The effect of various gonadotrophin-releasing hormone regimens on gonadotrophins, follicular growth and ovulation in deeply anoestrous mares.

Gonadotrophin-releasing hormone (GnRH) was used in several regimens in anoestrous mares in an attempt to investigate the effects of frequency and dose on follicle-stimulating hormone (FSH) and luteinizing hormone (LH) release, and consequently on ovulation and corpus luteum establishment. Thrice daily injections of GnRH induced development of pre-ovulatory follicles, but hourly injections or constant infusions were required to induce ovulation. Hourly injections induced a much higher LH:FSH ratio in the presence or absence of ovarian hormones. When anoestrous mares were given an implant that released a potent GnRH analogue at a rate of 120 micrograms/day, 55% of mares ovulated and established corpora lutea compared with 77% at 180 micrograms/day. Even at the higher dose, all mares ovulated during or within 3 weeks of the end of treatment. Conception rate and mean day of conception were the same as in controls indicating absence of adverse effects on the reproductive axis. Although pulsatile administration of GnRH is the most effective regimen for induction of an LH surge and ovulation, GnRH analogue implants are much more easily given and are sufficiently effective to be the method of choice in many practical situations.