A late-season heat wave is accelerating corn maturity across much of the Midwest, while also making disease issues and field-to-field yield variability more apparent.
For farmers trying to determine how much crop they have — and how much they can afford to sell into a strengthening market — the window to get an accurate read on yield is narrowing.
Rapid Drydown Tightens Decision Timeline
“The heat is turning and burning, and it’s pushing this crop along at a quicker pace,” says Ken Ferrie, Farm Journal Field Agronomist, in this week’s Boots in the Field report.
Many Illinois fields that have already reached black layer are losing more than a percentage point of moisture per day, Ferrie says. April-planted corn that was testing 24% moisture last week is now down to 18% in some fields.
That rapid drydown is tightening the timeline for both harvest and marketing decisions.
Disease Pressure Building in Fields
Disease pressure is also building in some corn crops, with Ferrie seeing more tar spot and southern rust. But most affected fields have corn that’s already past half milk line, making a fungicide application unlikely to add yield.
It could, however, improve standability.
The more significant visual concerns he’s seen on recent farm visits are top kill and crown rot. Ferrie notes that both can reduce yield, but infection timing plays a key role.
If top kill develops around half milk line, kernel depth can be affected because the upper leaves are still responsible for much of the kernel fill in the ear.
“But if top kill shows up at black layer or near the finish, it’s not going to have that big an effect on yield,” Ferrie says.
Crown rot follows a similar pattern: The later it develops, the less yield loss it causes. Ferrie is seeing more crown rot in Illinois fields that were particularly wet in June.
Plants with crown rot typically die prematurely, with ears that will hang down. When those ears are twisted, they can feel rubbery, and the kernels eventually may develop what Ferrie describes as reaching gray layer rather than reaching a solid black layer.
Stalk Rot and Soybean Disease Concerns
Another concern now is Physoderma stalk rot. Affected plants can become extremely brittle when the disease develops at the nodes. Severely affected stalks can snap in two during a push test.
Ferrie recommends performing a push test in susceptible fields every week and updating harvest crews on which fields will need to be combined first as harvest gets underway.
In soybeans, sudden death syndrome (SDS) and red crown rot are the primary diseases Ferrie is finding in fields.
As soybeans begin to mature, disease-affected areas are becoming easier to spot, even from the road. Plants in those areas are dying and turning faster, which can affect bean size.
“This is a good time to get some drone shots of the field,” Ferrie recommends. “They’ll help explain what we’re seeing in the yield maps later this fall.”
Accurate Yield Estimates Critical for Marketing
The combination of rapid maturity, uneven fields and the current market rally makes accurate yield estimates in corn especially important.
The question Ferrie says growers need to answer is, how big is the crop?
“Am I 50% sold, or could I be 75% sold?” he says.
For April-planted corn that is already mature, one of the best ways to answer that question may be to pull the combine into representative areas and get an actual yield number.
For May-planted corn still working toward black layer, growers can get a more accurate estimate by counting ears and kernels — while accounting for differences among hybrids.
Hybrid Differences Impact Final Yield
Ferrie says his hand-harvest data and hybrid plots show that hybrids can range from 55,000 to 110,000 kernels per bushel. That wide range underscores how much kernel fill and genetics can influence the final yield.
Hybrids that generate more of their yield through kernel fill need a strong finish. Hybrids that don’t flex as much in kernel depth can better tolerate a tougher finish.
July’s heat caused tip abortion in many fields because the crop was at a vulnerable growth stage when temperatures spiked. Hybrids with greater kernel-depth flexibility may be able to recover some of that lost yield through deeper kernels. Those with less kernel flex are likely to take a larger yield hit.
Using Yield Formulas for Better Estimates
Recent heavy rains are making accurate yield estimates even more challenging in some fields. Ferrie is finding more severe crown rot and top kill in those areas, along with greater variability in ear size.
Rather than relying on a quick ear count, he recommends taking a more detailed look and using a corn yield formula to estimate what you have in the field.
Here is a corn yield formula from Iowa State University Extension and an example provided for consideration:
Estimated yield (bu./acre) = Harvestable ears per acre × average kernels per ear ÷ kernels per bushel
For a quick field estimate, 90,000 kernels per bushel is the traditional assumption:
Yield = (ears/acre × kernels/ear) ÷ 90,000
If you’re using a 1/1,000-acre sample, the shortcut becomes:
Number of ears counted × average kernels per ear ÷ 90 = Yield (bu./acre)
Example:
- Suppose you count: 30 harvestable ears in 1/1,000 acre; Average of 500 kernels per ear
- The calculation and answer would be: 30 × 500 ÷ 90 = 166.7 bu./acre
Only count the ears that are likely to make it into the combine. Down plants that won’t be picked up should not be included in the ear count.
The process takes some time, Ferrie says, but it can give growers a realistic picture of the crop — and potentially change their marketing position.
“You may think you’re 50% sold and find out you’re closer to 75% sold,” he says.
Prepare for Challenging Harvest Conditions
Wind-damaged fields deserve particular attention as harvest approaches.
Ferrie recommends preparing farm employees and family members for what could be a long harvest, particularly in fields with damage. Fatigue and frustration can cause people to overlook details or take unnecessary risks.
“Everyone needs to pay extra attention to details and be extra careful,” he says, so harvest fatigue doesn’t lead to accidents.
In addition, the stronger market only makes harvest losses more costly.
“This crop is worth more with the rally. Let’s watch that harvest loss,” Ferrie says.
In some fields, that may mean taking extra time to adjust the combine. “Some of these ears are going to need a little grinding to get everything off,” he notes.
You can listen to the complete Boots In The Field report at the link below:
