Environmental and public health groups have filed a lawsuit seeking to force the government to stop farmers from adding antibiotics to livestock feed to boost animal growth. The Union of Concerned Scientists, the Natural Resources Defense Council and others say the U.S. Food and Drug Administration’s allowance of antibiotics in animals ends up weakening their effectiveness in humans. The suit alleges that “superbugs” are becoming more difficult to treat because the human body is less responsive to the antibiotics they have already consumed through animal products. An estimated 80 percent of antibiotics used in the United States are consumed by farm animals.