Are you having trouble with a moody mare? Hauling a mare can be complicated if she exhibits physical or behavioral changes when she’s “in heat” that affects her performance ability. This article by our very own Ricci Karkula, DVM, MS, cVMA, will focus exclusively on managing estrus in performance mares.


The Reproductive Cycle
A mare’s reproductive cycle is approximately 21 days long. Estrus is the time in which a mare is “in heat.” Estrus usually lasts 5-7 days, followed by 14-16 days of diestrus or “out of heat.” Behavioral changes during estrus are associated with estrogen production from follicular development on the ovaries. After a dominant follicle has been established and ovulates, a corpus luteum (CL) forms on the ovary. The CL produces progesterone and is responsible for diestrus, in which the mare does not exhibit any abnormal behavior. After 14-16 days, if the mare is not pregnant, the CL will regress, and the mare will come back into estrus.

Owners and trainers often describe a change in a mare’s behavior during estrus by poor performance, kicking, swishing her tail or aggression. There are several approaches to managing estrus when hauling a mare that is performing, all while considering her value as a broodmare after her performance career.

It is also important to rule out any underlying lameness or health issues that are not related to your mare’s reproductive cycle. A complete physical exam and thorough lameness exam are valuable tools. Back or sacroiliac (SI) pain, or gastric ulcers, may manifest as behavioral changes as well that will not resolve with management of your mare’s reproductive cycle.


An Exam
An ultrasound exam of a mare’s reproductive tract is a helpful diagnostic to determine if the ovaries are normal in size and shape. The presence of an abnormally large or cystic appearing ovary may be indicative of a Granulosa cell tumor (GCT). GCT are the most common tumors affecting the equine ovary. These tumors are often benign (do not spread) but can be performance-limiting due to their enlarged size and hormone production. Mares may exhibit stallion-like behavior, prolonged or intermittent estrus, or prolonged periods of anestrus (not cycling). The opposite ovary is usually small and inactive due to hormone production. A blood test for anti-mullerian hormone, inhibin, and testosterone is also recommended for the diagnosis of GCT. Ovariectomy (surgical removal of the affected ovary) is the treatment of choice and the unaffected ovary should return to normal reproductive function following removal.


Once a mare has been examined and any underlying issues have been addressed or ruled out, oral altrenogest is commonly used to suppress signs of estrus in mares. It reduces undesirable behavior while eliminating hormonal distractions. Oral altrenogest is administered by mouth or on the feed once daily and should be started 3-5 days before an event. Once oral altrenogest is discontinued, mares typically return to estrus within 4-5 days. The use of altrenogest does not have any long-term effects and mares can still have a successful breeding career after competing. (It is contraindicated for use in mares with a previous or current history of uterine inflammation; avoid skin contact and wear gloves when administering.)

Acupuncture has also been shown to modify a mare’s behavior during estrus and is growing in popularity as a treatment for behavioral estrus. It has been used in conjunction with Western medicine or when a mare does not respond to conventional treatments.

A previous study reported the insertion of glass marbles into the uterus to suppress estrus. However, only a limited percentage of mares remained out of heat when using this technique. A case report published in 2016 evaluated 5 mares following long-term (>1 year) use of intrauterine marbles. It reported complications of chronic endometritis and pyometra. Although it was a small sample size, this case report highlights potential complications and subsequent risk for reproductive consequences.

Bilateral ovariectomy (removal of both ovaries) has been described as a treatment for excessive behavioral signs of estrus if the mare is not intended for use as a broodmare. Unfortunately, this procedure does not always achieve the desired outcome of behavior modification. Progesterone production from the corpus luteum on the ovaries, or an exogenous source, is needed to stop behavioral estrus. In the absence of progesterone, mares may continue to exhibit a low level of estrus behavior.

Controlling the behavioral signs of estrus in the performance mare can be a source of frustration. Consult with your veterinarian and based on the mare’s discipline and work schedule, there are several options to explore.