Both grains are exported around the world, raising concerns about global food shortages and inflation. The impact on American grocery and meat case prices may take time to be felt but will likely be seen in inflation in coming months.
Last week, U.S. Agriculture Secretary Tom Vilsack designated more than 1,000 counties across the country as natural disaster areas due to the drought conditions, the largest single designation in the history of the USDA loan aid program.
In Nebraska, where most farmers irrigate their corn, flows in streams and rivers had dropped so much that the state on Monday asked 1,100 of Nebraska's 48,000 farmers and ranchers to stop pulling water from the waterways and use wells instead.
Iowa and Illinois produce a third of U.S. corn and soybeans. But prospects there have turned down sharply, raising fears losses will be the worst since 1988, the last major drought.
Prospects for the later-developing Midwest soybean crop were better than that for corn, though substantial rains were needed during the next three weeks to salvage Indiana's crop, Vyn said.
"The window for soybeans is closing," he said.
Soybeans usually go through their key growth period of flowering and pod-setting in August, a few weeks after corn in the Midwest. Soy is used in scores of products, from paints and feeds to edible oils and increasingly for soy-based diesel fuel.
"We need soaking rains now. We need 2 to 3 inches and that's not in the forecast," AgResource Co analyst Dan Basse said.
AccuWeather meteorologist Erik Pindrock said a seemingly immovable ridge of high pressure on Monday kept much of the central Corn Belt in a dome of heat, and he predicted the hot, dry weather would persist through July and possibly into August.
Monday's heat matched high temperature records for the date in many locations including Flint, Michigan, where it was 97 Fahrenheit (37 Celsius), and in Indianapolis, where it was 98 F, he said.
"We've seen Raleigh, North Carolina, tie its all-time record of 105 (F) degrees three times ... so this is definitely a country-wide heat wave," Pindrock said.