Mares Behaving Badly: Is it Estrus or Something Else?

Last Updated on February 22, 2022 by Allison Price

Learn how to control your mare’s antics and determine if they are caused by estrus.

Normal vs. abnormal Mare Behavior

Mares are seasonal breeders and cycle between May and October. A normal cycle includes approximately seven days of estrus followed by a period of 14 days of diestrus (when she’s not in heat).

Ryan Ferris DVM, MS and Dipl., states that a mare in diestrus, which is the active rejection of her stallion, can be trained or ridden. ACT is an equine reproduction specialist at Summit Equine in Newberg, Oregon. “The mare might exhibit an unattractive attitude during the seven-day period. She may squeal at other horses, urinate small amounts often, (becoming) easily distracted (she’s not in the arena but her mind is), etc. These are all normal behaviors associated to estrus.

He continues, “Mares don’t cycle all year.” “Most mare owners/riders are happy with their mares behavior in the late fall, winter and early spring.” These mares show no signs of estrus as their ovaries shrink and become inactive.

This is normal behavior. However, it can be difficult if the mare is in training or showing.

Mares Behaving Badly

Is it Estrus?

Sheerin also wants to know the details of the behavior of the naughty mare. He says that mares can urinate and swish their tails when they are angry or in pain. This is what people mistake for being in heat. A mare in heat will have a different posture and appearance than a mare who is upset. A mare in heat will sit down with her tail up and pass some urine. She may also ‘wink’ the clitoris.

The angry mare, on the other hand, swings her tail like an engine and passes more urine. She may also clamp her tail.

He says, “After evaluating the behavior, we examine the mare’s reproductive tract through palpation and ultrasound in order to determine if there are any abnormalities.” Do the ovaries appear normal? During breeding season, her ovaries should have the correct structures for the stage she is in. When she is in heat, her cervix should be open and have a large follicle. 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. You can also draw blood to check progesterone levels. If a mare is not in heat, it should have higher progesterone.

Mares can be made moody by problems

Ovarian tumors, bladder infections, and musculoskeletal problems are all possible causes of mimicking estrus behavior.

Ovarian tumors

Ferris says that these tumors can secrete hormones that could cause the mare signs of persistent estrus. Or she might never show any. Ferris says that some mares with these tumors can show stallionlike behavior, such as aggression toward other horses, and may try to herd other horses in the pasture or mount others.

Similar Content: Common and Uncommon Ovarian Anomalies

Affected mares often have cresty necks and act like stallions. Ferris says that these traits would indicate a granulosa or granulosa Theca cell tumor. To determine if the mare has an ovarian tumor, it is important to examine her ovaries with ultrasound and blood hormone testing.

If the mare has a tumor, the affected ovary can be removed. Sheerin states that the tumor-effected ovary secretes testosterone and other hormones. If we remove it, we will not see aggressive behavior.

Ferris says that depending on her plans to breed, the owner may opt to have one or both of her ovaries removed. The mare can reproduce normally with one ovary if the owner wishes to have future foals. The effects of the hormones from the tumor can take a while to heal, but it is possible for the mare to reproduce normally even if the affected ovary is absent.

Ferris says that a mare will not cycle again until six to nine months after a granulosa-cell tumor has been removed.

The mare should be able to resume normal cycle after the remaining ovary has healed, according to our sources. She should behave the same way as before the tumor.

Bladder/urine infection

This condition can cause irritation and discomfort in the bowel. After she examines her, the veterinarian will administer the necessary antibiotics to treat the infection.

Sheerin says that a mare with a urinary tract problem may have an inflamed genital area. This could cause them to urinate and posture frequently.

Mares who pool urine in their vagina or uterus also do this, 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. Poor perineal conformation can lead to mares aspirating air into their vulva (windsucking which invites contamination, vaginal, and cervical inflammation), and if there is a recessed anus, the vulva could allow feces to fall into it.

Sheerin says, “We consider what percent of vulva is below or above the brim” The pelvic brim is almost like a shelf. It has the anus on top and the folds below. Normal, healthy mares have the anus align vertically. He says that only one-third (the vulva’s) should be at the brim and two-thirds at the bottom. To reduce the tilt, poor seal or excessive vulva, we do a Caslick’s procedure. We suture the lips together to fix the vulva.

Lameness or pain

Sheerin says that discomfort can cause mares to be cranky. Sheerin says that mares who are in pain will display this behavior more often or when she has to work than the cyclical pattern of their estrous cycle.

Some mares experience colic symptoms due to painful ovulation. Sheerin says that it could happen repeatedly or occasionally. It helps to keep track of the times it happens to help you figure it out. Rectal palpation is not always used to diagnose colic in mares.

Non-steroidal anti-inflammatory medications that your veterinarian may prescribe to ease colic pain can also be used to alleviate ovarian pain. If the mare “colics”, it could be a sign of ovulation. Sheerin claims it is possible to relieve the pain by suppressing your estrus.

How to Stop the Estrus

There are many ways that Equine Veterinarians can prevent their horses from becoming pregnant. The most common method is to give the progestin altrenogest daily orally (e.g. Regu-Mate). Ferris says that this is the best and most reliable method. Ferris says that Regu-Mate is a simple and affordable way to test if a dog’s behavior is related to estrus.

If the mare continues to behave, it is likely that her problem is not related to her estrous cycle.

Other options include injectable estrogen products. Ferris says that these can be effective, and are similar to Regu-Mate. The injection would be given weekly or daily depending on the formulation to inhibit estrogenic signs. As with all injections, there is a risk that mares may develop soreness around the injection sites. This could hinder performance and make it difficult to train.

Depo-Provera is another synthetic progestin. Researchers at CSU found that the human birth control drug did not prevent mares from showing signs of estrus behavior or cycling normally in a study. Ferris said that progesterone and other progestins have a calming effect. However, this might make the behavior more tolerable.

Horse owners and trainers have tried cattle growth hormone implant to stop estrous behavior. These implants contain progesterone which is slowly released into the body over several weeks at a low level. Ferris says that cattle implants do not work well in stopping estrus and modifying behavior. The implant releases only a few milligrams per day. To modify estrous behavior, 150 mg progesterone (or equivalent) is needed.

Putting a glass marl in the mare’s uterus is an option that is not recommended. The theory was that the marble would make the mare think she is pregnant. This will allow her to keep and reform CLs. Ferris states that only a few mares have experienced long-term problems after marbles were left on the ground for years. These include the marble breaking and the accumulation of uterine fluid from the uterus trying the clear the foreign body. This can negatively impact their chances of becoming broodmares.

Some mares will eject the marbles to prevent their estrus. Ferris states that this method has fallen out favor for all of these reasons.

“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 keep her CL for 60-90 days, secreting her progesterone and keeping her from returning back to estrus.” Ferris states that 75-80% of these mares have CL.

He says that the downside of this protocol is the need to pinpoint the day and time of ovulation. This means that these mares would have to undergo frequent ultrasound examinations, which increases the cost.

I often recommend that owners who are unsure if their behavior is due to estrus try Regu-mate for a 30-day period to determine if it stops.Dr. Ryan FerrisAnother option to administer the hormone is Dr. Ryan Ferris Intramuscular injection of oxytocin . Ferris says that this is now very popular because mare owners can administer daily injections. There are no adverse effects to the mare. However, you will still need to determine when the mare ovulates in order to determine when to begin daily injections.

Ferris says that one problem with the peanut oil or oxytocin protocol is that the persistent CL each induces can last between 60 and 90 days. Veterinarians are unable to predict how long it will last. He says, “It would be a terrible thing to assume that a show horse will keep her CL through an important show–and then have the mare come into estrus.” Clients who use these protocols often ask me to recommend that Regu-Mate be used for major shows and events. I suggest it for a brief time starting one day before the show and continuing throughout the show.

Regu-Mate is a substance that horses owners are trying to avoid because it is easily absorbed by the skin of humans. It can disrupt women’s periods and cause miscarriages.

Although a vaccine is available to prevent mares from going into estrus, it has not been approved by the FDA and is not being marketed in the United States. “Equity is an anti-GnRH (gonadotropin-releasing hormone) vaccine available in Europe and Australia,” says Ferris. “This product blocks the action of GnRH and stops the growth of follicles. It can be used for up to two years.”

Researchers developed a slightly different product to use in feral horses herds. This was done to reduce the population explosion in Western rangelands. However, the commercial porcine zona-pellucida vaccine is only available in Europe or Australia.

Ovariectomy or ovary removal are other options to end estrous behavior. Ferris says, “If someone is serious about this option, we work together to evaluate the mare in anestrus, which typically occurs in January or February.” “Her behavior when she isn’t cycling is very similar to her behavior as an ovariectomized mare. Ovariectomy might not be able to change the mare’s behavior if she isn’t cycling.

These are the main ways to suppress estrogen, but some owners turn to other supplements to reduce estrogen-related behavior. These supplements can have calming effects.

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

Take-Home Message

Discuss any undesirable behavior with your veterinarian. You can also review the treatment options available to you. You shouldn’t choose a method just because your neighbor or barnmate has used it well. It might not be right for you.

“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. Ferris says, “Usually I have a conversation with the owner/trainer to help them understand what they are trying to achieve and what their goals for their mare.” It is possible that a different program may be assigned to each mare if they are working with multiple mares. This is based on their long-term goals.

Allison Price
Allison Price

I’m Allison, born and raised in San Diego California, the earliest memory I have with horses was at my grandfather’s farm. I used to sit at the stable as a kid and hang out with my Papa while he was training the horses. When I was invited to watch a horse riding competition, I got so fascinated with riding!