Corn Size Restriction for POST Herbicides

June 14, 2008

By Mike Owen and Bob Hartzler, Department of Agronomy

Weeds have continued to grow thus making the application of POST herbicides imperative despite the slow development of the corn crop. However, it is also important to follow the size restrictions that exist on the herbicide label, particularly given that corn is under stress due to the poor growing conditions. 

Recognize that taller corn is generally more sensitive to potential herbicide injury, particularly when nozzles are applying the herbicide directly into the whorl of the plant. If possible, herbicides should be applied directed POST whenever corn exceeds 12 inches tall in order to improve coverage of weeds and reduce exposure of the corn. Furthermore, be particularly cautious about tank mixing herbicides. Follow the most restrictive label and be sure that the adjuvants are appropriate for all herbicides in the mixture. 

In addition to size restrictions, consider the impact of late applications on the potential for carryover injury to rotational crops. For example, products containing atrazine state only to rotate to corn or sorghum if applied after June 10.

Table 1.   Application restrictions for selected corn postemergence herbicides1. 

1 Where both height and leaf stage are provided, use most restrictive classification.

2Not all POST herbicides registered for corn are included; refer the label for specific application restrictions.


Mike Owen is a professor of agronomy with research and extension responsibilities in weed management and herbicide use. Bob Hartzler is a professor of weed science with extension, teaching and research responsibilities.

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