Young broilers nibble feed at a chicken farm in Luling, Texas. The Food and Drug Administration has issued new guidance on how drug companies label antibiotics for livestock. Bob Nichols/USDA/Flickr hide caption

itoggle caption Bob Nichols/USDA/Flickr

Russ Kremer with some of his hogs on his farm in Frankenstein, Mo., in 2009. Instead of buying conventional feed, Kremer grazes his hogs in a pasture, and grows grains and legumes for them. Jeff Roberson /AP hide caption

itoggle caption Jeff Roberson /AP

A truckload of live turkeys arrives at a Cargill plant in Springdale, Ark., in 2011. Most turkeys in the U.S. are regularly given low doses of antibiotics. Danny Johnston/AP hide caption

itoggle caption Danny Johnston/AP

Cows wait to be milked at a California dairy farm. Justin Sullivan/Getty Images hide caption

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The FDA's latest effort to end the use of antibiotics as growth promoters in animals is getting mixed reviews from activists. Rob Carr/AP hide caption

itoggle caption Rob Carr/AP

Pigs take a mud bath at the De Jofrahoeve pig farm in Esch, Netherlands. Dutch farmers treat their animals with almost three times the antibiotics that their Danish neighbors use. Robin Utrecht/AFP/Getty Images hide caption

itoggle caption Robin Utrecht/AFP/Getty Images

Breeding sows in crates at a subsidiary of Smithfield Foods in 2010. The photo was shot by the Humane Society as part of an undercover investigation. Humane Society/Associated Press hide caption

itoggle caption Humane Society/Associated Press

Many livestock groups say there's no evidence that antibiotics in livestock feed have caused a human health problem, but researchers beg to differ. Scott Olson/Getty Images hide caption

itoggle caption Scott Olson/Getty Images