Understanding The Three Key Principles Of Glyphosate Resistance

A weed’s potential for developing glyphosate resistance is primarily guided by three factors: weed biology, intensity of glyphosate use, and glyphosate rate, according to Facts About Glyphosate Resistant Weeds on the Purdue Extension website. Here is a look at those three principles:

Weed Biology. It is likely that certain weed species have greater genetic diversity, so there is a greater risk that they will develop herbicide resistance. Weed species that have already developed resistance to other herbicides may have a greater probability of developing glyphosate resistance. Species that may be prone to glyphosate resistance based on resistance to other herbicide modes of action include pigweed species (including waterhemp and Palmer amaranth), common lambsquarters, common and giant ragweed, kochia, and ryegrass. Since the trait for glyphosate resistance can spread by pollen or seed, the spread of resistant populations will be faster for some weed species than others.

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Intensity of Glyphosate Use. Increasing the intensity of glyphosate use (frequency and number of acres treated) increases the probability of selecting an herbicide-resistant plant. With continued glyphosate use, the number of resistant plants will continue to multiply and create a resistant population. Herbicides do not cause the mutations that result in resistance. Rather, an extremely rare genetic trait that allows a weed to survive glyphosate may exist in the natural population. It is more likely to be found and increase when glyphosate is used frequently.

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Glyphosate Rate. It is less clear how rate affects glyphosate resistance development in weeds. Several known glyphosate-resistant weeds require eight to 10 times more glyphosate to be controlled than the normal, sensitive biotypes. This level of resistance means that labeled glyphosate rates will not control these weeds, and that making applications at labeled rates probably will not prevent resistance. There are other cases of glyphosate-resistant weeds that appear to have resistance at two to four times greater than normal biotypes. It is unknown if reduced glyphosate rates contributed to these plants becoming resistant, or if resistance would have occurred at labeled rates. Regardless, it is wise to use glyphosate at labeled rates to ensure good control and prevent hard-to-control weeds from increasing in any field.

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