For the second year in a row, Natural Resources Defense Council and several other organizations rated the 25 largest American fast-food and fast-casual restaurants on their policies toward antibiotics use. While the majority got failing grades in the Chain Reaction" report, many chain restaurants have made progress, especially when sourcing antibiotic-free chicken - though Chipotle, Panera and now Subway also have strong policies on beef and pork. "We were really encouraged to see that twice as many restaurant chains had adopted new policies," said Kari Hamerschlag ... one of the report's authors. An estimated 70 percent of medically important antibiotics are used in livestock production, often given routinely to healthy animals to prevent illness or stimulate growth rather than to cure disease. Between 2009 and 2014, livestock and poultry farms increased their use of these antimicrobial drugs by 23 percent, according to the Food and Drug Administration. The FDA discourages producers from using antibiotics routinely in feed to promote growth, but does not prohibit the practice. McDonald's grade improved from a C last year to a C-plus because it announced it had fully switched over to antibiotic-free chicken in its U.S. stores. Subway, based in Milford, Conn., which leapt from an "F" grade last year to a "B" grade this year, was listed as the only chain to adopt policies that apply to all types of meat sold. The report said now roughly 67 percent of its chicken is now raised without antibiotics, with turkey to follow.