Alternative solutions to hCG induction of ovulation in the mare

Injection of hCG (2000-2500 i.u., i.v.) to mares when a follicle reaches 35 mm induces ovulation between 24 and 48 h. However, repeated injections induce antibodies against hCG. We report attempts to induce ovulation without this inconvenience. We called 'response' an ovulation between 24 and 48 h after treatment. The typical response to hCG was obtained in 73% (N = 145) of treated mares. After immunization against hCG, the response (0%, N = 10) was less than in nonimmunized controls (100%, N = 9). Simultaneous injection of dexamethasone and hCG resulted in induction of ovulation (71%, N = 14). However, simultaneous hCG + dexamethasone resulted in antibody formation similar to that induced by hCG alone, when injected repeatedly every 21 days. Neither GnRH (2 mg i.m.) nor partly purified pig LH (26 mg i.v. or s.c.) induced ovulation consistently (40%, N = 30 and 31%, N = 16). Crude horse gonadotrophin (60 mg i.v. or s.c.) induced ovulation (86%, N = 14). Fertility was not different from control (61%, N = 13 vs 40%, N = 10). Crude horse gonadotrophin also induced ovulation in mares previously immunized against hCG (78%, N = 9): 50 or 25 mg gave satisfactory response (86%, N = 29 and 57%, N = 40). We conclude that crude horse gonadotrophin is a good alternative to hCG for the induction of ovulation in mares.