One of the most daunting challenges facing agronomic crop production is the continuing evolution of weeds resistant to herbicides. The magnitude of herbicide resistance is best measured on a global scale. The most recent summary indicates 520 unique cases of herbicide resistance—encompassing 268 species—occur globally. Approximately 11–12 cases of unique resistance are discovered each year. In contrast, our understanding of how and why weeds are evolving various resistance mechanisms is evolving much slower. This introduces a somewhat precarious situation: if we do not fully understand how/why these resistance mechanisms evolve, how do we develop sound herbicide recommendations to better manage this? Despite these scientific uncertainties, a few “themes” (i.e., recommendations) about how to manage herbicide resistance by using more herbicides or different herbicide use patterns continue to perpetuate among some weed management practitioners.
Recommendations to slow the evolution of herbicide-resistant weeds have been promoted by university and industry scientists for many years. A common element of these recommendations has been to diversify the herbicide modes of action (MOA) to which weed populations are exposed. Herbicide rotations (within and between years) and tank-mixtures are two strategies recommended to achieve this diversification, but few quantitative data described the effectiveness of these approaches at a landscape level. In April 2015, USDA/ARS and University of Illinois weed scientists published the results from a project that studied the evolution of target site-based glyphosate-resistant waterhemp (Evans et al. 2015). The research examined factors related to landscape, weed, and management from 105 Illinois grain fields, including over 500 site-years of herbicide application records. This research discovered that simply rotating herbicide modes of action actually increased the frequency of resistance. In contrast, exposing populations to multiple MOA through tank-mixtures greatly reduced the selection for glyphosate-resistant waterhemp. A field in which 2.5 MOA per application were used was 83 times less likely to select glyphosate-resistant waterhemp within 4–6 years than a field in which only 1.5 MOA per application were used.
What became of this (at that time) new information? By in large, the crop protection industry “signed on” and began campaigns to encourage growers to use multiple herbicides/multiple, effective herbicides/herbicide mixtures (add whichever other cliché is your favorite) more than promoting rotating from Herbicide X this season to Herbicide Y next season. But has this message now become overused? It does seem somewhat simplistic, that all you need to do to solve the challenges of weed resistance to herbicides is to continue using herbicides but in a slightly different way than we have historically. A recent industry podcast highlighted a prominent concern of many weed scientists; that all some want to do is use herbicides to solve a problem created by using herbicides.
Is it more likely weeds will evolve resistance to soil- or foliar-applied herbicides? Many will assert it’s easier to evolve resistance to foliar-applied herbicides than to soil-applied herbicides, but are there data to support this? A recent statement that “…there is much less potential to develop resistance [to soil-residual herbicides] begs the following question: does anyone actually know how common resistance is to soil-residual herbicides? The simplest answer to this question is no. Remember, most of the research done at the University of Illinois is with waterhemp and not with other weed species. But what we do know about waterhemp is that if a plant is resistant to a foliar-applied herbicide from herbicide Groups 2 (ALS inhibitors), 14 (PPO inhibitors) or 27 (HPPD inhibitors), the plant also is resistant to soil-applied herbicides from these herbicide groups. Add in Group 15 (VLCFA inhibitors) resistance in waterhemp and the frequency of resistance to soil-applied herbicides becomes even more concerning.
It is critically important to note the work we did demonstrating herbicide mixing was a more effective strategy to mitigate the evolution of resistance compared with herbicide rotation was based on a target site resistance mechanism. Much of the research our group has undertaken the past five to eight years has not involved target site-based resistance mechanisms, but rather nontarget site-based resistance mechanisms, most notably the ability of resistant waterhemp to rapidly metabolize a herbicide before it causes a lethal effect. To our knowledge, there are no similar data that describe if herbicide mixtures are the most effective way to mitigate the evolution of nontarget site-based resistance mechanisms. In fact, research published from Europe a couple years ago suggested herbicide mixtures might actually favor the evolution of “generalist resistance mechanisms” (i.e., enhanced herbicide metabolism) instead of the “specialized resistance mechanisms” (i.e., target site resistance) often selected when single herbicide active ingredients are used repeatedly (Comont et al. 2020).
We have identified waterhemp populations resistant to herbicides to which the population had not been previously exposed. How is that possible? In every instance to date, the resistance mechanism has not been a change in the herbicide target site, but rather a nontarget site mechanism (usually enhanced herbicide metabolism). So then, how does one know which herbicides remain effective against any waterhemp population? The simple answer is there is no simple way to know. Simply adding soil-residual herbicides to your weed management program, or “layering” residuals with your postemergence application, or tankmixing two or more herbicides may seem to be the solution, but we must admit we do not fully understand how these tactics will impact the future evolution of resistance mechanisms. At this time, the only certainty we have is that if there are no weed seeds produced at the end of the growing season, there is no change in the frequency of any resistance mechanism. Anything short of that is not much more than speculation. Those few, scattered waterhemp remaining in the field when the combine arrives probably won’t do much to slow the harvest operation, but seeds from those few females just might contain the next and newest herbicide-resistance mechanism. We have no doubt that herbicides will continue to be valuable tools to help preclude crop yield loss, but we also must consider additional tactics to ensure no weed seed production during every growing season. This point is worth repeating: consider additional tactics to ensure no weed seed production.
Dr. Patrick Tranel recently discussed aspects/challenges of metabolic herbicide resistance for a recent “War Against Weeds” podcast. You can listen to this informative conversation from April 19, 2023 at: https://waragainstweeds.libsyn.com/
Evans, J.A., P.J. Tranel, A.G. Hager, B. Schutte, C. Wu, L.A. Chatham, and A.S. Davis. 2016. Managing the evolution of herbicide resistance. Pest Management Science 72:74–80.
Comont, D., C. Lowe, R. Hull, L. Crook, H.L. Hicks, N. Onkokesung, R. Beffa, D.Z. Childs, R.P. Freckleton, and P. Neve. 2020. Evolution of generalist resistance to herbicide mixtures reveals a trade-off in resistance management. Nature Communications 11:3086.