Because of the detail that the U.S. Department of Agriculture (USDA) provides in its September crop report, the potential for the crops in any given year gains some clarity. Still, because their process includes both a producer survey and data collected “in the field,” there’s still a lot of uncertainty about the actual production.  

The recent report was a “mild” surprise because the USDA raised its yield forecasts slightly for corn and soybeans. The trade was expecting small reductions because of...