There are many different weeds and germination times that can cause problems for corn growers. The key to a successful weed control program is correctly identifying the weeds in your fields and selecting a herbicide program that best fits a specific weed infestation.
"Problem weeds we commonly find in the Corn Belt include grassy weeds such as foxtails, barnyardgrass and fall panicum," says Jeff Rectenwald, agronomist with Mycogen Seeds. "Broadleaf weeds, such as velvetleaf, lambsquarters, redroot pigweed and large-seeded weeds like giant ragweed and bur cucumber, continue to be major concerns as well."
He advises that before making a herbicide application, growers should know their weed spectrum and timing of weed germination in order to take advantage of their herbicide dollars. "I recommend farmers adopt an integrated pest management approach tailored for their specific weed issues. This includes scouting and knowing weed height before spraying postemergence herbicides."
Preemergence vs. postemergence
Farmers may employ different strategies for controlling weeds in corn, depending on type of weeds present, weed growth stage and stage of crop development. Most grassy weeds require a foundation herbicide application either at preplant or during planting.
"In corn, I recommend applying a preemergence foundation herbicide, like Keystone herbicide, to control grassy weeds and small-seeded broadleaf weeds until a postemergence herbicide can be used," says Rectenwald.
Broadleaf weed treatments are typically applied postemergence, but can be tank-mixed with a grass herbicide if the weed spectrum favors a one-pass program. In addition to a foundation herbicide, Rectenwald recommends a postemergence treatment with a broad-spectrum herbicide.
"Farmers who are considering switching to a postemergence-only program for corn should be aware that early weed competition in corn reduces yield faster than early weed competition in soybeans," advises Rectenwald. "Just because a postemergence-only treatment worked in a grower's soybean field, doesn't mean it will work in corn. Corn and soybeans differ in their growth, development and ability to compete."
Consult the experts in your area
He adds, "With soybeans, you can wait for weeds to germinate and then spray with a postemergence herbicide just before the canopy closes. This same application technique in corn won't work because corn is more sensitive to weed competition early in the season."
Rectenwald says glyphosate-based postemergence herbicides have no residual activity and frequently miss late-germinating weeds, such as bur cucumber and giant ragweed. "Growers should consider applying 1 pound to 1.5 pounds of an atrazine-based product preemergence to help with late-season weeds."
Rectenwald recommends growers consult with local resources such as trusted retailers or university Extension specialists to fine-tune their herbicide selections. "University Extension Web sites are excellent resources for growers to access updated weed control and herbicide information specific for their geography," he says. "Consult with local weed control experts to tailor a herbicide program to match the specific weed pressures on a field-by-field basis."