Roger Elmore, Iowa State University Extension corn agronomist, has compiled data from a multi-year study on date of planting for corn, based on results from six ISU research farm locations from around the state. He has used that data to determine the optimum corn planting dates for three regions in Iowa. The data were collected in 2006, 2007 and 2009.
The plots were planted at various dates and yield was calculated for each planting date. Then a date range was determined for the three regions in Iowa as indicated on a map that he has published on his website www.agronext.edu/corn.
Then a date range was determined for the three regions in Iowa on the map that would correlate to achieving 98% of maximum yields.
Planting corn in mid-April proved successful in this study
To achieve 98% of the corn yield potential, the data shows you should plant between April 12 and April 30 in northeast Iowa. In northwest, west central, central, and east central Iowa the best time to plant is from April 15 to May 9. In the remainder of the state (the bottom three tiers of counties in Iowa) to reach the 98% yield window you need to plant corn April 17 to May 8.
Click on the following link to check on soil temperatures by county in Iowa at the 4-inch depth. This ISU Extension site also gives you the weather forecast.
Iowa Soil Temperatures by County . For the three day history of Iowa soil temperatures by county, visit the Soil Temperatures for Ag Web page.
What about crop insurance? You can't plant before April 11?
There have been a few reports on the radio of a few Iowa farmers this week who planted a little corn prior to rains across the state on April 7. "In Iowa we don't see a lot of people who violate the replant provision of their crop insurance policy," says Steve Johnson, an ISU Extension farm management specialist.
If you have a farm level insurance product like Revenue Protection or Yield Protection, as long as you plant corn after April 11 in Iowa, and soybeans after April 21, you still have the ability to claim replant coverage, if you have to replant.
That doesn't mean you can't plant corn before April 11 or soybeans before April 21. But if you do, you may lose your crop insurance coverage for a replant. Check your policy and talk to your crop insurance agent to make sure you understand the possible consequences before planting that early.
Should you go ahead and plant corn early in cold soils?
Soil conditions can be the best ever during the first week of April. But the question is, should you plant corn early when the soil is still cold? University of Illinois agronomist Emerson Nafziger says yes, but with a few cautions.
"We should not expect yields of corn planted in the first week of April to be higher than those of corn planted the third or fourth week of April," he says. "We have had a few instances when corn planted in early April yielded less than corn planted later in April. This doesn't happen often enough to rule out early planting, but it does mean the main reason to plant in early April is to get done by late April and avoid late-planting yield loss."
Another caution is to plant early only when seedbed conditions stay favorable; if it rains or is still wet, growers should not try to get back in the fields too soon.
"It typically requires about 110 to 120 growing degree days (GDD) for corn to emerge," he says. "With highs in the mid-60s and lows in the 40s to low 50s, we accumulate less than 10 GDD per day, so it can easily take two to three weeks for the crop to emerge."
Typically, this isn't a problem. But it is a long time, and problems can develop to hinder emergence. Early-planted corn should be watched carefully, especially when GDD accumulations pick up and the crop approaches emergence.
Low soil temperatures aren't the major risk factor for corn
Low soil temperatures are not the major risk factor that planted corn faces. Instead, heavy rainfall soon after planting, with seeds or seedlings dying from lack of oxygen, is the major cause of replanting. Chances of this happening are no higher for early than for later planting, he says.
Planting into cooler soils may even improve chances for emergence following rainfall. Seeds are not triggered to germinate and emerge as rapidly in cool soils, so they often survive longer in cool, wet soils than in warm, wet soils. There is some risk of damage from frost after plants have emerged, but that's fairly rare.
"While we hope we won't need to replant, another advantage of very early planting is that if we do need to replant it, the replanting can be done early enough to avoid large penalties from late planting that you often incur when you have to replant corn," adds Nafziger.