Superovulation and Embryo Transfer

It's no secret that embryo transfer is expensive, and that its success rates are lower than those of some other methods of equine reproduction. "Commercially, single embryo recovery attempts have a 50% recovery rate, and there is a 50-65% pregnancy rate per transferred embryo," said Tal Raz, DVM, a graduate student in theriogenology at the University of Saskatchewan's Western College of Veterinary Medicine. "Economically, there is a need to increase the success rate with embryo transfer."

Raz presented a study on superovulating embryo donor mares. Superovulation simply means inducing the mare to ovulate several eggs instead of one at a time, so that more eggs can be fertilized and more embryos can be recovered. Ideally, this increases the pregnancy rate. It might also have a place in other assisted reproductive techniques such as oocyte transfer and gamete intrafallopian transfer (combining eggs and sperm outside the body and then placing them in the fallopian tubes to achieve fertilization), he said. Currently, single-embryo recovery attempts are more common.

In this study, equine follicle-stimulating hormone (eFSH) was used to superovulate donor mares. Twelve donor mares (four to 17 years of age) and 37 young, fertile recipients were evaluated. The donors were untreated on the first cycle (which served as the control cycle), were treated with eFSH for the second cycle, and were injected with human chorionic gonadotropin (hCG) on the last day of treatment. They were bred via artificial insemination, and recipients' estrous cycles were synchronized as needed.

Raz reported that eFSH significantly stimulated follicular development, but the number of ovulations per cycle (2.3 in treated mares) was not significantly different from controls (1.5). "This was an unexpected finding," he commented. He noted that during eFSH-treated cycles, mares had more follicles larger than 30 mm per cycle, but only 70% of those ovulated compared to 100% of follicles larger than 30 mm in control cycles.

However, eFSH did significantly increase embryo recovery rate (1.3 embryos per cycle vs. 0.5 in untreated cycles). Mean embryo age was significantly lower in eFSH-treated cycles, as additional ovulations occurred 24-48 hours later during those cycles.

Embryo quality was scored on a 1-4 scale, with 1 being excellent and 4 being poor. Embryos from eFSH-treated cycles were of poorer quality (average score 2.7) than control embryos (average score 1.8). This is also seen in superovulating cattle and might be a reason why post-transfer recipient pregnancy rates were lower when embryos were obtained and transferred from eFSH-treated donor mares (33% compared to 67% of controls). However, this difference was not statistically significant, possibly because of the small number of mares in the study.

Raz noted that some other studies have not found this negative effect of eFSH on embryo quality and pregnancy rate. He concluded that more study of eFSH with more mares is needed to clarify its effects in different situations. Age and genetic makeup of donor mares might affect its success, as could estrous synchronization protocols and embryo handling techniques. Also, observing the "coasting" period now recommended by Bioniche, the company that produces eFSH, might give different results than those found in this study. In this protocol, mares receive their last eFSH treatment when the majority of follicles are around 35 mm in size, then 36 hours elapse before an injection of hCG is given to induce ovulation.

In summary, under the experimental conditions Raz reported, eFSH increased the number of preovulatory follicles in treated mares, did not affect ovulation rate, increased embryo recovery rate, and did not increase pregnancy rates.

Get research and health news from the American Association of Equine Practitioners 2006 Convention in The Horse's AAEP 2006 Wrap-Up sponsored by OCD Equine. Files are available as free PDF downloads.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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