With big harvests in the bins and prices down significantly from last year, U.S. corn and soybean exports are surging back in markets all over the world. The latest government estimates project that U.S. corn exports for the 2013-14 marketing year will almost double to 1.45 billion bushels and soybeans will rise more than 10 percent to 1.475 billion bushels.

But U.S. crop exports are facing tougher competition from both traditional and newer players in global grain markets, according to Chad Hart, Iowa State University Extension crop market specialist.

At the same time, said Hart and others, the Chinese market continues to be very unpredictable for U.S. exporters, adding uncertainty to the markets for both corn and ...