by Casie Bazay Collaborator

Debunking the Mare Heat Cycle

Debunking the Mare Heat Cycle

Published: July 2020 | Updated: October 2022

As far as reproductive status goes, mares typically mature somewhere between 12 and 15 months of age. This is when their “heat” cycle begins, in which the mare ovulates and is able to conceive. The actual time period when a mare comes into heat is known as estrus, and this is the specific stage in the horse’s cycle when a mare will be receptive to a stallion. 

Mares are seasonally polyestrus, which means that they have regular estrus cycles during late spring, summer, and again in early fall and do not experience estrus during the winter. 

Other terms you might hear relating to the mare’s heat cycle include:

  • Estrous (oestrous in many parts of the world): the mare’s entire cycle;
  • Diestrus (dioestrus): the period between the estrus phases when the mare is not receptive to the stallion;
  • Anestrus (anoestrus): the complete absence of estrus. 

Your Mare’s Estrous Cycle

How often do horses have a heat cycle? 

 The mare’s normal cycling period begins approximately in March and lasts through September. During this period, the mare undergoes a series of cycles. The average horse heat cycle (period from one estrus to the next) lasts 21 days, but it can vary from 19 to 26 days in some horses. Estrus typically lasts five to seven days (six being average), but could be as short as two days or as long as ten days. Ovulation (release of the egg for fertilization) typically occurs 24 to 48 hours before the end of estrus.

Diestrus, the period when a mare is not actively in heat, typically lasts two weeks.

Each stage of a mare’s cycle is controlled by hormones produced in the pituitary gland, ovaries, and uterus, and these hormones are affected by external factors such as day length, temperature, and nutrition. 

Because a mare’s reproductive status is affected by the length of daylight, they experience their first heat cycle in spring, with cycles lasting into late summer or fall. As the length of daylight increases in spring, a mare’s ovaries become active with three to four waves of numerous large follicles, which each contain an immature egg. 

Mares have two follicular waves each cycle. The first wave occurs during diestrus and these follicles resorb before becoming mature. The second wave occurs after the corpus luteum dissolves and is associated with estrus. 

Most mares do not cycle during the winter months unless kept under artificial lights in a barn (more on that below). 

How do you know when a horse is in heat? 

A mare’s behavior often changes while she is in heat, and some signs that indicate estrus is occurring include restlessness, hyperactivity, and, in some cases, less interest in eating and sleeping. Your mare may also exhibit frequent urination, straddling posture (squatting), and clitoral “winking”. However, most mares will not show overt signs of estrus unless a stallion is present.

If your mare shows signs of being in estrus on a frequent basis, physical problems could be present, such as ovarian tumors, urinary tract or bladder infections, or even musculoskeletal pain (especially along the back). In cases such as these, have her checked by your veterinarian. 

Hormones involved in the mare’s heat cycle

As daylight hours increase in spring, ovarian hormonal secretions also increase and the mare begins the estrous cycle again. A tightly knit feedback system exists between the reproductive hormones and levels of specific hormones fluctuate as needed in order for reproduction to be possible.

Some of these hormones include:

  • Gonadotropin Releasing Hormone (GnRH), a hormone secreted by the hypothalamus of the brain, governing the activities of other reproductive hormones. Light (either natural or artificial) causes a reduction of melatonin secretion, which then allows GnRH to be secreted and stimulate the production of other hormones. 
  • Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH): Together, these two hormones
  • Estrogen: Several forms of estrogen are present throughout the mare’s cycle. Estradiol is most active in mares that aren’t pregnant. The presence of estradiol is also associated with behavioral displays common to estrus and causes relaxation of the cervix during estrus. In pregnant mares, several forms of estrogen are active, most importantly estrone sulphate. 

Artificial Manipulation of Light to Affect A Mare’s Heat Cycle

Because light affects the mare’s heat cycle, the breeding season can be lengthened by exposing a mare to 16 hours of light per day in late winter to trigger earlier heat cycles. However, 8-10 of weeks of artificial light use are needed in order for mares to respond. For example, if the breeding season is scheduled to begin February 15, mares should be exposed to daily artificial light starting on December 1. 

Artificial light needs to be provided beginning at dusk and can be added gradually or abruptly to a 16-hour day for 60 days. An alternative is to fit mares with a commercial facemask that emits a low level of blue light from 4 pm to 11 pm, mimicking long summer daylight hours. 

Foal Heat

The first heat cycle occurring after a mare foals is known as ‘foal heat’, and this happens approximately 7-14 days postpartum. In order to keep with a 12-month foaling schedule, many breeders choose to breed the mare back during this time period. Though this works well for many mares, other mares may not be physically ready for a foal heat breeding. 

How to Support Your Mare’s Reproductive Status

Most younger mares have relatively few issues when it comes to their reproductive health, however, as a mare begins to age, her reproductive health may decline. Supporting your mare with proper nutrition, including high quality forage, and ensuring vitamin and mineral needs are met will help to foster good reproductive health. 

A recent study in dairy cows showed that probiotic supplementation had a positive effect on breeding, leading to larger ovulatory follicles, shorter estrous cycles, and improved reproductive performance. This may very well be true in horses, as well, since we know that gut health is connected to overall health, which would include reproductive health.  

Supplementing your mare’s diet with high quality probiotics for horses can support every aspect of health and reproduction. 

Read More:

Effects of Live Yeast Supplementation on Hormonal Profile and Reproductive Performance

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