Timing of breeding a mare can be critical, especially when using shipped semen from a performance stallion that has a limited schedule for collection, or when foaling time dictates maturity of young horses for sale or competition. Ovulation timing is also relevant to financial costs of breeding a mare—the more accurate the time of ovulation, the fewer veterinary visits and ultrasound exams required to get a mare in foal. Historically, human chorionic gonadotropin (hCG) has been the mainstay for inducing ovulation, but in recent decades, an alternative GnRH agonist, buserelin, has been used.
A GnRH analogue to induce ovulation in mares is reported to have reduced efficiency in the spring transitional phase when the mare is coming out of winter anestrus. An Italian study compared the use of hCG with buserelin during the first estrus cycle of the season. [Fanelli D, Tesi M, Rota A, et al. hCG is more effective than the GnRH agonist buserelin for inducing the first ovulation of the breeding season in mares. Equine Veterinary Journal Apr 2021; DOI: 10.1111/evj.13455]
In the three-year study, 62 mares were pre-treated with sulpiride as a dopamine-2 antagonist to remove the inhibitory effects of dopamine, normal to the anovulatory season. The study was divided into two phases related to day length to review effects based on either early or late transition periods. Ovarian activity is stimulated best with 14.5 – 16 hours of daylight. Transrectal ultrasound was performed twice a week to measure follicles and grade uterine edema in correlation to mare responses when teased by a stallion.
A mare showing full estrus posture for three consecutive days and with grade 2 or 3 uterine edema was considered to be in full estrus. Mares with a follicle diameter of > 35 mm were assigned to either an hCG (2500 IU IV) group or a GnRH agonist (GnRHa; 1 mg subQ) group. Ovulation was induced the following day using the medication assigned to that horse’s group. It was expected that ovulation occurs within 24-48 hours following treatment.
Ovulation rates were comparable through all three years of the study: Mares treated with hCG ovulated 90.9% of the time compared to GnRHa 37.9% of the time. As might be expected, more mares (80%) ovulated after treatment in the late transitional phase compared to 53.1% in the early transitional phase. In addition, more mares identified to be in full heat ovulated more than mares not in full heat—78% versus 42.9%.
The two agents work differently according to the authors: “hCG has an LH-like activity and induces the final follicular maturation and ovulation by acting directly on follicular receptors, whereas GnRH (and exogenous GnRHa) stimulates LH release from the adenohypophysis which, in turn, stimulates final follicular maturation and ovulation.” Transitional mares have a lower LH content in the pituitary gland so are less responsive to GnRHa.
The authors conclude: “hCG was found to be much more effective than GnRHa in inducing the first ovulation of the breeding season in transitional mares.”