Acetochlor binds less tightly to soil colloids than S-metolachlor, and thus would be expected to require less rain for activation. Research at the University of Illinois showed relatively small differences in the rainfall requirement between two Group 15 herbicides (Table 1). Chemical properties of the herbicide, primarily the affinity of the herbicide for soil colloids, affect how much rain is required for activation. If the upper inch or two of soil is dry, the quantity of rain for activation will be slightly higher since the soil first must be wetted before water will move the herbicide into the profile. It typically takes at least a half-inch of rain to activate the herbicide, meaning, moving it to the depth where germinating seeds are located. Most farmers rely on rain to move the herbicide into the soil profile. Preemergence herbicides need rain to be effective. Both factors can be adversely affected by limited rain early in the season. In addition, there must be sufficient water to keep the herbicide dissolved in soil water. To be effective, the herbicide must be located within the soil profile at the depth where weed seeds germinate (primarily the upper inch of soil). Preemergence herbicides kill weeds by being absorbed into the seed as the seed imbibes water. The potential for continued dry weather across the state raises concerns about the impact of dry soils on preemergence herbicides.
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