Hormonal therapy is often used in performance mares to prevent estrous behavior. A wide variety of options are currently available, including hormonal, nonhormonal, and immunologic methods to modulate estrous behavior in mares.

Induction of a Prolonged Luteal Phase

Hormonal therapy includes the induction of a diestrous ovulation using 3000 IU of hCG when a diestrous follicle reached >30 mm, which results in a prolonged luteal phase. In one study not all mares developed a diestrous follicle of that size and not all treated mares ovulated.⁴¹ Similarly the prevention of luteolysis using twice daily treatments with 60 IU oxytocin from day 7–14 also creates a prolonged luteal phase in a high proportion of mares.⁴²

An indirect hormonal method to prevent estrous behavior is the use of intrauterine devices. A 20 mm diameter water-filled ball introduced into the uterus on day 2–4 post-ovulation resulted in a prolonged luteal phase in 75% of mares, for a mean diestrous period of 57 days. The mares with the prolonged luteal phases did not release prostaglandin at the normal time. Glass balls (marbles) 35 mm in diameter have also been used to induce a prolonged luteal phase in 22%–40% of mares. The authors described the insertion of the glass ball during estrus. Some mares reportedly expelled the glass balls, while others retained the glass balls and continued to cycle. Insertion during early diestrus may improve the success with glass ball devices.^43,44

Hormones, Implants, and Depot Injections

The use of oral progestagen altrenogest (0.44 mg/kg) is the common method of preventing estrous behavior. Altrenogest causes essentially no side effects in mares, and it may be used for prolonged periods. The intravaginal device (CIDR) can be used for short-term estrus suppression. Daily injections of progesterone (150 mg/mare) or a long-acting progesterone preparation may be administered once weekly. All of these treatments should be initiated at least 3 days before the time the mare is required to be out of heat, because if the mare is in estrus at the time of treatment, it takes a few days for the behavioral suppression to occur.¹⁶ Hormonal implants such as Synovex S, which contains 25 mg of progesterone and 2.5 mg of estradiol benzoate, have been administered in mares to suppress estrous behavior. In one study 80 implants were administered to mares, and all returned to estrus on time. There is no evidence to suggest that these implants alter the expression of estrus; therefore, if there is a beneficial effect on mares, it is exerting its effect through some other means.⁴⁵ Similarly, long-acting hormonal products such as medroxyprogesterone acetate (Depo-Provera) are not effective in regulating estrous behavior.

Anti-GnRH Vaccines

Immunologic means of preventing estrous behavior include the use of anti-GnRH vaccines. The hormone GnRH is a small decapeptide and is non-antigenic, so the GnRH is conjugated to another molecule to which antibodies are produced when injected. Two vaccinations are given 2 weeks apart. This protocol resulted in 98% of vaccinated mares becoming anestrus or transitional by 4 weeks after the second vaccination. In the breeding season after vaccination, 88% of mares had estrous cycles, and in the year after that 98% had estrous cycles.³⁸ Behavior of the mares vaccinated with anti-GnRH is often described as passive, similar to that of anestrous mares.^46,47