The U.S. has launched a challenge to China’s use of tariff-rate quotas for rice, wheat and corn at the WTO, charging that China’s administration of the program breached its WTO commitments and hurt U.S. farm exports. The tariff-rate quotas for rice, wheat and corn were worth $7B-plus in 2015, and China would have imported up to an additional $3.5B of the crops last year if the quotas had been fully used, the Office of the U.S. Trade Representative said. It was the second challenge to China’s agricultural policies by the U.S. Trade Representative since September and the latest in an escalating number of trade disputes between the two countries.