Effect of exogenous melatonin and exposure to a ram on the time of onset and duration of the breeding season in Icelandic sheep.

Icelandic ewes, living under natural photoperiod conditions, were treated with daily melatonin (2.5 and 10 mg day-1) for 75 days from the autumn equinox. Ewes were also exposed to a ram from the autumn equinox until the end of June, for 30 days from the end of the anoestrous period (30 October), and from 11 December for 30 days during the oestrous season. Blood samples were collected three times per week from September until the cessation of seasonal ovarian activity and assayed for progesterone. Melatonin treatment did not significantly alter the time of onset or the duration of the breeding season compared with control ewes. There were no significant changes in luteal activity due to melatonin. Ewes exposed to a ram from the autumn equinox had a longer breeding period than did control ewes owing to earlier onset of ovarian activity. Exposure to a ram had a significant effect on luteal activity during the first seasonal ovarian cycle. It is suggested that insignificant response to melatonin treatments and slow response to ram exposure in Icelandic sheep were caused by entrainment of the reproductive endocrine system by the large seasonal changes in the photoperiod.