For months, Illinois farmer David Kellerman held out hope for rain, even as the worst drought in nearly 25 years spread across the country.

He finally gave up when the temperature hit 108 three days in a row. Corn won't develop kernels if it gets too warm during pollination, and Kellerman knew the empty cobs in the fields where he works would never fill out. Just after the Fourth of July, he and the neighbor he farms with took an extraordinary step: They cut down the entire crop and baled the withered plants to use as hay for their cattle.

Almost a third of the nation's corn crop has been damaged by heat and drought, and a number of farmers in the hardest hit areas of the Midwest have cut down their crops just midway through the growing season. But the nation could still see one of the largest harvests in U.S. history, thanks to new plant varieties developed to produce more corn per acre and better resist drought.