The U.S. Department of Ag­­riculture (USDA) last week slashed almost 5 bushels per acre from its earlier trend-based projection of U.S. average corn yields for 2021 as extreme drought conditions hammer crops in the western Corn Belt, offsetting record yields east of the Mississippi River.

While projections for 2021 top last year, the uneven impact of weather on this year's crop is substantial.

“It’s amazing. You’re talking about in a year where we’ve got Illinois, Indiana and Ohio all at record yields, and yet USDA had to pull their numbers down from trend to below trend,” said Chad Hart, Iowa State University crop markets specialist. “That tells you how many bushels are being lost out in the west. It also points to...