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.
Statewide growing degree day accumulations since silking this year are averaging 82% of normal. These range between 78% and 87% of average depending on the crop reporting district; the highest GDD accumulations relative to normal occurred in East Central and Southeast Iowa.
What does this slower than normal crop progress mean for yield potential?
Here's how Elmore answers that question. "First, our slow planting progress no doubt compromises yield potential," he says. "The USDA's monthly Crop Production report issued August 12 gave this season's first yield estimate based on conditions in the field. That USDA yield forecast released August reflect 12th USDA yield forecast reflects this year's lagging crop. Iowa's USDA August forecast yield of 163 bushels per acre is almost 9% below 30-year trend line yields."
The second thing to consider, says Elmore, is that cool temperatures after silking not only slow growing degree day accumulation--thus slowing crop development--but also can increase yield potential, given specific conditions. For example, the record corn yields of 2009 resulted from slow growing degree day accumulation after silking--coupled with a late frost that year. On the other hand, warm temperatures after silking in 2010 reduced corn yield potential.
With a late crop, 2013 corn yields will depend on timing of first killing frost. "The bottom line this season will be the timing of the first killing frost this fall," says Elmore. By killing frost, he refers to a 28° F frost. A later than normal frost encourages a longer seed-fill period for the corn plants and higher yields. What if we have an earlier than normal frost? "Well, let's just hope that doesn't happen," says Elmore.