Tackling the Behavior of Mares in Heat

Some horse owners say that if you earn the trust of a mare, she will try harder for you than any other animal. In a social world where “ISO geldings only” advertisements rule and only “non-mareish mares” need apply, those that have truly loved a mare will agree to disagree. People with true-blue mares know that it’s an honor to be chosen as “her person,” but one will likely have to put in the time. Mares have a ton of heart. They’re passionate. They’re special. And they often have hormone-related opinions throughout parts of the year.

Mares are seasonally polyestrous breeding animals with a cycle dependent on the length of daylight. In the Northern Hemisphere, mares will have a natural estrous cycle from late spring to early fall. The ovulatory season is when a mare is cycling and producing ovarian follicles, and receptive to the stallion for breeding on a regular, cyclical basis. Anestrus occurs in late fall through winter when the mare does not cycle and is not receptive to breeding. The ovaries are smaller and inactive due to hormonal changes induced by a shortened amount of daylight.

A filly will become sexually mature and start cycling between 1 and 2 years old. During the breeding season, she will have a series of cycles — about 21 days (+/- 3 days) in length — with alternating phases: estrus and diestrus. Estrus (in heat) averages about six days but can be longer or shorter depending on the mare. Diestrus is roughly 15 days of the three-week estrous cycle. Mares are typically non-receptive or actively reject a stallion during this time. Behaviorally, diestrus is often when owners notice their mare returns to acting “normal” for riding and handling.

Estrus is when a mare is ready to breed and receptive to the stallion. During this reproductive stage, a follicle develops and matures, culminating in ovulation — the release of the egg for fertilization — occurring 24 to 48 hours before the end of estrus. Conception rates are highest if the mare is bred 36 hours before and up to ovulation. Understanding estrus is extremely important to the reproductive industry as it must be accurately determined for the best chance of conception.

Mares in heat can have affected behavior, and individual mares can show more intense behavior changes than others. Common behaviors include heightened sensitivity, nervousness, excitability or irritability. Mares in heat may seem more distracted and neigh, or whinny, to other horses. Being in heat can be accompanied with ovarian pain. This can make her more sensitive and respond unpredictably when handling, grooming, tacking up or riding.

Natural treatments, including herbs and botanicals that support anti-inflammatory or anxiolytic effects, may support mares in heat. Many supplements marketed for mare health focus on calming benefits. Magnesium supplements may offer benefits particularly for tense, crampy muscles. Medical intervention including hormone treatments exist that regulate or even stop a mare’s estrous cycle.

Signs & Symptoms of Estrus

Estrus is a normal and completely natural part of the reproductive cycle. Mares are individuals, and some may show bold behavioral changes while others may not show any changes at all. Some mares may only display overt signs of estrus when in the presence of a stallion. If a mare tends to be more challenging or sensitive one week out of three from spring through fall, her behavior is likely estrus related.

Major Hormones of the Estrous Cycle

The estrous cycle is dependent on daylight hours. As daytime increases in the spring time, hormones secreted by the ovaries signal the end of anestrus and the restart of the estrous cycle. A very specific feedback system with close interactions between various hormones regulates the reproductive cycle.

Melatonin: As daylight increases with spring (or under artificial conditions), the pineal gland is triggered to block melatonin production.

Gonadotropin-releasing hormone (GnRH): The decrease of melatonin signals the release of GnRH from the hypothalamus in the midbrain tissues. Located at the base of the brain, the pituitary gland is stimulated by GnRH to produce follicle stimulating hormone and luteinizing hormone. These two hormones act on the ovaries to prepare an oocyte (egg) for ovulation.

Follicle stimulating hormones (FSH): A gonadotropic hormone that travels to the ovaries via the bloodstream where it stimulates the development of one or more ovarian follicles.

Luteinizing hormone (LH): A second gonadotropic hormone, LH supports maturation and ovulation of the developing, egg-bearing follicle.

Estrogen: The developing follicle secretes estrogen that is released into the bloodstream. Different forms of estrogen exist in the mare depending on the phase of the cycle. Estrogen has several effects including the stimulation of estrous activity, relaxing the cervix and allowing sperm to enter the uterus, stimulating smooth muscles in the reproductive tract for transport of sperm, egg and more. When blood estrogen reaches a certain threshold, the pituitary gland triggers the release of LH. This surge causes the follicle on the ovary to rupture, thus causing ovulation. Estrogen affects behavioral centers in the brain. With the growth of the follicle around the maturing oocyte, more estrogen is produced alerting the mare’s brain that she needs to be bred in the next few days. This hormonal feedback causes the classic behavioral changes of a mare “being in heat.” Ovariectomized mares, in which one or both of the ovaries have been removed, can have relatively high levels of estrogen, thought to be produced by the adrenal cortex, which may cause her to show persistent heat behavior.

Testosterone: Testosterone can lead to aggressive behaviors in normal mares. Blood testosterone levels increase along with estrogen as mares come into heat, peaking around the time of ovulation. Testosterone can also be produced by granulosa-theca cell tumors — the most common equine ovarian tumor — which can sometimes lead to significant aggression.

Progesterone: The ruptured blood-filled follicle luteinizes to develop the corpus luteum, which produces progesterone. This key hormone responsible for maintaining a uterine environment conducive to fetal development, inhibits the secretion of LH and FSH. Via progesterone, the mare will not display estrus and not be receptive to the stallion. Progesterone is responsible for maintaining the first 70 days of pregnancy, after which dihydroprogesterone (DHP), produced by the placenta, maintains pregnancy to term. Synthetic progesterone, such as altrenogest, is often used and is very important in artificially manipulating the estrous cycle. It is used to suppress estrus in mares, particularly performance horses, and to synchronize estrous cycles for breeding purposes.

Prostaglandin: If there is no embryo present, prostaglandin is released from the inner layer of the uterus (endometrium), causing a regression of the corpus luteum and reducing progesterone levels. By removing the inhibition of progesterone, FSH rises and starts the cycle starts over again.