Hot conditions continuing in the South and expanding to much of the nation’s Corn Belt this week have raised concerns about the potential impact on corn crop yields as a large part of the crop enters the reproductive stage of development, says University of Illinois agricultural economist Darrel Good.

Key factors combining to determine U.S. corn yields include temperature and precipitation during July, he says.

“Crop yield models have long confirmed the large yield 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% above average,” he says.

Those kinds of conditions existed in 2009 and they contributed to the record U.S. average yield that year – but historically such conditions over large areas have been rare, Good notes.

“Weather conditions in July and earlier in 2011 have been far from ideal in many areas,” he says. “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 USDA’s Crop Progress reports. As of July 10, crop conditions rated the lowest in Texas, North Carolina, Kansas and Ohio. The highest crop ratings were in Iowa, Kentucky, Nebraska and Tennessee.

“There is some indication that 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,” Good says.

The impact of the 2011 U.S. corn yield is supported by USDA’s projection of record consumption of corn during the 2011-12 marketing year. Projections on July 12 forecast consumption at 13.5 billion bushels, 195 million bushels above expected consumption during the current marketing year, he says.

Stocks at the end of the 2011-12 marketing year are projected at 870 million bushels or 6.4% of projected use. If the forecasted 84.9 million acres are harvested, a yield below 156.5 bu. would force a reduction in the projected level of consumption.

Corn prices continue to trade about 50 cents below the high reached on June 9 and will continue to reflect weather conditions, weather forecasts and crop condition ratings, Good says.

USDA’s August crop production forecast will establish a benchmark for production expectations, and this year’s projections may have added impact due to possible adjustments in the harvested acreage forecast, he says.