Regumate for Horses

This helpful hormone therapy can increase your horse-breeding success.

By Dr. Patrick McCue in The American Quarter Horse Journal

The health of your broodmares is crucial to the health of their offspring. Journal photo

Regumate is the trade name for a synthetic hormone called altrenogest. It is the only such medication proven to have progesterone-type biologic activity in the horse. Clinical uses of Regumate include management of the transition period, suppression of heat in performance mares and maintenance of pregnancy in problem mares.


A majority of mares have limited ovarian follicular development and do not exhibit estrus, or heat, during the winter months. Follicle growth begins in the late winter or early spring in response to increased length of days. Mares can develop successive waves of follicles that grow and regress without ovulating during this transition between winter anestrus and the physiologic breeding season. Transitional mares can exhibit irregular and/or prolonged periods of sexual receptivity associated with each follicular wave.

The goal of Regumate therapy during the transition period is to suppress the long erratic estrous periods and to advance the first ovulation of the year. It has been reported that treatment is more effective late in the transition period (i.e. after the middle of March) than early in the transition period or during deep winter anestrus when the ovaries are inactive. A 14- to 18-day treatment period is generally recommended.

How closely do you follow your mare's heat cycle? When managing your broodmare to get her ready for breeding season, it might be appropriate to use hormone therapy. Learn everything you need to know to get ready for breeding season in AQHA's free e-book Mare Care: Breeding Tips.

Behavior Suppression

Mares can be difficult to train or not perform up to their potential when they are in heat. Consequently, it might be advantageous to suppress the expression of estrous behavior in these mares during critical training or performance sessions. Regumate is the hormone most commonly prescribed for suppression of estrus in mares. Treatment should begin a minimum of three days prior to a show or event and must be continued daily to attain and maintain estrous suppression. Regumate may be administered for an extended period of time without adverse effects on future reproductive performance or fertility.

Ovulation Control

In an embryo transfer program, it might be necessary to synchronize the ovulations of a donor mare with a specific recipient mare. Similarly, it might be necessary to alter or manipulate the estrous cycle of a mare for a scheduled breeding due to stallion availability. Regumate can accomplish both tasks. The progestin is administered once daily for 10 to 14 days, and a dose of prostaglandins is typically administered on the last day of treatment. Mares should come back into heat in three to five days after the conclusion of therapy and are usually able to be bred seven to nine days after the end of treatment.

Pregnancy Maintenance

Progesterone production by the corpus luteum is required for maintenance of pregnancy during the first two or three months of gestation. After 90 days of pregnancy, the placenta takes over the role of progesterone production. Inadequate production of progesterone by the corpus luteum might be a contributing factor to early embryonic loss in some mares. Supplementation with exogenous progesterone is often suggested for mares with a history of repeated early pregnancy loss.

Breeding horses can be difficult, no matter if you've been a breeder for many decades or just a few weeks. Learn everything you need to know about getting your prized mare in foal with a few great tips from Mare Care: Breeding Tips.

Regumate is also effective in maintaining pregnancy in mares who appear to be coming back into heat even though an embryo can clearly be observed during an ultrasound examination 14 to 16 days after ovulation. In addition to problem pregnancies, Regumate is often administered to embryo transfer recipient mares, pregnant mares with medical issues (i.e. colic), pregnant mares subjected to lengthy transportation and after manual reduction of a twin pregnancy.

Prudent use of the progestin Regumate can be beneficial in the reproductive management of mares. However, the use of exogenous progestins is not warranted in many instances, and many mares are administered the drug without a clear medical indication. Please consult with your equine veterinarian regarding the safe and effective use of this product.

Dr. Patrick M. McCue is a reproductive specialist at the Equine Reproduction Laboratory at Colorado State University.