Chad Dechow, a geneticist at Pennsylvania State University who studies dairy cows, is explaining how all of America's cows ended up so similar to each other.

He brings up a website on his computer. "This is the company Select Sires," he says. It's one of just a few companies in the United States that sells semen from bulls for the purpose of artificially inseminating dairy cows.

Dechow chooses the lineup of Holstein bulls. This is the breed that dominates the dairy business. They're the black-and-white animals that give a lot of milk.

Dairy farmers can go to this online catalog and pick a bull, and the company will ship doses of semen to impregnate their cows. "There's one bull — we figure he has well over a quarter-million daughters," Dechow says.

The companies rank their bulls based on how much milk their daughters have produced. Dechow picks one from the top of the list, a bull named Frazzled. "His daughters are predicted to produce 2,150 pounds more milk than daughters of the average bull," he says, reading from the website.

Farmers like to buy semen from top-ranked bulls, and the companies keep breeding even better bulls, mating their top performers with the most productive cows. "They keep selecting the same families over and over again," Dechow says.

A few years ago, Dechow and some of his colleagues at Penn State made a discovery that shocked a lot of people. All the Holstein bulls that farmers were using could trace their lineage back to one of just two male ancestors. "Everything goes back to two bulls born in the 1950s and 1960s," he says. "Their names were Round Oak Rag Apple Elevation and Pawnee Farm Arlinda Chief."

This doesn't mean that the bulls in the catalog are genetically identical. They still had lots of different mothers, as well as grandmothers. But it does show that this system of large-scale artificial insemination, with farmers repeatedly picking top-rated bulls, has made cows more genetically similar. Meanwhile, genetic traits that existed in Holstein cows a generation ago have disappeared.

"We've lost genetic variation," Dechow says. "Now, some of that variation was garbage that we didn't want to begin with. But some of it was valuable stuff."

To see what might have been lost, Dechow decided to do an experiment. He located some old semen from other bulls that were alive decades ago, with names like University of Minnesota Cuthbert and Zimmerman All-Star Pilot. You might call them heirloom bulls. The U.S. Agriculture Department keeps samples of their semen in deep-freeze storage in Fort Collins, Colo.

Dechow used that semen to impregnate some modern cows. They gave birth, and now it's possible to see some lost pieces of the Holstein family tree come to life in a barn at Penn State — in the form of three cows.

Dechow leads the way to the barn. He points toward a cow that eyes us suspiciously. "Here is our old genetic lineage, [cow] number 2869," he says.

To the untrained eye, this cow looks pretty much like all the others. But Dechow sees things that others can't. "If you notice, if you look over her back — see how that cow to her left is a little more bony?" he says.

Once Dechow points it out, the difference is plain to see. "So she definitely carries more body condition. She's a little bit fatter," he says.

Traditionally, dairy farmers didn't like cows with extra body fat. They thought the ideal cow was a skinny one, because she was turning all her feed into milk, not fat. So farmers chose bulls that tended to produce that kind of daughter.

"We've kind of selected for tall, thin, cows," Dechow says. "And that's a really bad combination. They're infertile, unhealthy. So we need to get away from that."

Dechow thinks the frozen semen from those long-forgotten heirloom bulls can bring back valuable genes that went missing — maybe genes that would allow cows to thrive in warmer temperatures, for instance.

For this to work, though, farmers actually have to use those bulls, and they'll only do so if they're persuaded that the daughters will also produce lots of milk.

So Dechow is carefully monitoring his experimental cows. So far, he says, it's going pretty well. Two of the three cows are producing at least as much milk as the industry average.

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