The United States has expanded its search team in Oregon as it hunts for the source of unapproved genetically modified wheat found growing wild on a farm there in April.
An Agriculture Department spokesman told Reuters on Monday that a team of 15 is now collecting evidence and information, versus nine investigators on the ground last week. He said there are "no indications that there is any GE (genetically engineered) wheat in commerce."
Preliminary tests on U.S. wheat and wheat flour imports by South Korea did not detect any GE wheat; final results were expected on Wednesday. The country's Ministry of Food and Drugs said it planned to test all U.S. shipments.
Asian and European buyers have backed away from U.S. wheat following Wednesday's announcement of the discovery of the unapproved wheat. The USDA said the wheat was an experimental strain developed by Monsanto Co more than a decade ago.
U.S. wheat exports, forecast this year at about $9 billion, could be at risk if unapproved wheat is found to be more widespread than the few plants in Oregon.
A European Union health official said it could be two weeks before a validated method of testing for GE wheat is available. Monsanto gave a testing method to the EU Joint Research Center over the weekend. The EU has said it would test incoming shipments and reject any with GE wheat.
The unapproved wheat was found sprouting in April on an 80-acre field in northeast Oregon, a lightly populated region with a semi-arid climate of cool winters and hot summers. Annual precipitation is around 13 inches.
"We continue to be focused in Oregon," said USDA spokesman Ed Curlett, with 15 employees from a biotechnology investigative and enforcement unit in the state.
Oregon produces predominantly winter wheat and smaller amounts of spring wheat. The GE strain was discovered as unwanted "volunteer" seedlings that unexpectedly survived spraying by the weed killer glyphosate in a winter wheat field that was held fallow this year.
Monsanto developed GE spring wheat to tolerate the widely used weed killer, often sold under the brand name Roundup Ready, but stopped work in the face of worldwide opposition to GE wheat. The last U.S. field trial was in 2005.
Curlett said the USDA was testing to determine if the volunteer wheat was a winter or spring variety. The USDA used an array of tests, including genome testing, to identify the wheat as containing Monsanto's genetic alteration, known as MON71800.
USDA's announcement on Wednesday, after sophisticated tests to identify the wheat strain, prompted Japan to shun wheat from the Pacific Northwest at its weekly tender on Thursday, while some Asian countries ramped up inspections and the European Union said it would step up testing.
"Although our preliminary test result shows no genetically modified wheat was found, we are aiming to test all samples of wheat and flour imported from the U.S.," said an official at Korea's Ministry of Food and Drugs.
South Korea - which last year sourced roughly half of its total wheat imports of 5 million tonnes from the United States - has also raised quarantine measures on U.S. feed wheat.
The United States has for several years embraced genetically modified crops such as soybeans and cotton. However, GM (genetically modified) wheat has never been approved in the United States or anywhere else.