Ninety percent of the nation’s dairy cows—some 8.2 million animals—belong to a single breed: Holstein. Owing to the dairy industry’s extensive reliance on artificial insemination using semen from only the choicest bulls, this Holstein population is heavily inbred. “Today, it’s as if there were only about 35 unrelated cows [contributing genes to] our national Holstein herd,” explains geneticist Harvey D.
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