While conditions during much of April were conducive for planting, these same conditions were NOT conducive for good performance of soil-residual herbicides. Many surface-applied herbicides received neither timely precipitation nor mechanical incorporation to move the applied herbicide 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. The amount of precipitation required to move the herbicide into the soil and how soon after application the precipitation is needed are difficult to define and can vary by herbicide, but surface-applied herbicides generally require 0.5 to 1.0 inch of precipitation within 7 to10 days after application for optimal incorporation. Factors such as soil condition, soil moisture content, residue cover, and the chemical properties of the herbicide influence how much and how soon after application precipitation is needed. Weed control could be reduced if precipitation less than these amounts was not received until later than 10 days after application.
A few callers have indicated broadleaf and grass weeds have emerged in fields where the soil-residual herbicide was applied to the surface with no subsequent precipitation or mechanical incorporation. Will the soil-residual herbicide control these emerged weeds once sufficient precipitation is received to move the herbicide into the soil solution? The simple answer is that there is not a simple answer. There are a few instances where the soil-residual herbicide might “reach back” to control small, emerged weeds following precipitation. For example, we have observed emerged velvetleaf (up to about 1-inch tall) turn white following precipitation in research plots treated with isoxaflutole. However, there are many other instances in which the emerged weeds are likely to survive a recently “activated” soil-residual herbicide. Annual grass weeds that have emerged in fields treated with soil-applied chloroacetamide herbicides will not be controlled by the chloracetamide and will have to be controlled with a postemergence herbicide. It is advisable to take steps to control emerged weeds taller than 1 inch, rather than wait several additional days after precipitation to see whether or not the soil-applied herbicide will control them. Even though the soil-applied herbicide might not control emerged weeds following the next precipitation event, it could still provide residual weed control once sufficient precipitation is received.