Nearly all mares are seasonally polyestrous and cycle when day length is long. Anestrus is seen during the winter when day length is short. During anestrus, the uterus is flaccid, and the ovaries are inactive with no significant follicles or corpora lutea. The cervix may be closed but not firm and tight, or it may be thin, short, and dilated. As day length increases, mares undergo a vernal transition and the ovaries become active, with numerous large (>25 mm) follicles. The cervix and uterus have minimal tone. Mares have 3–4 prolonged intervals of estrus (sexual receptivity) during the vernal transition, but ovulation does not occur. The end of vernal transition is marked by a surge of luteinizing hormone and subsequent ovulation. After this first ovulation, the first 21-day interovulatory period occurs and a regular estrous cycle is established.

Although the mare continues to ovulate regularly every 21 days throughout the breeding season, the length of estrus (receptivity to the stallion) varies, ranging from 2–8 days, and the length of diestrus varies accordingly. Early in the breeding season, estrus tends to be longer, whereas around the summer solstice the mare may be sexually receptive for only 2–3 days.

Follicles enlarge on the ovary during estrus. Usually, one follicle becomes dominant and ovulates when it is ≥30 mm in diameter. The dominant follicle becomes tense 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 simulates closure of the cervix and increases uterine tone. This corpus luteum will be mature and become responsive to prostaglandin at ~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

After the winter anestrus and the vernal transition, cyclicity naturally commences some time in the spring, when breeding can begin. Because changes in the mare's genital tract are seen in response to day length, the onset of regular estrous cycles—and thus, the onset of the breeding season—can be hastened by exposing the mare to 16 hr of artificial light per day; 8–10 wk are required for mares to respond. If the breeding season is scheduled to begin February 15, artificial lighting should be started December 1. It is recommended that mares experience a natural photoperiod of decreasing day length in the fall. Mares can be exposed to light abruptly for a 16-hr day, or the light can be gradually increased to 16 hr over 60 days. In a constant program, light is added from 4:30 pm until 11:00 pm daily. In a less expensive stepwise program, 3 hr of light are added in the evening during the first week, increasing by 30 min each additional week. An automatic timer aids compliance and saves on labor.

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. A 3.7 × 3.7 m stall may require one 200-watt bulb or two 40-watt bulbs. Mares can be stimulated as a group in a lighted paddock.

Manipulation of Ovarian Activity

Ovarian activity is frequently manipulated to facilitate scheduling of breeding appointments and to allow many mares and stallions to remain in competition during the breeding season. Breedings should be spaced for stallions with large books of mares so that semen use is optimized. Geographic locations and transportation constraints also may necessitate scheduled breedings. Many situations can benefit from an ovulation control program. (Also see Hormonal Control of Estrus.)

Administration of prostaglandin (PG) F_2α, IM, to a mare in diestrus causes luteolysis and allows a follicle to develop and ovulate. The corpus luteum must be 5–14 days old to respond. The mare will come into estrus 2–5 days after administration of PGF_2α. Time to ovulation is variable (3–10 days) and depends on the size and character of follicles at the time of PG administration.

Dinoprost, a naturally occurring PGF_2α (1 mg/45.5 kg, IM), may cause transient side effects such as lowered body temperature, increased heart and respiratory rates, sweating, muscle cramping, colic, ataxia, and weakness. Signs are seen within 15 min and usually subside within 1 hr. Synthetic preparations, eg, cloprostenol sodium (0.55 μg/kg, IM) have fewer side effects.

Human chorionic gonadotropin 2,500–5,000 IU, IV or IM, can be administered to hasten ovulation of a dominant follicle during estrus. If the mare has a preovulatory follicle ≥35 mm diameter, ovulation occurs within 36–48 hr after injection. Deslorelin (1.5 mg, IM), a gonadotropin-releasing hormone analog, causes ovulation of a 30-mm follicle in ~40–48 hr.

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

Altrenogest is a synthetic progestin that suppresses the receptive sexual behavior that develops during estrus. It 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

A successful breeding management program revolves around a good estrus detection program. The mare should be presented to a stallion (teaser) daily or every other day during the breeding season, and an accurate interpretation and record should be made of her response. 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. Mares in seasonal anestrus may remain passive.

Some anestrus mares will be receptive when confronted by a stallion and will tolerate a stallion's sexual advances. This tolerance seems to be due to a lack of progesterone, similar to the tolerance seen in an ovariectomized mare used as a stimulus for mounting. Adequate exposure to and contact with the teaser should be allowed to thoroughly evaluate the mare's response; a mare in estrus may initially not appear receptive due to nervousness or inexperience. Some mares with foals by their side may not exhibit estrus to the teaser due to 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 if estrus has begun and indicate when a mare should be palpated and bred. Failure to return to estrus 2–3 wk after breeding may suggest that the mare is pregnant.

Mares will normally have 3–4 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 occur during the autumnal transition between the breeding season and winter anestrus.

Last full review/revision July 2011 by Patricia L. Sertich, MS, VMD, DACT