Genetically modified wheat created by Monsanto Co. (MON) that wasn’t approved for use turned up on an 80-acre farm in Oregon last month, threatening the outlook for U.S. exports of the grain that are the world’s largest.     A farmer attempting to kill wheat with Monsanto’s Roundup herbicide found several plants survived the weedkiller. Scientists found the wheat was a strain field-tested from 1998 to 2005 and deemed safe before St. Louis-based Monsanto, the world’s largest seedmaker, pulled Roundup Ready wheat from the regulatory approval process on concern that importers would avoid the crop.