Cool temperatures result in slow heat unit accumulation for Iowa corn

Cool temperatures across Iowa not only refresh us but also slow down corn heat unit accumulation – growing degree days or GDDs. Our very late planting dates obviously affect the crop as well (see figure 1 in this ICM News). How will these two factors affect yield?

Season-long GDD accumulations

Growing degree day accumulations lag behind normal. The figure attached to this blog posting shows GDD accumulation as a percent of normal for each of the Crop Reporting Districts (CRD) as well as for the state average. The column to the left of the pair for each CRD shows GDD as a percent of normal since May 1st. I realize this is early for most of our corn but it provides a good benchmark – we finally had half of Iowa’s corn planted by mid-May this year. On average 2013 GDD accumulation was 93% for the state with a range of 90 to 95% in the different CRD’s. The three eastern CRD’s received higher GDD accumulations than the other CRD’s.

GDD accumulations since silking

Accumulations since silking lag even further behind normal. The right-most column of each pair in the figure shows GDD accumulation since July 28th. According to the USDA, July 28th is the date when half of our corn crop silked – that contrasts dramatically with last year’s 96% silked and the five-year average of 77% silked on July 18th. Statewide GDD accumulations since silking average 82% of normal. These range between 78 and 87% of average depending on the CRD; the highest GDD accumulations relative to normal occurred in East Central and Southeast Iowa.

What does this mean?

First, our slow planting progress no doubt compromises yield potential. The August 12th USDA yield forecast reflects this; Iowa’s USDA August forecast yield of 163 bushels per acre is almost 9% below 30-year trend line yields.

Second, cool temperatures after silking not only slow GDD accumulation – thus slowing crop development, but also can increase yield potential given specific conditions. The record yields of 2009 resulted from slow GDD accumulation after silking….coupled with a late frost. On the other hand, warm temperatures after silking in 2010 reduced corn yield potential (Link to see an analysis on this).

The bottom line of this whole season will be the timing of the first 28⁰ F frost this fall. A later than normal frost encourages longer seed-fill period and higher yields. An early frost … well let’s hope it doesn’t happen!

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