The effect of endometrial biopsy on ovulatory interval in mares

Luteolysis with subsequent shortening of diestrus has been reported to follow equine endometrial biopsy performed early in the luteal phase of the estrous cycle, but a comprehensive study of the effect of endometrial biopsy on cycle length at different times of the cycle has not been reported. In this study, endometrial biopsies were obtained from four different pony mares on each of Days 0, 4, 8, 12, 16 and 20 after ovulation as determined by serial palpation per rectum. Compared with control cycles in the same mares, the ovulatory interval was shortened in mares from which biopsies were obtained on Days 4 and 8, it was lengthened in from which samples were taken on Days 16 and 20 and it remained unchanged by biopsies performed on Days 0 and 12. Although the changes were less consistent and smaller than those observed in cows, our findings are similar to those reported for effects of endometrial manipulation or uterine infusion on the estrous cycle of cows.