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.
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.
Original podcast: Most U.S. Dairy Cows Are Descended From Just 2 Bulls. That’s Not Good