Size Restrictions for Postemergence Herbicides in Corn

June 8, 2009

By Bob Hartzler, Department of Agronomy

While most farmers and applicators have good intentions to apply herbicides in a timely fashion, it isn’t uncommon for equipment limitations or weather to result in delayed applications. Nearly all postemergence herbicides have restrictions on how late in the season they can be delayed. 

The most common reason for the restriction is the reduction in crop tolerance to the herbicide as corn size increases. For some products the application timing is restricted to prevent herbicide residues from exceeding tolerance levels in the grain. Regardless of the reason, it is important to follow the restriction.

Timely applications not only avoid problems with label restrictions, but they also provide more effective control due to treating weeds at a more susceptible stage and at a time when the crop canopy does not interfere with coverage, and  also are more effective at protecting yields from early-season competition than late applications.

Bob Hartzler is a professor of weed science with extension, teaching and research responsibilities. He can be contacted by email at or phone (515) 294-1164.

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