Cool temperatures that have occurred across Iowa this summer have not only refreshed us but have also slowed down corn heat unit accumulation—known as growing degree days or GDDs. Iowa's very late planting dates due to the wet spring in 2013 have obviously affected the progress and development of this year's crop as well (see the graph accompanying this article). How will these two factors affect yield this year?
Iowa State University Extension corn agronomist Roger Elmore writes the ISU Corn Source column each month in Wallaces Farmer magazine. He points out that growing degree day accumulations this summer are lagging behind normal as of August 19.
Growing degree day accumulation since beginning of 2013 growing season is lagging behind normal
Look at the season-long degree day accumulations for 2013 that Elmore has compiled in the graph. It shows the growing degree day accumulation as a percent of normal for each of the Iowa Crop Reporting Districts as well as for the state average. The column to the left in the pair for each crop reporting district (blue column) shows GDDs as a percent of normal since May 1.
"I realize this is early for most of our corn but it provides a good benchmark," notes Elmore. "We finally had half of Iowa's corn planted by mid-May this year. On average the 2013 growing degree day accumulation was 93% for the state with a range of 90% to 95% in the different crop reporting districts. The three eastern districts—Southeast, East Central and Northeast--received higher growing degree day accumulations than the other crop reporting districts in Iowa."
Growing degree day accumulation since silking is lagging even further behind normal
Accumulations of growing degree days since silking this summer have lagged even further behind normal. The right-hand column of each pair (the red column) in the accompanying graph shows GDD accumulation since July 28. According to USDA's weekly statewide crop condition survey, July 28 is the date when half of Iowa's 2013 corn crop silked. That 50% mark on July 28 contrasts dramatically with last year's 96% silked and the 5-year average of 77% silked as of July 28.