Gary Rogers is driving down a highway in Tennessee pointing out the dairy farms and cows that dot the landscape. He has a bit of data that a Northeast city dweller finds eye-opening: "All of the black and white cows around the world are related, and I don't mean 500 years ago," he says. In fact, today, most of the nine million dairy cows in the United States share just a handful of paternal grandfathers.
That has resulted in intense...
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