Horse-Breeding Chemistry

Sometimes, synthetic progestins that are used on mares are also used on unruly stallions to calm their demeanor. Does this affect their performance in the breeding shed?

Regumate or altrenogest, a synthetic progestin approved for suppression of estrus in mares, is also used to help maintain pregnancy in mares determined to be at high risk. Regumate is occasionally administered off-label to stallions to suppress undesirable male behavioral characteristics that interfere with the horse's training or athletic performance. In horses, products can be used off label if there is a valid veterinary-client-patient relationship and there is no licensed product on the market that treats the condition you’re trying to treat. Any liability for the product’s safety and efficacy lies with the prescribing veterinarian. So does Regumate have adverse effects on reproductive performance in stallions? Progestins are known to inhibit pituitary luteinizing hormone secretion. In the male, LH stimulates Leydig cells of the testes to produce testosterone for spermatogenesis and libido.


Three controlled studies have been published regarding the effects of altrenogest administration on horse reproductive behavior, physiology and semen characteristics of stallions. In one study, 15 stallions received altrenogest at twice the label dose once daily for either 150 or 240 days, depending on the treatment group. Altrenogest treatment resulted in a dramatic suppression of LH concentrations within eight days. Concentrations of testosterone in the blood also declined significantly during the treatment period.

If you’re looking for more information on breeding horses, try AQHA’s Equine Breeding Techniques report. It includes helpful information on how to care for your mare during the breeding and foaling process.

Changes in reproductive behavior or libido were noted during altrenogest treatment and included an increased time to erection, increased time to ejaculation and an increase in the incidence of ejaculation failure. Altrenogest therapy was associated with a decrease in total scrotal width, which is an indicator of testicular size. It was also associated with a decrease in the stallion's daily sperm output and a decrease in the percentage of morphologically normal sperm in the ejaculate. In addition, some stallions exhibited a decrease in progressive motility of spermatozoa during altrenogest therapy. Cessation of therapy after either 150 or 240 days resulted in an increase in blood levels of LH, total scrotal width and daily sperm output. However, concentrations of testosterone in blood, libido and percentage of morphologically normal spermatozoa in the ejaculate did not return to normal by the end of the 90 day observation period. A subsequent study evaluated the effect of short-term altrenogest therapy (30 days) at the label dosage on behavior and reproductive function in four stallions compared to four untreated controls. Treatment resulted in reduced hormone levels, minor alterations in stallion behavior and no adverse effects on semen quality or quantity. Testosterone levels remained suppressed until the end of the observation period, 60 days after treatment ended.

To learn more about horse breeding, download AQHA’s Equine Breeding Techniques report. In it, you’ll find helpful horse breeding information to help you through the breeding and foaling process with your mare.

A third study evaluated the effects of altrenogest on reproductive parameters in young stallions. Five stallions between the ages of 2 and 4 years were administered altrenogest at twice the label dose once daily for eight weeks. Treatment resulted in decreased daily sperm production, a decrease in the percentage of morphologically normal sperm and some alterations in horse breeding behavior. Again, some of these parameters did not return to normal within eight weeks after the end of treatment.


In summary, giving stallions altrenogest suppresses LH, testosterone and other reproductive hormones. Short-term treatment with the label dose of altrenogest for suppression of estrus in mares apparently has less adverse effects on behavior and semen parameters than prolonged administration of a higher dose. A stallion's testosterone might remain suppressed for a prolonged period of time at either treatment dose. The long-term effects of altrenogest administration on future testicular function are unknown. Owners and trainers should use caution and discretion when contemplating the use of altrenogest in young performance stallions destined for a future reproduction career.