Effect of constant administration of a gonadotropin-releasing hormone agonist on reproductive activity in mares: preliminary evidence on suppression of ovulation during the breeding season.

The study explored the impact of administering a gonadotropin-releasing hormone agonist consistently on the reproductive activity of mares during the breeding season. Notably, preliminary evidence displayed ovulation suppression in mares during the breeding season.

Study Design and Methodology

• Twenty-four mares exhibiting estrous cycles were randomly divided into six groups. Four mares were selected for each group.
• On the designated day (May 29), mares in different groups received different treatments. While the control group did not receive any treatment, the others received varying doses (ranging from 120 micrograms to 1,200 micrograms) of a gonadotropin-releasing hormone (GnRH) agonist per day for 28 days through a subcutaneously implanted depot. The final group received 120 micrograms/day of the GnRH agonist for 84 days, administered three times at 28-day intervals.
• A pretreatment period was observed from April 19 to May 29. After this period and for the subsequent 90 days, researchers monitored the follicular development and ovulation using transrectal ultrasonography at every 2 to 3 days interval.
• Blood samples were collected on each occasion to evaluate the luteinizing hormone (LH) and progesterone levels.
• Estrous behavior of the mares was monitored by teasing with a stallion.
• The stage of the estrous cycle at the time of initiating the agonist treatment was random, and all mares in the study ovulated either 11 days before or after the treatment.

Observations and Findings

• Three out of four mares in the control group exhibited estrous cycles throughout the study duration, with their interovulatory intervals ranging from 18 to 26 days.
• In contrast, one mare from the control group had an elevated progesterone concentration noticeable after asynchronous dual ovulation during the pretreatment phase. This particular mare suggested the presence of a persistent corpus luteum, pointing towards ovarian conditions.

Significance of the study

• The research sheds light on how the administration of a GnRH agonist can influence the reproductive activity of mares during the breeding season.
• The preliminary findings suggest that constant administration of a GnRH agonist could suppress ovulation in mares during the breeding season. This could lay down the foundation for developing reproductive control methods in equine species. However, further thorough research would be required to confirm these initial observations.