Making sure that cows bear cows Experimental procedure sorts sperm to ensure sex of animal offspring

April 11, 1993|By Arthur Hirsch | Arthur Hirsch,Staff Writer

The Utopian dairy farm would be an exclusive female club: cows giving birth to cows, bulls represented solely by their sperm.

It would be more efficient and would make possible more rapid improvements in genetics, the sort of changes that have already tripled the average American cow's milk production in the last 50 years.

,.5l The trouble is, nature likes a balance in births, roughly half male and half female. And no one's been able to figure out how to change that.

Until now.

At the National Agricultural Research Center in Prince George's County, scientists have devised a high-tech answer to the ancient question of how to select the sex of offspring. It's not ready for the farm, nor advanced enough to be applied to humans, but the technique is the closest thing yet to a method that could give people the power to alter the natural ratio of the sexes in animals.

"The impact could be tremendous," said Ralph Robertson Jr., a third-generation dairy farmer in Westminster, whose farm milks about 100 cows in Carroll County. "It would give you the ability to fine-tune your genetic selection."

Since the ancient Greeks, folks have speculated about how to select the sex of offspring. One theory in the 18th century held that sperm from the right testes produced more males, sperm from the left testes, more females.

In the 1920s, research focused on the distinctions between X spermcells, which produce females, and male-producing Y cells. Sperm determine the sex of offspring, as eggs contain X chromosomes only.

"Since the early 20th century it's been known there was a difference in DNA" between X and Y sperm cells, said Lawrence A. Johnson, the Ph.D. in animal science and erstwhile Wisconsin dairy farm boy who headed the Beltsville project.

"But not until the 1970s was there the technology to look at a cell in enough detail" to fathom the distinctions between X and Y cells, he said.

The difference is the amount of DNA, the stuff of chromosomes, the chemical that carries the genetic code.

In mammals, the X sperm cells have a bit more DNA than the Y. In cattle, the DNA difference is 3.8 percent; in pigs, 3.6 percent. In humans, the difference is 2.9 percent.

The system devised by Mr. Johnson and his team of scientists is designed to spot the difference and divvy up the cells accordingly. The equipment -- developed in collaboration with federal nuclear weapons laboratories in New Mexico and California -- uses a laser beam, light sensor and a computer.

It works like this: Sperm cells are treated with fluorescent dye that binds to the DNA. The cells are then drawn through a super-fine bevel-tipped needle and allowed to flow, single file, past a laser beam. The beam activates the dye, which causes the larger X cells to glow brighter than the Y.

A light sensor picks up this distinction and triggers a computer, which zaps the cells with an electrical charge: negative for Y, positive for X, allowing them to be separated into two glass tubes.

"It's pretty complex, but it works," said Mr. Johnson.

They did it with rabbits and pigs, surgically implanting the sorted sperm cells and producing live young. In rabbits, Beltsville experiments produced 94 percent female offspring with sperm cells sorted for females and 81 percent males with sperm sorted for males. The success rate for pigs was about 75 percent.

In January, six calves -- three males, three females -- were born in Cambridge, England, the product of in-vitro fertilization using sperm sorted by the method devised by Mr. Johnson's team. The calves' sex ratio was just what the sperm sorting ordered.

But the process is slow -- it takes about an hour to sort a half-million cells. This is a problem, as it takes about 20 million sperm to impregnate a cow through artificial insemination, the method used on virtually all U.S. dairy farms.

Mr. Johnson figures it will take five to 10 years before the process is ready to produce presorted doses of male and female sperm for farms.

"The genetic impact would be substantial" in the dairy industry, said Steve Kerr, secretary-treasurer of the Purebred Dairy Cattle Association, a national organization based in Brattleboro, Vt.

Mr. Kerr says the technique would allow the dairy farmer to choose from a much larger pool of females, meaning a higher quality breeding stock and more rapid genetic improvement.

As it stands, a farmer seeking, say, improved milk protein might breed his best dairy cow to a high quality bull. If the cow bears a bullock, the farmer has to sell the male calf and wait another year to try again with the same cow.

Selective breeding in the American dairy herd has bumped the average cow's annual production from about 5,000 pounds of milk in the 1940s to about 17,000 pounds today, Mr. Kerr says.

There's also a potential economic impact, as "there are costs in breeding and raising males. Every dairy farmer would like to eliminate those costs," said Mr. Kerr, who also heads the Holstein Association of America.

The downside for the farmer, Mr. Kerr said, could be the increased supply of cows and milk and reduced milk prices.

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