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The Equine Estrous Cycle

The reproductive cycle of the mare is controlled by a series of hormones, influenced by daylight, that ebb and flow in sequence. Horses are seasonal breeders, usually cycling from late spring to early fall each year.

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During the colder months of the year, when the days are shorter, mares will be in the anestrus, or non-cycling phase and the ovaries will be small; mares will not be fertile or capable of breeding during this period. As the days get longer in springtime, the ovaries become slightly larger and more active, then transition into the cycling phase due to signals they receive from the brain.

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Each estrous cycle lasts 21-22 days, and is broken down into two major parts:

Estrus (6-8 days in length)

At the beginning of the breeding season, increased sunlight exposure signals the hypothalamus in the brain to secrete a hormone (GnRH, or gonadotropin releasing hormone), that signals the pituitary gland to fire up so it can produce the two major hormones that influences the ovaries: follicle stimulating hormone (FSH) and luteinizing hormone (LH). FSH encourages the ovaries to develop follicles at the beginning of the cycle, then LH encourages the growth of one (or sometimes 2-3) dominant follicle(s) to become mature and ovulate. This dominant follicle is what creates the egg that will be released into the fallopian tubes for potential ovulation. As they get closer to ovulation, the ovaries begin to secrete estrogen. At this time, mares may exhibit signs of receptivity that a stallion would note, such as:

Squatting and urinating frequently
Raising the tail
“Winking” (everting of the clitoris when squatting)
Squealing
Stud-like behavior

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As the mare becomes behaviorally receptive under the influence of estrogen, it will also make the cervix relaxed and open to allow for breeding. As the follicle ovulates late in the estrus phase, it releases an egg into the uterine body, where it will await sperm for fertilization.

Diestrus (14-16 days in length)

The diestrus phase begins 1-2 days after ovulation and completes the second half of the estrous cycle. Estrogen levels decrease because there is no longer an active mature follicle; behavior will regress to normal, the mare will not be receptive to the stallion, and the cervix will close and tighten. The empty follicle regresses and turns into a corpus luteum (CL), which will secrete progesterone for 12-14 days. Progesterone is the hormone responsible for maintaining pregnancy by halting the production of hormones that would continue the repeating estrous cycle (FSH and LH). If no embryo is present at the end of that 12-14 days, the uterus will secrete prostaglandin, which will destroy the CL on the ovary. Once that occurs, the progesterone supply will cease, and the cycle will restart with increasing levels of FSH. Monitoring a mare’s behavior and cycle is important for managers to breed at appropriate times to increase chances of pregnancy.

Gestation

If the mare has been bred successfully either naturally or through artificial insemination, the fertilized egg will form an embryo, and will enter the uterus about six days after ovulation. At that point, the egg will traverse the uterus, and eventually attach to the uterine lining at about 17 days. This time frame is critical for the mare’s body to recognize a viable embryo. If the embryo isn’t acknowledged, the uterus will secrete prostaglandin which will destroy the CL, shed the uterine lining, and begin the cycle over again. If the CL is no longer viable, progesterone won’t be produced, the uterus will not hold the embryo attachment, and the pregnancy will be lost.

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The length of pregnancy in horses is typically 340 days, although this can vary slightly. Frequent examinations by your veterinarian are important to monitor fetal viability and check for any issues with the mare or placenta such as placentitis, that could become dangerous for one or both the mare and foal. Common milestones during the 11-month pregnancy include:

14-16 days after ovulation: check for pregnancy and potential twins. Because twinning is often fatal for one or both of the fetuses, and potentially the mare, this is avoided at all costs. If twin embryos are present, they can be more easily manipulated before settling on the uterine wall, and the chances of successfully terminating just one are much greater.
26-30 days: fetal heartbeat and viability are determined at this stage. If the embryo is non-viable, the empty/false pregnancy can be terminated, and the mare may resume her normal estrous cycle activity.
At around 45 days, endometrial cups are created by the fetal tissue and create attachments between the placenta and uterine wall. They release a hormone called equine chorionic gonadotropin (ECG), that aids with additional progesterone production to sustain pregnancy. These cups will shed on their own eventually at around 120 days of gestation, whether the mare has a viable pregnancy or not. Due to the increased progesterone, if a pregnancy is lost during this period, it becomes very difficult to rebreed the mare during that season.
60-70 days, or 110-140 days: these timeframes are the only point where the sex of the foal can be determined. This process can only be attempted at this point before the fetus gets too large and moves into a more difficult position. The foal’s genital area of development is evaluated relative to the position of the hind legs.
Repeat optional ultrasounds may be performed at five, seven, and nine months of gestation when the Pneumabort vaccine is administered, to monitor fetal growth and placental health to try to catch any abnormalities early. This vaccine is given to help prevent Equine Herpesvirus-1, a common cause of late-term abortion in mares.
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Additional ultrasounds or exams should be performed by your veterinarian right away if you notice any vaginal discharge, behavioral changes, premature mammary development, or any other concerns.

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Signs that your mare is getting close to her foaling date include:

Enlarged mammary glands
Waxing of the teats
Relaxation of the muscles around the tail and vulva
Isolation from herd members
Slightly decreased appetite

Post-Partum Cycling

Mares typically have a short cycle immediately after giving birth. This is termed “foal heat,” and is a finnicky time for those trying to keep their mare on a tight yearly breeding schedule. Depending on how the mare’s uterus is recovering, they may or may not be in ideal condition to rebreed this early. Mares typically ovulate around seven days after giving birth.

If the mare’s uterus is not recovering in a timely manner, or if there is any infection post-partum, this cycle will likely not be viable time for breeding. Your veterinarian will help you determine whether to try to breed based on ultrasound findings.

If a mare is not bred at this cycle, or does not become pregnant, she will then resume her normal 21-day estrous cycle activity.

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