Breeding horses is a complex process that relies on a deep understanding of the mare’s reproductive cycle. From recognizing the signs of estrus to managing gestation and manipulating cycles for optimal timing, breeders must be attuned to the mare’s biological rhythms. This guide provides a comprehensive overview of the equine estrous cycle, gestation, and the methods used to enhance breeding success.
The reproductive cycle of the mare is a finely tuned biological process influenced by hormonal signals and environmental factors, primarily daylight hours. Horses are seasonal breeders, typically cycling from late spring through early fall. During the shorter days of winter, mares enter anestrus, a non-cycling phase where ovaries are inactive and fertility is absent. As daylight increases in spring, hormonal signals from the brain stimulate ovarian activity, initiating the breeding season.
The Equine Estrous Cycle Explained
The equine estrous cycle spans approximately 21-22 days and is divided into two main phases: estrus and diestrus.
Estrus: The Receptive Period
Estrus, lasting about 6-8 days, is the period when the mare is receptive to the stallion. Increased daylight triggers the hypothalamus to release gonadotropin-releasing hormone (GnRH), stimulating the pituitary gland to produce follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH promotes the development of ovarian follicles, while LH induces the maturation and ovulation of a dominant follicle, releasing an egg. During this phase, rising estrogen levels cause mares to exhibit behavioral signs of receptivity, including squatting, urinating frequently, raising their tail, “winking” the clitoris, squealing, and sometimes displaying stud-like behavior. The cervix also relaxes and opens, allowing for breeding. Ovulation typically occurs towards the end of the estrus phase.
Diestrus: The Non-Receptive Period
Following ovulation, the mare enters the diestrus phase, which lasts 14-16 days. Estrogen levels drop as the mature follicle regresses and forms a corpus luteum (CL). The CL secretes progesterone for 12-14 days, a hormone crucial for maintaining pregnancy by suppressing further estrous cycles. If no embryo is detected by the end of this period, the uterus releases prostaglandin, destroying the CL, causing progesterone levels to fall, and initiating a new estrous cycle.
Gestation and Foaling
Successful breeding, whether through natural cover or artificial insemination, leads to pregnancy. The fertilized egg implants in the uterus around 17 days after ovulation. For a pregnancy to be sustained, the mare’s body must recognize the viable embryo, which is critical for continued progesterone production.
The gestation period for horses is approximately 340 days. Regular veterinary examinations are essential to monitor fetal health and placental function, screening for potential complications such as placentitis. Key milestones during pregnancy include:
- 14-16 days post-ovulation: Pregnancy and potential twin detection. Early detection of twins is crucial as multiple fetuses can pose risks to the mare and foals.
- 26-30 days: Fetal heartbeat and viability are confirmed. Non-viable embryos can be terminated to allow the mare to resume her estrous cycle.
- 45 days: Endometrial cups form, supporting progesterone production. These cups shed around 120 days. Losing a pregnancy during this phase makes rebreeding difficult due to elevated progesterone.
- 60-70 days (or 110-140 days): Sex determination of the foal is possible through ultrasound before the fetus becomes too large.
- Five, seven, and nine months: Optional ultrasounds may be performed, coinciding with the administration of the Pneumabort vaccine to prevent Equine Herpesvirus-1 (EHV-1), a common cause of late-term abortion.
Signs that a mare is nearing her foaling date include enlarged mammary glands, “waxing” of the teats, relaxation around the tail and vulva, isolation from the herd, and a slight decrease in appetite.
Post-Partum Cycling
Mares often experience a short cycle, known as “foal heat,” shortly after giving birth, typically around seven days post-foaling. However, rebreeding at this time depends on the mare’s uterine recovery and the absence of post-partum infection. Veterinarians assess uterine health via ultrasound to determine the viability of rebreeding during foal heat. If not bred or if pregnancy doesn’t occur, the mare will resume her normal 21-day estrous cycle.
Reproductive Cycle Manipulation
For competitive or management reasons, breeders often aim for foals to be born early in the calendar year (January 1st is a common birthday for racing purposes). This can be achieved through:
Light Manipulation
Exposing mares to 16 hours of artificial light daily, beginning in late November or early December, can stimulate them to cycle earlier in the spring.
Hormonal Manipulation
Synthetic hormones can be used to influence the mare’s cycle:
- Altrenogest (Regumate®): A synthetic progestin used for estrous suppression, particularly in mares with extreme behavioral changes during cycling, or to initiate cycling in transitional mares.
- Prostaglandins (Lutalyse®): Used for “short-cycling” to break down the CL prematurely, inducing ovulation within 5-7 days. This is useful for missed ovulations or persistent CLs.
- Deslorelin (Sucromate™): Administered to mares with a follicle of at least 30 mm to induce ovulation within 36 hours, which is particularly helpful for coordinating artificial insemination schedules.
Pinpointing Estrus for Breeding
Accurate timing of breeding is critical for success. For live cover or fresh semen, breeding ideally occurs within 24 hours before ovulation. For frozen semen, artificial insemination is typically performed within 6 hours after ovulation.
Estrus can be monitored through behavioral observations (“teasing”) with a stallion. More precise tracking, especially for AI, involves serial reproductive ultrasounds by a veterinarian to assess follicle development, uterine edema, and cervical tone. Uterine edema appears as thick folds or “bicycle spokes” on ultrasound as ovulation approaches, and the cervix softens due to estrogen influence.
Mare Reproductive Cycle FAQs
What is the best time for breeding a horse in the reproductive cycle?
For live cover or fresh semen, breed within 24 hours prior to ovulation. For frozen semen, inseminate 6 hours after ovulation.
How long is the estrus cycle of a horse?
The estrus, or fertile period, lasts 6-8 days.
How often do horses go into heat?
Horses cycle every 21-22 days during the breeding season (early spring to late fall).
How do you get a mare out of heat?
Estrous suppression can be achieved using hormonal manipulation with altrenogest.
What do female horses do when in heat?
Mares in heat may squeal, squat, wink, or display stallion-like behavior.
