No. 6 Article 4/May 2, 2008

Weed Control in Corn and Wet Field Conditions

The persistent rains across much of Illinois have delayed corn planting and, in many areas, have also delayed implementation of weed control practices. The planting delays caused by wet soil conditions make it likely that some percentage of corn fields were (or will be) planted before any herbicide has been applied. It's somewhat difficult to consider all possible weed control questions or scenarios that might currently exist, but the following are some items for consideration with respect to weed management in corn.

Possible Scenarios

No herbicide applied, corn not planted. Existing vegetation should be controlled before planting. This can be accomplished by either preplant tillage or herbicide application. Tillage would provide the shortest interval between the weed control practice and planting, but if fields are tilled on the wet side, larger weeds may survive the tillage operation and continue to be problems after the crop emerges. If you opt for a herbicide to control existing vegetation prior to planting, consider several factors:

No herbicide applied, corn has been planted. It is likely that planting delays caused by wet soil conditions have resulted in some corn fields being planted before a soil-residual was applied. If you still intend to apply a soil-residual herbicide (now preemergence instead of preplant), a couple of considerations follow:

Corn farmers should be aware that many, but not all, soil-applied corn herbicides can be applied after corn emergence. Not all of these herbicides will control emerged weeds, so additional management procedures (such as the use of a rotary hoe or the addition of a herbicide that has postemergence activity) may be needed in situations where weeds also have emerged. Farmers are also cautioned about the potential for enhanced corn injury if these products are applied during periods of crop stress, such as stress caused by excessive soil moisture or cool air temperatures. Table 1 summarizes information about postemergence applications of the more traditional soil-applied corn herbicides. Consult the respective product label for additional information, such as the need for spray additives.

Herbicide applied, corn not planted. These fields, especially where the herbicide application was made several weeks ago, are excellent candidates for scouting prior to planting. If weeds are present, you should consider controlling them before you plant. Why not just wait and spray after planting? That may be a feasible option, but the planting operation will likely injure some of the weeds, and they will need time to recover before being sprayed. Waiting to control existing weeds after planting is also gambling that the weather will cooperate and allow you to make the application before the existing weeds begin to adversely impact the crop.

Herbicide applied, corn has been planted. Whether you initially planned to use a soil-applied program for weed control or a soil-applied followed by postemergence herbicide program, remain vigilant for weed emergence. The heavy precipitation in many areas of the state may have moved some soil-applied herbicides deeper into the soil profile than is conducive for good weed control. The less-than-ideal growing conditions may also increase the likelihood of corn injury from some soil-applied herbicides.--Aaron Hager

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