Mares are known to be seasonally polyestrous, meaning that the mare’s reproductive activity cycles through a reproductive season and a non-reproductive season.¹ The catalysts for both of these are predicated upon the amount of light that is available seasonally. Light sends a biochemical message to the mare’s pineal gland which results in the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus. GnRH then stimulates the pituitary gland to release two of the hormones that control her estrus cycles, follicle stimulating hormone (FSH) and luteinizing hormone (LH).²

The reproductive season begins in the Spring and continues through Summer when light is abundant. The non-reproductive season (known as anestrus) occurs during the Fall and Winter when there is little light. During anestrus, the mare will not respond to a stallion's attention, and her ovaries become inactive. 

During the mare’s reproductive season, her deportment changes along with the hormonal activity within her reproductive system. During this period, the mare experiences a series of estrous cycles. The ovaries swell in size and become active rather than dormant. These cycles repeat themselves at 21 to 23-day intervals until the mare either becomes pregnant or reverts to anestrus as light fails and late Fall and Winter arrive.³

Estrous Behavior 

Estrous behavior can be undesirable in performance and working horses. Even the quietest mare can display truly disruptive behavior during her estrous cycle, which can make riding, training, competing, or handling these horses a challenge.¹ Common complaints of mares in estrus include a poor attitude, difficulty in training, and a decrease in performance. A recent study revealed that 90% of veterinarians surveyed believe that the estrous cycle impacts an athletic mare’s performance.⁴

Manipulating estrus is essentially a method by which the mare’s hormonal balance is altered to “trick” her reproductive system into staying longer in a nonestrous phase or to bring on estrus faster and shorten its duration.² A delicate balance of progesterone and estrogens induce the various phases of the mare’s estrous cycle.^3,4 Estrogen is the predominant hormone in play during estrus. In diestrus—the period between estrous cycles— progesterone is the predominant hormone. Estrus makes a mare sexually excited, whereas progesterone tends to keep the mare more calm.

Altrenogest for Equines 

The reproductive system can be encouraged to alter her hormonal balance without drugs, most often through manipulation of the light cycle—but this method is most often used in the area of synchronizing estrous for breeding purposes. In the case of performance or working horses where suppression of estrus is desired, altrenogest remains today’s gold standard in estrus manipulation for performance horses in the U.S. and many other countries.⁴ 

Where to buy Altrenogest

Altrenogest is available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies. 

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.

¹Merck Veterinary Manual

²horseandrider.com.

³thehorse.com.

⁴aaep.org.