Estrus, ovulation, and serum hormones in mares given prostaglandin F2 alpha, estradiol, and gonadotropin-releasing hormone.

A gonadotropin-releasing hormone (GnRH) was injected in mares given prostaglandin F2 alpha (PGF2 alpha) to induce luteolysis in an attempt to sunchronize ovulation. Pretreatment with estradiol-17 beta (E2-17 beta) was used to determine whether or not estradiol would enhance the release of luteinizing hormone (LH) after treatment with GnRH. Twelve mares were used in a balanced Latin square crossover design. Mares were injected with PGF2 alpha, treatment A; PGF2 alpha mgnRH, treatment B; or PGF2 alpha me2-17 beta mgnRH, treatment C. The interval +/- SEM from PGF2 alpha injection to estrus was 3.3 +/- 0.2, 3.2 +/- 0.3, and 2.3 +/- 0.1 days for treatments A, B, and C, respectively. The mean interval in days from injection to first ovulation averaged 1 day less (P less than 0.10) for treatments B (6.7 +/- 0.6) and C (6.2 +/- 0.3) than for treatment A (7.6 +/- 0.7). The furation of estrus and the interval from ovulation to end of estrus were similar among the three treatment groups. Mares treated with GnRH (treatments B and C) had a significant (P less than 0.01) two- to threefold increase in LH concentrations when compared with controls. Pretreatment with estradiol-17 beta did not appear to enhance LH release after GnRH was given, and the time of ovulation was not significantly changed by GnRH treatment.