The drought and heat are also hitting just as the corn crop in many states enters the important tasseling, or pollination phase. It needs wetter, cooler weather, in order for kernels to form properly. Hot, dry conditions are affecting about two-thirds of the corn belt, in locations such as eastern Iowa, Illinois, Indiana, and Ohio. But the northern part of the corn growing region is doing fairly well.

Nelson said the crop is the worst since 1988, but not nearly as bad. Then 23 percent of the crop was good to excellent at this time of year, and the drought then was national. Nelson expects this year’s crop to yield 145.3 bushels per acre, well below the 166 expected by USDA. He said the year’s trend is 161, and that would be a 10 percent decline from trend. In 1988, there was a 28 percent decline from trend.

Ashley Gulke, analyst wth Gulke Group, was driving in Illinois Monday. She said some fields looked very dry, and there was talk of some farmers swapping out corn to plant soybeans.

Nelson said he heard the same and expects it will be a small amount of acreage that gets replanted, but only after rains start to fall.

Gulke said the conditions are very variable across the region. For example, a Missouri farmer told her earlier that his crop was fine, but fields 20 miles in either direction were not in good shape.

Gulke's road trip also took her into Wisconsin, where she took a look at other fields.

“Just saw a field, thought it was onions—nope corn,” Gulke sent in a quick email.

“Beans look consistently pretty good,” she noted in an email.

Follow Patti Domm on Twitter: @pattidomm

Questions? Comments? Email us at marketinsider@cnbc.com