The Mares Reproductive Cycle

Mare and foal in their stall.
Mare and foal in their stall.

The best time for mating of a mare and stallion is determined by several factors, including the length of daylight, the daily temperature, the mare's general nutrition, the amount of rainfall, the climate, and the latitude.

Generally in March, many mares begin to develop ovarian activity. This early activity may not be accompanied by ovulation, but beginning in May, with longer daylight hours, warmer temperatures, and the arrival of green grass, nearly all mares are cycling consistently, with conception rates peaking in June.

Mare in field grazing.

Mare in pasture

Mature mares come into season about every 22 days during the spring, summer and fall.

The equine estrous cycle

As increasing daylight stimulates the receptor centers in the brain to trigger reproductive hormones, these hormones begin the pattern of regular periods of estrous, also known as heat. The estrous cycle is the time period from one ovulation to the next. The average cycle is 22 days and this can vary by a few days especially at the beginning or ending of breeding season.

Sexual maturity Approx 18 months
Estrous cycle 22 days
Estrus (fertile) length 6 to 8 days
Diestrus (not fertile) length 14 to 16 days
Gestation period 340 days
Postpartum heat 7 days after parturition

The reproduction cycle of the mare is divided into two phases: the estrus phase, during which the mare is actively interested in and is receptive to the stallion; and the diestrus, which is a time of sexual disinterest that begins 24 to 48 hours after ovulation and lasts 14 to 16 days.

While fillies become sexually mature at around 18 months, they are still experiencing growth that may be hindered by pregnancy. The ideal age to begin foaling is around four years of age. Mares are capable of continuing to breed until late in life and do not usually suffer ill effects if nutrition and condition are maintained.

Estrus usually lasts for six to eight days, although during the summer it may decrease to four days. Estrus continues throughout the summer, but ceases during the fall. During estrus, the pituitary gland releases a follicle-stimulating hormone that causes egg follicles within the ovary to grow and produce increasing amounts of estrogen, which prepares the reproductive tract for mating and fertilization. The increased amount of estrogen also affects the behavior of the mare.

When the egg follicle approaches maturity, a second hormone is released that causes the follicle to ovulate, usually about 24 hours before the end of estrus. A normal pregnancy in horses lasts approximately 11 months or 340 days. Premature foals may be born and survive with intensive care after 310 days gestation. Some mares tend to carry foals by as much as three to four weeks longer.

Ordinarily, the ideal time for a foal to be born is May or June when grass is available to nourish the mare's milk supply. For this timing to occur, the mare needs to be bred sometime from June through August.

Manipulation of the estrous cycle

Breed registries and thoroughbred racing associations have designated January 1st as a universal date of birth for foals no matter when the foal is actually born.

In racing and other competitive events, a horse born in January has an athletic advantage over one born in June. For this reason, manipulation of the estrous cycle has lead to an operational breeding season that begins February 15 and ends July 15.

By exposing the mare to a longer period of daylight, in addition to using artificial light, the onset of regular estrous cycles can be hastened, making breeding possible much earlier in the year. If the breeding season is scheduled to begin February 15, artificial lighting needs to begin on December 1.

Use of artificial lights can be implemented abruptly, with mares being exposed to light for 16 hours a day, or the light can be increased gradually over a period of 60 days. With the increasing schedule, three hours of light are added during the evening of the first week, increasing by 30 minutes each additional week, until the goal of 16 hours of light is reached.

Research shows that the supplemental light must be added during the afternoon and evening, usually from approximately 4:30 PM to 11:00 PM. Light added in the morning, before dawn, does not achieve the wanted results. Installation of an automatic timer saves on labor and aids in consistency. The amount of light needed is enough to allow comfortable reading of newsprint.

Manipulation of horse ovarian activity



Regu-mate is a popular drug used to modify the natural cycling of a mares estrus.

Along with the manipulation of daylight, ovarian activity is often manipulated to allow mares and stallions to remain in competition during breeding season and to facilitate the scheduling of breeding appointments.

Veterinarians often administer prostaglandin or human chorionic gonadotropin, or use a Deslorelin implant to manipulate ovarian activity. Protocols have been developed in each case and a knowledgeable veterinarian can recommend what will work best for a particular mare.

Altrenogests (Regu-mate) are also used by veterinarians to shift or synchronize the time of estrus for the purposes of timing for artificial insemination. After withdrawal of this drug, a mare will predictably come into heat.

Pinpointing the time of equine estrus

Effective estrus detection results in successful breeding management.  Mares behave in distinct ways when they are responsive to a stallion. Mares in estrus raise their tail, squat, and may urinate in the presence of the stallion. This behavior is referred to as teasing and is a good indication that the mare is receptive and about to ovulate.

The estrous cycle produces observable changes in the mare's reproductive tract. Rectal palpitation by an experienced veterinarian or handler can detect changes in the uterus, ovaries, vagina, and cervix of the mare.


Estrous - Refers to the overall reproductive cycle.

Estrus - Refers to the phase when then mare is in heat and receptive to the stallion.

Ultrasonography may be used to confirm the findings of rectal palpitation. The uterus shows characteristic folds during estrus that disappear just before ovulation. The shape of a follicle about to ovulate changes from round to teardrop and leaks fluid toward the ovulation fossa. Ultrasonography will also reveal twin pregnancies, tumors, or other abnormalities.

Once it has been determined that the mare is in estrus, mating, also known as covering, should take place in an area free of distractions. For the safety of the stallion and handlers, the mare should be restrained. It is best to have experienced handlers involved in the mating. Usually, at least three handlers are required, one for the mare, one for the stallion, and another person for additional safety and control.

Some mares and stallions are kickers and biters and provisions need to be made to lessen any chances for injury to either horse.

Some breeders rely on artificial insemination for their mares, which is seen as having some advantages. An increase in pregnancy rates, the low risk of spreading reproductive infections, the prevention of breeding injuries make artificial insemination attractive.

Most mares do not show signs of abdominal enlargement until the last three weeks of pregnancy. Although mammary development is usually obvious in maiden mares during the last month, it may not be apparent in other mares until just before foaling.

Mare gestation

Gestation is the period between conception and birth. A mare immunological pregnancy test is used frequently to determine if the mare is pregnant. It is 95 percent accurate, inexpensive, and convenient.

Transrectal ultrasound is another option for determining pregnancy and is usually used after the 20th day, with the fetal heart becoming visible by day 30.

Rectal palpitation by a skilled examiner is another way to detect pregnancy. Usually it is done as early as 20 to 30 days after service. Transrectal palpitation performed between days 40 and 50 is 90 percent accurate when done by a skilled examiner.

Mare with foal.

A mare with her healthy foal

The result of a successful breeding and a thrilling sight for all horse lovers.

During gestation, the mare should be fed her usual ration of high-quality feed along with access to free-choice salt. During the last three months, the amount of feed should be increased approximately 15 percent since the mare will require an additional 15 percent in dietary protein and energy and twice as much calcium, phosphorus, and vitamin A.

Given good care, nutrition, and a proper amount of exercise, the mare should deliver a healthy foal after approximately 340 days. Postpartum heat usually occurs sometime between day 7 and 12 after foaling, with a second heat cycle at approximately 30 days postpartum. At this point, the mare's reproductive cycle starts over, making it possible to produce foals on a 12 month interval.

About the Author

EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.