The first survey-based corn and soybean yield estimate of this year’s crop, released last week, showed the effect of dry weather and other crop problems. But the reductions that showed up in the U.S. Department of Agriculture (USDA) report did not match traders’ expectations.

U.S. corn production for 2017 is forecast at 14.2 billion bushels, down 102 million bushels from the July projection, the USDA said in its report. The season’s first survey-based corn yield...