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Is your herbicide still working?

If you planted corn earlier and quicker than you usually do, your crop may be subject to greater-than-normal early weed pressures.

Farmers made massive planting progress over the last 2 weeks in some key Corn Belt states. Though it's got the overall pace way out in front of normal progress, it also may have gotten too far ahead of herbicide and nutrient applications for those products to effectively work, both on their own and together, says Iowa State University (ISU) Extension agronomist Bob Hartzler.

"The rapid pace of planting in late April followed by rain has resulted in many fields having emerged corn before preemergence herbicides and nitrogen applications have been completed," Hartzler says in a university report alongside fellow ISU agronomist John Sawyer. "Of particular concern are no-till fields where planting was completed prior to killing emerged weeds."

This calls for quick action in those fields where you think preemerge herbicides may have not done their job, for a couple of reasons. First, Hartzler says corn typically can stave off early weed damage until about the V3 growth stage...if it's planted in a clean seedbed. If you've got weed pressures this early in the season, though, "yields can be impacted much sooner," he says.

Then, there are weeds that preemerge herbicides control well when they're small in size but lose efficacy as they get larger. Failing to get a start on control early on can cause major issues later on.

"Several of the early-emerging weed species are some of the most difficult to control once they get a little size to them. These tough to control weeds include horseweed/marestail, giant ragweed and lambsquarters," Hartzler says.

When you do get out to knock down the weeds that escaped early preemerge applications, make sure the herbicide you're using will pack enough postemergence punch to get the job done. "Most preemergence herbicides also are registered for application after corn has emerged, however, their activity on emerged weeds varies," Hartzler says. "If weeds are present, determine whether the postemergence activity of the residual herbicides is sufficient to control the weeds present in the field or if an additional herbicide with better postemergence activity is needed."

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