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A Sobering Analysis of Herbicide-Resistant [HR] Weed Development

It is widely accepted that one of the most if not the most challenging production problems facing crop producers is that of weeds continuing to evolve resistance to herbicides. Thus, all parties–e.g. those that study weed biology, those that develop weed control chemicals and biologicals, and those that apply available weed control products–must always take this into account when thinking about weed control in crops.

Dr. Aaron Hager, Extension Weed Science/IPM Specialist in the Dept. Of Crop Sciences at the Univ. of Illinois, provides a sobering assessment of the future of weed control in the face of the continued development of HR weeds in his recent article titled “Recommendations to Manage Herbicide-Resistant Weeds: It’s Not as Easy as Some Believe”. Points to consider based on the content of his article follow.

•    Our understanding of how and why weeds are evolving various resistance mechanisms is low.

•    If the understanding of how and why resistance mechanisms evolve is minimal, how then can sound herbicide recommendations for weed control be developed?

•    A common element of recommendations to slow the development of HR weeds has been to diversify the herbicide modes of action [MOA] to which weeds are exposed.

•    There are few quantitative data to support the recommendation of using tank mixtures and herbicide rotations to achieve MOA diversification.

•    There is research to support that simply rotating herbicide MOA’s may actually increase the frequency of HR weed development in some weed species. However, there is also research to support that using more than two MOA’s per application in tank mixtures slows HR development in some weeds.

•    There is concern among many weed scientists that some practitioners want to use herbicides to solve a problem caused by using herbicides.

•    Weed species that are resistant to foliar-applied herbicides will also be resistant to soil-applied [residual] herbicides from the same herbicide group or groups.

•    There is little to no knowledge about how current tactics that are used to mitigate HR weed development–e.g. adding residual herbicides to a weed management program, layering residuals with POST herbicide applications, tankmixing multiple herbicides–will affect the future evolution of herbicide resistance mechanisms in weeds.

•    At this time, there is only one certainty, and that is if there are no weed seeds produced in a crop field at the end of the growing season, then there will be no change in the frequency of any resistance mechanism.

•    There is no doubt that herbicides will continue to be valuable tools in the weed control/management toolbox, but it is also important to realize that additional non-herbicide weed control tactics must be used as well so that there is no weed seed production during any growing season.

Soybean producers in the Midsouth should assume that it is not “if” a weed species will become resistant to a particular class of herbicide, but “when”. That is why it is of paramount importance that non-herbicide measures such as cover crops and harvest weed seed control be used now rather than later as part of a weed control/management program in soybeans. In other words, do not wait for a weed or weeds to become resistant to a class or classes of herbicides, but rather use non-herbicide tactics such as those listed in the previous sentence to minimize weed seed replenishment of the soil weed seedbank. According to Dr. Hager, this is the only certainty that is currently available.

Composed by Larry G. Heatherly, May 2023,