The continuation of high temperatures in southern areas and the expansion of hot weather to much of the Corn Belt this week raise additional concerns about corn yield.

The high temperatures in the Corn Belt are occurring during the reproductive stage for a large portion of the crop, said University of Illinois Agricultural Economist Darrel Good.

A number of factors combine each year to determine the U.S. average corn yield. Among those factors, temperature and precipitation during July are the most important.

"Crop yield models have long confirmed the large impact of July weather. The most favorable weather conditions in July in the heart of the Corn Belt consist of temperatures that are modestly below average and precipitation that is about 25 percent above average," he said.

These are the kind of conditions that were experienced in 2009 and contributed to the record high U.S. average yield that year. Historically, such conditions over large areas have been rare.

"Weather conditions in July, and earlier, in 2011 have been far from ideal in many areas," he said. "Planting was late in portions of the eastern and northern Corn Belt. Southern portions of the United States have experienced hot and generally dry conditions for an extended period. The central and northern growing areas have experienced widely varying weather conditions during planting and the early part of the growing season."

These widely varying conditions have been reflected in the USDA's weekly Crop Progress reports, which report crop condition ratings. As of July 10, the lowest crop ratings were reported in Texas, North Carolina, Kansas and Ohio. The highest crop ratings were in Iowa, Kentucky, Nebraska and Tennessee.

"There is some indication the intense heat will begin to moderate in many areas by the upcoming weekend. Still, average July temperatures in the Corn Belt may rank among the highest since 1960," he said.