Preemergence herbicides provide very early weed control, and it is early weed competition that often has the greatest impact on crop yield. The reduction in weed density resulting from the use of preemergence herbicides also reduces the selection pressure for resistance evolution from subsequent postemergence herbicide applications. Preemergence herbicides often have a different mechanism of action than postemergence herbicides, which, as discussed in this previous Weeders of the West blog post titled, “I Can’t Say This Enough!”, also reduces the risk for the development of herbicide resistance.
Due to the popularity of postemergence herbicides over the past two or three decades, a lot of people have forgotten or never learned how to optimize the performance of soil-applied herbicides. Several factors including soil texture, soil organic matter content, soil pH, cation exchange capacity (CEC), soil moisture, soil temperature, soil disturbance by tillage, crop residue type, abundance, and distribution all can affect herbicide efficacy. Growers need to be aware of these factors when using preemergence herbicides.
As resistance to postemergence herbicides continues to increase, growers should consider incorporating preemergence herbicides into their weed control programs. Doing so will not only improve weed control, but it will also extend the useful life of the few effective herbicides that we still have.