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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.”