The Effects of Cover Crops on early Corn growth

The Effects of Cover Crops on early Corn growth: A Polk SWCD Cover Crop Demonstration Project

Background

Polk Soil and Water Conservation District (SWCD) manages a ‘farming Demonstration Project’ north of White Oak to showcase farming practices that reduce nutrient pollution. The site is used for field days and soil health education. In 2024, Polk SWCD planted a series of eight different cover crops on a five-acre plot to demonstrate how different species and mixes germinate and grow through the fall and spring. All cover crops were planted using a seed in the fall with NRCS seeding rates.

In spring 2025, the site was transitioned into a Regenerative Farming Demonstration Plot to highlight how regenerative practices can reduce nutrient

pollution to surface waters. Corn was planted directly into the previous fall’s cover crop residue across each of the species rows, without adjusting planter settings between rows. By mid-summer, clear differences emerged in how corn responded to the different cover crops.

Corn growth following Winter-Killed Cover Crops

Corn following winter-killed species such as oats, radishes, and turnips grew quickly and showed vigorous early development. Much of the cover crop residue was still visible in Spring but did not interfere with planting. Soils under these covers were loose and friable, making planting easier. The decaying roots and residues likely helped recycle nutrients like nitrogen, sulfur, and potassium, giving corn an early boost.

Winter-killed species are an excellent, low-risk entry point into cover crops. They protect soil and water through the fall, scavenge nutrients, and naturally terminate with frost, clearing the way for spring planting without additional management.

Corn growth following Over-Wintering Cover Crops

Corn following overwintering species—rye, winter wheat, and triticale—showed much slower early growth. By July 1, 2025, height differences between winter-kill and overwintering cover plots exceeded 20 inches. Rye had the largest impact on corn growth, followed by winter wheat and triticale, though by mid-August the height gap had largely disappeared.

The overwintering cover crop plots were more difficult to plant. Their fibrous root systems and heavy spring biomass challenged the planter’s ability to open and close seed furrows and maintain consistent seed depth. Because the grasses were terminated with herbicide on April 30—the same day corn was planted—these management practices directly contributed to delayed corn growth.

Late termination likely intensified challenges with overwintering grasses

The specific reason for Rye and other grasses’ negative effect’ on corn is unknown(1) but several factors might be involved:

1) The presence of rye mulch or roots alters the soil environment in a way that inhibits corn growth. The mulch may delay soil warming and drying, creating a less favorable environment for corn. While it has been generally supposed that grass cover crops, particularly rye, release chemicals that have allelopathic effects, these effects are proportional to seed size. Since the corn seed is large, these chemicals likely have little effect on at the growth of a corn plant. (1)

2) Nitrogen tie-up: The high carbon-to-nitrogen ratio of grass residues encourages soil microbes to immobilize nitrogen during decomposition, leaving less available for young corn seedlings.

3) Rye may act as a ‘green bridge’ for plant pathogens. The dying rye could serve as a host for pathogens that move to corn seedlings after the rye dies. This hypothesis is currently being studied

Management Recommendations

Proper management reduces the risk posed to corn production:

Termination Timing

Terminate grass cereal cover crops at least 10–14 days before planting corn to allow allelopathic compounds to break down and residue to begin decomposing.

Starter Fertilizer

Apply a starter nitrogen source with corn to compensate for nitrogen tied up by decomposing grass residues. This helps young corn establish even if termination is later than ideal.

Key Takeaways

Winter-kill cover crops are simple, low-risk, and give corn a strong early start.
Overwintering cover crops provide more long-term soil and water quality benefits but require more management.
Termination timing of overwintering cover crops is critical—delayed termination decreases the effects of cereal crops to slow corn establishment.
With proper management (early termination and starter fertilizer), corn can perform well following even the most challenging cover crops.

(1) Kaspar, T. C., & Ercoli, L. (2015). Cereal rye cover crops, allelopathy, and corn. Integrated Crop Management, Iowa State University Extension and Outreach. https:// crops.extension.iastate.edu