Indian Quenn Basmati rice by Bharat Industrial Enterprises LimitedThe Obama administration announced a new challenge over Beijing’s barriers to American rice, wheat and corn, attacking a policy that U.S. officials say prevented as much of $3.5 billion in agriculture shipments to China and reported the case to the World Trade Organization. China, which had joined WTO 15 years ago, is already facing a previous U.S. challenge this year over its alleged subsidies for its domestic cereal production. U.S. trade lawyers accused China of improperly administering a program designed to put limits on grain imports to the country, violating international agreements.

United States accused Beijing of operating its complicated import barriers known as tariff-rate quotas, or TRQs, in ways that disadvantage U.S. growers through lack of transparency and unreasonable implementation. “China’s TRQ policies breach their WTO commitments and limit opportunities for U.S. farmers to export competitively priced, high-quality grains to customers in China,” U.S. trade representative Michael Froman said in a statement.

U.S. exports of wheat, rice and corn to China last year fell to $381 million, compared with $2.3 billion in 2013, according to the Census Bureau. By comparison, soybean exports to China, used to fatten Chinese hogs, were worth $10.5 billion last year. In its efforts to discipline China, the Obama administration has challenged China 15 times at the WTO, representing a majority of its two-dozen WTO legal actions against trading partners over the last eight years. President-elect Donald Trump, who made criticism of China’s trade practices a centerpiece of his campaign, could further step up pressure on Beijing next year.