Horse Breeding

Regumate for Horses

August 16, 2013

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

foal and grazing mare

Part of owning a mare is dealing with hormones. For some mare owners, hormone therapy can help with breeding a mare, maintaining a pregnancy or even balancing moody behavior. Journal photo.

By Dr. Patrick McCue in The American Quarter Horse Journal

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.

Management

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.

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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.

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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.