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课时作业1 Friendship Ⅰ.阅读理解 (2017·江苏省南通中学测试) Why Are Pig Farmers Still Using GrowthPromoting Drugs? It's one of the most controversial practices in agriculture: feeding small amounts of antibiotics to animals in order to make them grow faster. But what if the drugs don't even work very well? There's some good evidence that they don't, at least in pigs. They used to deliver a boost in growth, but that effect has disappeared in recent years or declined greatly. The reason for this is interesting and even paradoxical. Researchers think the antibiotics used to work by suppressing lowgrade infections. In recent years, however, pork producers found other ways to accomplish the same thing through improved hygiene(卫生). As a result, the drugs have become largely superfluous—yet many farmers still use them. To understand how this happened, you have to step back in time, says Steve Dritz, a specialist in pig nutrition at Kansas State University. Sixty years ago, when antibiotics were new, “people started treating animals, and feeding [the antibiotics], and finding that they had increased growth rates and feed efficiencies，” he says. Nurseryage pigs, for instance, grew 12 to 15 percent faster with antibiotics. The animals also needed less feed to reach full weight. Other studies showed similar results in chickens and cattle. In the 1980s, a new set of studies found similar effects. So the growthpromoting effects of antibiotics became standard practice among meat producers. Fast forward to the 1990s. Dritz was starting his career as a scientist at Kansas State University, and pork production was changing dramatically. | 
