Soybean futures declined on mounting speculation that a bumper crop in the U.S., the world’s largest grower, will overwhelm demand. Wheat dropped to the lowest level since 2010, and corn rose. Soybean prices tumbled 24% this year. The Department of Agriculture has forecast output of the oilseed crop at an all-time high of 3.913 billion bushels, up 19% from last year.