Undesirable behaviors clients often report in their mares, said Crabtree, include overt reproductive behaviors (e.g., squatting and peeing), aggression toward horses or people, and, in most cases, difficulty training or an unwillingness to respond to the rider.
The veterinarian must also rule out other sources of pain (e.g., back or sacroiliac problems, hind-limb lameness, saddle fit, dental pain, gastric ulcers, granulosa cell tumors, etc.) and perform a gynecological exam to detect abnormalities, along with an endocrine exam to measure hormone levels.
Once the veterinarian confirms hormones are to blame, “can we justify ovariectomy on grounds of behavior?” Crabtree asked.
To answer this question, he looked at the related literature from the past 30 years and identified four important papers that shine light on the topic:
- In 1993 Hooper et al. retrospectively evaluated 23 bilateral ovariectomy cases (both ovaries removed), 16 of which were for behavioral reasons, and found that 35% of the mares continued showing estrous behavior post-ovariectomy.
- In 2007 Kamm et al. performed a retrospective study of 35 bilateral and unilateral ovariectomies, 23 of which were for behavioral reasons. Based on survey results, 22% of owners were dissatisfied with the procedure, primarily due to a lack of behavior changes.
- Also in 2007, Hedberg et al. looked at the effect of administering adrenocorticotropic hormone on hormone levels in five mares before and after ovariectomy. As part of the study, they teased ovariectomized mares (exposed them to stallions) and found that they all still demonstrated sexual receptiveness. The ovariectomized mares also showed more days of estrus than when they were intact.
- In 2015 Roessner et al. completed a retrospective study of 20 bilateral ovariectomies performed for behavioral or performance reasons and compared each horse’s behavioral improvement from altrenogest therapy alone to that post-ovariectomy. While the ovariectomies resulted in significantly better outcomes than therapy using the progestogen (hormone that regulates pregnancy) altrenogest, 30% of the mares still showed estrous behavior after surgery.
Based on these study results, ovaries are not essential for estrous behavior, said Crabtree. “Ovariectomy might even make the situation worse, as estrous behavior may become irregular or persistent,” he said.
While it’s not completely clear why ovariectomized mares continue demonstrating reproductive behavior, Crabtree said it’s most likely due to estrogen originating from the adrenal gland.
“If the goal is to change aggressive behavior, an ovariectomy might be more successful,” he noted. “It’s also more likely to be curative if the cause is ovarian pain.”
Crabtree then listed alternatives to ovariectomies to control undesirable behavior in mares:
- Progestogen supplements, such as oral or injectable altrenogest (e.g., Regu-Mate).
- Prolonging the lifespan of the corpus luteum (the structure responsible for producing progesterone—which suppresses estrous behavior—post-ovulation) with intrauterine glass balls, coconut oil infusions, administering the hormone oxytocin, or pregnancy termination. Crabtree cautioned against using glass balls, however, because veterinarians have reported associated complications. Instead, he uses sterile polymethylmethacrylate spheres. Crabtree said he’s seen a high level of client satisfaction (about 75%) after administering pharmaceutical-grade coconut oil 10 days post-ovulation, contrary to recent reports that it’s ineffective.
- Inducing late diestrus (the period between estrous cycles) ovulation.
- Administering a gonadotropin-releasing hormone (GnRH) vaccine or overdose to suppress the hypothalamic-pituitary-testicular axis and, thus, ovarian activity and sexual behavior. The vaccine, however, is only available in Australasia.
Overall, he said, “undesirable behavior might be better managed with medical therapy or cycle manipulation. Aggressive mares might be candidates (for ovariectomy), but explore management changes or other causes first.”
And regardless of the reason for performing an ovariectomy, owners should be aware that estrous behavior can persist, he said.