New Hormone Therapy to Enhance Fertility in Mares
Mare pregnancy rates are only 60-80% due to factors ranging from age to stress to poor uterine environments. As such, veterinarians have turned to a variety of advanced reproductive technologies (ART) to enhance mare fertility.

As one of the keynote speakers at the 2019 Equine Science Society Symposium, held June 3-6 in Asheville, North Carolina, Janet F. Roser, PhD, professor emeritus in the University of California, Davis, Department of Animal Science, described current technologies, as well as a new hormone therapy with the potential to make them even more effective: recombinant equine gonadotropins.

First, Roser listed ART techniques and their limitations.

  1. Embryo transfer (ET), which involves removing an embryo from a donor mare and placing it in a recipient mare’s uterus to carry to term. It’s a common way breeders get foals from performance mares or mares with health problems.

“One of the problems with ET is obtaining one if not more embryos from the donor mare,” Roser said. “Whereas a single ovulating mare results in a 50% embryo recovery rate, double or triple ovulations would result in a higher recovery rate.”

  1. Intracytoplasmic sperm injection (ICSI), the direct injection of sperm into a mare’s mature oocyte (egg). Veterinarians might use this technique in mares with reproductive abnormalities or disease. More oocytes would enhance the procedure’s success, said Roser.
  2. Oocyte transfer (OT), which works like embryo transfer, but with an egg rather than an embryo. Veterinarians might use this technique in mares with oviduct problems, uterine infections, and torn cervices, among other conditions.

“As in the case of ET and ICSI, a good number of immature oocytes for OT is essential for a successful procedure,” Roser said.

  1. Gamete intrafallopian transfer (GIFT), during which the veterinarian transfers both the sperm and the mature oocytes into the recipient mare’s oviduct. This procedure is useful for mares with reproductive problems and stallions with low sperm numbers.
  2. Cloning, or producing genetically identical horses. Again, more follicles to release more oocytes would enhance its success, said Roser.

What do these advanced reproductive technologies have in common? They’d all benefit from multiple ovulations. One way to achieve this is through superovulation, in which the veterinarian administers specific hormones to the mare that stimulate her ovaries to produce multiple follicles.

A variety of hormone therapies (human chorionic gonadotropin, gonadotropin-releasing hormone, etc.) currently exist to induce ovulation, but they face issues such as inconsistent performance, significant expense, and low stability, and they produce variable results.

“With the advent of recombinant technology, two relatively new stimulating agents, reFSH, with or without reLH, have been developed and reported to superovulate the ovaries of the cycling mare,” with efficient production and high stability, said Roser. She described a sampling of studies confirming its efficacy.

  • In a 2007 study on reLH, researchers found that a 0.75 milligram dose had a 90% ovulation rate within 24-48 hours, confirming it could stimulate timely ovulation in mares.
  • In a 2010 study on reFSH, researchers achieved 100% ovulation rate in a group of mares receiving a 0.5-milligram dose.
  • In a 2011 study comparing reFSH and an reFSH/reLH combination, researchers achieved 100% ovulation rate but recovered more embryos using the combination.
  • In a 2012 study researchers administered reFSH to mares in deep anestrus (not cycling) and noted a “very good pregnancy rate (56-75%) in recipient mares after transfer, considering the time of year,” said Roser.

So in conclusion, said Roser, the unique benefits of reFSH and reLH on assisted reproductive technologies are that:

  • reLH induces timely ovulations and increases the number of ovulations;
  • reFSH increases the number of follicles, ovulations, and embryos for transfer, allowing mares with fertility and reproductive issues to get pregnant; and
  • reFSH induces superovulation in cycling mares as well as those in deep anestrus, allowing mares to get pregnant or produce multiple embryos in the nonbreeding season.

“It has been demonstrated that these recombinant gonadotropins can successfully induce multiple preovulatory follicles, ovulations, and embryos in both cycling and deep anestrus mares,” Roser said. “The question now remains as to whether these recombinants will enhance reproductive efficiency by increasing the number of viable oocytes for ICSI, OT, GIFT, and cloning.”