Effects of melatonin treatment on the onset of ovarian activity, reproductive parameters and PRL plasma levels of immature ewes

The aims of this study were to investigate the effect of chronic treatment with melatonin on prolactin plasma profiles, the onset of ovarian activity and the fertility of immature ewes. Beginning in late June, 30 maiden ewes were administered 2.5 mg melatonin (i.m.) daily until mid-September. Progesterone and prolactin plasma concentrations were determined by validated radioimmunoassays on samples collected every 5 days from 17 June until 31 October. Prolactin (PRL) plasma concentrations in the control animals were highest at the beginning of the experiment and lowest towards its conclusion. In melatonin-treated ewes, PRL levels dropped just after treatment began and were similar to those observed in the control animals at the end of the experimental period. The ovarian activity was advanced by approximately one month by the administration of melatonin, while the mean date of lambings was advanced by about two weeks compared with the controls. Fertility in the treated animals was very similar to that of the controls; the prolificacy (lambs/pregnant ewe) was 2 in the treated and 1.62 in the control ewes. © 1987.