A great deal of the equine industry centers around breeding mares, and a great deal of time and money is spent on this pursuit. Said economic considerations in the equine breeding business make it necessary for horse owners, breeders and managers to understand the mare’s reproductive cycle and the best management techniques in this area.1 This is important regardless of whether a horse owner is breeding a mare privately or if they send her to a breeding facility.

Mares have an estrous cycle that is referred to as seasonally polyestrous. What this means is that the mare is usually receptive to the stallion and ovulates during specific times of year, and only at these times. During the anestrus (non-ovulating) part of the cycle, the mare will show no signs of sexual receptivity and will not develop ovulating follicles. In the northern hemisphere, this typically occurs during the winter months. During the ovulatory season (spring through late fall), the mare cycles regularly, exhibits sexual receptivity to the stallion, produces follicles that ovulate and can become pregnant. 

Management of the Mare’s Estrous Cycle

Given the aforementioned economic considerations, management of the mare’s estrous cycle is often performed by veterinarians and broodmare managers. A great deal of research has been done in this area, and has allowed veterinarians to manage the mare’s cycle in order to provide optimal breeding times, effectively use artificial insemination, induce ovulation, synchronize mares, induce superovulation, advance the onset of the breeding season, terminate pregnancy, and manage a variety of other reproductive conditions and diseases.2

Management of the mare’s ovulatory cycle and season requires mare owners, breeders and managers to understand the physiological determinants of the mare’s reproductive status. Follicular development and ovulation are controlled by various hormones. An understanding of normal hormonal patterns is important, because management practices used to increase conception rates must revolve around upon these patterns.

The artificial advancement of the ovulatory season in open and bred mares helps maintain yearly foaling intervals and produce older foals at the breed association’s January 1 universal birth date.2 A number of pharmaceutical products are currently being used in the equine industry to maximize conception rates in cycling mares. Hormone therapy is among these; this not only serves to help manage the mare’s estrous cycle, but can aid in mitigating various reproductive disorders.

For example: Under normal circumstances, a non-pregnant mare will return to estrus (heat) after being out of heat for approximately two weeks. Unfortunately, it doesn’t always work out this way. Ovulation failure is a common cause of reproductive inefficiency in the mare and subsequent economic loss. “Since affected mares do not ovulate, fertilization and pregnancy cannot occur. In addition, ovulation failure results in a prolonged interovulatory interval, which translates into more days open and higher subsequent costs.”3 Older horses tend to have a higher incidence of ovulation failure, with approximately 13% of mares greater than 15 years of age experiencing ovulation failure at least once during a given breeding season. Further, a high percentage of mares that experience one ovulatory failure will often experience another during the same breeding season.

Progesterone and Estradiol Therapy 

As knowledge of the mare’s reproductive cycle has improved, veterinarians have become better able to exert some control over their cycling. Progesterone and estradiol (P&E) therapy has been used since the 1980’s to synchronize the ovulation of mares, manage timed breeding, and to stimulate the cycle in mares with aberrant and irregular cycles.3

The two most common uses of progesterone and estradiol therapy are:

• Programming mares to ovulate at a specific time (generally, when a stallion is available)

• To synchronize mares for embryo transfer or to maximize the usage of semen 

Estradiol (also estradiol cypionate) is a natural estrogen salt used primarily to induce estrus. Indications for the use of estradiol include enhancing estrus behavior and receptivity in ovariectomized mare. Estradiol is the most active endogenous estrogen and possesses the pharmacologic profile expected of the estrogen class. Estradiol is necessary for the normal growth and development of female sex organs and, in some species, contributes to the development and maintenance of secondary female sex characteristics.

Progesterone, a key reproductive sex hormone in the mare is the substance that causes the mare to come out of heat after ovulation.4 It is also required for the maintenance of pregnancy. During ovulation, the ovarian follicle ruptures and releases the egg, which is then transported along the oviduct where fertilization can occur if the mare has been bred. After ovulation, the collapsed follicle releases hormones which cause progesterone to cease. The absence of progesterone and increase in levels of estrogen produced by the next follicle cause the mare to return to estrus. 

Where to buy Estradiol + Progesterone

Estradiol + Progesterone is available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies. ESTRADIOL 3.3 MG/ML / PROGESTERONE 50 MG/ML by NexGen Pharmaceuticals is indicated for the management of cyclic activity in the mare.

Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

1okstate.edu.

2selectbreeders.com.

3colostate.edu.

4colostate.edu.