Japan and South Korea are still testing the U.S. wheat they buy to make sure the grain is not contaminated with an experimental genetically modified version developed by Monsanto Co., but can stop the practice, the head of a U.S. wheat association said Thursday.

The two countries, which are among the top purchasers of U.S. wheat, have been sampling and testing all U.S. wheat they have purchased since last year, when news broke that a farmer in Oregon had found Monsanto’s unapproved biotech wheat growing in his field, according to U.S. Wheat Associates, which markets American wheat to international buyers.

All of their test results on more than 5 million tons of wheat have been negative, it said.

The testing requirements are a competitive disadvantage for American wheat, said U.S. Wheat President Alan Tracy. But both countries now should be able to stop the testing after assurances last week from the U.S. Department of Agriculture that the Oregon event was isolated and no biotech wheat had made its way into commercial supplies.

“We’re hopeful that they’ll be able to suspend the testing. It is not something they really want to keep doing indefinitely,” Tracy said. “They approach things cautiously. They felt they had no choice until this report (from USDA) was out.”

U.S. Wheat Associates will meet representatives from Japan, Taiwan and South Korea later this month, he said.

According to U.S. export sales data, Japan bought about 3 million tons of U.S. wheat in the 2013-2014 marketing year, which ended May 31, making it the fourth-largest buyer of U.S. wheat for that period after Brazil, China and Mexico. So far this marketing year, Japan has bought 1.4 million tons.

Japan has the sampling and testing of the wheat it buys handled in the United States before it is shipped, Tracy said.

South Korea bought 1.3 million tons during the last marketing year, making it the seventh-largest buyer. So far this year, it has purchased 749,000 tons.

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 be hurt.

The fact that experimental wheat was found growing uncontrolled in Oregon last year led to fears the biotech wheat might be in commercial supplies. The unapproved wheat was also found this summer in Montana, growing in a research plot where field trials of the GMO wheat were conducted from 2000 to 2003.

The USDA said Friday that it is investigating the Montana situation, but like Oregon there are no indications the unapproved wheat entered commercial supplies.

Tracy said Asian markets remain very cautious about GMO grains in general.

“It’s not up to us to tell them what to do,” he added.