It’s all in the timing: ovulation induction in the mare.

This research article focuses on the challenges in predicting ovulation timing in mares and the importance of accurate prediction for efficient horse breeding management. The paper also discusses the use of pharmacological induction or ‘programming’ of ovulation to optimize pregnancy rates.

Challenges in Predicting Mare Ovulation

• Compared to other species, predicting ovulation in mares is difficult due to the uniquely protracted luteinising hormone (LH) surge in mares.
• Accurate prediction of ovulation is critical in modern horse breeding, especially considering efficient breeding management systems using either natural cover or artificial insemination (AI).
• Optimally, each mare would be covered or inseminated once per year to achieve pregnancy. Limiting the number of breeding instances helps conserve the resources of extensively booked stallions or semen supply, and reduces the damage to the mare’s uterus that can occur with breeding.
• Minimising breeding instances also reduces the chances of developing breeding-induced endometritis, thereby increasing the pregnancy rate.

Pharmacological Induction of Ovulation

• Pharmacological induction or ‘programming’ of ovulation is usually used in breeding mare practice to better manage the timing of breeding.
• Ovulation induction agents are used to narrow the ovulation window, allowing for optimised pregnancy rates by scheduling insemination or covering at the most ideal time in relation to ovulation.
• In the UK, the two commonly used ovulation induction agents are human chorionic gonadotrophin (hCG) and a deslorelin acetate implant.
• hCG mimics endogenous LH and, if given at the appropriate time, it triggers ovulation within 36 (+/- 4) hours. Deslorelin is a GnRH agonist and triggers ovulation by stimulating the release of LH and follicle-stimulating hormone (FSH) from the anterior pituitary gland. When given at the correct stage of oestrus, deslorelin induces ovulation within 40.7 (+/- 3.2) hours.

Challenges with Ovulation Induction Agents

• Despite their efficacy, there has been a considerable interest in understanding why ovulation induction agents occasionally do not trigger ovulation within the expected timeframe.
• The most common reason is inappropriate administration timing, which is often due to over-reliance on the dominant follicle’s size and inadequate consideration of other factors like cervical relaxation, uterine oedema, and teasing patterns.
• There is ongoing debate in the literature whether repeated usage of hCG can result in less reliable responses, potentially due to the formation of anti-hCG antibodies.