Owing to the wide individual variation in estrus duration and ovulation interval in the mare, pharmacologic induction of ovulation is important for breeding situations. Since the 1970s, human chorionic gonadotropin (hCG) has been demonstrated consistently to induce ovulation and has been used routinely in broodmare practices.1-3 Numerous studies have reported that a high percentage of mares with a follicle J 35 mm in diameter will ovulate within 48 hours of hCG tratment.1,4-9 Since hCG is a large glycoprotein, repeated doses over the course of a breeding season can result in high antibody levels against hCG.10,11 Some investigators have deported decreasing ovulatory response rates with repeated hCG administration, while others have failed to demonstrate an adverse effect.2,4,11 One advantage of the use of a gonadotropin-releasing hormone (GnRH) agonist to hasten ovulation is that repeated use should not diminish efficacy.12 The GnRH agonist deslorelin (Ovuplant, Fort Dodge Animal Health, Overland Park, KS) was approved for use in mares in the United States in 1999 in the form of a controlled-release subcutaneous implant. When administered to mares with an estral follicle J 30 mm diameter, Ovuplant induces ovulation within 48 hours on over 88% of cyclic mares. 13,14 Using frequent (ie, every 2 h) examinations, McKinnon et al5 reported hCG injection resulted in a shorter interval to ovulation than deslorelin, whereas Samper et al9 reported administration of deslorelin implants resulted in a shorter interval to ovulation. One potential disadvantage to the use of deslorelin implants has been the need to remove implants at the time of ovulation in order to prevent follicular suppression and delayed return to estrus.15,16 Removal of deslorelin implants once ovulation is confirmed, while a relatively straightforward process, may be objectionable to some practitioners or mare owners.
Recent research demonstrated that deslorelin implants was effective for inducing ovulation when administered intramuscularly in a short-term biodegradable liquid.17 In 2003, a short-term-release (less than 24 hr) deslorelin (BioRelease Deslorelin injection, BET Pharm, Lexington, KY) product in a biocompatible liquid vehicle became available. This product is administered in a single dose by intramuscular injection. The goal of this study was to compare the clinical efficacy in inducing ovulation among short-term-release deslorelin injection, deslorelin implants, and hCG in cyclic mares examined once daily.