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.
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 14, 2008. The information contained within may not be the most current and accurate depending on when it is accessed.