Mares Behaving Badly: Is it Estrus or Something Else?

Find out if your mare’s frustrating antics are due to estrus, and learn about ways to keep them in check.

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Mares Behaving Badly: Is it Estrus or Something Else?
Figuring out the root of the behavior is the first step in trying to address it in a positive way. | Photo: iStock

Is it estrus or something else?

Rides on Ruby haven’t been much fun lately. She swishes her tail, screams to horses near and far, refuses to respond to cues, and just has a generally unpredictable attitude.

It’s not unusual for mares to exhibit behavior changes related to their estrous cycles. Sometimes the behavior interferes with their management, training, or performance, which can be frustrating for owners, handlers, and riders. It’s no surprise, then, that when a mare is particularly moody or distracted, we blame her demeanor and actions on estrus. In some instances, however, the unwanted behavior stems from some other problem entirely. Figuring out the root of the behavior is the first step in trying to address it in a positive way.

Normal vs. Abnormal Mare Behavior

Mares are seasonal breeders that cycle from about early May through October. A normal cycle consists of roughly seven days of estrus and a 14-day period of diestrus (when she is not in heat).

“The behavior of a mare in diestrus (which involves active rejection of the stallion) generally is not objectionable for training or riding her,” says Ryan Ferris, DVM, MS, Dipl. ACT, an equine reproduction specialist at Summit Equine, in Newberg, Oregon. “During the seven-day period of estrus, the mare may show an undesirable attitude—squealing at other horses, urinating small amounts frequently, (becoming) easily distracted by other horses (she’s in the arena, but clearly her mind isn’t), etc.” These are all normal behaviors associated with estrus.

“Mares generally do not cycle year-round,” he continues. “Often mare owners/riders are quite happy with their mares’ behavior in late fall, winter, and early spring. The ovaries become smaller and inactive, and these mares don’t show signs of estrus during that time.”

While this is normal behavior, it can be challenging if the mare’s in training or showing.

Is it Estrus?

Peter Sheerin, DVM, of Nandi Veterinary Associates, in New Freedom, Pennsylvania, says it’s important to determine whether unwanted behavior is or isn’t related to estrus. “Often clients don’t keep track of (behavior) to see if it is actually happening on a cyclical basis,” he says. “If it is related to the estrous cycle it should be happening every 21 days or so. I usually advise clients to start writing it on a calendar. And if the bad behavior continues in winter, it’s likely not related to the reproductive cycles.”

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Sheerin also requests a detailed description of the naughty mare’s behavior. “Many mares, when they are in pain or angry, may urinate and swish their tails repeatedly, and people think they are in heat,” he says. “The posture and presentation of a mare that is urinating when she is in heat is different from a mare that is upset. The mare in heat will squat, with tail raised, and pass a little urine and may also ‘wink’ the clitoris.”

By contrast, the angry mare usually swings her tail like a windmill and passes much more urine. She might also clamp her tail.

“After evaluating behavior, we look at the mare’s reproductive tract via palpation and ultrasound to see if there are any abnormalities present,” he says. “Do the ovaries look normal? During the breeding season … her ovaries would contain the appropriate structures related to the stage of her cycle. If she is in heat, she should have a big follicle and uterine edema (fluid swelling), and her cervix would be relaxed. If she is out of heat she should have a corpus luteum (CL, the progesterone-producing structure formed after the follicle releases the egg) in the ovary and her cervix should be tightly closed, with no edema. We can also pull blood and look at progesterone levels. A mare that is not in heat should have elevated progesterone.”

Problems That Can Make Mares Moody

Physical problems that can cause behaviors mimicking estrus include ovarian tumors, urinary tract/bladder infections, and musculoskeletal pain.

Ovarian tumors

“These tumors secrete a range of hormones that may cause the mare to exhibit signs of persistent estrus, or she may never show any signs of estrus,” says Ferris. “Some of these mares may show stallionlike behavior and become aggressive toward other horses.” These mares might try to herd horses around in the pasture and form a harem, for example, or they might mount other horses.

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Affected mares not only act and sound like stallions but also often develop a cresty neck. “With those traits we would suspect a granulosa cell tumor or a granulosa theca cell tumor,” says Ferris. “The mare’s ovaries should be evaluated with ultrasound and blood hormone testing to determine if that mare actually has a tumor on her ovary.”

If it turns out the mare does have a tumor, removing the affected ovary will fix the problem. “The ovary with the tumor (secretes) testosterone along with other hormones, so if we take that away we won’t see those aggressive behaviors,” Sheerin says.

The owner might opt to have both ovaries removed, depending on plans to breed her, says Ferris. If the owner wants future foals, the mare can reproduce normally with just one ovary. It takes a few months to recover from the effects of the tumor’s hormones, however, even in the absence of the affected ovary.

“Usually a mare won’t start cycling again for at least six to nine months following removal of a granulosa cell tumor,” says Ferris.

Once the remaining ovary recovers, the mare should resume normal cycles, say our sources, and behave as she did before the tumor.

A urinary tract/bladder infection

This condition might cause perineal discomfort and irritation. After the veterinarian examines her, he or she administers the appropriate antibiotics to resolve the infection.

“Mares with a urinary tract infection may have an inflamed vagina,” says Sheerin. “This may cause them to posture and urinate frequently.”

Mares that pool urine in the vagina or uterus do the same thing—again, due to inflammation. “Checking them with a vaginal speculum can help rule these problems in or out, along with looking at the perineal conformation—whether the vulva is normal (relative to the anus) with a good seal to the lips,” says Sheerin. A mare with poor perineal conformation might aspirate air into the vulva (windsucking, which also invites contamination and vaginal, uterine, or cervical inflammation), and if the anus is set back, or recessed, from the vulva, feces might fall into the vulva.

“We look at what percent of the vulva is above or below the brim of the pelvis,” says Sheerin. The pelvic brim can serve almost like a shelf, with the anus above it and the folds of the vulva below. In a normal, healthy mare, the anus and the vulva align vertically. “Only about one-third of (the vulva) should be above the brim, and two-thirds below,” he continues. “If there is excessive tilt or not a good seal, or too much of the vulva above the brim of the pelvis, we perform a Caslick’s procedure and suture the lips of the vulva together, to see if that helps diminish the behavior.”

Lameness or pain

“Discomfort can make her cranky,” says Sheerin. “If a mare is in pain, she will show that behavior frequently or whenever she has to work, versus the cyclical pattern of her estrous cycle.”

Some mares show colic symptoms because of painful ovulation. “They may do it repeatedly, or just now and then,” Sheerin continues. “It helps to keep track of when it occurs to figure it out. Not all mares that colic are examined via rectal palpation, so it may be hard to link (painful ovulation and colic) if the mare is simply treated for colic and her ovaries are not examined.”

The non-steroidal anti-inflammatory drugs your veterinarian might prescribe to relieve colic pain also generally ease ovarian pain. But if the mare “colics” again in three weeks, you could suspect it’s related to ovulation. Sheerin says it’s possible to resolve the pain by suppressing estrus.

Mares Behaving Badly: Is it Estrus or Something Else?
Another option is an injectable progesterone product. The downside, as with any injection you have to give repeatedly, is the risk that some mares could develop soreness at the injection sites. This could inhibit performance or the ability to train. | Photo: Erica Larson/The Horse

How to Halt Estrus

Equine veterinarians have several ways to prevent estrous behavior. The classic method is daily oral administration of the progestin altrenogest (e.g., Regu-Mate). “This is the tried-and-true method that works,” says Ferris. “Often what I recommend to an owner who is wondering if behavior is associated with estrus is to do a 30-day trial of Regu-Mate, to see if that behavior ceases.” He adds that this method is both easy and inexpensive.

If the behavior doesn’t stop, then the mare’s problem is probably not associated with her estrous cycle.

Other options include injectable progesterone products. “These can work well, similar to what we can achieve with Regu-Mate,” says Ferris. “To inhibit estrous signs you would give the injection daily or weekly, depending on the formulation. The downside, as with any injection you have to give repeatedly, is the risk that some mares could develop soreness at the injection sites. This could inhibit performance or the ability to train.”

Another synthetic progestin is Depo-Provera. In a study at CSU, researchers showed that this human birth control drug does not stop mares from cycling normally or showing signs of estrus behavior to a stallion. Progesterone and progestins do have a calming effect, however, which Ferris says might alter the behavior enough that it becomes more tolerable.

Some horse owners or trainers have tried to use cattle growth hormone implants to inhibit estrous behavior. These implants contain progesterone, which is released gradually into the body at a low dose over several weeks. “The cattle implants are ineffective at stopping estrus or modifying behavior, as the implant only releases a few milligrams of progesterone a day, and 150 mg of progesterone or equivalent is required to modify estrous behavior,” says Ferris.

An option that is no longer recommend for use is placing a glass marble into the mare’s uterus. The idea was that the marble causes the uterus to think the mare is pregnant, which causes her to maintain and reform CLs. Ferris says that a small percentage of mares have suffered major long-term ramifications after these marbles were left in place for several years (such as the marble breaking or chronic uterine fluid accumulation from the uterus trying to clear the foreign body), negatively affecting their ability to become broodmares later in life.

Some mares eject the marbles before they can prevent estrus. For all these reasons, Ferris says, this procedure has fallen out of favor.

“Another option stems from a study in England showing that infusion of peanut oil into the mare’s uterus on Day 10 post-ovulation also mimics the maternal recognition-of-pregnancy signal,” he says. “She would then maintain her CL for 60 to 90 days, secreting her own progesterone, keeping her from returning to estrus.” This accomplishes what the marbles do, only more safely and reliably—Ferris says 75-80% of these mares maintain a CL.

“The downside to this protocol is that you need to pinpoint the day of ovulation, so these mares would have to have frequent ultrasound examinations,” which adds to the expense, he says.

Another option is to administer the hormone oxytocin intramuscularly. “This is now quite popular and easy, since owners can administer the daily injections and there are no adverse effects on the mare,” says Ferris. The downside is that you still have to determine when she ovulates, to know when to start daily injections.

Ferris adds that one issue with the oxytocin or peanut oil protocol is that the persistent CL that each induces has a life span of between 60 and 90 days, and veterinarians can’t predict exactly how long it will last. “You would hate to assume that a show mare will maintain her CL through a certain important show—and then have her come into estrus during that show,” he says. “What I often recommend to clients who are using these protocols is that for any major shows or events they should put the mare on Regu-Mate for a short time, starting a day or two before the show and continuing with it daily during the show.”

Some horse owners try to avoid using Regu-Mate because it is readily absorbed by human skin and might disrupt women’s menstrual cycles or cause miscarriages.

A vaccine exists to keep mares out of estrus, but it is not currently FDA-approved or marketed in the United States. “Equity is an anti-GnRH (gonadotropin-releasing hormone) vaccine available in Europe and Australia,” says Ferris. “This product stops the action of GnRH, halting the growth of follicles for one to two years.”

Researchers have studied a slightly different product for use in feral horse herds, to help curtail population explosion on Western rangelands. The commercial porcine zona pellucida vaccine, however, is only available in Europe and Australia.

Another option to eliminate estrous behavior is ovariectomy, or ovary removal. “If someone is seriously considering this option, I work with them to evaluate the mare during anestrus, which is typically January or February,” says Ferris. “Her behavior during anestrus, when she is not cycling, is similar to what her behavior would be as an ovariectomized (“spayed”) mare. If the mare’s behavior is not satisfactory when she is not cycling, then ovariectomy may not change her behavior after the surgery.”

While these are the main methods used to suppress estrus, some owners also turn to a variety of supplements to help reduce estrus-related behavior. Some of these are associated with potential calming effects.

“There is a lot of anecdotal evidence and owner comments, but there is very little science, if any, that has evaluated these products to know if they actually stop the estrous cycle or reduce some of the behaviors associated with estrus,” Ferris says.

Take-Home Message

If your mare is exhibiting less-than-desirable behaviors, discuss these problems with your veterinarian and review your treatment options to determine which one might be best for her. Don’t select a method because it’s what your barnmate or neighbor has had success with; it might not work for your particular situation.

“When you consider the various options—looking for something that works, is easy to give, and cost-effective—there are some drawbacks to every product,” says Ferris. “Usually I have a discussion with the owner or trainer to try to define what they want to accomplish and what their goals are for that mare. If they are working with several mares, there may be a different program assigned to each mare, based on the long-term goals.”


Written by:

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey’s Guide to Raising Horses and Storey’s Guide to Training Horses. Besides having her own blog,, she writes a biweekly blog at that comes out on Tuesdays.

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