Use of a GnRH antagonist, antarelix, associated or not with hCG, to control ovulation in cyclic pony mares.

The research article investigates the impact of the GnRH antagonist antarelix, either alone or alongside hCG, on ovulation control in cyclic pony mares.

Objectives and Methodology

• The primary goal of the study was to assess the effects of the GnRH antagonist, antarelix, with or without the hormone human chorionic gonadotropin (hCG), on ovulation control in mares during their periovulatory period. This was done by conducting two separate experiments to determine impacts on ovulation timing, the LH surge (a surge in luteinizing hormone leading to ovulation), and fertility.
• In the first experiment, 20 mares were divided into treated and control groups. The treated mares received antarelix for three days from the day when the dominant follicle reached a specific size (32mm).
• In the second experiment, 10 mares were divided into treated and control groups. Treated mares received antarelix for six days from D0 (the day with the first large dominant follicle) and all mares received hCG on the second day (D1).
• In both experiments, the timing of ovulation, LH surge and fertility were monitored and compared between the treated and control groups.

Key Findings

• Results from both experiments demonstrated that antarelix disrupted or entirely abolished the LH surge.
• In the first experiment, half of the treated mares displayed simultaneous ovulation with the control group, while the other half, with lesser LH concentration, ovulated about 13.4 days later.
• In the second experiment, all the treated mares ovulated at the same time as the control group.
• For the mares that ovulated during the treatment, the levels of progesterone and fertility rates did not vary from control mares.

Implications of the Findings

• These findings show that even a slight increase in endogenous LH can trigger ovulation in mares. Hence, using antarelix can be a way to control or delay ovulation.
• The study also showed that ovulation can be postponed approximately 13 days following a 3-day antarelix treatment if the treatment is initiated just before the preovulatory LH surge.
• The injection of hCG can induce ovulation even when the levels of endogenous LH are low.
• Inhibiting the post-ovulatory LH surge using antarelix does not affect the corpus luteum (the temporary endocrine structure involved in ovulation and early pregnancy stages) or fertility in the mares.