A new cover crop or just another learning opportunity from 2025?

While harvest is slowing down across the state, many have noticed new plant growth in the field: volunteer corn. While volunteer plants aren't unusual, it is rare to see so many fields with substantial unwanted plants this fall. Let’s examine why and what it might mean for 2026.

The crop

Corn and soybean suffered from numerous stressors this year, resulting in a relatively quick maturing crops throughout Iowa. Several issues come to mind when thinking about why crops matured so rapidly in 2025:

1. Early planting. Approximately 1/3 of the corn crop was planted before April 27.
2. Excessive rainfall. Too much water caused nutrient leaching and other stresses in some cornfields, particularly in central and north central Iowa.
3. Hot temperatures. Warm, humid conditions in July hastened reproductive development, and created ideal conditions for southern rust to develop and spread.

Ultimately, the environmental conditions resulted in corn that was more challenging to harvest. Challenges ranged from threshing corn grain from rubbery cobs to variable test weights and seed size. These issues create problems getting grain into the hopper rather than spreading it on the ground behind the machine. An average of just 20 kernels per 10 square foot can equal approximately 1 bushel lost per acre, so fields with substantial volunteer corn represent significant yield loss from harvest operations.

For those fields yet to be harvested, performing a power shutdown in the field and examining where the grain is coming from (header, thresher, separator) is useful. The ISU Digital Ag team has a webpage dedicated to combine setting adjustments that may be useful if field checks confirm significant loss of grain behind the machine. 

The weather

Harvest started with remarkable speed this fall, with an estimated 8% of corn harvested for grain across the state by September 21 and 15% harvested by September 28. This harvest rate was several percentage points ahead of the 5-year average and fall 2024. Conversely, the soybean harvest rate across Iowa lagged behind the 5-year average. 

In addition to the early harvest, fall temperatures have been above normal, with much of the state averaging nearly 150% of normal for growing degree-day accumulation. The average temperature statewide ranged from the mid-to-high 60s between September 15 and October 23.

Beyond warm temperatures, there were several timely rainfall events in the latter part of September and early October. Moisture has been beneficial for cover crops seeded this fall but also germinated corn and soybean left from harvest activities. Some fields have volunteer corn nearing 12 inches in height, though much of that died by frost on October 23.

What does this mean for next year?

Insects:

• Having volunteer plants, or any green plant tissue, within and around crop fields in the fall can be attractive to insects. They could be searching for refuge or places to overwinter (e.g., crop residue or weeds, or under the soil surface) or laying eggs near the growing vegetation. A common example is female corn rootworms that lay eggs near the bases of corn plants.
• If the volunteer seed from a Bt corn hybrid germinated the following spring, it will likely express reduced levels of Bt toxins. The reduced expression could allow corn rootworm larvae to survive exposure to the toxin.
• Management implications for fields with significant volunteer plants is not fully understood for pests that are active before planting and in the early-vegetative stages (e.g., grubs, wireworms, millipedes, isopods, seedcorn maggot, etc.). 

Plant diseases:

• Volunteer corn currently growing could be infected with pathogens that have been present in this year’s crop.
• Infected volunteer plants could serve as a source of inoculum of tar spot and northern corn leaf blight for next year’s crop.
• Remember that southern rust does not overwinter on crop residue or dead volunteer corn plants in Iowa.

Nutrient cycling:

• Sprouting crops this fall means soil nitrate can be taken up and protected from potential loss.
• Like winterkill cover crops, nutrients should be available in the spring for the next crop.
• Corn less than 6-leaf stage has a very low C:N ratio and decomposes very quickly
• Residue from this volunteer corn may not even be visible come spring planting.

Weed management:

• It is possible that we will also see higher levels of volunteer corn next year in some fields. Monitor fields for the presence of volunteer corn and adjust herbicide programs to manage this issue in a timely manner.
• If fields are returning to corn in 2026, advice compiled in an ICM blog following the 2020 derecho may be useful.