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 hartzler@iastate.edu or phone (515) 294-1164.

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on June 8, 2009. The information contained within may not be the most current and accurate depending on when it is accessed.

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Bob Hartzler Professor of Agronomy

Dr. Bob Hartzler is a professor of agronomy and an extension weed specialist. He conducts research on weed biology and how it impacts the efficacy of weed management programs in corn and soybean. Dr. Hartzler also teaches undergraduate classes in weed science and weed identificatio...