Issue No. 8, Article 7/May 18, 2007
Dry Soils and Soil-Applied Herbicides
In some areas of Illinois, preplant and preemergence corn herbicides have been on the ground for anywhere from a few days to several weeks without having received adequate precipitation to move the herbicides into the soil solution. Herbicide effectiveness can be significantly reduced when a soil-applied herbicide is sprayed on a dry soil surface with no incorporation (mechanical or by precipitation) for several days following application. How much rainfall is required to move the herbicide into the soil, and how soon after application is precipitation needed? While it's not possible to define an amount applicable to all possible scenarios, surface-applied herbicides generally require 1/2 to 1 inch of precipitation within 7 to 10 days after application for optimal "activation." Factors such as soil condition, soil moisture content, residue cover, and the chemical properties of the herbicide influence how much rain is needed and how soon after application it's necessary.
If weeds have begun to emerge before the herbicide has been moved into the soil solution, it may be time to consider additional management options. Certain soil-applied herbicides may still provide some control of emerged weeds if precipitation occurs soon, but if emerged weeds exceed 2 to 3 inches, a postemergence herbicide application may be necessary to control them. Don't wait too long to see if the soil-applied herbicide will control emerged weeds (especially if dry soil conditions persist), as these weeds are competitive with the corn.
Rotary hoeing can control small emerging weeds and give surface-applied herbicides some incorporation (though usually only minor). Rotary hoeing is most effective while weeds are still in the "white stage," following seed germination but prior to emergence. Once weed seedlings have emerged, the effectiveness of rotary hoeing is diminished, since the weed's rapidly developing root system helps "anchor" the plant. Hoeing is generally most effective when done at speeds of 8 to 12 miles per hour. A second rotary hoeing 7 to 10 days after the first might improve weed control. Hoeing may also aid crop emergence by breaking soil crusts that can develop after planting.--Aaron Hager