Based on recent study results, the suspense of waiting to know a new foal’s sex could be a thing of the past: Breeders might soon be able to choose ahead of time whether they’d like a colt or a filly out of a specific mating.

As reported by Juan Samper, DVM, MSc, PhD, Dipl. ACT from JCS Veterinary Reproductive Services Ltd., in British Columbia, Canada, and colleagues, a Texas-based company (Sexing Technologies) could offer "sex-sorted stallion semen" that allows horse owners to pick the sex of their future foals. However the current system has several limitations and substantially more research is needed before the process is implemented worldwide, Samper said.

"There are many companies and patents claiming that they can alter the sex ratio of the offspring, but only a technique called high-speed flow cytometry can separate X- and Y-bearing sperm," explained Samper.

High-speed flow cytometry involves taking a fresh ejaculate and running the sample through the high-tech cell-sorting machinery, which separates the X (female) sperm from the Y (male) sperm. The resultant samples are between 90% and 95% pure, meaning that about 5% of the sorted sperm won’t produce a foal of the desired sex.

The other, bigger problem with sex sorting is that although the process is called "high speed," it still takes around two to three hours to sort enough sperm to produce one insemination dose of 20 to 40 million sperm, which is a much lower number of sperm typically used to inseminate mares.

"This requires changes in insemination techniques, such as deep horn insemination, which adds another level of reproductive management," noted Samper.

Nonetheless, sex-sorted semen has been used successfully in horses. For example, 173 mares in two embryo transfer programs were inseminated with 18 to 80 million sex-sorted sperm, and 109 embryos were recovered. Those embryos were transferred to recipient mares, resulting in an average pregnancy rate of 60.4% (an acceptable pregnancy rate). Further, the desired gender was achieved in 97.8% of the foals.

"Those data, however, were generated using fresh, not frozen, sex-sorted semen. Similar positive results by freezing sex-sorted semen have not been achieved," relayed Samper.

Although it remains unclear why fresh sex-sorted semen "works" better than frozen-thawed sex-sorted semen, it means that owners wishing to breed mares with sex-sorted semen need to have the mares located at the facility where the sex-sorting is performed to breed the mares immediately after the flow cytometry has sorted the semen.

Despite these positive results, Samper cautioned, "The full impact of this technology will not be realized until a system can be developed to cool and freeze the sex-sorted semen. Further, more research needs to be done regarding the impact of the sorting on the structure and function of the sperm."

The article, "The use of sex-sorted stallion semen in embryo transfer programs," appeared in the July 2012 edition of the Journal of Equine Science. The abstract is available online.