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Reproductive technology: Embryo transfer technology offers benefits to equine industry

by Wyoming Livestock Roundup

“Embryo transfer is quite common in the equine industry and has been for the last 20 years. It is really a routine procedure now,” states Dr. Patrick McCue, Professor of Equine Reproduction at Colorado State University. 

            In fact, Patrick notes the first successful embryo transfer (ET) in horses was reported in the early 1970s, following the first successful ET in rabbits in 1890, rats and mice in 1930 and cattle, sheep and pigs in the 1950s. 

             “Since the first foal produced by ET was born in 1974, there has been a tremendous amount of basic and applied research performed which has allowed ET to become a relatively routine procedure in equine reproduction,” he says. 

Equine ET

“ET involves the removal of an embryo from the uterus of one mare, and the transfer of that embryo into the uterus of another mare,” Patrick explains. “The best embryo donors are mature, reproductively healthy mares. 

He continues, “Optimal breeding management of the donor mare and attention to detail prior to and after breeding is essential for embryo collection success.”

            Patrick notes mares are monitored via palpation and ultrasonography to determine the exact day of ovulation and optimal day of breeding.

            “They are then bred with fresh, cooled or frozen semen and ovulation is confirmed,” Patrick says. “Seven or eight days after ovulation, the uterus is flushed and hopefully an embryo is recovered.”

 “A sterile catheter is inserted through the cervix and the uterus is filled with embryo flush media. The media is then allowed to flow back out through the catheter and it is passed through an embryo filter,” he explains “Contents of the filter are poured into a search dish and examined for the presence of an embryo.”

He further explains embryos are recovered around 50 to 70 percent of the time and embryos collected on day seven or day eight are generally less than one millimeter in diameter. 

            “In horses, unlike cattle, we do not have the ability to induce multiple ovulations so superovulation is not currently possible with ET in the horse,” Patrick explains. “Therefore, we are looking for only one or two embryos in mares on each estrus cycle.” 

“Recovered embryos are washed and maintained in a holding media until transferred into a recipient mare,” he says. 

             He continues, “The embryo is then transferred non-surgically into a synchronized recipient mare, typically a mare who has ovulated one or two days after the donor mare.” 

            Patrick notes 70 to 90 percent of recipient mares become pregnant.

Differences and downfalls

            “The technique is generally similar between horses and cattle,” Patrick notes. “In both species, embryos are flushed from the uterus, and the embryo is then transferred to a recipient, which needs to ovulate close to the same time as the donor mare or cow.” 

            However, Patrick explains there are some differences when performing ET in horses.

“The largest issue with ET technology in horses is the fact that horses don’t have the ability to superovulate,” he notes. “It is a standard practice in cattle, which allows an opportunity to flush multiple embryos, but we can’t do that with horses.” 

“ET is a common, clinical practice in the equine industry today, so most of the challenges with ET have been worked out over the past two or three decades,” Patrick states.

            Despite this, Patrick says the technology still possesses the same potential pitfalls as breeding a mare to carry a foal.

            “As mares age, their fertility declines,” he explains. “There might also be some issues with ovulation failure in the mare or other reproductive issues that occur in horses.” 

            “Challenges with stallions might also be the same, such as getting quality semen at the correct time,” he adds. 

            “Whether it is artificial insemination, ET or other reproductive technologies, a lot of the reproductive challenges are the same in mares as they are if a mare was bred to carry her own foal,” Patrick continues. 

Benefits of equine ET

            As for the benefits of utilizing ET technology in horses, Patrick says there are several. 

            “ET allows a donor mare to continue to be a performance horse, meaning she can stay in training, go to shows and still be able to have a foal, which is carried by a recipient mare,” he says. “With ET, there is also the possibility to have more than one foal per mare in a given year.” 

            “Another benefit of ET in horses, which is possible only through this technology, is that some mares with challenging medical conditions who should not be allowed to carry their own foal, can have a younger, healthier mare carry the baby for her to protect the health of the donor mare,” Patricke explains. 

            Hannah Bugas is the assistant editor for the Wyoming Livestock Roundup. Send comments on this article to

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