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The Reproductive Cycle of Horses


Patricia L. Sertich

, VMD, DACT, Department of Clinical Studies-New Bolton Center, School of Veterinary Medicine, University of Pennsylvania

Reviewed/Revised Feb 2021 | Modified Nov 2022
Topic Resources

Most mares are seasonally polyestrous and cycle when the length of daylight is long. Anestrus occurs during the winter when daylight length is short. During anestrus, the ovaries are inactive with no significant follicles >10 mm or corpora lutea, so plasma levels of estrogen and progesterone are low. Therefore, the uterus is flaccid and the cervix may be closed but not firm and tight, or it may be thin, short, and dilated. As the length of daylight increases, mares undergo a vernal transition and the ovaries become active, with 3–4 waves of numerous large (>25 mm) follicles. The cervix and uterus have minimal tone. Mares have 3 or 4 prolonged intervals of estrus (periods of sexual receptivity to the stallion) during the vernal transition, but ovulation does not occur. The end of vernal transition is marked by a surge of luteinizing hormone that stimulates ovulation, after which a regular, 21-day interovulatory estrous cycle is established.

Although the mare continues to ovulate regularly every 21 days throughout the breeding season, the length of estrus (sexual receptivity) varies from 2–8 days, and the length of diestrus varies accordingly to maintain a 21-day interval. Early in the breeding season, estrus tends to be longer, whereas around the summer solstice estrus may last for only 2–3 days.

Mares have 2 follicular waves each cycle. The first wave of follicular development occurs during diestrus, and these follicles become atretic. The second wave occurs after luteolysis and is associated with estrus. Early in estrus, the endometrial folds of the uterus are edematous, but the edema wanes as ovulation approaches. Usually, one follicle becomes dominant and ovulates when it is ≥30 mm in diameter. The dominant follicle enlarges and then softens just before ovulation. The oocyte is released through the ovulation fossa. A corpus hemorrhagicum and subsequent corpus luteum form and produce progesterone, which stimulates closure of the cervix and an increase in uterine tone. This corpus luteum will be mature and become responsive to prostaglandin in ~5 days. If pregnancy is not established, luteolysis occurs at 14 days, and the mare returns to estrus and continues to cycle.

Artificial Manipulation of Photoperiod in the Reproductive Cycle of Horses

After winter anestrus and the vernal transition, cyclicity naturally starts sometime in the spring, when pregnancy can be established. Because changes in the mare’s genital tract are seen in response to an increase in the length of daylight, the onset of ovulation and subsequent regular estrous cycles—and thus, the onset of the breeding season—can be hastened by exposing the mare to 16 hours of light per day; 8–10 weeks are required for mares to respond. If the breeding season is scheduled to begin February 15, mares should be exposed to daily supplemental artificial lighting starting on December 1. Mares need to experience a natural photoperiod of decreasing length of daylight in the fall.

The supplemental light must be added at dusk; light added in the morning before dawn is not effective. A minimum of 10 foot-candles (107 lux) of incandescent or fluorescent light is necessary. (The amount of light should allow one to comfortably read newsprint.) Mares can be stimulated individually in a stall or as a group in a lighted paddock.

Mares can then be abruptly exposed to 16 hours of light each day, or the supplemental light can be gradually increased to a 16-hour day throughout 60 days. In an abrupt lighting program, mares living in natural daylight are exposed to supplemental light from ~4 pm until 11 pm daily. In a less expensive, energy conserving, stepwise program, mares can be exposed to 3 hours of supplemental light in the evening the first week of December, and then the supplemental light is increased by 30 minutes each week until mares are exposed to 16 hours of light each day. An automatic timer aids compliance and saves on labor. Mares can also be fitted with a commercially available facemask on December 1 that is programmed to emit a low level of blue light to one eye from 4 pm to 11 pm, mimicking long, summer days.

Manipulation of Ovarian Activity in the Reproductive Cycle of Horses

Ovarian activity is frequently manipulated by administration of hormones to facilitate scheduling of breeding appointments and to limit the number of breedings per estrus. Breedings should be spaced for stallions breeding large books of mares by natural cover so that semen use is optimized. Geographic locations and transportation constraints may also necessitate scheduled inseminations. Many situations can benefit from an ovulation control program. (Also see Hormonal Control of Estrus Hormonal Control of Estrus .)

Administration of prostaglandin (PGF), IM, to a mare in diestrus causes luteolysis and allows a follicle to mature and ovulate. The corpus luteum must be 5–14 days old to respond to PGF. The mare will come into estrus 2–5 days after administration of PGF. Time to ovulation is variable (3–10 days) and depends on the stage of the mare's current follicular wave and on the size and character of follicles at the time of PGF administration. It is recommended that the mare's ovaries be examined by palpation and ultrasonography just before PGF administration to optimize the prediction of ovulation.

Dinoprost, a naturally occurring PGF (1 mg/45.5 kg, IM), may cause transient adverse effects such as lowered body temperature, increased heart and respiratory rates, sweating, muscle cramping, colic, ataxia, and weakness. Signs are seen within 15 minutes and usually subside within 1 hour. Synthetic preparations, eg, cloprostenol sodium (0.55 mcg/kg, IM), have fewer adverse effects.

The sustained-release GnRH analogue deslorelin acetate (1.8 mg, IM) will cause ovulation within 48 hours of administration to an estrous mare with a developing 30–40 mm follicle. This FDA-approved preparation of deslorelin eliminates the need to use human chorionic gonadotropin (HCG) off-label, which has been administered at a dose of 2,500–5,000 IU, IV, or IM to cause ovulation in 36–48 hours when a preovulatory follicle ≥35 mm was present.

Ovulation can be timed accurately using the following protocol (not FDA approved): On days 110, 10 mg of estradiol 17-beta and 150 mg of progesterone are administered IM daily. On day 10, dinoprost (1 mg/45.5 kg, IM) is also administered. On day 16, mares come into estrus, and insemination should be performed on day 19 or 20. Most (85%) mares ovulate on day 20, 21, or 22. This regimen is effective at any time in cycling mares except when a large, dominant follicle < 48 hours from ovulation is present. If a mature follicle is present, initiation of the protocol should be delayed until after ovulation of the dominant follicle.

Altrenogest is a synthetic progestin that suppresses the receptive sexual behavior of estrus. Altrenogest is administered at 0.44 mg/kg, PO by dose syringe or top-dressed on feed for 12–15 days. Estrus occurs 4–5 days after treatment ends, with variable timing of ovulation (8–15 days). Although altrenogest effectively suppresses estrus, it does not consistently control the time interval to ovulation.

Estrus Detection in the Reproductive Cycle of Horses

Frequent palpation and ultrasonography of the genital tract, excellent record keeping, and administration of hormones are used to intensively monitor and manipulate a mare's estrous cycle. Breeding management can be optimized if a good estrus detection program is in place. A mare detected in estrus will prompt the breeding farm manager to examine and prepare the mare for breeding. Estrus may be the first indication that a previously pregnant mare has experienced early embryonic death or an abortion Abortion in Horses Twin pregnancy is the most common noninfectious cause of abortion in mares. In most cases, uterine capacity and subsequent placentation are inadequate to support two fetuses to term. Although... read more .

The mare should be presented to a stallion (teaser) daily or every other day during the breeding season, and the mare's behavioral response observed, interpreted, and recorded. Mares in estrus raise their tail, squat, urinate, evert the vulvar lips to expose the clitoris, and ultimately tolerate copulation. Mares in diestrus usually squeal, kick, bite, and reject the stallion’s advances. Adequate exposure to and contact with the teaser may be needed to elicit the mare’s response; a mare with a dominant follicle may initially not appear receptive because of nervousness or inexperience. Some mares with foals by their side may not exhibit estrus to the teaser because of their protective nature. The mare’s behavior when teased should be consistent with the findings on examination of the genital tract. Response to teasing can determine whether estrus has begun and indicate when a mare should be palpated and bred. If the mare does not return to estrus 2–3 weeks after breeding, she may be pregnant.

Mares in seasonal anestrus tend to be passive in the presence of a stallion.

Some anestrus mares will be receptive when confronted by a stallion and will tolerate a stallion’s advances. This tolerance seems to be due to a lack of progesterone, similar to the tolerance seen in an ovariectomized mare that is frequently used as a stimulus for semen collection from a stallion. Anestrous mares do not ovulate.

Mares normally have 3 or 4 prolonged periods (7–14 days) of sexual receptivity during the vernal transition before the first ovulation of the breeding season occurs. Similar long periods of sexual receptivity normally occur during the autumnal transition between the breeding season and winter anestrus.

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