About a third of the ground beef feared to have been contaminated with a carcinogenic pesticide that poisoned dairy herds in Arkansas, Missouri and Oklahoma has been cleared and will be released to schools for lunch programs, U.S. Department of Agriculture inspectors reported. They said tests on 105 lots of ground beef produced by five packers, including Service Packing Co. of Los Angeles, revealed no traces of heptachlor, which had been used to treat grain that was fed to cattle killed at 18 slaughterhouses. USDA inspectors said testing will continue. Holds were placed on the beef on March 25, but Los Angeles Unified School District was unaffected because it did not get supplies from any of the five packers. In Orange County, at least 10 of 27 school districts producing school lunches pulled the federally supplied beef off menus after the USDA warning.