Attempt to control the day of ovulation in cycling pony mares by associating a GnRH antagonist with hCG.

This study examined whether the precise timing of ovulation in cycling pony mares could be controlled using a combination of hCG and a GnRH antagonist, with the aim of achieving optimal breeding conditions and improving fertility.

Understanding the Experiment Design

• The researchers conducted the study on 40 mares which were divided into a control group and three treatment groups – A3d, A4d, and A5d. The treatment groups were given different regimens of a GnRH antagonist (antarelix) for 3, 4, or 5 days once the dominant follicle reached a size of 28 mm. They were also given an injection of hCG on the first, second, or third day after the dominant follicle reached a particular size respectively.
• The control group, on the other hand, was only given an injection of hCG when the follicle size reached 35 mm.
• The researchers measured plasma LH, FSH, progesterone, and total estrogens in all the mares.

Key Findings of the Research

• The study found that in the treated groups (A3d, A4d, and A5d), 90%, 60% and 50% of mares respectively ovulated as expected (i.e., between 24 and 48 hours after the injection of hCG). In the control group, 70% of mares exhibited the typical response to hCG.
• In groups A4d and A5d, in some cases, the dominant follicle did not ovulate and ovulation was delayed for between 11 and 15 days after treatment ended.
• The LH surge usually seen before ovulation was suppressed in the treated mares and their estrogen levels decreased during the preovulatory peak. However, the levels of FSH hormone were not changed.
• The study found no difference in fertility between the treated and the control mares implying the treatment did not impact fertility negatively.

Conclusions of the Study

• The results suggest that with a combined treatment of antarelix and a single dose of hCG, ovulation can be planned to occur on a specific day in a 3-day period with about a 67% success rate.
• Sometimes, the dominant follicle might regress and not ovulate accounting for about 20% of cases. For these mares, ovulation is delayed by about 2 weeks.
• The growth of the preovulatory follicle needs a low concentration of the LH hormone, but the atresia induced by the GnRH antagonist is significant if the treatment continues for over 18 hours.