Q: Is there a reliable way to control estrous-related behavior in mares? My otherwise easy mare becomes very difficult for a few days every month or so through the spring and summer. She is pushy with her herdmates and grumpy around people—squealing when she’s groomed and shutting down when I try to ride. I’ve given up trying to work with her on certain days. I’ve heard about medications and even spaying, but I don’t really know the pros and cons of each. What are my options?

A: This is a common question, for sure. And the short answer is yes! The signs of estrus in mares vary from none at all to those you describe and more, such as kicking, reluctance to move and even signs of colic.  There are ways to suppress estrus in your mare, but before considering those options, it is important to determine if her unwanted behavior is indeed due to being in heat. The periodic display of mare-ish behavior seemingly associated with her heat certainly suggests a close association. If an investigation led by your veterinarian connects your mare’s undesirable behavior to her estrus cycle, you will want to explore the following alternatives.

Click here to learn how to read equine body language.

Several progesterone compounds—oral and injectable—will suppress the signs of estrus. I suggest you begin by testing the effect of oral altrenogest (a synthetic progestin marketed commercially under several brand names including Regu-mate, Ovamed and Altren) for two to four weeks during the breeding season. With this regimen, it is unlikely that your mare will show signs of heat, and hopefully the treatment will suppress the other marish behaviors you describe. With any progesterone compound, there are additional considerations. First, progestins lower the immunoglobulin content in the uterus, which can increase changes of a uterine infection. Second, use of these products is barred in some equestrian sports.

Nonetheless, many mare owners opt to suppress estrus with a regimen of daily oral altrenogest or by administering injectable altrenogest at 14- or 30-day intervals. The advantages of the daily administration are several: first, progestin levels stay high with daily oral dosing as opposed to a possible tapering effect of time-release injections, and there is no painful injection site or swelling that may be associated with the injectable form. However, if using several of these products, care must be taken to avoid human skin contact during its administration.

Other progestin formulations have also been tested in mares. Synovex C, a cattle hormone implant designed to promote weight gain, is touted to release progesterone and estradiol for 60 to 90 days. Research shows, however, that Synovex C is not only ineffective in suppressing estrous behavior in mares but can cause significant swelling at the injection site. Anecdotally, some horse owners and trainers have reported that suppression of undesirable behavior following its administration of this product. 

In addition, the estrus-control potential of progesterone-releasing intravaginal devices (PRIDs) has also been explored but they have been found to cause profound vaginitis and discharge.

Another hormone, oxytocin, has been shown to suppress estrus in mares for two to three months when administered seven to 14 days after ovulation or daily for 30 days. In effect, oxytocin encourages the mare’s body to release of progesterone. 

Likewise, a gonadotropin-releasing hormone (GnRH) vaccine has been shown to be effective for years at a time in suppressing estrus behavior and ovulation in mares but is not available at this time in the US. Stay tuned.

The non-hormonal therapy of placing a glass marble in the uterus was once widely popular. This technique has fallen into disfavor due to the uterine damage too often seen with the imbedding of the marble into the endometrium and/or its fracturing.

Finally, the removal of the ovaries (ovariectomy) represents a permanent solution that some owners have been pleased with the outcome. However, following ovariectomy, mares often display a low grade of estrous behavior at all times due simply to the lack of endogenous progesterone in the body. 

All options certainly have drawbacks to consider. Altrenogest remains the gold standard at present for suppressing estrous behavior in mares. A thorough discussion with your veterinarian will likely reveal the best course for you and your mare.

Lisa Metcalf, MS, DVM, DACT
Honahlee PC
Research Assistant Professor, Oregon Health Science and University
Sherwood, Oregon

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