Derecho. Another one of those words we wished we hadn’t heard in 2020 but are quite certain we won’t forget about the results from its occurrence. Millions of corn acres were damaged, and there are many questions about the lasting impact of the recent widespread windstorm that tore across central and eastern Iowa and into Illinois. 

“The closer to upright that the corn plant remains following a windstorm, the less long-term damage occurs,” notes Mark Licht, Iowa State University Extension cropping systems agronomist. “As a general rule, corn that is leaning less than 45 degrees from its normal upright position can usually recover. However, the corn that is flat on the ground will stay there.”  

Wide range of corn damage 

For the cornfields struck by the derecho windstorm on Aug. 10, the damage varies considerably. Licht, along with ISU Extension field agronomists Joel DeJong and Meaghan Anderson, break the current corn crop injury situation into three categories and offer recommendations for harvesting the wind-damaged crop: 

Slightly root-lodged or leaning. Plants are only slightly root-lodged or leaning at a 45 degree angle or greater. These plants are leaning over no more than half-way toward the ground from the normal upright or 90-degree position.  

There is some research that has looked at impacts of lodged corn on yield. But most of it was done on corn before or shortly after tasseling. Most of this year’s derecho-affected corn was further along. It was in the dough stage (R4) or early dent stage (R5) when the windstorm struck.  

In a University of Wisconsin study in 1988, researchers manually pushed the base of corn plants perpendicular to row direction to cause root lodging. They noted that within two days after lodging, the upper portion of plants became upright and subsequent timing of plant development was not impacted. However, more barren plants were observed when lodging occurred at later development stages, impacting yield. Corn that was lodged after the V17 stage of growth resulted in a 12% to 31% yield reduction.  

There are a couple distinctions to consider. The ability of corn plants to recover and become upright is much less likely when plants are lodged at the R4 to R5 stages than in the late vegetative or early reproductive stages. Yield loss would be more when lodging at V17, since no grain dry matter accumulated at that time. Grain dry matter at R4 stage of growth is about 20%, and at the beginning of R5, it is 25%. This is explained in ISU Extension publication PMR 1009, Corn Growth and Development. Availability of soil moisture, severity of root system damage by lodging, and other issues can impact the extent of yield loss in this situation. 

Pinched but not wilted. These corn plants are pinched over but not wilted yet, or are broken above the ear, or are severely root-lodged (less than a 45-degree angle). The severely root-lodged plants are laying on the ground or near the ground. 

Kinks in stalks restrict movement of resources within the plant, similar to kinking a hose while filling a water tank. If all plants are flat and still rooted into the soil, certainly they are not intercepting as much sunlight and, therefore, not filling grain as normal. If roots were damaged significantly and the weather continues to be dry, additional yield reductions will occur because nutrient and water uptake efficiency will be compromised. Plants with broken stalks above the ear will continue to produce photosynthates using intact leaves, but the available resources to fill the grain will be greatly limited.  

Best-case scenario would be the yield loss when plants are broken above the ear, with more yield loss possible based on these other potential issues. Test weight will also be compromised. Also, these stresses might cause premature death of some plants. Hopefully, the plants continue to live and produce marketable grain, although they will mature at a slower pace. 

Broken and wilting. The plants are broken off below the ear and are now wilting or dead above the breakage site. These corn plants can no longer add dry matter to the grain, so you are left with about 25% of the potential dry matter accumulated. Test weight will be very low, and the ability to store grain killed in the dough stage is extremely limited — if it is at all even possible.  

If your corn made it to the early dent stage when the derecho struck, yield loss is still likely over 40%, and test weight and storage capability remains very low. Low test weight corn grain is more likely to have quality problems during storage. The best use of low-test weight corn is silage, if a use for that much silage can be found and it can be properly ensiled. 

All of this discussion deals with what happens to the ear on the stalk. The ISU agronomists note that future news articles on the ISU Integrated Crop Management website will discuss the problems and offer suggestions for harvesting the compromised 2020 corn crop and dealing with potential grain quality issues.