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 if too many corn plants act as weeds and competes with the crop. The other downside is more subtle, but could be just as damaging in the long run.
Fortunately you can use grass herbicides to remove glyphosate or glufosinate resistant corn out of soybeans the following season. But if you choose to let if it grow or don't get a complete kill, there could be consequences.
Bill Johnson, Purdue University weed specialist, reports that studies over the past two seasons attempted to determine just how much volunteer corn could affect yield potential if left growing in soybeans. In a 2008 study yield was significantly lower if 2 plants per square foot were present. Yield dropped another level of significance at 4 plants per square foot. In a 2009 study, it took 4 plants per square foot to produce significantly different yield performance vs. a clean soybean field. Obviously, there's variability from year to year. But the point is valid- a modest stand of volunteer corn left to compete with soybeans chews up yield potential.
The even tougher problem is in corn after corn, Johnson observes. If the original hybrid that generates the volunteer corn the next year happened to be a quad-stack, with resistance to both glyphosate and gluforsiante, then there's virtually no effective way to remove volunteer corn in a stand of continuous corn.
Decreased yield due to competition with the growing crop is the short-term effect of volunteer corn. Aiding development of resistant beetles to Bt corn could be the long-term effect.
'Entomologists noted that volunteer corn where Bt rootworm corn was planted the spring before were 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 as strongly 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 is roughly 64% effective.
What that means, Krupke explains, is that volunteer corn contains what may be a sub-lethal 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," Obermeyer stresses. "The bottom line is that you really need to control volunteer corn by mid-June. That's typically when rootworm larvae start emerging."
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 stands? So far, there doesn't appear to be a satisfactory answer to that question.