Effect of GnRH treatment during the anovulatory season on multiple ovulation rate and on follicular development during the ensuing pregnancy in mares.

This study examined how treatment with a GnRH analogue (GnRH-A) influences ovulation and follicular development during the non-breeding season in mares. A significant increase in ovulation and multiple ovulation rates were observed with the treatment, and the effectiveness was dependent on the initial size of the largest follicle.

Introduction

• The research was designed to investigate the impact of Gonadotropin-releasing hormone analogues (GnRH-A) treatment on mares during the seasonally anovulatory period. Mares are seasonally polyestrous, meaning they experience multiple estrus cycles and typically do not ovulate during winter months.
• The researchers administered GnRH-A and Human Chorionic Gonadotropin (hCG) when the largest follicle reached a diameter of 35 mm.

Experiment 1

• The aim of the first experiment was to confirm if GnRH-A treatment could trigger ovulation in mares during the anovulatory season.
• In this experiment, treatment with GnRH-A was started on December 23, when the diameter of the largest follicle per mare was less than or equal to 17 mm. The study found that 57% of the treated mares ovulated within an average of 15.8 days as opposed to the control group where the shortest duration to ovulation was 57 days.
• This result shows that the treatment can significantly enhance ovulation during the anovulatory season in mares.

Experiment 2

• The second experiment aimed to investigate the relationship between the initial size of the largest follicle and the success of the GnRH-A treatment.
• The treatment was started on January 15, and the mares were categorised depending on the initial size of the largest follicle. A significant correlation was found where the likelihood of ovulation within 21 days increased with the size of the initial follicle. For example, in mares where the initial follicle diameter was less than or equal to 15 mm, 36% ovulated; however, when the initial follicle diameter was greater than 25 mm, 100% of mares ovulated.

Summary of Findings

• Mares treated with GnRH-A exhibited multiple ovulations at a higher rate than control mares.
• For treated mares with the largest follicle greater than or equal to 25 mm at the start of the treatment, the multiple ovulation rate was higher than those with follicles of a smaller diameter.
• The pregnancy rate for single ovulators was similar for both control and treated mares.

This research has demonstrated that GnRH treatment can induce ovulation in seasonally anovulatory mares, and that the response is influenced by the initial size of the largest follicle. This has potential implications for the management of horse breeding practices during non-breeding seasons.