A 1970 study reported that only 55% of mares bred annually produce live foals.1 More recent data revealed that only 57.7% of thoroughbred mares bred during 2005 produced live foals, a considerably lower foaling rate than other farms which typically report foaling rates of 71% to 85%.2 Still more research showed that the extent of reproductive management used in the latter facilities was much greater than that used on farms with lower foaling rates. Thus, it was concluded that there was a need to develop controlled breeding programs for the horse industry based on recent advances in the understanding of cost-effective hormonal control of reproduction in breeding horses.  

Some experts contend that equine reproductive management has not improved very much overall over the last 30 years.2 The mare’s long estrus period (with an ovulation anytime from 1 to 10 days after the beginning of estrus) has continued to confound breeders and managers by making the reproductive management of cyclic mares time-consuming, expensive and inefficient. Added to this is the uncertainty associated with the long and variable transition from anestrous to cyclicity in mares, which increases the complexity of efficient reproductive management for mares. 

As long as breeding associations continue to use January 1 as the official birth date for foals born during the same season, economic incentives to breed mares as early as possible will continue to come into play. Thus, treatment protocols for the induction of ovulation in anestrous or transitional mares are sought-after commodities. 

Estrogen Therapy for Mares

Estrogen therapy is widely used by veterinarians to regulate the cyclic activity of mares, as well as to treat horses exhibiting clinical signs of intermittent upward patellar fixation (IUPF) or Proximal Patellar Hesitation (PPH) and to enhance estrus behavior and receptivity in ovariectomized mares. Unwanted behavior in mares is a commonly presenting encountered by veterinarians. This may range from a mare being uncooperative or aggressive when handled, kicking, bucking or rearing when ridden or being aggressive towards other horses. In some cases, ovarian neoplasms cause a mare to change behavior, but in other cases, there is no apparent reason for the unwanted behavior.3 Occasionally, an owner will determine that ovariectomy is the most prudent solution.

Estrogens have effects on the skeletal system, increasing calcium deposition, accelerating epiphyseal closure, and increasing bone formation. Estrogens have a slight anabolic effect and can increase sodium and water retention.4 Estrogens also affect the release of gonadotropins from the pituitary gland. This can cause inhibition of lactation, ovulation, and androgen secretion. 

Estradiol for Horses

Estradiol 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, contribute to the development and maintenance of secondary female sex characteristics. It can cause increased cell height and secretions of the cervical mucosa, thickening of the vaginal mucosa, endometrial proliferation, and increased uterine tone.4

Estradiol Warnings and Contraindications

Estradiol is contraindicated during pregnancy as it can cause fetal malformations of the genitourinary system and induce bone marrow depression in the fetus. In cases of prolonged corpus luteum in cows, a thorough uterine exam is recommended to determine if endometritis or a fetus is present.4

Estradiol has been associated with severe adverse reactions in small animals. In cats and dogs, estrogens are considered toxic to the bone marrow and can cause blood dyscrasias. Estrogens have also been shown to induce mammary neoplasia. When used chronically in male animals, feminization may occur. In females, signs of estrus may occur and persist for 7-10 days.4

Where to buy Estradiol

Estradiol is available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies. ESTRADIOL 2 MG/ML by NexGen Pharmaceuticals is an excellent solution for managing the cyclic activity of 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.

1Loy, R.G. (1970).  Management and other factors affecting breeding efficiency in mares. Proc. 3rd Techn. Conf. Artif. Insem. Reprod., Chicago, Illinois.

2Burns, P.J., et. al. (2006) New Pharmacological Treatments for Equine Reproductive Management. BioRelease Technologies 1st  Annual Review.

3Melgaard, D. et. al. Moody Mares-Is Ovariectomy a Solution? Animals: An open access journal from MDPI vol. 10,7 1210. 16 Jul. 2020.

4Plumb’s Veterinary Drugs.