A Breeder's Dream
By Kristen Tribe

The Cattleman
October 1999

Sexed Semen has been a dream of man for thousands of years, once surrounded by myth and folklore, but by the year 2001 science could make it commercially available around the world.

     It's a common practice for beef cattle producers to breed for gender-specific traits such as carcass composition or mothering ability. But even if you put together a dynamic duo for early growth and weight strain there is no ancient fertility dance or special request line to guarantee you a bull calf. Putting all wishful thinking aside, a 50-50 chance has always been your best shot, but with the help of the new sexed semen technology, your chances just got better.
     It is now possible with the help of a fluorescent dye and a cell sorter machine to separate sperm into male- and female-producing groups and then use the chosen sexed semen in artificial insemination with an approximate 90-percent chance of getting the desired sex.

Applications for beef producers

     "In my opinion, the most important use of sexed semen for beef cattle producers will be to breed their heifers to have heifer calves,'' says Dr. George Seidel Jr., physiology professor, Colorado State University. This would greatly improve calving ease because heifer calves weigh an average of five pounds less than bull calves, and since you selected these heifers for their genetics as replacements, they naturally would be the perfect mothers for the next generation of replacements."
     Those producers who already use AI methods in their breeding systems will be the first ones to benefit, but its availability may encourage others to begin an AI program. This technology will enable the beef industry to create more efficient breeding systems while making genetic improvements quicker than ever before. For example, in a rota-terminal system, producers use the rotational system to provide replacement females and the terminal side is used to produce males for market animals. Typically, you have to breed twice as many females as the number of replacements you need because 50 percent of those calves will probably be bulls, which then possess traits chosen for replacement heifers, not meat-producing calves. And on the terminal side, traditional breeding will produce a calf crop of about 50 percent females that aren't as profitable as their male counterparts in the production of beef. Heifers may have lower growth rates and feed efficiency, and, in turn, bring less on the market.
     But by using sexed semen, you only have to use half as many cows to get the desired number of replacements, and you're able to transfer more mother cows to the terminal side to produce more steer calves each year.
     Your imagination could go wild with the possibilities. Seed- stock producers would be able to produce the best proportion of males and females to meet the needs of their systems and clientele. Seidel even went on to explain that another potential result for producers could be the elimination of the cowherd. He says each heifer would replace herself. A producer would use sexed semen and AI for a heifer calf, and maybe breed her a little early. He would wean the calf early and then fatten the cow for sale. The weaned calf would grow with a little supplemental feeding and the cycle would start all over again. "You don't have the cost of maintaining a cow herd, and everything you're feeding is growing,'' says Seidel.

Developing the technology

     Sexed semen has been a dream of man for thousands of' years, once surrounded by myth led folklore, but by the year 2001, science could make it commercially available to producers around the world. During this century, many efforts have been made to develop such technology but none have proven as successful as the one invented by Dr. Lawrence Johnson, animal physiologist with the agricultural Research Service, Beltsville, Md., and patented by the U.S. Department of Agriculture in 1992.
     "I grew up on a dairy farm in Wisconsin. So I knew what it was to breed back the top 40 percent of your herd for milk production and then end up with bulls we had to sell for nothing," Johnson says or his interest in the subject. "I started work on this particular project in 1982, and initially. Nobody thought that it could be done."

Hundreds of animals have been born using sexed semen and all have been born normal and healthy, including cattle, sheep, rabbits and pigs, and most recently, horses.

     Three years ago, XY Inc. in Fort Collins, Colo., received a license from USDA to sex semen in non-human mammals. XY Inc. was formed as a joint venture between Colorado State University and a local instrumentation company known as Cytomation, a world leader in high-speed cell sorting.
     Dr. Mervyn Jacobson, XY Inc. president and chief executive office, says the formation of XY Inc. and bringing together numerous teams from around the world, including thc transfer of the USDA license, enabled XY Inc. to complete the development work that had been going on in different parts of the world during the prior 20 years.
     "We are the ones who will commercialize sexed semen worldwide, but we do not intend for there to be little XY boutiques in the future here you send semen, get it sexed and sent home to you in a nice little box with a ribbon on it" says Jacobson. ''We intend to develop the know-how, and then we will license throughout the world to the appropriate people in each country. It may well be one major license per species, per country.
     "The technique is sophisticated so we must ensure that he people who will be doing it are very serious and professional and have the resources, funding and scientists to do this properly."
     Hundreds of animals have been born using sexed semen and all have been normal and healthy, including cattle, sheep, rabbits and pigs, and most recently, horses. In late May, three female dairy calves in Britain were the first born outside he United States using sorted semen with AI. Jacobson says these births are significant because it proves this semen-sorting technique also works reliably outside the research lab where it was developed.

Sexing the semen

     To produce female calf, a sperm with an X chromosome must fertilize the egg, which always has an X chromosome. When a sperm with a Y chromosome fertilizes an egg, a male calf is produced. These are often referred to in conversation and publications as the X and Y sperm. "DNA content is the only real known difference between X and Y sperm, besides of course, half have a Y chromosome and half have an X," Johnson explains. The DNA is located in the chromosomes, and in mammals, the X sperm contains more DNA than the Y sperm, but the difference does vary among species.
     In cattle, the X sperm has about 3.8 percent more DNA than the Y sperm. In the pig, the X sperm has about 3.6 percent more DNA, in the chinchilla it's 7.5 percent and in the human it's 2.8 percent. The larger the difference, the easier it is to sort the sperm at a higher purity. The difference in DNA between X and Y sperm is the basis for sexed semen technology.
     To begin the process, the semen is stained with a fluorescent dye, which binds to each individual sperm cell based on its amount of DNA. Sperm cells then pass through the cell sorter in a fine stream, which uses a laser beam that essentially illuminates the dye. Since the X sperm have more DNA, they in turn have more dye and ''glow'' brighter. Based on this, the computer categorizes the sperm as X-bearing sperm, Y-bearing sperm or a mixed population of X and Y. Thc stream breaks into droplets which contain a sperm, and a positive or negative electrical charge is then assigned to each droplet based on the above categories. They are passed through a magnetic field whore the positives are pulled to the negative side, and the negatives are pulled to the positive side. Once it is sorted, fresh semen should be used within 24 hours, but it is also possible to freeze the sorted sperm for much later use.
     Johnson says that his early method of sorting sperm only produced 300,000 an hour. Three years ago, he and his co-workers developed a new nozzle for the cell sorter that increased the efficiency of sorting by nearly three times.
     After adapting it for a high-speed cell sorter, the current production rate in his laboratory is as many as 12 million sperm per hour. This combination has resulted in a much broader application of sexed semen in conjunction with AI.

CSU'S research

     "If it only takes one sperm to fertilize an egg, why do we need 20 or 30 million?" Seidel asks. ''The answer is one doesn't need to put in 20 or 3 0 million if you do certain things correctly. We've been trying to develop methods to breed animals with fewer sperm and equipment to sort sperm faster."
     One result of their efforts has been deep uterine AI, requiring only 1,000,000 sperm for heifers vs. the traditional 20 million. Sexed semen is deposited deep in the uterus, half into each uterine horn.
     Colorado State has also run numerous field trials in conjunction with XY. For the past three years, CSU has bred its 45O-cow purebred Angus herd with both sexed semen and normal semen to compare conception rates, predicted sex ratio and effects on fertility and embryo viability.
     The first year, CSU re-searchers inseminated only heifers with one dose of about half a million sperm in fresh, sexed semen and they used normal frozen semen for their control group. Expecting conception rates of only 20 percent to 30 percent with the sexed semen, they were pleasantly surprised with a rate of 42 percent and when the calves were born, 95 percent of the calves were heifers.
     By the next year, technology had advanced to allow the freezing of sexed semen and heifers were inseminated with a dose of one million frozen sperm with a 51-percent conception rate.

In June 1999, CSU had just finished breeding the herd again. By this time, improved efficiency in sorting made more sperm available for use, which in turn made it possible to compare doses of 1 million and 3 million sperm.

     "Unfortunately, the only bad result we've had throughout the years was the conception rates last year. The cows were considerably lower than the heifers. The heifers were about 50 percent and the cows were about one-half of that" says Dr. Ronnie Green, professor of animal breeding and genetics, CSU. "We did expect it to be a little lower due to greater difficulty in synchronizing mature cows.
     "We really don't know what caused it, but weather conditions were worse when we bred the cows than heifers. We're anxious to see this year how we come out given that we had more normal conditions with our cows."
     Each year the ratio of calves was as predicted, and re-searchers saw no more early embryonic loss from using the sexed semen.
     Besides conducting field trials with their own herds, CSU researchers are conducting some experiments with computer modeling. Patrick Doyle, a doctoral student at CSU, is using the Colorado Beef Cattle Production Model to investigate the impacts of using sexed semen in commercial production systems under various breeding systems. "There's really two things we're after," says Doyle. "No. 1, what are the benefits for the producers? And No. 2, how cheap does that technology have to be before it's adopted? We'll be looking at various mating systems that are often affected by herd size as well as the use of AI vs. natural breeding. We need to find out how this will affect the bottom line."
     A target does exist to provide sexed semen at a cost no more than $10 above that of unsexed semen. Doyle is currently working on this study and expected to have the results at the beginning of this month.

A global project

     Jacobson says that even though there have been hundreds of successful inseminations by the university and XY, that is not enough to go commercial yet. This year XY is planning 10,000 breedings in England, 10,000 in Switzerland and 5,000 in the United States.
     XY is also working with other species and delivered the first sexed foal in the world in August 1998. Horses are much more biologically complicated requiring 500 million sperm by traditional AI methods. Foals expected to be born in late summer 1999 will be the first group conceived using sexed semen and artificial insemination with no surgical intervention.
     The USDA has demonstrated the effectiveness of the sexing technology for swine, and Johnson has produced numerous litters of pigs using sexed semen with surgical insemination and invites fertilization of pig eggs with a 90-percent accuracy of the desired sex. Use of sexed semen with pigs is currently not feasible because 3 billion sperm are required for a single AI.
     XY is also now looking at pigs and endangered species. Jacobson says XY is in discussion at the moment with associations, zoos and conservation organizations regarding several species.
     "This is global technology, and we think that the whole world should be invited to play a part" says Jacobson. XY has a six-man board with members from Switzerland, England, Australia and the United States. Stockholders and financial support are also coming in from all over the world.
     "This is not just a local project in Colorado," Jacobson continues. "What we're doing is good for the breeder, the rancher, the animals and the planet, but we're not rushing to market. We want to be very professional, very responsible and very careful about this. We believe it works and the producers are asking for it now, but we're saying 'not just yet' because we feel we have a certain responsibility to make sure this product is certainly reliable and safe when it is ultimately released to the market".