EMBRYO LOSS FOLLOWING GNRH-INDUCED OVULATION IN ANOVULATORY MARES

Two experiments were conducted using a 21-day GnRH analogue treatment regimen to induce ovulation in seasonally anovulatory mares. In Experiment 1, nontreated (n=20) and treated (n=83) mares were defined as having inactive ovaries (largest follicle less-than-or-equal-to 15 mm) at the start of treatment (February). In Experiment 2, nontreated (n=7) and treated (n=10) mares were defined as having active ovaries (largest follicle 21 to 29 mm) at the start of treatment (April). Fewer mares ovulated that had inactive ovaries at the start of treatment compared to mares that had active ovaries (26% versus 70%). In mares that had inactive ovaries at the start of treatment, the pregnancy rate on Day 11 (ovulation=Day 0) was not different between treated (14/22; 64%) and nontreated (14/20; 70%) mares. However, the embryo-loss rate between Days 11 and 40 was significantly higher in treated (9/14; 64%) than in nontreated (1/14; 7%) mares. Losses tended to occur more frequently between Days 15 and 25. Averaged over Days 3 to 18, there was a significant main effect of group for the diameter of the largest follicle and for circulating concentrations of FSH, LH and progesterone in mares that had inactive ovaries at the start of treatment. The group effect was attributable to decreased mean values for each end point in treated mares, especially in treated mares that had embryo loss. A significant decrease in progesterone on Day 15 in treated mares with embryo loss corresponded to the first day that embryo loss was detected. In mares that had active ovaries at the start of treatment, pregnancy rate, embryo-loss rate, and ovarian and hormonal end points were not significantly different between the treated and nontreated mares. The results indicated that hormonal capacity was reduced and embryo-loss rate was high following GnRH-induced ovulation in mares that had inactive ovaries at the start of treatment.