News Release

Study: Weather better for growing in last 40 years than in previous 60

Peer-Reviewed Publication

University of Illinois at Urbana-Champaign, News Bureau

CHAMPAIGN, Ill. -- Illinois farmers have had almost 40 years of "infinitely better weather" for growing than they did in the previous 60 years, and improving technology and agricultural practices have helped to change the weather standard for high yields, according to a review of the 20th century.

A detailed study -- published by the Illinois State Water Survey -- identified how various weather conditions and their effects on Illinois corn yields have shifted over time between 1901-1997. The state was divided into four regions based on soils and climatic conditions and their impact on crops. The 20 highest and 20 lowest yields were classified for each region, each being based on the departure from each year's expected yields, a formula designed to account for changing technologies. There were 19 different kinds of weather identified.

As would be expected by farmers, the lowest yields -- 65 percent of the time -- followed hot and dry Julys. However, 25 percent of bad-yield years came with near normal July rainfall and temperatures but with a hot and dry June, said Stanley Changnon, chief emeritus of the water survey.

"Interestingly, all of these hot, dry June seasons occurred in the years before 1960," he said. "Regional differences were notable, too. Two weather types produced six of the 20 lowest yields in Southern Illinois, but these types never resulted in low yields in northern or east central Illinois."

Changnon and Derek Winstanley, chief of the water survey, conducted the study. Both researchers hold faculty appointments in the U. of I. geography department.

The highest yields of each region came in years that had cool and wet Julys. Such conditions occurred in 50 percent of the high-yield years. Seasons with near normal July temperatures and rainfall were linked with 20 percent of the high yields.

A weather type with below normal July rainfall but above normal rains in May and June produced 20 percent of the higher yields in Northern and East Central Illinois, where soils are deeper and store moisture longer than soils in Western and Southern Illinois. This type of weather led to high yields during the first half of the century, but the eight times it has occurred since 1970 did not result in one of the century's highest 20 yields.

"Such temporal changes reflect how shifting agricultural technologies, new corn varieties and farming practices collectively have altered how certain seasonal weather conditions affect yields," Changnon said. "The shifts in how differing weather conditions can affect corn production during the past 100 years send us an important signal that efforts to project how future weather will influence yields are fraught with dangers. No one in 1960, using weather and yield data from 1901 to 1960, could have accurately predicted the shifts in how weather would influence yields today."

The trend of fewer seasons of lower yields since 1960 agrees with other studies that show that Illinois has experienced greater precipitation and cooler temperatures since the 1940s, Changnon said.

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