Volunteer Corn

Volunteer Corn

August 7, 2009

If soil moisture loss and yield reductions weren't enough reason to provide timely control of volunteer corn, researchers have now found another motivator. A recent Midwest study suggests that rootworm feeding of Bt volunteer corn could lead to the development of corn rootworm resistance to this technology.

In the most recent Agronomy Journal, four researchers from Purdue University reported on a study where volunteer corn plants in eight soybean fields across northern Indiana were sampled and tested for the presence of glyphosate resistance (GR) and the Bacillus thuringiensis protein, Cry 3Bb1, the Bt protein active against corn rootworm larvae.

These soybean fields had been planted the year before to a GR Bt corn hybrid expressing the MON88017 event developed and marketed by Monsanto. Between 81 and 141 volunteer corn plants were collected from each soybean field. The roots were washed and rated for rootworm feeding using the 0-3 root injury rating scale.

Overall, 86.95% of the volunteer corn plants tested positive for glyphosate resistance, 64.93% tested positive for Cry 3Bb1, and 60.28% tested positive for both traits. There were no significant differences in root injury ratings between plants expressing the Cry3Bb1 toxin and those that did not. However, there was significant root feeding injury (> 0.5 on 0-3 scale) on 25.78-40.97% of the volunteer corn plants sampled. The authors suggest that this most likely was due to reduced levels of Cry3Bb1 protein in the volunteer corn.

The authors note that corn rootworm larvae feeding on volunteer corn expressing a sub-lethal dose of Cry3Bb1 may be selected for resistance to the Bt protein, and that this could increase the risk of resistance to Bt developing in corn rootworms. They also note that survival of rootworms on volunteer corn from Bt hybrids could be important in continuous corn systems as well as corn-soybean cropping systems.

Remove Volunteer Corn Early

Impact of Volunteer Corn on Soybean Yield

Volunteer corn is a highly competitive weed in soybean. It grows taller than soybean early in the season, shading surrounding soybean plants and competing for nutrient and water resources. The yield effect of volunteer corn depends on its density.

South Dakota State University conducted studies in 2007 and 2008 where they established volunteer corn densities of 0 to 17,800 plants/ac in soybean (Alms et al. 2008). The corn was allowed to compete for the entire growing season and soybean yields were measured. A density of 5,000 volunteers/ac reduced soybean yield approximately 20% — a 12 bu/ac yield loss in 60 bu/ac soybean. With a density of 5,000 plants/ac, there would be a volunteer corn plant every 3.5 feet of row.

Researchers in Minnesota (Andersen et al. 1982) and Illinois (Becket and Stoller 1988) conducted studies in the 1980s to measure the effect of clump density on soybean yield. Clumps of corn (7 to 10 plants per clump) were established at different densities. Depending on the location and year, soybean yield was reduced 1% for every 75-115 clumps per acre. A clump of corn will be more competitive than an individual plant.

Many soybean growers do not control volunteer corn early enough. Many fields with severe volunteer corn infestations have corn 24-36 inches tall and are free of other weeds, suggesting that at least one application of glyphosate was applied early in the season to soybean. We expect that some growers do not notice the volunteer corn before the first glyphosate application. In other cases, growers choose to wait until the second application because they know that some volunteer corn will germinate after the first glyphosate application, and they only want to apply a postemergence grass herbicide once. However, not controlling volunteer corn with the first application of glyphosate poses several risks/costs:

  1. Even though all the weeds in a field may be dead at the end of a year, allowing them to grow for a long time along with the crop costs you yield. Let's use volunteer corn as an example and use WeedSOFT to estimate yield losses. Given a moderate density of 1300 volunteers/ac, we would expect a yield loss from season long competition to be about 4.5 bu/ac in 60 bu/ac soybean. If the volunteer corn was removed by the V3 growth stage, yield loss from early season competition would be about 0.2 bu/ac. However, if the corn was allowed to compete until flowering, the yield loss would be 2 bu/ac. At $10/bu soybean, the delay would cost $18/ac of potential yield.

  2. By delaying the application of the postemergence grass herbicide from 6-12 inch corn to 12-24 inch corn or taller, higher herbicide rates are required. As a result, the cost of control increases by 50% or more.
  3. Delaying the postemergence herbicide application reduces the effectiveness of rotating between corn and soybean by allowing disease and insect pests to survive in the field. In addition, it may jeopardize the insect resistance management strategies that have been adopted by the industry to reduce the risk of corn rootworms from developing resistance to Bt.

Managing Volunteer Corn Infestations

Table 1. Labeled rates of ACCase herbicides to control volunteer corn, based on plant size.


Active Ingredient

Corn Size


Assure II or Targa


0-12 in
12-18 in
18-30 in

4 oz
5 oz
8 oz

Fusliade DX


0-12 in
12-24 in

4 oz
6 oz


Fluazifop + Fenoxaprop

0-12 in
12-24 in

4 oz
6 oz

Select or Arrow


0-12 in
12-24 in

4 oz
6 oz

Select Max


0-12 in
12-24 in
24-36 in

6 oz
9 oz
12 oz

The best way to manage volunteer corn infestations is to avoid them. Select corn hybrids that are not likely to lodge and set combines to minimize harvest losses.

Volunteer corn is less of a concern in no-till fields than in tilled fields. In tilled fields, tilling in the fall may allow the corn to germinate and be killed by a frost, or to emerge early enough in the spring to be controlled by a second tillage operation or an effective herbicide application before soybean planting.

If the volunteer corn comes from a non-Roundup Ready® hybrid, the simplest control method is to apply a glyphosate herbicide (in Roundup Ready soybean).

If the volunteer corn comes from a Roundup Ready hybrid, it is critical to add a postemergence grass herbicide (ACCase inhibitor) to a glyphosate application. Many of the available products and use recommendations are listed in Table 1. Two non-ACCase inhibitor herbicides can be applied postemergence to volunteer corn. Ignite 280 can be used in LibertyLink Soybean, and provides fair to excellent control, depending on the corn size and environmental conditions at application. Products that contain imazethapyr (Pursuit and Extreme) will control 70-80% of the volunteer corn when it is small.


For more information on the research referenced in this article, see:

Bob Wright
Extension Entomologist
Mark Bernards
Extension Weed Scientist
Lowell Sandell
Extension Weed Science Educator

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A field of corn.