The U.S. Department of Agriculture next month will tighten regulations for field trials of genetically modified wheat, the government said in a notice on Friday.
The USDA's Animal and Plant Health Inspection Service (APHIS) said it will require developers to apply for a permit for field trials involving biotech wheat planted on or after Jan. 1. Since 1997, such trials have been authorized by notification, a less stringent process.
The rule change follows the discoveries in 2013 and 2014 of unauthorized genetically modified wheat growing in Oregon and Montana.
Field trials of biotech wheat were never authorized at the Oregon site, and the last authorization for a field trial at the Montana site expired 10 years earlier.
"Using permits for field trials of GE (genetically engineered) wheat provides an additional level of safeguarding based on, and consistent with the biology of wheat," USDA said in a report accompanying its notice.
"It became clear to us following the GE wheat incident in Oregon that the detection of regulated GE wheat where it was not authorized, had great potential to disrupt wheat markets globally," USDA said.
With permits, USDA can require a longer period to monitor so-called "volunteer" plants, which can emerge following the harvest of a crop.
There is no commercially approved biotech wheat, but Monsanto's herbicide-tolerant "Roundup Ready" wheat was near commercialization a decade ago before the company shelved the project amid fears that export sales would suffer.
The USDA said it authorized 572 requests for field trials of various crops in 2014, including 21 for wheat. Most of the rest were for corn, soy and cotton.
"Research in wheat has ramped up quite a bit in the past few years, including trials with biotech traits," said Steve Mercer, a spokesman for U.S. Wheat Associates, a trade group that markets American wheat to international buyers.