"A couple of big issues are staring at us--stalk rots and corn dry down," says Clarke McGrath, Iowa State University Extension field agronomist. "These issues are tied together and corn growers may have some tough decisions to make. Artificial drying can certainly test a grower's patience. As energy prices (especially propane or LP gas) climb and/or availability of LP becomes an issue, costs can really add up. All this makes us appreciate any natural drying of corn in the field that Mother Nature can provide. This fall, however, leaving corn in the field to dry naturally for too long may be problematic in some areas of the state— for a couple reasons."
McGrath, who writes a column each month in Wallaces Farmer magazine, says "The stalk rot issues we've discussed in recent weeks have only gotten worse. In scouting, in field visits and while putting in four cover crop plots I've spent a lot of time in corn fields recently," says McGrath, who is based at Harlan in western Iowa.
Stalk rot and weak corn stalks aren't hard to find this year
"Stalk rot and the resulting standability issues are not hard to find in most of southwest and west central Iowa," he says. "If you can scout your corn well and find areas that have good stalk quality, then some field drying of the corn may be in order. However, for fields with significant stalk rot and risk of lodging, the risk/reward tradeoff of field drying the corn vs. harvesting it wet and artificial drying is worth a good look."
Unfortunately, this time of year the risk (stalk lodging) escalates and reward (natural field drying) typically decreases. "We have had tough luck getting a lot of growing degree days for speeding along maturity and drying in the last month—we're around 20% below average in heat unit accumulation. And as we head into October the field drying of corn usually slows down even more. Some good news is that the 7 to 10 day weather forecast does show some above average temperatures, so if we can dodge the showers we'll gain some ground in getting corn to dry down in the field a little more."
Estimated field drying, how fast can corn dry down in the field?
McGrath and other ISU Extension field agronomists are getting a lot of questions about how much, and how fast, corn grain will dry down as it stands in the field, especially as the crop heads into October. That's a really good question, with a typical agronomic answer… it depends.
"My understanding—based on good information from Dr. Roger Elmore who is now at University of Nebraska and previously was at ISU—is this," says McGrath. "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 October as temperatures drop, "we accumulate around 5 to 10 GDDs per day," he says. Calculating the math on all this, McGrath continues: "From the few harvest reports, some hand shelling, and listening to experienced growers as they chew on recently black layered corn (and when we double check their estimates with a grain moisture meter they are usually pretty darn close), there is a lot of corn currently sitting out there at around 30% grain moisture content, so we'll use that as an example."
A lot of corn in Iowa now has 30% grain moisture content
"With 30% moisture content for corn this morning, September 26, and looking through next Wednesday, October 1, based on the weather forecast, we'll accumulate approximately 120 GDD's. The math tells us this will bring that 30% moisture corn down to around 26%. Given the winds and the more 'open' canopy in many fields as a result of the foliar leaf disease, experience leads me to think we'll gain a little more ground than that so we'll go with 25% moisture corn. That's 25% corn starting October 2 and looking through October 5, and the forecast gives us around 42 GDD's."
"So our rough approximation puts us at around 23% moisture content or so for corn grain in the field as we start the week of October 6. That's not dry enough to make many of us very happy given the cost of drying corn with an artificial dryer—considering the time and the fuel and electricity cost."
Today's 30% moisture corn may be 17% to 18% by end of October
If "average" conditions return for mid-October, you recall that we get around 5 to 10 GDD's per day, and it takes approximately 45 GDD's to drop a point from 25% on down, notes McGrath. "So we are projected to drop maybe a couple points a week through mid-October. As we look towards averages in late October and into early November, we may drop a point of moisture a week, and if we head into mid-to-late November field drying often slows to a crawl."
So based on these calculations, today's 30% corn might be around the 17% to 18% moisture range by Halloween time. Or wetter… or drier…. It depends. "We're all no doubt on board with hoping for a warm, dry run that has us finished harvesting by the end of October and patiently waiting for fall anhydrous ammonia application season to start," he notes.
Of course, weather during October can make a big difference
Remember all the above is "ballpark" figuring, says McGrath. "As mentioned earlier, a warm windy day like today (September 26) may pull more moisture out of the corn that's standing in the fields than our GDD estimate would indicate, since we have a nice breeze. On the other hand rainy, cloudy days may see less drying than the accumulated GDD's would indicate."
Husk and ear characteristics will also affect the rate of corn grain dry down in the field. Ears with the following characteristics will allow for faster drying: fewer and thinner husk leaves; early husk leaf senescence; ears with tips that protrude beyond the husks; looser husk leaves; early ear drop from an upright position; thinner or more permeable pericarp. Dr. Bob Nielsen, agronomist at Purdue University, has a more complete discussion of this online.