Effects of melatonin implants in pony mares 1. Acute effects

The effects of melatonin implant treatment over a four week period on LH, estradiol (E 2) and progesterone (P 4) secretion during the breeding season were studied in ovary-intact and ovariectomized pony mares. Mares with melatonin implants had significantly higher daytime melatonin concentrations than mares with sham implants (P=0.0065). In ovariectomized mares, LH secretion did not differ between mares with melatonin and sham implants. In ovary-intact mares, melatonin implants altered the pattern of LH secretion (P=0.0023) in such a way that an increase in LH secretion was observed during the periovulatory period. Estradiol and P 4 secretion were unaffected by melatonin implants. These results suggest that constant administration of melatonin may enhance the secretion of LH during the periovulatory surge but does not adversely affect E 2, P 4or basal LH secretion in mares during the breeding season.