Reproductive behavior in sport horse mares can range from annoying to performance-limiting. For this reason, many mare owners and their veterinarians try to control or eliminate it. However, how do you really know if her reproductive system is to blame?

Dirk Vanderwall, DVM, PhD, Dipl. ACT, set out to address this concern and offer ways to manage undesirable behavior in his presentation at the 2017 American Association of Equine Practitioners convention, held Nov. 17-21 in San Antonio, Texas. Vanderwall is a professor of horse reproduction at Utah State University’s School of Veterinary Medicine, in Logan.

At the start of his presentation, he polled the veterinary audience and found that 79% believed that estrous behavior affects some mares’ performance. These results were similar to those from a survey of more than 750 veterinarians in which 90% believed the estrous cycle affects mares’ performance.

Based on the latter survey, said Vanderwall, the most frequently reported performance-inhibiting clinical sign associated with the estrous cycle was attitude change. Others included tail-swishing, difficulty training, squealing, excess urination, kicking, and diminished performance.

But before getting into the possible causes of these behaviors, Vanderwall reviewed mare reproductive physiology and seasonality.

The key ovarian hormones regulating reproductive behavior, he said, are estrogen and progesterone. “The primary hormonal product driving the display of estrous behavior is estrogen,” he said. “Progesterone is key to blocking estrous behavior.”

When a mare is in heat, her estrogen levels are high and her progesterone low. Post-ovulation, her progesterone levels rise.

Like many mammals, horses are seasonal breeders. In winter, mares’ hypothalamic-anterior pituitary-ovarian axis (which regulates her reproductive processes) is inactive, while in summer it’s at peak activity. During the spring and fall its activity increases and decreases, respectively.

So, many owners will see irregular or prolonged periods of estrous behavior during the spring transition, he explained, followed by more regular estrous cycles in the summer. Mares will again display irregular ovarian activity in the fall before ceasing it altogether going into winter.

During estrus, said Vanderwall, the mare’s high estrogen levels generally cause her to stand base-wide behind, lean toward stallions or other stimuli, raise and hold her tail to one side, pass small amounts of urine frequently, and display clitoris “winking.” During diestrus (the phase between periods of heat), high progesterone levels generally trigger behaviors such as agitated movement, tail-swishing, ear-pinning and tense facial muscles, urine-squirting, aggression, and vocalization toward stallions.

“In addition to their effects on mare behavior, estrogen and progesterone profoundly affect the physical and ultrasonographic characteristics of the tubular genitalia,” said Vanderwall. Thus, ultrasound is a critical tool in determining a mare’s hormonal state.

Going back to the initial list of performance-affecting behaviors veterinarians noted in the survey, it’s evident that it includes behaviors associated with both estrogen and progesterone. Furthermore, said Vanderwall, some problematic behaviors that get blamed on estrus are in fact signs of submissive behavior, urogenital discomfort, or stallionlike behavior.

So, “when evaluating an owner/trainer complaint of an estrous-cycle-related behavior or performance problem in a mare, the veterinarian should first determine whether the problematic behavior is or is not related to the reproductive status of the mare,” he said.

This includes a complete physical exam to rule out any other medical conditions, then gathering a history from the owner to determine if the behavior correlates with specific reproductive events. Then the veterinarian can perform a transrectal palpation and ultrasound examination to evaluate the reproductive tract.

If the mare is, indeed, cycling, Vanderwall recommends tracking her reproductive status over one or more estrous cycles. This involves veterinarians performing transrectal palpation and ultrasound a couple of times a week for three to four weeks. They might identify signs of pain during manual evaluation of the ovary, back sensitivity, or coliclike pain. In the latter two scenarios, the mare might benefit from an ovulatory agent to reduce the time the follicle is present on the ovary or administration of hormones to suppress ovarian follicular activity, he said.

If these serial exams confirm that the mare’s hormonal status correlates with her problematic behavior, Vanderwall said the veterinarian might prescribe hormone therapy to mimic diestrus (e.g., altrenogest), prolong diestrus (e.g., oxytocin), mimic/induce the winter anovulatory state (e.g, via ovariectomy), or mimic/induce estrus (e.g., the prostaglandin PGF2α).

Summing up, Vanderwall said veterinarians should perform a careful assessment of a mare’s hormonal status/reproductive state to determine whether there’s a definitive, repeatable relationship between it and her undesirable behavior. If so, “it behooves the practitioner to use an evidence-based approach when attempting to regulate reproductive behavior and to choose the most appropriate treatment to eliminate the underlying causes of the issue.”