Mares behaving badly: when to get help

Placing glass marbles in a mare's uterus is one method used in veterinary medicine to prevent a mare from cycling.
Placing glass marbles in a mare’s uterus is one method used in veterinary medicine to prevent a mare from cycling.

It is not uncommon for mares that are usually gentle or well behaved to suddenly act up at times. Often, these occurrences seem to coincide with their breeding (estrous) cycle. A mare’s disposition may also change if she becomes pregnant, or soon after foaling.

Although behavioral problems are often reported in performance mares, they may also be present in pleasure horses.

Undesirable behaviour in mares varies widely. The most common problems are overt displays of “heat” or estrus behaviour, including squatting, frequent urinating, vocalizing, and tail raising/twitching, which owners and trainers may view as too distracting for a performance mare. Others refuse commands, won’t respond and exhibit a lower performance level. In extreme cases, which are fortunately the rarest, a mare becomes aggressive toward other horses and humans.

Sometimes, mares become so dangerous that they must be euthanized. While hormonal changes related to a mare’s estrous cycle or pregnancy can cause a mare to misbehave, it can be a mistake to assume that is the sole reason, even if the behaviour occurs during her breeding cycle or heat.

“In many cases, the reasons for the sudden change in behaviour will remain unknown,” said Dr Ahmed Tibary, a WSU professor and large animal theriogenologist or reproductive specialist.

“It may be due to subtle changes in hormonal levels. There can also be an obvious physical problem like an ovarian tumor which causes the production of male hormones. Ovarian bleeding, urinary tract infections, back pain, and vaginitis (vaginal inflammation) can be other reasons a mare may alter her behaviour.

“Unfortunately, we don’t know the mind of a horse well enough to know what is going on when a physical problem can’t be found, or about the many receptors in the brain and central nervous system that trigger behaviour,” he said.

“It’s possible that some mares at certain stages may develop migraine-type complications during phases of their cycles. If this happens, they are certainly not going to be in a good mood. Others may experience muscular changes that make them hypersensitive during their cycle.”

To make a diagnosis, veterinarians often begin with a mare’s complete history, physical examination, and possibly a thorough reproductive examination. If a physical problem can’t be found, the mare’s owners, trainers, or caretakers will have to precisely define the unwanted behaviour and then keep a daily journal and perhaps video of any reproductive events and changes in the mare’s behaviour or performance. This should help define whether estrous is the cause of the unwanted behaviour or if something else is at play.

“The communication between a veterinarian and the owner, breeder, or trainer is imperative to making a diagnosis,” Dr. Tibary said.

Mares who are normally well behaved can act up when their breeding (estrous) cycle occurs.
Mares who are normally well behaved can act up when their breeding (estrous) cycle occurs.

“A lot of horse owners expect to figure out the problem in a single visit to their veterinarian, but it is usually not that simple. A lot of people like to link unwanted behaviour to breeding times, so it would be ideal to examine the mare when she is displaying the objectionable behaviour and then do a thorough reproductive examination. In a way, the easiest thing to deal with is a gross abnormality like a tumor that can be removed or at least be established with precision as the problem. In many cases, a lot of time and documentation is required before an appropriate treatment plan can be implemented.”

If estrous is determined as the cause, there are several options available to stop a mare from cycling or suppress the behaviour of heat, including hormonal treatments. “Progesterone or altrenogest are the only hormonal treatments known to stop a mare from cycling,” Dr. Tibary said.

“Another treatment is to place glass marbles in the uterus. Nomads in Africa discovered this method thousands of years ago by placing date pits in the uterus before glass marbles were found as an alternative. Some may also turn to chiropractic techniques and feed management and supplements, although these options are considered alternative medicine and lack some evidence of efficacy. I do not recommend owners try these techniques without veterinary supervision.”

Another method commonly used to control a mare’s breeding cycle is surgery to remove the ovaries. This procedure, called an ovariectomy, is commonly performed by equine surgical specialists at WSU.

“Unfortunately, this may not be a good treatment for every mare that has behaviour problems related to her cycle,” Dr. Tibary said.

“Sometimes, taking the ovaries can exacerbate or increase estrus behaviour. Up to 30 percent of mares will act constantly in heat because the organs regulating their cycles were removed.”

For more information about undesirable behaviour in mares and estrus management, contact Dr Ahmed Tibary at 509-335-1963 or, or the WSU Veterinary Teaching Hospital at 509-335-0711.

This article, which first appeared on in July 2009, is courtesy of the Washington State University College of Veterinary Medicine.

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One thought on “Mares behaving badly: when to get help

  • August 3, 2015 at 1:56 am

    I am curious as to how a mare with out ovaries can display heat. Where are all those hormones now coming from?


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