Disease Du Jour: Subfertility in Mares

In this episode of the Disease Du Jour podcast, Benjamin Espy, DVM, DACT, joined us to discuss subfertility in mares. He explained common causes of subfertility, strategies for addressing them, alternative reproductive techniques, and more. 

This episode of Disease Du Jour is brought to you by Equithrive.

Risk Factors for Subfertility

Espy explained that one major risk factor for subfertility in mares is poor body condition. “You want fat coverage on mares to allow them to cycle correctly,” he said. “Mares that are too skinny will not cycle correctly because their caloric expenditure is focused on survival, not necessarily reproduction.”

Skinny horses are also more likely to have external problems that negatively impact fertility, such as a sunken anus and tilted vulva, which can result in urine pooling and uterine infection. 

Espy said the ideal body condition score for a broodmare is 5-6/9. Obese mares are more likely to struggle with the foaling process, he explained. Therefore, he feeds pregnant mares the same as geldings until the last three months of their pregnancy. 

Age is another major determinant of fertility because mares are born with their lifetime supply of oocytes. “The oocytes that a mare ovulates at 20 are literally 20 years old,” Espy said. This aging of genetic material reduces the likelihood of successful fertilization and early embryonic development. Uterine health also declines with age. When mares remain open for long periods, they accumulate fibrosis and scarring in the endometrium. Older maiden mares therefore face multiple obstacles at once, including aged oocytes, vulvar conformation that favors contamination, and a uterus that has not been kept pregnant and has developed fibrotic changes.

Strategies for Addressing Subfertility in Mares

Management strategies that improve reproductive outcomes focus on supporting the mare’s normal physiology. Artificial lighting remains one of the most reliable tools. Mares’ breeding season is determined by day length, and lighting them for 60 days helps them cycle earlier in the year. Espy recommends starting lights around Dec. 1 so mares begin cycling by February.

Endometritis is another common cause of subfertility. Espy said reproductive specialists today are very cognizant about treating biofilm, which prohibits antibiotics from reaching bacteria. Treatment often involves mechanical flushing and the use of solutions such as hydrogen peroxide, DMSO, EDTA, or kerosene. 

Biologic therapies can also be useful. Espy employs PRP, but often he simply collects whole blood, allows it to settle, and uses the serum portion. “We don’t necessarily think it’s the actual plasma spinning to harvest the PRP like you would on a tendon or a joint,” he said. “We think that those same beneficial elements of the blood products are in the serum.”

Advanced Reproductive Techniques

Embryo transfer or intracytoplasmic sperm injection (ICSI) can sometimes be helpful when traditional reproduction methods fail. The choice depends on which part of the reproductive tract has pathology: the ovaries, the uterus, the cervix, or the vulvar seal. 

For embryo transfer to be successful, the mare must have a good ovary, a normal fallopian tube, and a uterus that can maintain a pregnancy for a week. In contrast, ICSI bypasses most of the reproductive tract. Oocytes are harvested directly from the ovaries, and a single sperm cell is injected into each oocyte. However, Espy noted that outcomes are still restricted by the quality of the mare’s genetic material. 

Listen to the podcast episode to learn more about addressing subfertility in mares. 

About Dr. Ben Espy

Benjamin Espy, DVM, DACT, received a biology degree from Duke University in 1991. In 1996, he became a Doctor of Veterinary Medicine at Texas A&M University.

After completing a residency in Lexington, Kentucky, in 2005, Espy became recognized as a board-certified specialist in equine reproduction, infertility, and neonatology. During his career, Espy has practiced equine medicine in Brenham, Texas; Lexington, Kentucky, at Hagyard Equine Medical Institute; and most recently in San Antonio, Texas. 

He is the past chairman of the AAEP Owner Education Committee, served on the AAEP Board of Directors, the Board of Directors for the American College of Theriogenology, and on the King Ranch Board of Directors. Espy has also served as the AAEP liaison for the Board of Directors of the Texas Equine Veterinary Association. He serves on the ethics and grievance committee for the Texas Veterinary Medical Examiners, teaches in the Texas A&M University System, and is on the AVMA board of directors.

Besides being the King Ranch veterinarian since 2005, Espy currently practices sports and regenerative medicine almost exclusively on professional roughstock and rodeo timed event horses, as well as being their reproductive consultant. Because of this unique skill set, he is licensed in six states. He has been the Head of Veterinary Services at the San Antonio Stock Show and Rodeo since 1996.

Since 2011, Espy has been a consultant on the Animal Care and Welfare Committee for the Professional Rodeo Cowboy Association. He was a finalist every year from 2016 to 2021 for the PRCA Veterinarian of the Year and won the award in 2021.

Related Reading

• Laser Ablation of Endometrial Cysts Can Improve Mare Pregnancy Rates
• Associations Between Cytology/Bacteriology Findings and Live-Foal Rates in Broodmares
• Responsible Use of Antibiotics in Equine Reproduction

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