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Herbicide Mixtures for Resistance Management? Well, It Is Complicated!

Posted by Albert Adjesiwor, University of Idaho | October 24, 2022

If you have ever listened to any presentation or read any scientific, extension, or fact sheet on herbicide management, chances are that you would have heard about the use of herbicide mixtures for resistance management. For years, weed scientists continue to recommend that farmers utilize herbicide mixtures to slow down the rate of herbicide resistance. Well, this is backed by science and practice. We know that one of the main causes of herbicide resistance is the reliance on one herbicide site of action over and over again. In other words, if you spray any herbicide (be it glyphosate, 2,4-D, dicamba, etc.) over and over again, you will eventually select for resistance to that herbicide. The rationale behind using herbicide mixtures for resistance management is that any weed(s) that escape control from herbicide A will be controlled by herbicide B, thereby preventing any weeds from escaping control and producing seeds. The key thing here is that, both herbicides A and B must be effective on the target weed. We know from experience that this is a very high bar. There’s also the problem of economics. If herbicide A will provide 90% control of the target weed, why should I spend additional money on an extra herbicide? This is a question probably every weed scientist had to answer at some point or maybe on multiple occasions. More than two decades ago, one researcher pointed out that farmers are businessmen who take decisions in the context of their own enterprises. Thus, unless they can be convinced that herbicide-resistant weeds will significantly affect their profits, they won’t adopt any proactive measures to prevent or delay the evolution of herbicide resistance (Orson, 1999). This was probably not the first time someone pointed this out, but I will say this rings true today just as it did two decades ago. It is common knowledge that the costs of weed control programs recommended by academics and professionals tend to be higher than standard farmer’s practices. For example, one study has found that weed control costs were about 30% higher for best management practices recommended by academics compared to standard practices used by farmers (Edwards et al. 2014).

In a recent publication, Kniss et al. (2022) saw that effective herbicide mixtures that incorporate different sites of action for resistance management were at least 2-fold as expensive as one effective herbicide site of action (Figure 1). In other words, if the focus of weed control is on proactive resistance management, it would cost twice as much each year to control weeds. Is all hope lost? Definitely not! What is clear, though, is that we cannot use herbicides alone to manage resistance. We are lucky in the PNW that we have very diverse cropping systems. Let us continue to leverage these diverse crop rotations to slow down the rate of herbicide resistance and manage weeds that are already resistant to certain herbicides.

(Select photo to enlarge)

Figure 1. The cost of implementing effective herbicide programs for proactive resistance management. Adapted from Kniss et al. (2022).

The second thing we can do is to start thinking ahead as opined recently (Swanton 2022). We know from experience that resistance is not a question of IF, but a question of WHEN. Maybe this is the time for us to start research projects focusing on proactive recommendations for reactive resistance management. This is confusing, I know. The point is, if our best options for managing weed “A” in wheat are herbicides “X” and “Y”, we should start projects evaluating the future scenarios where those herbicides are no longer effective. What are the alternatives? In fact, this will even aid our messaging for proactive resistance management. In effect, we are letting stakeholders know that the cost of not doing anything will be very high in the future. I believe this was what Orson (1999) meant by saying that “unless we can convince farmers that herbicide-resistant weeds will significantly affect their profits, they won’t adopt any proactive measures to prevent or delay the evolution of herbicide resistance.”

Additional Reading

Edwards CB, Jordan DL, Owen MD, Dixon PM, Young BG, Wilson RG et al. 2014. Benchmark study on glyphosate-resistant crop systems in the United States: economics of herbicide resistance management practices in a 5 year field-scale study. Pest Manag Sci. 70(12):1924-9 Available from:

Kniss AR, Mosqueda EG, Lawrence NC, Adjesiwor AT. 2022. The cost of implementing effective herbicide mixtures for resistance management. Adv Weed Sci. 40(Spec1):e0202200119.;40:seventy-five007

Orson JH. 1999. The cost to the farmer of herbicide resistance. Weed Technol. 13(3):607-11. Available from:

Swanton C. 2022. Weed science and the clock of the long now. Weed Science, 70(4), 369-369. doi:10.1017/wsc.2022.25

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