The journey of equine reproduction is a fascinating interplay of hormones, seasons, and careful timing. For breeders and horse enthusiasts, understanding the mare’s reproductive cycle is paramount to successful breeding and the healthy development of foals. This guide delves into the intricacies of the equine estrous cycle, gestation, and reproductive manipulation, providing essential knowledge for those involved in equine breeding.
The Equine Estrous Cycle: A Seasonal Dance
Horses are seasonal breeders, meaning their reproductive cycles are largely influenced by the length of daylight. Typically, mares enter their anestrus (non-cycling) phase during the shorter days of winter, characterized by small, inactive ovaries. As spring approaches and daylight hours increase, hormonal signals prompt the ovaries to become more active, initiating the estrous cycle.
This cycle, lasting approximately 21-22 days, is divided into two main phases:
Estrus: The Receptive Phase (6–8 days)
As daylight extends, the mare’s brain releases gonadotropin-releasing hormone (GnRH), stimulating the pituitary gland to produce follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH encourages follicle development on the ovaries, while LH drives the maturation and eventual ovulation of a dominant follicle, releasing an egg. During this phase, mares experience rising estrogen levels, leading to characteristic signs of receptivity that a stallion would recognize. These may include frequent urination, tail raising, “winking” (clitoral eversion), squealing, and stud-like behavior. The mare’s cervix also relaxes and opens, preparing for breeding.
Diestrus: The Maintaining Phase (14–16 days)
Following ovulation, the diestrus phase begins. Estrogen levels drop as the dominant follicle transforms into a corpus luteum (CL), which secretes progesterone for about 12-14 days. Progesterone is crucial for maintaining pregnancy by preventing further estrous cycles. If no embryo is detected, the uterus releases prostaglandin, destroying the CL and lowering progesterone levels, thus restarting the estrous cycle. Careful monitoring of a mare’s behavior and cycle is vital for optimizing breeding opportunities.
Gestation: Nurturing the Future Foal
If fertilization occurs, the embryo enters the uterus about six days after ovulation and implants in the uterine lining around 17 days. Successful implantation is critical; if the mare’s body doesn’t recognize a viable embryo, the pregnancy can be lost. Gestation in horses typically lasts around 340 days, with veterinarian examinations playing a key role in monitoring fetal health and placental well-being. Potential complications like placentitis can pose risks to both mare and foal.
Key milestones during pregnancy include:
- 14–16 days post-ovulation: Pregnancy and potential twins are checked. Twinning is often managed early to improve survival chances.
- 26–30 days: Fetal heartbeat and viability are assessed. Non-viable embryos can be terminated, allowing the mare to resume her cycle.
- Around 45 days: Fetal tissues form endometrial cups, aiding progesterone production. These cups shed around 120 days. Losing a pregnancy during this period can make rebreeding difficult.
- 60–70 days (or 110–140 days): Sex determination of the foal is possible via ultrasound before the fetus becomes too large.
- Five, seven, and nine months: Optional ultrasounds monitor fetal growth and placental health, coinciding with the administration of the Pneumabort vaccine to prevent Equine Herpesvirus-1, a common cause of abortion.
Approaching foaling, mares may exhibit enlarged mammary glands, waxing of the teats, relaxation around the tail and vulva, and isolation from the herd.
Post-Partum Cycling: The “Foal Heat”
Mares often experience a “foal heat” cycle shortly after giving birth, typically around seven days post-foaling. Whether this is an optimal time for rebreeding depends on the mare’s uterine recovery and absence of infection. Veterinarian assessment is crucial to determine suitability for rebreeding. If not bred during foal heat, the mare will resume her regular 21-day estrous cycle.
Reproductive Cycle Manipulation: Precision Breeding
For competitive purposes, such as racing, foals are often assigned an official birthday of January 1st, regardless of their actual foaling date. This practice encourages breeders to aim for early-year foals to maximize their developmental advantages.
Light Manipulation
To encourage earlier cycling, mares can be exposed to artificial light, simulating longer days. Starting in late November or early December, 16 hours of light per day can help mares transition into the breeding season earlier, around February.
Hormonal Manipulation
Synthetic hormones can be used to manipulate the mare’s cycle.
- Altrenogest (Regumate®): This synthetic progestin can be administered for 10-14 days to mares experiencing transitional periods or exhibiting extreme behavioral changes during estrus, often inducing cycling within one to two weeks.
- Prostaglandins (e.g., Lutalyse®): Used for “short-cycling,” these injections help break down the CL earlier, typically leading to ovulation 5-7 days later. This is useful if an ovulation is missed or a persistent CL needs regression.
- Deslorelin (Sucromate™): Administered to mares with a follicle at least 30 mm in size, Sucromate can induce ovulation within 36 hours, which is particularly helpful for coordinating breeding with artificial insemination schedules.
Pinpointing Equine Estrus for Optimal Breeding
Accurately identifying estrus is critical for maximizing breeding success, especially with artificial insemination (AI).
Behavioral Monitoring: Observing the mare’s reactions to a stallion during teasing can indicate her stage in the cycle. As estrogen rises and ovulation nears, receptive behaviors become more pronounced.
Reproductive Ultrasounds: For precise timing, especially with AI, veterinarians perform serial ultrasounds. This allows for assessment of follicle development, uterine edema (thickening of uterine folds), and cervical tone, all indicators of approaching ovulation.
Mare Reproductive Cycle FAQs
What is the best time for breeding a horse in the reproductive cycle?
For live cover or fresh semen, breeding ideally occurs within 24 hours prior to ovulation. With frozen semen, the target for AI is approximately 6 hours after ovulation.
How long is the estrus cycle of a horse?
The estrus, or fertile period, lasts for 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 altrenogest.
What do female horses do when in heat?
Mares in heat may exhibit squealing, squatting, winking, or stallion-like behaviors.
