While recent rainy conditions have prevented farmers from using soil-applied herbicides, one Purdue Extension specialist reminds corn growers that many products can be applied postemergence.
"Almost all chloroacetamide/atrazine premix products can be applied to emerged corn," said weed scientist Bill Johnson.
But he also said growers need to take precautions to prevent plant injury.
"Atrazine and chloroacetamide injury is more common in cool, cloudy weather and wet soils because these conditions slow a corn plant's ability to metabolize or detoxify herbicides," Johnson said. "Corn will be stressed after coming through a cool, wet period and will be more susceptible to herbicide injury."
Once corn has emerged, farmers should be careful not to apply chloroacetamide/atrazine premixes in nitrogen solutions. Most labels state that atrazine premixes should be applied only with water once corn has emerged.
"Nitrogen solutions are effective in promoting herbicide uptake and causing necrosis on leaves by themselves, resulting in the burning and yellowing injury we occasionally see," Johnson said. "A few products do allow postemergence applications in nitrogen solutions, but growers should consult the label if they have questions about a specific product."
Tank mixes of 2,4-D with chloroacetamide/atrazine premix applied postemergence also can injure crops if applied improperly. Acetochlor (Harness, Surpass, Volley and others) labels indicate that 2,4-D should not be applied within 7-14 days before or 3-5 days after planting, but it should be applied before crop emergence.
Johnson said applications within 7-14 days before planting can injure corn by being washed down into the corn seed germination zone, or seed furrow. Applications after planting can cause injury if the combination contacts corn foliage.
"If in doubt about the crop injury potential of a specific chloroacetamide/atrazine premix, consult the label to see if it is allowed, or do not tank-mix 2,4-D with chloroacetamide/atrazine premixes and apply to emerged corn," he said.
Should farmers have fields where corn is yellow and stressed from cool, wet weather, they should wait a few days to allow corn to recover from cold stress before applying herbicides.
Treatments containing atrazine will control many small, emerged broadleaf weeds. But, according to Johnson, emerged grass weeds tend to be more of an issue.
"Atrazine has activity on emerged grasses, and it is most effective when applied at high rates to very small grasses," he said. "Products containing rimsulfuron will provide some foliar residual control of grass weeds and can be mixed with either the atrazine premixes or with glyphosate or glufosinate in Roundup Ready or Liberty Link corn.
"Larger grasses will require the addition of postemergence herbicides."
For a table with a full list of corn products that have both some burndown capability and can be applied postemergence, visit the Purdue Extension weed science website at http://www.btny.purdue.edu/weedscience/2011/ColdWetCorn.html.