A quick look at corn planting progress by mid-May, especially in major corn-growing states, indicates a significant number of acres could be planted after May 15. Corn planting was 50% or more completed in only five of the 13 states listed in the table below. Corn planting trailed the five-year average in several key states, including Illinois, Indiana, Iowa and Nebraska.

Bob Nielsen, retired Purdue Extension corn specialist, took a closer look at whether planting progress by mid-May means state corn yields will be lower. He arbitrarily selected May 15 as a reference point to characterize “early” and “late” planting. He considered years with a higher percentage of corn planted after mid-May as late planting seasons and years with a lower percentage of corn planted after May 15 as early planting seasons.

Nielsen looked at the relationship between “percent departure from trend yield” and “percent of corn acres planted after May 15” for Indiana over the past 30 years.

“The data show there is indeed a tendency for lower yields statewide when more acres are planted after May 15,” Nielsen reports. “However, this relationship is not perfect.

“There were three late-planted years — 2009, 2013 and 2022 — where 76% to 89% of the state’s corn crop was planted after May 15, but statewide average yields ended up 4.5% to 8% above trend. There were also two earlier-planted years — 1997 and 1999 — where only 31% to 38% of the crop was planted after May 15, but statewide average yields ended up 6% to 10% below trend.”

Related:Will planting delays sink 2024 corn yields?

In fact, Nielsen determined that planting date effect only describes about 13% of overall year-to-year variability in yield.

More corn planting data

University of Nebraska researchers Roger Elmore and Jenny Rees documented the same absence of a strong relationship between statewide corn planting progress and departures from trend yield in Nebraska, summarizing it in a 2019 report.

“Such a weak relationship reflects the fact that many other factors besides planting date influence yield,” Nielsen says. “The bottom line is that statewide averages for planting progress and yield are not strongly related.”

Scott Irwin, University of Illinois ag economist, performed multivariate regression analysis involving yield and weather data collected from 10 major corn-producing states. In his work, he defined May 20 as the planting date after which substantial yield losses occur.

“His analyses concluded that late planting was the third most important weather-related variable influencing average U.S. corn yields, behind July rainfall and temperature,” Nielsen says.

Nielsen notes, however, that summer rainfall and temperature are not independent of planting date. Crop growth stage influences the effects of extreme weather.