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Article originally posted on the Bulletin.

Decades ago it was very common for the majority of corn and soybean acres in Illinois to be treated with one or more soil-residual herbicides before crop/weed emergence. During the 1980s, commercialization of broad-spectrum, postemergence herbicides began the shift away from widespread use of soil-residual herbicides; products such as Basagran, Classic, Accent and Pursuit contributed to the early adoption of postemergence weed control programs. The era of total postemergence weed control reached its zenith following the widespread adoption of glyphosate-resistant crops and the concomitant use of glyphosate. However, the evolution of glyphosate resistance in several weed species has heralded a shift back to the use of soil-residual herbicides, especially in soybean.

Soil-residual herbicides can provide many weed management benefits, but several factors influence their effectiveness. Factors such as product selection, application rate, and when the herbicide is applied in relation to crop planting are largely under the control of the farmer, whereas soil moisture content at the time of application and the interval between application and the first precipitation event are factors largely beyond the farmer’s control.

In order for a soil-applied herbicide to be effective, the herbicide needs to be available for uptake by the weed seedling (usually before the seedling emerges, but some soil-applied herbicides can control small emerged weeds under certain conditions). Soil-applied herbicides have an Achilles heel: when applied to the soil surface they require mechanical incorporation or precipitation to move them 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 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. If no precipitation is received between application and planting, mechanical incorporation, where feasible, can still help move the herbicide into the soil solution.