U.S. Dairy Cows Are Very Genetically Similar. That's Not Good : The Salt The drive to make more milk has had an unsavory side effect: Cows have become more genetically similar and less fertile. Scientists are trying to recover valuable genetic variation that was lost.

Most U.S. Dairy Cows Are Descended From Just 2 Bulls. That's Not Good

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There are millions of dairy cows in the U.S. producing an amazing amount of milk, but the drive to maximize production has come with an undesirable side effect. The cows have become more and more genetically similar. In fact, the vast majority of them are descended from just two bulls. NPR's Dan Charles has the story.

DAN CHARLES, BYLINE: Chad Dechow is sitting in his office at Penn State University, explaining how dairy cows ended up so much alike.

CHAD DECHOW: This is the company Select Sires.

CHARLES: We're looking at the website of a company that sells semen from bulls.

DECHOW: So if we just look at their Holstein lineup...

CHARLES: Holstein cows are the black and white ones that make a lot of milk. They dominate the dairy business.

It says loading America's finest bulls there on the screen.

DECHOW: That's right.

CHARLES: There are just a few companies like this with hundreds of stud bulls. Dairy farmers go online, pick a bull and the companies ship doses of semen to impregnate their cows.

DECHOW: In fact, there's one bull who - we figure he has well over a quarter million daughters.

CHARLES: The companies rank their bulls based on how their daughters perform, how much milk they produce.

DECHOW: This is a bull named Frazzled. His daughters are predicted to produce 2,158 pounds more milk than daughters of the average bull.

CHARLES: Farmers pay extra for semen from top-ranked bulls, and the companies keep breeding even better ones, mating their top bulls with the most productive cows.

DECHOW: They keep selecting the same families over and over again.

CHARLES: Well, a few years ago, Chad Dechow and some other scientists at Penn State made a discovery that shocked a lot of people. All the Holstein bulls that farmers are using can trace their lineage back to just one of two male ancestors.

DECHOW: Everything goes back to two bulls born in the 1950s and '60s.

CHARLES: And their names?

DECHOW: Their names were Round Oak Rag Apple Elevation and Pawnee Farm Arlinda Chief.

CHARLES: Now, this doesn't mean that all the bulls in the catalog are genetically identical. They had lots of different mothers and grandmothers. But Chad Dechow says it does show this system of large-scale artificial insemination with farmers picking top-ranked bulls has made cows genetically less diverse. Traits that you'd have found in Holstein cows a generation ago have disappeared.

DECHOW: We've lost genetic variation. Some of that genetic variation was garbage that we didn't want to begin with, but some of it was valuable stuff that will be gone.

CHARLES: 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 Zimmermann All Star Pilot - heirloom bulls. The U.S. Department of Agriculture keeps samples of the semen in deep freeze storage in Fort Collins, Colo. Dechow used it to impregnate some modern cows. They gave birth, and now you can see some lost pieces of the Holstein family tree come to life in a barn at Penn State - three cows.

DECHOW: Right here is our old genetic lineage. This is No. 2869.

CHARLES: She doesn't look any different, does she?

DECHOW: Well, you'll notice if you look kind of over her back. You see that cow on her left is a little bit more bony over her back.

CHARLES: Yeah, absolutely.

DECHOW: So she definitely carries more body condition, a little bit fatter.

CHARLES: Keeping more fat is a genetic trait that dairy farmers didn't want in their cows. They thought the ideal cow was a skinny one because she was turning all the feed she ate into milk, not fat, so farmers went for bulls that had that kind of daughter.

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

CHARLES: Dechow is thinking maybe the frozen semen from those long-forgotten heirloom bulls can bring back valuable genes that went missing, like genes for extra body fat, better health. For this to work, though, farmers actually have to use those bulls, and they'll only do it if they're persuaded that their daughters also will make lots of milk. So Chad Dechow's carefully monitoring his experimental cows. So far, he says it's going pretty well. Two of the three are producing as much milk as the industry average.

Dan Charles, NPR News.

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