Low temperatures have been well above normal across most of the Midwest this summer, which is causing concern for the U.S. corn crop.

“Since June 1, highs have been only 1-3 degrees F above normal, but lows have been 3-7 degrees F above normal, especially across the southern Midwest.” said Kyle Tapley, Agricultural Meteorologist for MDA EarthSat/CropCAST.

The warm low temperatures are a concern since the corn crop develops best when nighttime temperatures drop into the low 60s.

“We took a look at Iowa and Illinois corn yields over the past 40 years and found that an average low of 64 degrees F from June 1 to Aug. 31 appears to be the magic number for corn yields,” said Tapley. “Corn yields were generally above trend when lows were lower than 64 degrees F on average, but were generally lower than the trend yield when lows were greater than 64 degrees F.”

Since June 1, Des Moines, Iowa, has seen an average low of 67.5 (second highest since 1970), while Springfield, Ill., has seen an average low of 68.7 (highest since 1970), both well above the critical 64 threshold.

“Above normal temperatures are expected to continue across the Midwest, so corn yields will likely be negatively impacted due to a shortened kernel filling period,” added Tapley.