Pam Smith, Farm Journal Crops & Issues Editor
The first day of the Midwest Crop Tour was enough to make Pro Farmer's Chip Flory exclaim that he's "never seen a crop this late the third week in August.”
He's not the only one—with a few exceptions where it's been dry or a garden spot in terms of timeliness, everyone seems to have the same question this fall. What is the likelihood of these fields making it to maturity prior to a killing frost?
Several years ago, Purdue University collaborated on field research that investigated the effects of delayed planting on the Growing Degree Day (GDD) needs of different hybrid maturities. That research indicates delayed planting actually decreases hybrid GDD requirements from planting to maturity.
According to the Purdue study, after May 1, the number of GDDs from planting to kernel black layer (physiological maturity) decreased by about 6.8 GDDs per day. In other words, a hybrid that normally requires 2700 GDDs from planting to maturity would require slightly less than 2500 GDDs if planted on May 31 rather than May 1 (30 days of planting X 6.8 GDD-2700).
Purdue University corn specialist Bob Nielsen reports that hybrids commonly grown throughout his state will mature safely when planted throughout most of the month of May. However, things get sketchy when planting is delayed further as the ever-shortening growing season finally exceeds the fuller season hybrids' abilities to adjust their developmental GDD needs.
Iowa State University plant pathologist Rich Pope notes that despite the tumultuous planting season, Mother Nature has done a pretty good job this summer getting corn to reproductive stages. However degree days in Iowa still remain behind long-term averages (as of Aug. 17). See Iowa's accumulated degree days and departure from averages at: www.extension.iastate.edu/CropNews/2008/0818pope.htm.
One thing to keep in mind is traditional relative maturity ratings of hybrids (days to maturity) are of little help in determining whether a hybrid will safely mature before a killing frost (light fall frosts injure corn leaves, but usually do not kill the plant). Maturity ratings of hybrids are simply indicators of relative grain moisture difference among hybrids of a given company at harvest and do not refer to absolute calendar time from planting to maturity.
Purdue work indicates corn planted in late April and early May will reach maturity in approximately 55 to 65 days after silking. That's when kernel dry weight usually reaches its maximum and kernels are said to be physiologically mature and safe from frost. This estimate puts the maturity date in mid-September.
The number of days from silking to maturity increases with later planted corn because GDD accumulation decreases dramatically toward late summer and early fall. Corn planted in early June requires 67 to 71 days after silking to reach maturity. That puts corn maturity estimates in mid- to-late October and dangerously close to the average first fall freeze dates in much of the Cornbelt. Normal killing frost dates in Iowa are around the first of October in the northern part of the state and about the 10th of October in the south.
Now is a good time to get out in the field and plan a strategy. While you can't control the weather, knowledge of what stage your crop is in and the average expected frost dates for your area. It might be better to salvage some late maturing fields at risk for frost injury for silage and harvest the earlier maturing corn for grain.
For More Information
You can a photo story detailing kernel development at www.agry.purdue.edu/ext/corn/news/timeless/GrainFill.html.