The U.S. Department of Agriculture shocked traders by increasing its corn crop estimate from last month. The USDA now pegs the U.S. corn yield at 169.5 bushels per acre and crop size at 13.9 billion bushels.

While the yield was 3.5 bushels higher than the July estimate, crop size only increased 26 million bushels due to a reduction in acres.

Official corn acreage is now at 90 million for planted acres this year, down 1.7 million from the previous...