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.