“This is a moving target,” said Darrel L. Good, a professor emeritus of agricultural and consumer economics at the University of Illinois at Urbana-Champaign. “But what we know is this: There’s been some permanent and substantive yield reduction already, and we’re on the cusp, depending on the weather, of taking that down quite a bit more.”In its most recent assessment, released on Monday, the Department of Agriculture reported that 48 percent of corn crops nationally were in good or excellent condition, a drop from 56 percent of crops a week earlier. In some states, though, the circumstances were far worse. In Indiana, half of corn crops were designated poor or very poor, and in Illinois, another state among the nation’s top corn producers, only 26 percent of crops were considered good or excellent.

John Hawkins, a spokesman for the Illinois Farm Bureau, said those in the southernmost sections of his state “are close to or past that point of no return,” while elsewhere, “there’s a lot of praying; it’s hanging on by a thread.”

“These 100-degree temperatures are just sucking the life out of everything,” he said.

American farmers had high expectations for corn this year, planting 96.4 million acres of it — a number 5 percent more than the previous year. High prices and an expectation of strong returns made this year’s planting the largest corn acreage in 75 years. Those were heady times in farm country, with farmland prices rising on and on, even as the recovery moved sluggishly in other realms. An uncharacteristically warm March in the Midwest sent hopes still higher, allowing farmers to plant corn weeks earlier than usual. For some crops, including some cherries in Michigan and apples in Indiana, unexpected April frosts then caused damage, but the corn, said Randy Anderson, a farmer in Southern Illinois, went right along beautifully.

And then very little rain fell, and temperatures soared. By last week around corn country, scores of triple-digit heat records were being broken: Jefferson County, Mo., 111 degrees; Evansville, Ind., 107 degrees. That left corn, including Mr. Anderson’s crop, shriveling.

“We’re talking five-feet-tall corn with no ears, no shoots and no tassels,” he said. “It wears on your nerves to even look.”

For much of the region, the next few weeks — as the plants’ tassels shed pollen to fertilize the silks and create kernels — are crucial. The endless fields of soybeans are at risk in the Midwestern heat, too, though they are seen as more resilient and able to pollinate later.

But a stressed, withered corn plant may not pollinate at all. “This is a very narrow window for corn, and there’s little room for error,” said Brad Rippey, an agricultural meteorologist for the United States Department of Agriculture. “Whatever happens in that window, it is what it is — that cob is made or broken.”