U.S. corn yields could fall by more than 20% from trend this year based on a historical model comparing this year to previous years with abnormally late planting, according to Iowa State University (ISU) crop markets specialist Chad Hart.

That would put the national yield “in the 135 bushels per acre range, with a yield loss similar to the drought of 2012,” Hart wrote in the ISU Iowa Farm Outlook June newsletter. Weather patterns in June and July will influence whether yield losses are actually that large, but...