With the continued wet weather, weeds have continued to grow, therefore making the application of postemergence herbicides necessary despite the slow development of the corn crop. However, it is also important to follow the size restrictions that are explained on the herbicide label, particularly given that corn is under stress due to the poor growing conditions.
That advice comes from Mike Owen and Bob Hartzler, Iowa State University Extension weed control specialists. They offer the following guidelines.
Follow restrictions on herbicide labels
You need to 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. Go to www.extension.iastate.edu/CropNews/2008/061201.htm to view a table the ISU weed management specialists have compiled, listing the application restrictions for various corn postemergence herbicides.