By Carey Gillam

(Reuters) - The U.S. Agriculture Department, probing how a genetically engineered trait entered an Oregon wheat field and the extent of the contamination, has tested a dozen wheat samples and is looking at a national seed storage facility in an effort to resolve the mystery and calm a troubled market for U.S. wheat exports, a spokesman said on Thursday.

The government has tested eight samples of seed and four grain samples and none of the more than 100 tests conducted have turned up positive for the experimental genetically engineered trait that was found contaminating the Oregon wheat field this spring, said Ed Curlett, a spokesman for the USDA.

Curlett said the government tested nine "pools" from each of the 12 samples for detection of as small a contamination level as 0.003 percent, or roughly one in about 30,000 kernels.

"Our ongoing investigation seeks to determine how the incident with GE wheat in Oregon occurred, and all leads are being pursued," he said.

Industry players remain on edge as the wheat harvest in the Pacific Northwest nears, and farmers fear they will have problems selling their wheat as the mystery of how the contamination happened and how widespread it is remains unsolved.

"People are so concerned," said Blake Rowe, chief executive of the Oregon Wheat Commission. "Until the customers are back, that won't change."

Rowe and another Oregon wheat representative met Tuesday with USDA officials in Washington and are writing a letter laying out for USDA what the industry and customers need to restore market stability, including a rapid test to assure supplies are free of the GMO trait. More detail from USDA on exactly what its investigation is revealing is also needed, he said.


USDA has yet to specify details on the type of wheat under suspicion after the rogue biotech wheat plants were found in April in a farm field in Oregon. The farmer who discovered the plants had been trying to kill them off with Roundup weed-killer, and when they would not die, he had them tested at Oregon State University. Those tests, and subsequent government tests, confirmed the presence of the genetically modified herbicide-resistant trait for wheat developed by Monsanto Co more than a decade ago.

Monsanto had hoped to commercialize its "Roundup Ready" wheat, but broad market opposition to biotech wheat led the company to stop field tests in 2005.

The USDA said Thursday it is investigating whether any of the experimental wheat was shipped to a government seed storage facility in Colorado. Monsanto officials said earlier this month that when the company closed its field trials, it told participants they could ship some seed materials to the Colorado facility, called the National Center for Genetic Resources Preservation.

Claire Cajacob, head of Monsanto's wheat project, said it is routine for Monsanto to store experimental seed both at its own facilities and at the government location.

The national center is designed to preserve a collection of genetic resources for U.S. agriculture and includes a small catalog of genetically altered and patented seed material from companies like Monsanto, as well as a wider selection of publicly available materials.

Officials with the center said Thursday they were uncertain whether or not they had ever received the experimental wheat in question, and the USDA's Curlett said how much, if any, seed was sent there, and if it was all accounted for, was one part of the government's investigation.

As the probe continues, some foreign buyers continue to shy away from the western white wheat variety grown in Oregon and other states. This week marked the fourth in a row that Japan's Ministry of Agriculture avoided the U.S. western white grade in a regular weekly wheat purchase.

Japan was one of first nations to shun U.S. white wheat imports after the May 29 USDA announcement of the discovery of the genetically engineered wheat.

USDA maintains that there is no evidence that any biotech wheat has entered commercial supplies.

(Editing by Dan Grebler and Eric Walsh)