GnRH Administration Increases Synchronisation of Ovulation in Mares

The objective of this study was to evaluate the effects of exogenous GnRH administration at the beginning of estrus synchronisation in mares during the spring transitional period. Estrus was synchronised using a progesterone releasing intravaginal device (PRID). The PRID was left in the vagina for 10 days, followed by an injection of 0.4 mg of cloprostenol at PRID removal. The GnRH group (n = 32) was subjected to intramuscular administration of 100 μg of the GnRH agonist triptorelin at PRID insertion, while the control group (n = 32) received 1 mL of sterile physiological saline solution. Ovulation was induced by an intramuscular injection of 3000 IU of human chorionic gonadotropin until the dominant follicle reached a diameter of 35 mm. The mares were examined and insemination was performed. Subsequently, insemination was carried out every 12 h until ovulation. Transrectal palpation and ultrasound were carried out 15 days after ovulation to confirm the presence or absence of an embryonic vesicle. The days of ovulation induction and insemination in the control group were more dispersed than in the GnRH group. Compared with the control group, the time of ovulation induction and insemination in the GnRH group were accelerated and concentrated. In summary, GnRH given at the beginning of the estrus synchronisation program significantly increased synchronisation of ovulation in mares; however, it did not increase pregnancy rates.