Located at Harlan in western Iowa, Iowa State University Extension field agronomist Clarke McGrath notes -- "so, we planted late in 2013, which set us up for wet corn; we followed this with a hot, dry spell that sped things along at the wrong time, and the last couple weeks we've been all over the board with temperatures. Many farmers are wondering about dry down of the stressed corn, 'and I wish I had a rock solid answer'. There are slight differences in opinions on how much, and how fast, our corn will dry in the field, especially as we head into October," says McGrath.
He explains that "My understanding (and I get the best info from our own Dr. Roger Elmore at ISU) is this; estimating dry down rates can be considered in terms of Growing Degree Days, or GDDs. Generally, it takes around 30 GDDs to lower grain moisture each point from 30% down to 25%. Drying from 25% to 20% requires about 45 GDDs per point of moisture. In September we typically average about 10 to 15 GDDs per day; this year we averaged nearly 18 GDD's per day in September, putting us closer to where we want to be even with a late planted crop."
In October as temperatures drop, the rate drops to five to 10 GDDs per day.
However, both of these estimates are based on generalizations, and it is likely that some corn hybrids vary from this pattern of drydown, as hybrid factors impact drydown rates some, says McGrath.
We still need the warm pattern to stay around and we need a dry fall, too
Some past research evaluating corn drydown provides more insight on the effects of weather conditions on grain drying. During a warm, dry fall, grain moisture loss per day ranged from 0.76% to 0.92%. During a cool, wet fall, grain moisture loss per day ranged from 0.32% to 0.35%. "So, let's hope the warm pattern holds for the later planted corn," says McGrath.