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Splitting Heirs

Sperm-sexing technology will soon offer producers important choices for their AI program.

Today, systematic crossbreeding, artificial insemination (AI), heat synchronization and embryo transfer are routine livestock breeding practices that once seemed revolutionary. The next quantum leap - sex selection - is about to follow the same story line.

Semen products sorted by sex will soon be commercially available for extensive use by the U.S. beef cattle industry. As scientists fine-tune sex-sorting technology, the advantages of being able to pick the sex of a calf quickly come into focus.

For both seedstock and commercial beef cattle producers, sexed sperm will allow breeding replacement cattle to have heifers specifically designed for the breeding herd. Ranchers using AI will be able to breed for a higher percentage of male market calves from terminal sires.

These and other benefits are just around the corner for the U.S. beef industry. The technology is now being used in the United Kingdom (UK), where 600,000 male calves born each year in commercial dairies represent a tremendous financial burden.

In the UK, selected dairy cows are being bred with beef-type sexed semen to produce more commercially viable beef-cross male calves. Other dairy cows are bred with semen to produce female offspring for replacements.

This summer, XY Inc., Fort Collins, CO, joined with Cogent - a British cattle breeding cooperative - to make sexed sperm available in the UK. The sperm sexing process patented by USDA is licensed to XY, which is recognized as the world leader in the research, development and commercialization of techniques to pre-select the sex of non-human mammals.

"Sex-selected sperm will allow production of offspring of a desired sex, taking advantage of differences in value of males and females for specific markets," says reproductive physiologist John Schenk, manager of sperm processing and flow cytometry laboratories for XY.

Using a computerized cell-sorting machine called a MoFlo, XY technicians separate bovine sperm that carry the X chromosome and produce females from sperm that carry the Y chromosome and produce males. Females are then bred with the desired sperm using AI.

This sex-sorting technique should be available for commercial use in the U.S. in about two years, says Schenk. "We've shown this method of sorting bovine sperm by sex can finally be introduced into the commercial world," he says.

Impact On The Industry

George Seidel, Jr., a Colorado State University reproductive physiologist, has worked closely with XY to make sperm sorting a viable practice for both the dairy and beef industries.

"The efficacy of this practice has improved markedly over the past several years, and improvements continue to be made," says Seidel. "As happened with embryo transfer, success rates will stabilize as experience accumulates."

Since the mid 1990s, several hundred calves have been born from sperm sorted by sex - and hundreds more are on the way.

Due to the cost of sexing semen and AI, it's reasonable to assume, for now at least, only semen from superior and proven sires would be sexed, sorted and inseminated into heifers, says Ron Torell, University of Nevada Extension livestock specialist at Elko.

"For the registered producer especially, the use of sexed semen certainly makes a lot of sense," he says.

For example, producers could breed replacement heifers to have all heifers.

"We could essentially produce our replacements from our replacements," explains Torell. "We usually have less dystocia in first-calf heifers that have heifer calves."

The technology will allow commercial beef producers to breed their cows to terminal-cross sires resulting in calves that would perform more efficiently in the feedlot and on the rail.

"Down the line, we'll find more applications for sexed semen, especially in making better use of heterosis in crossbreeding programs," says Torell. "There's no question, once the process is perfected and the costs go down, it will have quite an impact on the cattle industry."

The cell-sorting technology used by XY now provides about 90% sexing accuracy. Current pregnancy rates using sexed semen are about 80% of what can be obtained through normal AI procedures.

While some sperm are damaged in the sorting process, the damage is relatively minor compared to that caused by semen freezing practices. New methods for freezing and handling sex-sorted semen, along with new AI techniques using fewer sperm, are being investigated, Seidel says.

Expanding The Process Since freshly collected semen is necessary for the sexing process to work well, commercial sorters will likely be placed at or near bull stud facilities. Estimated costs for sexing sperm in the U.S. will be about $50/dose initially - not counting the cost of the semen, predicts Seidel.

"But, once the process is streamlined to produce millions of doses per year for the cattle industry, the cost will probably be in the $20 per dose range," he says.

The advent of sexed semen will also open doors for the AI industry, says Willie Altenburg, beef program manager for Alta Genetics Inc.

"Sexed sperm would make AI more feasible, more practical and more affordable for the cattle business," explains Altenburg.

To date, the purebred and AI industries have found it necessary to expend a great deal of emphasis on calving ease traits for use on heifers. Sexed semen will allow for added attention on sire traits, which could effectively "up the bar" on growth and carcass traits, says Altenburg.

Depending on how widespread the technology becomes, it could ultimately bring enough breeding efficiency into both the dairy and beef industries that producers may find they don't need as many cows to produce the world's beef.

"Sexed sperm could potentially change what type of genetics a dairy cow is exposed to and make every other dairy cow a beef producer," says Altenburg. "It certainly has the potential to change the face of the cattle breeding business."

Sexed sperm has also been used to produce offspring in five other mammalian species - humans, horses, swine, sheep and rabbits.

"It appears that the process will work for nearly all mammals," explains Seidel. "A major constraint with horses and pigs is that many more sperm are required to obtain acceptable pregnancy rates than with cattle."

The technology also may prove to be useful in propagating endangered species and conserving rare genetic material.

Interest in sperm sexing technology is expanding geographically as well. XY is working with Goyaike S.A., a leading Argentine cattle breeding firm, and two cattle breeding companies in Japan. Each has entered into research agreements with XY and secured rights to commercialize the sperm-sorting technology in their respective countries.

Schenk warns, however, that using flow-sorted sperm to produce sex-selected offspring is not a panacea for all applications or cattle producers. Nor is it a process that lends itself to anything but well-managed herds and the best-trained breeding technicians.

"Attention to detail will be mandatory," he advises. "And, producing and marketing flow-sorted sperm will require considerable investments in time and money. There's every sign, though, that this technology truly offers a world breakthrough in animal breeding."

The sex of offspring in mammals is determined by either the X- (female) or Y- (male) chromosome-bearing sperm. Because more DNA is contained within the X-chromosome of bovine sperm, live sperm can be separated by sex using a specialized cell-sorting technique developed by USDA reproductive physiologist Larry Johnson, Beltsville, MD.

During sex sorting, a fine stream of sperm is stained with a DNA-specific dye. The X-chromosome-bearing sperm containing more DNA bind more dye than Y-chromosome-bearing sperm and thus glow brighter when exposed to laser light.

Properly positioned detectors on the sperm sorter quantify and discriminate between the X and Y brightness differences emitted from the DNA and transfer that information to a computer for processing.

As sperm flow through the sorter, a vibrating crystal breaks the stream into small droplets (Figure 1). A positive or negative electrical charge is assigned to droplets according to the sperm's brightness. Droplets then pass an electric field where the respective droplets are deflected away from each other depending on their charge.

Streams of droplets containing selected sperm are collected into a test tube for further processing.

Only live, membrane-intact sperm are sorted; dead sperm and sperm with abnormal chromosome content are removed from the sperm population during sorting - a side-benefit of cell sorting. Presently about 10 million sperm of each sex can be sorted/hour. This is roughly the typical semen dose for AI. Researchers understand that the procedure must be sped up considerably for commercial use.

The cell sorters, which currently cost $280,000, are built by Cytomation Inc., Fort Collins, CO. XY Inc. has exclusive rights to the patent for sexing sperm through flow cytometry in non-human mammals. They intend to license this technique to bull studs and livestock genetics companies with royalties being paid to the U.S. government and the Colorado State University Foundation.

The equipment cost is expected to decline as sperm sorters are simplified and mass-produced. XY Inc. would like to have at least one cell sorter for commercial use in the U.S. by early 2001.