpartum/non-lactating, n=5) and 3 (postpartum/lactating, n=5) ewes were fed 4 mg melatonin daily for 60 days from 26 June (Day 1), and control ewes in Group 4 (postpartum/lactating, n=3) were fed vehicle pellets not containing melatonin. The plasma concentration of progesterone was monitored to assess the resumption of ovulatory cyclicity. The ewes were teased by a vasectomized ram daily on Day 4 onward, and introduced to fertile rams on Day 41. The resumption of ovulatory cyclicity in postpartum ewes fed melatonin were significantly earlier than that in postpartum ewes in the control (p<0.05). However, there was no difference in the timing of the onset of ovulatory cyclicity and return to estrus between non-lactating and lactating ewes treated with melatonin. The interval from the start of melatonin feeding to the onset of ovulatory cyclicity in postpartum ewes was longer than that in barren ewes (p<0.05). These results suggest that the physiological status in early postpartum ewes results in a delay of the reproductive response to melatonin administration mimicking the short-day pattern, but that the lactational suppression of gonadal function could be overcome by the melatonin treatment.
