Suppressing Estrus in Mares

Even the quietest of mares can turn into a proverbial fire-breathing dragon during her estrous cycle, which can make riding, training, competing, or handling these horses a challenge for any equestrian. Fortunately, veterinarians have effective methods by which to prevent or lessen the effects of estrus in mares while still allowing them to have a second career as a broodmare later in life.

At the 2012 Hagyard Bluegrass Equine Symposium, held Nov. 1-4 in Lexington, Ky., Ryan Ferris, DVM, MS, reviewed current options for estrus suppression in mares.

"This is a common request from owners and trainers," he relayed. "One of the more common phone calls we get at the repro center is: How can we suppress estrus in this mare?"

Ferris, a clinical instructor in the Colorado State University Veterinary Teaching Hospital Department of Clinical Sciences, said that in most cases, the behaviors or issues owners and trainers are hoping to eliminate with estrus suppression are actually things veterinarians and breeders look for in a broodmare, including tail swishing, squealing, and excess or frequent urination.

Other common complaints of mares in estrus include a poor attitude, difficulty in training, and a decrease in performance, he said. Owners and trainers aren't alone in observing the latter issues, Ferris said--a recent study revealed that 90% of veterinarians surveyed believe that the estrous cycle impacts an athletic mare's performance.

Ferris shared four factors that owners and veterinarians should consider when implementing an estrus suppression treatment. The treatment should be both safe for the mare and effective; however, it should also be reversible (so owners can breed the mare when her athletic career ends, or if she requires extensive time off due to injury) and cost-effective for the owner.

Keeping that in mind, Ferris discussed four treatment methods and more specific options within those groups.

Exogenous progesterone/progestin administration

When many horse owners think of estrus suppression, they think of exogenous progesterone or progestin administration, whether they know it or not. One of the most common estrous control drugs--altrenogest--falls into this category, Ferris said.

This orally administered drug can be used safely for extended periods of time without adverse effects on ovarian function or future fertility, Ferris said. Alternatively, owners might elect to only treat the mare with altrenogest three to four days prior to the start of a competition to ensure estrus behavior doesn't interfere with her performance.

Ferris relayed that one drawback to altrenogest is the caution required when administering the drug. Because altrenogest can be absorbed through the skin, it's recommended that humans wear protective gloves when handling the drug and that those with certain medical conditions and women who are or could be pregnant avoid handling the medication.

For that reason, some owners look for other estrus suppression options for their mares, he said.

Also falling into this category is medroxyprogesterone acetate (marketed for humans as Depo-Provera). Study results, however, showed that administering this drug to mares with the goal of suppressing estrus wasn't effective, Ferris said. Mares still showed typical estrus behavior, and the drug did not appear to inhibit follicular development or prevent ovulation, he relayed.

Similarly, researchers have found that hydroxprogesterone caproate, hydroxyprogesterone hexanoate, and melengestrol acetate--all of which are effective in controlling estrus for extended periods in humans--were not effective in doing so in horses, Ferris said.

Extending Mares' Corpus Luteum Function

Another option for suppressing estrus is extending the mare's natural corpus luteum (the structure formed after the follicle releases the egg, or ovulates, and then produces progesterone) function.

A common method to extend the function of the corpus luteum is use of an intrauterine marble. As the name suggests, when veterinarians place a glass marble in the mare's uterus, pregnancy recognition occurs and estrus is suppressed due to persistent function of the corpora lutea.

Ferris said this technique only suppresses estrus in about 40% of mares for 60 to 90 days without additional medical treatment. No significant uterine problems have been noted during treatment with this technique, he said, and mares return to estrus after the marble is removed.

Ferris also noted that research is under way evaluating using a polypropylene ball instead of the glass marble. Early research has suggested this technique is effective in developing a persistent corpus luteum, as well; however, more research is needed in the area. Additionally, these polypropylene balls are not commercially available at this time, he said.

Next, Ferris discussed infusing plant oil in the uterus to extend corpus luteus function and suppress estrus.

"We don't know exactly how plant oils result in a persistent corpus luteum, but it works reliably," he relayed.

Ferris explained that to use this method, a veterinarian infuses 1 to 1.5 milliliters of sterilized peanut oil in the mare's uterus on Day 10 post-ovulation. This technique has been shown to prolong the corpus luteum's function in about 90% of mares. The one downside to using this method for estrus suppression is the additional management required to know when to infuse the plant oil (on the day of ovulation), he said.

Oxytocin administration during diestrus has also been shown to increase the duration of mares' corpus luteum. Ferris said that injecting 60 international units of oxytocin intra-muscularly from Days 7 to 14 post-ovulation can extend the corpus luteum's function for 60 to 90 days in about 70% of mares. Oxytocin administration is not as reliable as the plant oil administration, but requires less mare management, he noted.

And finally, Ferris discussed using pregnancy to maintain the function of the corpus luteum and suppress estrus. Getting a mare pregnant and then eliminating the pregnancy at 16 to 20 days of gestation will suppress estrus reliably for 60-90 days, he said. Ferris noted that owners might be against the idea of eliminating a pregnancy and that this technique requires a large amount of mare management.

The only downside with using a persistent corpus luteum for estrus suppression, Ferris said, is that there's no way to predict when exactly during the 60-90-day period the corpus luteum will stop producing progesterone. To combat this, mare owners might opt to supplement these treatment options with altrenogest in the three to four days preceding a competition and continue throughout the duration to ensure the mare doesn't exhibit estrus behavior during that time.

Suppressing Ovarian Follicular Activity

Another method of estrus suppression is to minimize the mare's ovarian follicular activity. One option that falls into this category is to down regulate activity using analogs of gonadotropin-releasing hormone (GnRH), the hormone responsible for initiating release of both follicle stimulating hormone and leuteinizing hormone that stimulate the ovaries into producing other hormones necessary for breeding behavior. GnRH analogs might prevent mares from cycling, causing a period of low progesterone, Ferris relayed, but some mares still exhibit behavioral estrus when on this treatment modality.

GnRH antagonists are another option for suppressing ovarian follicular activity. These substances are essentially designed to nullify the GnRH's effects. While antagonists have been proven effective in suppressing follicle stimulating hormone and leuteinzing hormone, no research has been performed to determine if they suppress estrus behavior, and currently the drugs are cost prohibitive to use in equine medicine, Ferris relayed.

Another option is to "vaccinate" mares against GnRH, which stops a mare from cycling by inactivating the GnRH she produces. Simply put, the whole hormonal cascade is interrupted temporarily and the ovaries are essentially deactivated. While most mares in one study returned to cyclicity and became pregnant the following year, some young Thoroughbred mares never returned to cyclicity, Ferris noted. Several factors could have contributed to that, he noted, including whether the drug was given prior to sexual maturity, which is not recommended.


The last resort in attempting to control estrus in mares is an ovariectomy, Ferris said. While it might sound like a sure bet, he noted, simply removing the ovaries isn't a sure thing.

"Mares are still capable of behavioral estrus when there is a lack of progesterone and minimal production of estrogen from the adrenal glands," he said. In one study he described, 10 out of 10 ovariectomized mares exhibited behavioral estrus, and all 10 mares stood to be mounted by a stallion.

That said, another study showed that roughly 70% of owners were happy with the outcome of an ovariectomy, Ferris relayed. The others were dissatisfied because the mares continued to show signs of estrus following surgery, he said.

Herbal Supplements

Finally, Ferris briefly discussed using supplements or herbs to control mare behavior. Many products, he relayed, have label claims such as "formulated for the ... nutritional maintenance of a healthy hormonal and nervous system," or "specialized ingredients to support normal hormone levels and a balanced temperament." He urged caution as to these products' efficacy, as none of them have been tested for label claims.

Take-Home Message

"There's no single dose medication to suppress estrus for an extended period of time," Ferris concluded; however, there are several options available. It's recommended to consult a veterinarian to determine the ideal method of estrus control in specific mares.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.