University of Illinois

No. 12/June 12, 1998

Corn Herbicide Reminders

1. Note corn size limits on postemergence herbicide labels. Determining corn size by counting the number of visible leaf collars is a more reliable method than using a ruler (that is, measuring the height in inches). Height based on inches does not take into account corn internode length. The limits for broadcast over-the-top, as well as drop nozzle postemergence, herbicide applications are specified on the respective herbicide labels and appear in tabular form in Table 15, Chapter 2 (p. 76) of the 1998 Illinois Agricultural Pest Management Handbook.

2. Note weed size limits. Delayed postemergence herbicide applications will often translate into oversized weeds, but the recent periods of cool air temperatures may have limited weed growth. Unfortunately, the cool air temperatures may have also limited the corn's ability to rapidly metabolize postemergence herbicides, so injury symptoms may soon develop.

3. Minimize herbicide injury to corn. Using the proper herbicide, application rate, and spray-additive selection can all help reduce corn injury potential. Crops under stress (environmental, biological, or chemical) are often more susceptible to herbicide injury than "normal," nonstressed crops. Try to minimize postemergence herbicide applications when corn is in its rapid-growth phase.

4. Rootless corn? Cultivation may preclude the need for additional herbicide applications and may throw additional soil around the base of the plants. Extra soil at the base of plants may be needed to better support the plant as top growth increases.

Marshal McGlamery ( and Aaron Hager (, Crop Sciences, (217)333-4424