Use of gonadotropin-releasing hormone (GnRH) treatment to induce multiple ovulations in the anestrous mare

A two-year study was conducted to evaluate the effectiveness of pulsatile gonadotropin-releasing hormone (GnRH) administration to induce multiple ovulations in the anestrous mare. With one exception, all doses of GnRH utilized (2, 20 and 100 g GnRH/hour) induced ovulation within a period of 11.4 ± 1.0 days from the start of infusion (overall mean ± SEM), whereas control mares (saline-infused and uninfused) did not show follicular development or ovulate within the experimental period. The exception was a single mare in the 2 g GnRH/hour treatment group which showed significant follicular development by Day 15 of infusion, but did not ovulate. This mare was subsequently induced to ovulate a single follicle by the administration of 1000 U human chorionic gonadotropin. Mares in the 20 and 100 g GnRH/hour treatment groups ovulated 2.9 ± 0.5 and 3.5 ± 0.9 follicles, respectively and this response was greater (P<.05) than in the group treated with 2 g GnRH/hour (1.3 ± 0.2 follicles ovulated). Peak pre-ovulatory concentrations of FSH, following the initiation of GnRH infusion, occurred 5 to 6 days prior to ovulation, and peak concentrations in mares across treatments were correlated with the number of follicles subsequently ovulated (R=.51, P=.013). By contrast, serum LH and estradiol-17β levels peaked one day prior, to 2 days following ovulation, and one to 2 days prior to ovulation, respectively, and these levels were unrelated to the number of follicles ovulated (P>.50). We conclude that the use of GnRH may be useful for inducing multiple ovulations in the seasonally anestrous mare, however, the extent of response is much less than that observed using conventional superovulation techniques in other domestic animal species.