By Janet Raloff
Each year, U.S. farmers raise some 36 million beef cattle. Farmers fatten up two-thirds of these animals by using hormones.
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Many cattle are fed the same muscle-building androgens–usually testosterone surrogates–that some athletes consume. Other animals receive estrogens, the primary female sex hormones, or progestins, semiandrogenic agents that shut down a female’s estrus cycle. Progestins fuel meat-building by freeing up resources that would have gone into the reproductive cycle.