by Margaret Smith • Forage Agronomist &
Bairet Eiter • Cover Crops Lead

Interest in seeding a range of cover crop species into corn early in the growing season is increasing among both conventional and organic farmers. Why interseed cover crops in corn early, rather than into maturing corn or after corn harvest?

  • Bigger window (more growing season) for cover crop growth
    • Increase growth once corn starts to dry down with biomass potential for fall grazing
    • Increase carbon capture
    • Improve ground cover and erosion control
  • Better seed-to-soil contact and ‘catch’ compared with aerial seeding into later standing corn
  • Suppress weeds
  • Sequester nitrogen later in growing season
  • Use excess moisture during growing season
  • Moderate soil temperatures during hottest part of growing season
  • Create better footing for combine traffic if season is wet

Researchers also are working to define whether legume covers planted early can contribute nitrogen to the existing corn crop and if they can contribute nitrogen and/or improve mineral cycle for a following crop.

Won’t Early-Planted Cover Crops Compete with Young Corn?

Researchers  identified the critical weed-free period for corn, defined as the ‘window’ in the crop growth cycle during which plant competition must be minimized to prevent unacceptable yield losses (usually less than 5%). They found s that the  critical period is between a range of vegetative stages from V2 through V10. Why the variation in observed critical periods? The critical weed-free period varies with time of corn planting, the weed community present, nitrogen status of the soil, and rainfall. The generally accepted critical weed-free period for corn is V2 through V6.

This critical weed-free period, combined with the need for sunlight to get an  early cover crop established, has led to the timing of seeding into corn at vegetative growth stages V4-V7. Much earlier and you may hurt corn yield; much later and you may not be able to get cover crops established.

Herbicide Use in the System

Herbicides may be a constraint to making this system work. Herbicides and methods of application may need to be adjusted to allow for early cover crop growth. Strategies include using:

  • Herbicides with no residual
  • Herbicides with no effect on selected cover crops
  • Preplant herbicides only, with short residual time
  • Banded residual herbicide over the row, where cover crops will not be affected
  • Combination of these methods

What herbicides can work in this system? Seven Iowa farmers conducted on-farm interseeding trials into corn and used a range of herbicides and tank mixes, including: Liberty and atrazine; Status and Roundup PowerMaxx; Buccaneer Plus, Calisto, and atrazine; Status and Tomahawk; and Liberty.

Researchers at Pennsylvania State University examined a wide range of cover crop and a number of herbicide options. They predicted herbicide damage potential for three plant families of cover crops: grasses, legumes and brassicas, then classified herbicides by their expected effect on early-season planted cover crops in those plant families. For this information and to help plan your herbicide program, see references.

Seeding Methods

Cover crops seeded into early-season corn have been most successful when the seed was covered and the soil firmed after planting. Pennsylvania State successfully tested a high-clearance drill for seeding into V7-V8 corn, but it is an expensive piece of equipment. Farmers and other researchers have modified commercial no-till drills for early cover crop seeding. Removing drill openers that would run over the corn rows while leaving three drill row units between 30” spacings is a simple adjustment and highly effective in V4-V6 corn. Conventional drills may be less effective, depending on the preplant tillage for corn. Other farmers have broadcast seed with spinner-type seeders. Rotary hoes or tine weeders, modified to exclude the corn row, have been used for incorporation.

Still others have used air seeders and drop tubes, some while tine weeding or cultivating, while others sidedress liquid N or anhydrous. Farmer ingenuity rules on this practice! Check out YouTube for great ideas on seeding methods.

Timing seeding before a rain will increase cover crop germination and establishment.

Cover Crop Selection and Seeding Rate

Cover crop species with the best survival in this planting window include: annual ryegrass; red clover; and several brassicas, including rapeseed, kale, and radishes, likely due to their greater shade tolerance than most species. Other cover crop species with good shade tolerance include forage collards and hairy vetch. Crimson, berseem, and balansa clovers are somewhat tolerant of 50 percent shade, but much less tolerant than these other species under heavy shade, like a corn canopy later in the growing season.

Winter cereal rye often (but not always) survives the corn-growing season until after harvest. A wider variety of species tend to survive if corn is chopped off for silage. South of the Minnesota border, cowpeas are a traditional option that still works. Other cover crops that have shown some success in this system include: crimson clover, penneycress, and winter camelina. There are indications that mixing species improves the survival rate, but most research has been done with single species.

Different seeding rates for each cover crop have not been widely tested, but some rates that have been used successfully are included below in Table 1.

Fall biomass production has been measured usually at less than 1 ton dry matter per acre following corn grain harvest.

Improving Cover Crop Survival and Biomass Production

Limiting factors for cover crop establishment in early-season corn and biomass production include:

  • Adequate moisture for establishment

  • Adequate moisture throughout the growing season, particularly during August Shallow-rooted, small cover crop plants may die during even minor drought stress

  • Adequate sunlight for growth

Not much can be done to influence available moisture during the growing season, unless a field is irrigated. There are ways, though, to increase sunlight penetrating into the corn canopy. Do hybrids with more upright leaf orientation allow more sunlight to reach interseeded cover crops? Preliminary data in 2019 from the University of Wisconsin did not show any effect of leaf orientation, either in sunlight penetration or in cover crop biomass production.

Reducing corn plant population will increase sunlight penetration into the corn. Reducing corn plant populations to 22,000 to 26,000 plants/A improved cover crop establishment and increased fall cover crop biomass in Pennsylvania, but corn yield was decreased.

Planting corn in wider rows also help increase light penetration into the canopy and more than doubles cover crop biomass production. There is a currently a lot of work going on with 60” corn row spacings and this practice – stay tuned!

ValueMax Cover Crop Mix

Albert Lea Seed offers these species of cover crops and a custom mix for this planting window, ValueMax (CC2), comprised of annual ryegrass, crimson clover, rapeseed, and radish which add diversity in both leaf growth habit and rooting structure. We can also custom mix other seed combinations.


Cover Crop Interseeder: Improving the Success in Corn, Pennsylvania State University

Using Herbicides and Cover Crops in Corn and Soybeans, University of Minnesota

Early-season cover crop seeding into corn, M. Scott Wells, University of Minnesota

Planting Corn in 60-in. Row-Widths for Interseeding Cover Crops, Practical Farmers of Iowa

Planting Corn in 60-in. Row-Widths for Interseeding Cover Crops (2019), Practical Farmers of Iowa

Cover Crop Seeding: No-till drill modification [VIDEO], University of Wisconsin

Cover Crop Interseeding – Stearns County, MN [VIDEO]

Interseeding Cover Crops into Corn with Producer Rob Olson and Daughters [VIDEO]

Managing Residual Herbicides in Cover Crops, Farm Progress

Interseeding Early Cover Crops into corn Ontario, Canada

Managing residual herbicides with cover crops, Corn and Soybean Digest

Control Weeds Early in Corn, Manitoba, Canada

Determination of critical period for weed control in the second crop corn under Mediterranean conditions [Turkey]

Interseeding Cover Crops into V5 Corn, University of Wisconsin

Interseeding Cover Crops in Michigan Corn Rotations

Interseeding Cover Crops into Corn or Soybeans, University of Nebraska

Interseeding early cover crops into corn, [Ohio] Field Crop News

A Performance Evaluation of Three Cover Crop Species Interseeded into Continuous Corn Via High Clearance Drill and Broadcast Seeding Method [Ohio]

Critical Period for Weed Control: The Concept and Data Analysis [Nebraska]