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 Midwest study suggests that rootworm feeding of Bt volunteer corn could lead to the development of corn rootworm resistance to this technology, University of Nebraska agronomists reported in a recent issue of CropWatch.

In an Agronomy Journal article, 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 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 glyphosate-resistant 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-to-3, root-injury rating scale.

Overall, 87% of the volunteer corn plants tested positive for glyphosate resistance, 65% tested positive for Cry 3Bb1 and 60% 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 (greater than 0.5 on 0-to-3 scale) on 26% to 41% 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 sublethal 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

Many soybean growers do not control volunteer corn early enough, the Nebraska agronomists report. Many fields with severe volunteer corn infestations have corn 24 to 36 inches tall and are free of other weeds, suggesting that at least one application of glyphosate was applied early in the season to soybeans.

"We expect that some growers do not notice the volunteer corn before the first glyphosate application," says Mark Barnards, University of Nebraska weed scientist. "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 post-emergence grass herbicide once."

However, Barnards says not controlling volunteer corn with the first application of glyphosate poses several risks and associated 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 yield. Given a moderate density of 1,300 volunteers per acre, Barnards expects a yield loss from season-long competition to be about 4.5 bushels per acre in 60-bushel-per-acre soybeans. If the volunteer corn was removed by the V3 growth stage, he says yield loss from early season competition would be about 0.2 bushels per acre. However, if the corn was allowed to compete until flowering, the yield loss would be 2 bushels per acre. At $10-per-bushel soybeans, the delay would cost $18 per acre in potential yield.
2. By delaying the application of the post-emergence grass herbicide from 6- to 12-inch corn to 12- to 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 post-emergence herbicide application reduces the effectiveness of rotating between corn and soybeans by allowing disease and insect pests to survive in the field, Barnards adds. 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

The best way to manage volunteer corn infestations is to avoid them, says weed scientist Lowell Sandell. 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," Sandell says. "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 soybeans, Sandell says.

If the volunteer corn comes from a Roundup Ready hybrid, it's critical to add a post-emergence grass herbicide (ACCase inhibitor) to a glyphosate application.

Two non-ACCase inhibitor herbicides can be applied post-emergence to volunteer corn. Ignite 280 can be used in LibertyLink soybeans, 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% to 80% of the volunteer corn when it's small.