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Freezing Equine Embryos to Maximize Transfer SuccessBy Kentucky Equine Research Staff · December 11, 2017

Victory! You finally have viable embryos from your favorite mare that can be stored for future transfer. Now what? How should these embryos be preserved for maximal viability?

Embryos are fertilized eggs that are flushed from a mare’s uterus and either transferred immediately to a recipient mare or frozen for future use. Embryo transfer has several benefits, such as preservation of valuable genetics, especially in mares unable to maintain a pregnancy, and production of foals while a mare continues to compete.

“Whether breeding occurs as natural cover, artificial insemination, or embryo transfer, consistently producing live foals in horses remains notoriously challenging compared to other domestic species. Mares have lower initial pregnancy rates and a high occurrence of early embryonic death,” said Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research (KER).

She added, “Up to 30% of mares lose their foals within the first 35 days of pregnancy, and 15-20% lose their foals within the first 50 days.” Breeding challenges explain, at least in part, why harvesting embryos from valuable mares has become popular. While fresh or cooled embryos used for embryo transfer reportedly have good success rates, freezing the embryos prior to transfer continues to be troublesome.

According to a recent study*, slow freezing embryos remains relatively unrewarding, whereas ultrafast freezing, known as vitrification, appears more promising, yet imperfect. For example, high concentrations of cryoprotectants are required for ultrafast freezing, which can be toxic for the embryos. Further, the liquid nitrogen needed for freezing can potentially contaminate or infect embryos.

To gain an improved understanding of cryopreservation techniques for equine embryos, one research group* froze equine (both horse and donkey) embryos using either a slow- or fast-freeze method and compared the health of the embryo to unfrozen, control embryos. They found:

  • A significant decrease in embryo quality was observed after cryopreservation, regardless of the method; and
  • Horse embryos suffered similar levels of damage after slow-freezing or vitrification.

Considering vitrification is less expensive, quicker, and easier to perform than slow-freezing, this may be the method of choice.

“As we know, nutrition plays a huge role in reproductive efficiency in mares,” noted Crandell. “In order to produce healthy embryos and ultimately healthy foals, all mares need to be in optimal nutritional status and body condition.”

In addition to consulting with a KER nutrition advisor, consider supplementing your mare with omega-3 fatty acids such as KER’s EO•3, a potent marine-derived oil rich in the omega-3 fatty acids DHA and EPA. EO•3 provides multiple benefits to broodmares, stallions, fetuses, and foals.

“In addition, supplementing mares with a natural vitamin E, such as Nano•E, provides antioxidant support to the reproductive tract,” Crandell added.

*Pérez-Marín, C.C., G. Vizuete, R. Vazquez-Martinez, et al. Comparison of different cryopreservation methods for horse and donkey embryos. Equine Veterinary Journal. In press.