While the technology behind sex-sorted semen is relatively new, the tool has become commonplace, particularly in dairy breeding. And as the process for sorting, processing and using sexed semen becomes more efficient, leaving a calf's sex up to chance could become obsolete.
"Sex is the most important genetic trait," says Colorado State University reproductive physiologist George Seidel, who recently presented a webinar on the topic, hosted by the Dairy Cattle Reproductive Council (DCRC).
Seidel, who helped pioneer the sorting process and application for sexed semen in cattle, says some of the original research took place in the 1980s at the Lawrence Livermore National Laboratory in California. Scientists there became concerned about possible effects of radiation on workers' offspring, and their studies of sperm led to discoveries regarding differences between sperm cells carrying X or Y chromosomes. In the late 1980s, USDA researchers found they could segregate X and Y sperm from rabbits, and in the early 1990s researchers in the United Kingdom began experimenting with producing male calves through in-vitro fertilization. In the late 1990s, Seidel and his team at CSU developed a process for creating sex-sorted cattle semen for freezing and use in artificial insemination (AI). Through the CSU Research Foundation, they formed a company, XY Inc., which was later sold to Sexing Technologies, based in Texas.
In 2006, Seidel says, about 1.5% of Holstein heifers and fewer than 1% of Holstein cows in the United States were bred using sexed semen. By 2008, sexed semen accounted for about 14.5% of breedings in Holstein heifers and around 2.5% in cows.
Sorting semen is complicated by the fact that X and Y semen are virtually identical in most respects. Their size, shape, weight, electrical charge and speed are essentially the same, which is nature's way of providing a 50/50 chance for male or female offspring. Scientists found however, there is slightly more DNA in an X sperm than a Y sperm - about 4% more in cattle. Using a DNA-binding dye, laser light and precise computer analysis, they found they could identify sperm cells based on differences in their fluorescence.
The computer-controlled sorting machinery, which costs about $500,000 for a basic unit, separates semen into microscopic droplets, each ideally containing one sperm cell, and tags X sperm with a positive charge and Y sperm with a negative charge. Sperm that are non-viable or are not clearly identified are discarded in the sorting process. The system can evaluate about 25,000 droplets per second, as they move through the machine at 50 miles per hour. In a given sample, the system typically will sort off about 30% X sperm, 30% Y sperm and 40% unidentified or non-viable sperm.
Purity of the sorted samples can reach 95%, and the industry standard is 90%, but sorting takes considerable time, and the process damages more sperm cells than conventional collection and freezing, adding to the cost of each dose. A conventional dose of frozen semen contains around 20 million sperm cells. Sexed-semen doses typically contain closer to 2 million sperm cells, which provide a reasonable balance between cost and fertility. Pregnancy rates with sexed semen typically are lower than with conventional AI, although well-managed estrus synchronization, with AI timed six to 12 hours later than with conventional semen can help optimize fertility. Higher sperm counts per dose can improve fertility with sexed semen, but not enough to justify the much higher cost of production, Seidel says.
Sexed semen generally costs about $15 more per dose than conventional semen. Lower fertility and less than 100% accuracy also contribute to the overall cost per breeding with sexed semen, Seidel says
Theoretically, as the technology improves, fertility rates with sexed semen in cattle could equal or exceed those with conventional AI, as it does today in sheep. The process already does a good job of sorting off dead or non-viable sperm cells, and could eventually provide more cells per dose at cost-effective prices. Accuracy also could be improved. Current technology can produce accuracy around 95%, but the cost is higher than that for the industry standard of 90%. Those costs could drop in the future.
Currently, dairy producers often use sexed semen to select for heifer calves from their most productive females, sometimes using conventional semen from beef sires to add value to their remaining calves as beef animals. They could eventually use sexed semen to produce female calves for replacements in the milking herd and male calves for beef production. Beef producers could use a similar process, breeding for maternal heifer replacements from their best females while using male sexed semen from terminal-cross bulls to produce feeder steers from the rest of the herd.
Potentially the technology could make the use of un-sexed semen obsolete, Seidel says, adding that depending on the production system, "one sex always is more valuable than the other."