Many producers may be in a position where they must unexpectedly change from a preemergence-based program for grass control in corn to a postemergence program due to the recent rains. Several products are available that provide effective control of emerged grasses in these situations. These products are based on the sulfonylurea herbicides nicosulfuron and rimsulfuron (Table 1). Nicosulfuron, the lone active ingredient in Accent, has little residual activity but has more foliar activity than rimsulfuron. Rimsulfuron has less foliar activity than nicosulfuron, but provides better residual control. The residual activity of rimsulfuron is not as good as provided by the acetamide herbicides (for example, Axiom, Dual II Magnum, Harness, Outlook, Surpass). Because the rimsulfuron is applied after weed emergence has begun, this product does not need to provide control as long as herbicides are applied near planting prior to crop and weed emergence.

Application windows and targeted foxtail heights of the products are provided in Table 2. Product selection should be based on crop and weed size and spectrum of weeds present in the field. The products with higher rates of nicosulfuron (Accent, Celebrity Plus, Steadfast) provide a slightly larger window of application due to better foliar activity. Several of the products include tank-mix partners that enhance control of broadleaf weeds.

In summary, several options are available to control emerged weeds in fields where planned preemergence herbicides could not be applied because of rain. As with any herbicide, crop tolerance with these products is affected by weather and other factors that may cause stress. If situations force applications to be made under less than ideal conditions, the herbicide treatments targeted primarily for grass control (Accent, Steadfast) should reduce the risk of crop injury compared with products including herbicides targeted at broadleaves. This approach will result in the need for a second application to obtain broad-spectrum weed control, but can lower the potential for crop injury.

Table 1. Composition of herbicides for postemergence grass control in corn.

^aEquivalent to 2.4 oz of Hornet.

^bEquivalent to 4 oz of Distinct.

Table 2. Recommended application timing for postemergence herbicides.

^aUse most restrictive corn measurement when determining corn stage, e.g., if corn was 16 inches tall but only had four visible collars, only Accent or Celebrity Plus should be used. The decision in this situation is based on the height rather than leaf number.