Bringing a new foal into the world is a significant event for any horse owner, and understanding the mare’s reproductive cycle is paramount to successful breeding. This comprehensive guide delves into the estrous cycle, gestation, and various management techniques to optimize breeding outcomes.
The Equine Estrous Cycle
Horses are seasonal breeders, with their reproductive cycles primarily influenced by daylight hours. Typically, mares begin cycling between one and two years of age, with the first heat cycle marking their biological eligibility for breeding. This cycle, which lasts approximately 21-22 days, is divided into two main phases: estrus and diestrus.
Estrus (6–8 days)
The estrus phase, often referred to as “heat,” is when the mare is receptive to breeding. As daylight increases in late spring and summer, hormonal signals from the brain (GnRH, FSH, and LH) stimulate the ovaries to develop follicles. A dominant follicle matures and releases an egg, preparing for ovulation. During this period, mares may exhibit specific behaviors such as frequent urination, tail raising, “winking” (everting the clitoris), squealing, and exhibiting stallion-like behavior. The cervix also relaxes, facilitating breeding.
Diestrus (14–16 days)
Following ovulation, the diestrus phase begins. Hormonal levels shift, estrogen decreases, and the mare is no longer receptive to the stallion. The ruptured follicle transforms into a corpus luteum (CL), which secretes progesterone. Progesterone is crucial for maintaining a potential pregnancy by preventing further estrous cycles. If no embryo is detected, the uterus releases prostaglandin, which destroys the CL, leading to a drop in progesterone and the initiation of a new cycle.
Gestation and Foal Development
Successful breeding, whether through natural cover or artificial insemination, leads to fertilization and the formation of an embryo. The embryo travels to the uterus and implants in the uterine lining around 17 days post-ovulation. The mare’s body must recognize this viable embryo to sustain the pregnancy.
Gestation in horses typically lasts around 340 days. Regular veterinary examinations are vital throughout this period to monitor fetal viability and placental health. Key milestones include:
- 14–16 days post-ovulation: Pregnancy and potential twin detection. Twin pregnancies are often managed to prevent complications.
- 26–30 days: Fetal heartbeat and viability are assessed.
- Around 45 days: Endometrial cups form, supporting progesterone production. These cups naturally shed around 120 days.
- 60–70 days (or 110–140 days): Sex determination of the foal can be attempted via ultrasound.
- Five, seven, and nine months: Optional ultrasounds and administration of the Pneumabort vaccine (to prevent Equine Herpesvirus-1) help monitor fetal growth and placental health.
Signs that a mare is nearing foaling include enlarged mammary glands, waxing of the teats, relaxation around the tail and vulva, isolation from the herd, and a slightly decreased appetite.
Post-Partum Cycling and Reproductive Manipulation
Mares typically experience a “foal heat” cycle shortly after giving birth, usually around seven days post-foaling. However, breeding during this time depends on the mare’s uterine recovery and absence of infection. If not bred or if pregnancy doesn’t occur, she will resume her normal estrous cycle.
For competitive and management purposes, breeders often aim for foals to be born as early in the year as possible. This can be achieved through:
Light Manipulation
Exposing mares to 16 hours of artificial light per day starting in late November or early December can stimulate them to cycle earlier, typically by February or March.
Hormonal Manipulation
Various hormones can be used to manage the mare’s cycle:
- Altrenogest (Regumate®): A synthetic progestin used for estrous suppression or to induce cycling in transitional mares.
- Prostaglandins (Lutalyse®): Used for “short-cycling” to expedite the breakdown of the CL, useful if ovulation is missed or the CL is persistent.
- Deslorelin (Sucromate™): Administered to mares with a mature follicle (at least 30 mm) to induce ovulation within 36 hours, beneficial for timing artificial inseminations.
Pinpointing Estrus for Breeding
Accurate timing of breeding is critical for success. This involves monitoring mare behavior, especially during teasing sessions with a stallion, to identify receptivity. For precise management, especially with artificial insemination, veterinarians perform serial ultrasounds to track follicle development, uterine edema, and cervical tone.
Mare Reproductive Cycle FAQs
What is the best time for breeding a horse in the reproductive cycle?
For live cover or fresh semen, breeding within 24 hours prior to ovulation is ideal. For frozen semen, insemination approximately 6 hours after ovulation is recommended.
How long is the estrus cycle of a horse?
The estrus phase, when the mare is fertile and receptive, lasts 6–8 days.
How often do horses go into heat?
Mares typically cycle every 21–22 days during the breeding season.
How do you get a mare out of heat?
Altrenogest can be used for estrous suppression to manage mares exhibiting extreme behavioral changes when in heat.
What do female horses do when in heat?
Mares in heat may squeal, squat, wink, urinate frequently, and display other behaviors indicating receptivity to a stallion.
