On a road trip last September through Minnesota, Wisconsin and Iowa, Jeff Gunsolus was struck by all the volunteer corn sticking up out of ripening soybean fields. “It seems that people aren’t valuing this as a weed-management issue," says the Minnesota Extension weed scientist. Many growers are not killing it soon enough as volunteer corn is very competitive with soybeans, he says.

The short-term penalty for ignoring just 2-4 volunteer corn plants/sq. meter (about 10 sq. ft.) is a soybean yield loss of 20%, according to research from Bill Johnson, Purdue University Extension weed scientist.

In a 50-bu. soybean crop, that loss is 10 bushels of $11 beans, or $110/acre.

But wait, there's more. Longer term, it could hamper insect-resistance management and blunt the usefulness of Bt technology, says Christian Krupke, Purdue Extension entomologist. Often, volunteer corn does not express the full dose of Bt toxins, so insects may survive exposure. Krupke and Johnson confirmed that Western corn rootworm beetles emerge from soybean fields after feeding on volunteer Bt corn roots. Exposure to sub-lethal rates of any toxin increases the risks of building the population of resistant insects.

Both yield competition and the threat of resistance give farmers good reasons to control volunteer corn early this season, Johnson says.

“We see volunteer corn every year to some extent,” says Jeff Nagel, field agronomist for Ceres Solutions, Crawfordsville, IN. Severe storms last year resulted in many pockets of lodged or downed corn in western Indiana and elsewhere around the Midwest, so there could be more volunteer corn than usual sprouting this spring.

A 2007 survey by the North Central Weed Science Society found that volunteer corn had become one of the top five weeds infesting Midwest soybean fields. About a quarter of the time, it was the only weed escape in the field, immune to glyphosate, the foundation of soybean weed management. The problem was worse in tilled fields than in no-till.

This trend has only increased as farmers plant more stacked corn hybrids tolerant to both glyphosate and glufosinate, Johnson and Krupke say. In 2011, herbicide-resistant corn was planted on 72% of U.S. corn acres, according to the USDA, and insect-resistant varieties on 65% of corn acres. “It’s an issue that certainly isn’t going away,” Johnson adds.

In 2008 and 2009, Johnson quantified the competitive effect of volunteer corn on soybean yields. In 2008, just two volunteer corn plants per square meter cut soybean yield by 11 bu./acre. Four plants per square meter cut yield by 18 bu./acre. In 2009, there was a significant yield loss with four plants per square meter.

Volunteer corn plants in soybean fields also feed rootworms. These plants are “like a bridge from one corn crop to the next,” says Ken Ostlie, University of Minnesota Extension entomologist.

“Five years ago, you’d expect zero rootworms to come out of soybean fields, because there was no corn there after glyphosate applications,” Krupke says. But eggs are always present in the soil. If volunteer corn grows with the soybeans, rootworm larvae “will survive in those little islands throughout the field.”

Krupke and Johnson found that Western corn rootworm beetles emerge from fields after feeding on volunteer Bt corn plants. In fact, about the same number of rootworms survived after chowing down on volunteer Bt corn roots as on non-Bt corn roots. This indicates that the Bt insecticide expressed by volunteer corn plants is less potent than commercial hybrids, and doesn’t suppress the pest, Krupke says.

Why is this a resistance-management problem?

Exposing rootworms to a less-than-fatal dose of Bt toxins “allows insects to survive that may not be resistant to the full dose of Bt toxin,” Krupke says. These insects pass on the genes that helped them survive, increasing the likelihood of offspring resistant to a higher dose. This can lead to insects that can withstand a full dose of the Bt toxin produced by corn plants.

Both herbicide and insect traits are carried over in the majority of volunteer corn plants, Johnson and Krupke found. About 87% of volunteer corn plants were resistant to glyphosate. The Bt protein Cry3Bb1 carried over to about 65% of volunteer corn plants. Both traits were expressed in about 60% of volunteer corn plants.

Controlling volunteer corn

Fortunately, it’s not hard to control glyphosate-tolerant volunteer corn in soybeans, says Nagel, the Indiana agronomist. “There are lots of grass herbicide options,” and more farmers are starting to tankmix them with their regular post-emergence glyphosate applications. Make sure you get the right adjuvant for the formulation you use, he adds.

Johnson tested Select, Fusilade and Assure individually and in combination with Pursuit, as well as Raptor alone. All provided excellent control of volunteer corn up to 12 in. tall.

Adding another grass herbicide to your glyphosate application will cost about $3-4/acre this year, Nagel says. Scout first to determine if volunteer corn densities exceed the yield-loss threshold of two to four plants per square meter, he suggests.

However, Minnesota’s Gunsolus notes that, in light of corn rootworm resistance issues, economic thresholds for removing volunteer corn from soybeans may be less relevant, and “removal would be justified based on the corn rootworm issue alone.”

Get rid of volunteer corn plants early, ideally by mid-June, Johnson says, both to minimize soybean yield loss and to shorten the amount of time that rootworms have to feed on Bt roots. It’s best to spray volunteer corn before it reaches 12 in. You’ll get more complete control and you can use lower herbicide rates. “We suggest putting a grass herbicide in with your first post-emergence Roundup application.”

Volunteer corn emergence is often extended, and the plants grow slowly early in the spring, so growers tend to delay treatment until the second post application. However, Johnson’s research found that volunteer corn is most competitive with beans when they emerge about the same time. Volunteer plants that emerge after the third trifoliate stage of soybean growth don’t cut into yields, he says. “Soybeans do a good job of smothering them.”

And keep in mind that if you wait to kill volunteer corn until July or August, Krupke adds, you give rootworm larvae time to develop into adults. “Although the weed is dead by harvest, you haven’t done anything to manage insect resistance.”

More glyphosate-resistant volunteer scenarios

Volunteer corn is prevalent in continuous corn rotations, where it’s harder to detect. Since most stacked corn hybrids are resistant to both glyphosate and glufosinate, there’s no practical way to eliminate volunteers, says Christian Krupke, Purdue University Extension entomologist.

These corn plants may produce ears and kernels, so growers won’t necessarily see a yield drop because of volunteer competition. The bigger question is whether these volunteer corn plants, which may also express variable levels of Bt toxin, allow more rootworms to survive and build up tolerance, Krupke says.

In another twist on the problem of herbicide-resistant volunteers: “In Minnesota, South Dakota and North Dakota, we also get questions about the reverse scenario – volunteer soybean in corn,” says University of Minnesota Extension weed scientist Jeff Gunsolus. Volunteer soybeans could present “another potential crop rotation bridge,” Gunsolus says, “but this time for soybean cyst nematode (SCN).” However, there hasn’t been any research on the impact of volunteer soybeans on SCN numbers or corn yields, he says.

For more on managing volunteer corn in soybeans go to: http://bit.ly/xL3dQ1.

For managing volunteer soybeans in corn, go to: http://bit.ly/xy3pZi.