Managing Volunteer Corn in Soybeans

Introduction

Volunteer corn is one of the most competitive weeds in soybean. Infestations can result from ear and kernel losses in the previous season’s corn crop. These losses can occur from lodged corn, dropped ears, or be caused by the combine while it is harvesting. Growing conditions that are conducive to certain stalk rot disease pathogens such as Fusarium, Gibberella or Diplodia may cause stalks to become weak and lodge, especially during a windstorm event. Dropped ears can also occur from European corn borer and/or Western bean cutworm feeding on ear shanks if the corn product is not protected by an above ground insect trait or with an insecticide. Additionally, high plant population or late season drought conditions may stress corn plants and cause ear loss.

Impact of Volunteer Corn on Soybean Yield

Research examining the impact of volunteer corn has demonstrated that even low densities of volunteer plants can substantially reduce soybean yields.¹ University of Nebraska research indicated that a volunteer corn density of 3,500 plants/acre resulted in a 10% yield reduction, and a 27% yield reduction was realized by doubling the density of volunteer corn to 7,000 plants/acre.² Similar research at South Dakota State University found that a 20% yield loss occurred with a volunteer corn density of 5,000 plants/acre, which equates to just one volunteer corn plant every 3.5 feet.²

Clumps of volunteer corn (clump meaning growing from an ear that is all or mostly intact) associated with dropped ears were also found to be more competitive than individual plants scattered throughout a field. The University of Nebraska research also found that 3,500 clumps/acre could result in a 40% yield reduction. Research results from Minnesota and Illinois indicated that soybean yield was reduced 1% for every 75 to 115 clumps of volunteer corn/acre (with 7 to 10 plants/clump).²

In addition to causing yield loss, volunteer corn can attract and encourage corn rootworm beetles to lay eggs in soybean fields, increasing the threat to corn the following season and limiting the benefits of a corn-soybean rotation.³ Additionally, if the previous year’s corn crop was a B.t. rootworm product, the volunteer corn will likely express a reduced level of B.t. toxin compared to its parent product, potentially selecting for resistance to that B.t. toxin in the local rootworm population. Therefore, it is important to control volunteer corn in soybean.

Preventing Volunteer Corn in Soybean

Management practices should be employed to help prevent infestations of volunteer corn in soybean.

• Choose corn products with above ground insect protection traits to manage ear drop problems caused by insects feeding on the ear shanks.
• Harvest corn fields in a timely manner to minimize lodged or downed corn that can result in harvest losses.
• Make sure combines are properly adjusted to reduce harvest losses.

Preventing volunteer corn in soybean fields is one of the best ways to avoid the need for control practices.

Preplant Control of Volunteer Corn

Volunteer corn can be controlled by tillage or by the use of postemergence herbicides prior to planting soybean. Strategies should be employed in the fall on corn fields where substantial lodging, ear drop, and/or harvest losses occurred. In areas where the soil is still warm enough after harvest for volunteer corn to germinate, a shallow tillage operation could be used to incorporate kernels and ears into the soil. Doing so may encourage the corn to germinate in the fall and then winterkill with freezing temperatures. However, allowing the seed to stay on the soil surface can also potentially reduce populations the following season through exposure to freezing temperatures, and seed left on the surface may be more susceptible to predation or decay. If possible, allowing cattle to graze field after harvest can also help reduce the number of seeds in the field. Alternatively, using no-till practices can minimize seed-to-soil contact and the germination of volunteer corn seeds.

Preplant tillage can control volunteer corn if the plants have emerged. Shallow tillage (field cultivation) can eliminate emerged volunteer corn, but it can also “plant” other volunteer seeds which could lead to heavier populations of volunteer corn later in the season. Where volunteer corn pressure is expected to be high, consider planting those fields last to allow as much of the volunteer corn to germinate before the final control measure (tillage, herbicide, etc.) prior to planting.

Preplant burndown herbicide applications can be used in no-till fields for control of emerged volunteer corn. Knowing the herbicide traits in the prior year’s corn is critical when selecting the burndown program. Roundup^® brand agricultural herbicides, containing glyphosate as the active ingredient, can help provide excellent control of volunteer corn that grew from conventional or LibertyLink^® (glufosinate-resistant) corn trait products. Liberty^® herbicide, containing glufosinate as the active ingredient, can be used to help manage volunteer corn that grew from conventional or glyphosate-resistant corn products. However, Liberty^® herbicide may not be effective on volunteer corn that contains the LibertyLink trait. The volunteer corn’s herbicide resistance is an important factor when considering control options.

Gramoxone^® herbicide, containing the active ingredient paraquat, can also be used for preplant control of emerged volunteer corn. However, much like glufosinate, paraquat is a contact herbicide and may not provide complete control if the seed has not germinated. The addition of a photosynthesis-inhibiting herbicide (metribuzin, linuron) in a tank mixture with Gramoxone^® herbicide is recommended to help improve the control of volunteer corn and provide residual weed control. Refer to herbicide product labels for use restrictions when applying prior to soybean planting.

Herbicide Resistance Traits Associated with Insect Protection Traits

The following herbicide resistance technologies are included in the listed traits for insect protection.

• Roundup Ready^® Corn 2 Technology (glyphosate resistant)
  • VT Double PRO^® corn
  • Trecepta^® corn
  • VT4PRO^™ corn
• Roundup Ready^® Corn 2 Technology and LibertyLink^® Technology (glufosinate resistant)
  • SmartStax^® corn
  • SmartStax^® PRO corn

Post Emergent Control Options

Postemergence grass herbicides in group 1, known as ACCase inhibitors and which contain active ingredients such as clethodim (e.g., Select Max^®) or fluazifop (e.g., Fusilade^® DX) can control emerged volunteer corn in soybean, either before planting or after the crop has emerged. These products can be tank mixed with permissible Roundup^® brand agricultural herbicides to help broaden the spectrum of weeds controlled. Always read and follow product labels for recommended additives.

It is important to be timely with herbicide applications to help limit volunteer corn competition to soybean. Delaying applications may lead to soybean yield loss and lead to more time being needed to control volunteer plants. Always refer to herbicide product labels for recommended application rates and additives, approved tank mixtures, use instructions, precautions, and restrictions.

Sources

¹Alms, J., Moechnig, M., Vos, D., and Clay, S. 2016. Yield loss and management of volunteer corn in soybean. Weed Technology. 30(1):254–262. https://doi.org/10.1614/WT-D-15-00096.1

²Jhala, A. and Rees, J. 2018. Got volunteer corn? Adapted from Cropwatch. Farm Progress and the University of Nebraska-Lincoln Extension. https://www.farmprogress.com/crops/got-volunteer-corn-

³Nicolai, D., Stahl, L., and Gunsolus, J. 2018. Managing the potential for volunteer corn in 2019. Farm Progress and the University of Minnesota Extension. https://www.farmprogress.com/crops/managing-the-potential-for-volunteer-corn-in-2019

Web sources verified 03/08/25. 1323_101321