Can you reverse herbicide resistance?

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Once you have herbicide-resistant biotypes in your fields, it is unlikely that they will revert to susceptible biotypes within a farming generation, even if you stop using the herbicides that they resist. This is especially true if the resistant biotype becomes established for several years and is producing seed annually. Once resistance is established, fields typically will have a mixture of susceptible and resistant biotypes (we’ll refer to them as S and R biotypes). The ratio of these individuals could change over time depending on herbicide use. With repeated use of the same herbicide group over time, the population of a resistant biotype will increase and be the dominant biotype.

There is a phenomenon in plants known as a “fitness penalty,” wherein the resistance trait comes with inferior growth and/or reproduction. But in most instances, herbicide resistance does not appear to introduce a substantial fitness penalty. As a result, even if the problem herbicide group is no longer used, and a portion of the susceptible seedbank remains, the population will likely continue to be a mixture of R and S biotypes. Depending on the species and other management inputs, weed seeds can persist in the soil for several years. Seeds buried deeper in the plow layer tend to remain viable longer than seeds closer to the soil surface. Of course, seeds buried below the depth they are capable of emerging pose little risk, unless they are brought back near the soil surface with tillage. The seeds of some species, such as giant foxtail and other annual grasses, generally survive for only a few years. Others with harder seed coats, such as common lambsquarters and velvetleaf, might survive for decades in the soil under the right conditions. Depending on the weed species, this could impact the longevity of both R and S biotype seeds in the soil. If there are no weed seeds produced at the end of the growing season, the size of the weed seed bank will decline as seeds die, and the frequency of R and S biotypes should remain the same.