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.