Horse Breeding Cycles: Everything You Need to Know

Fillies reach sexual maturity and begin cycling between one and two years of age (typically around eighteen months). The first heat cycle marks a mare’s first biological eligibility for breeding. A heat cycle can be influenced by many factors including seasonality, genetics, and breed.

When first breeding a mare, it is important to consider her current size, and the size of the stallion she’ll be bred to. It is often safer to wait until the filly has reached maturity, typically around four years of age, to give her the best opportunity at safely delivering a healthy foal.

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.

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.

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

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.

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.

• 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.

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.

Reproductive Cycle Manipulation

The mare’s reproductive cycle can also be manipulated. Many breeds consider foals' birthdays to be January 1, regardless of the actual foaling date, for racing or competition purposes. This means a foal born in February might be racing against a foal born in June; these few months can create significant advantages in skeletal maturity, muscle development, and strength in the first few years. Many breeders want foals to be born as early in the year as possible both for this reason, and to avoid mid-summer babies that must deal with hot weather.

Light Manipulation

To try to get mares cycling earlier in the year, we can manipulate their brain into thinking it is spring sooner. Placing mares under lights beginning in late November or early December can influence their bodies to begin transitioning into the breeding season in February, rather than March or April. Mares should be exposed to 16 hours of light per day; this is often done by placing them in stalls in the late afternoon and leaving a light on until 11:00 or midnight.

Hormonal Manipulation

During breeding season, the cycle can be manipulated (or attempted to be manipulated) using synthetic hormones.

Mares who are “transitional,” or not beginning to cycle appropriately in spring, might be started on altrenogest (Regumate®), a synthetic progestin, for 10–14 days, then pulled off it. Mares should start cycling one to two weeks later. Altrenogest is also used for estrous suppression in mares that exhibit extreme behavior changes when cycling.

Prostaglandins, such as Lutalyse®, can be used for “short-cycling” a mare, to help break down a CL earlier than would occur naturally. This might be injected into their muscles for 1–2 days in a row, and they typically ovulate 5–7 days later. This protocol may be used if a mare’s ovulation has been missed, or if they have a persistent CL that’s not regressing normally.

Desorelin, or Sucromate™, can be administered to influence timing of ovulation. If a mare has a follicle at least 30 mm in size, Sucromate can be administered intramuscularly, and this encourages the follicle to ovulate within 36 hours. This can be helpful when breeding via artificial insemination (AI), when stallion collection and semen delivery has set schedules.

How to Pinpoint Equine Estrus

Deciding when to breed a mare during the estrus phase of the cycle is crucial in optimizing potential success. Depending on the method of breeding (natural, artificial insemination with either fresh or frozen semen) can affect when to breed in relation to time of ovulation.

Estrus can be tracked by monitoring mare behavior changes. The teasing behaviors described above help reveal if a mare is receptive to a stallion and nearing ovulation. The mare is often walked in front of a stallion. If he smells her or begins whinnying at her, she will often kick, paw, or squeal angrily when she is non-receptive or too far away from ovulation. As she nears ovulation and estrogen levels increase, you will see her display squatting, winking, etc.

For more precise tracking, especially when breeding via AI, serial reproductive ultrasounds will be performed by your veterinarian. They will evaluate ovaries for follicle development.  Along with follicular changes, it is important to monitor uterine edema and cervical tone. Mares will develop uterine edema, which looks like thick folds or bicycle spokes on ultrasound as they get closer to ovulation, and the cervix will soften towards the end of estrus due to estrogen influence.