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Volunteer corn bad for crop ecosystem

Volunteer corn growing in soybean fields that originated from Bt corn the year before may be much more onerous than it looks. One possible negative effect is obvious: soybean yield loss. The other downside is more subtle, but could be just as damaging. It relates to promoting insect resistance.

Fortunately, you can use grass herbicides to remove glyphosate- or glufosinate-resistant corn in soybeans. But if you don’t get a complete kill, there could be consequences.

Bill Johnson, Purdue University weed control specialist, says studies over the past two seasons determined how much volunteer corn could affect yield potential in soybeans. In a 2008 study, yield was significantly lower if two plants were present per square meter. Yield dropped another level of significance at four plants per square meter. In a 2009 study, it took four plants per square meter to produce significantly different yield performance.

Obviously, there’s variability from year to year. But the point is valid — a modest stand of volunteer corn chews up yield potential.

The even tougher problem is in corn after corn, Johnson observes. If the original hybrid that generates volunteer corn happened to be a quad-stack, with resistance to both glyphosate and gluforsinate, then there’s virtually no effective way to remove it.

Key Points

• Soybean yields can suffer from volunteer corn.

• Volunteer corn expresses sublethal dose of a protein that kills rootworms.

• Killing volunteer corn in corn is a formidable challenge.

Insect angle

Decreased yield due to competition with the growing crop is the short-term effect of volunteer corn. It could dig into your pocketbook this season. Aiding development of rootworm beetles resistant to Bt corn could be the long-term effect.

“Entomologists noted that volunteer corn where Bt rootworm corn was planted the spring before was chewed up by larvae in some cases after rootworms hatched,” says John Obermeyer, a Purdue University entomologist. “So they started checking. They soon discovered that the protein that controls rootworms is not expressed in similar levels in the volunteer corn as in the original crop.”

Christian Krupke, a Purdue Extension entomologist, studied how potent various traits were in volunteer corn. Tolerance to Roundup carries over at about 87% in the volunteer crop. But the Bt trait carries over to about 64% of the volunteer plants.

Sublethal dose

What that means, Krupke explains, is that volunteer corn contains what may be a sublethal dose of the protein that kills rootworm larvae. It sets up a situation that could speed development of resistant rootworm beetles.

“We don’t want beetles surviving in fields where larvae feed on volunteer corn roots leaving the field later,” Obermeyer stresses. “The bottom line is that you really need to control volunteer corn by mid-June. That’s typically the latest date when you can kill the plant and still prevent rootworm larvae from developing into adults.”

The disturbing question for entomologists is the same as the disturbing question for weed control specialists. How do you kill volunteer corn in continuous corn?

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MAKE HIM HOMELESS: John Obermeyer doesn’t want to see this young corn rootworm larva find a home on volunteer corn. That could lead toward resistance someday. Photo provided by Purdue University

This article published in the March, 2010 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.