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 little data has described the effectiveness of these approaches at a landscape level. In April 2015, USDA and University of Illinois weed scientists published the results from a project that studied the evolution of target site-based glyphosate-resistant waterhemp. 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 was 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 new information? By and large, the crop protection industry “signed on” and began campaigns to encourage growers to use multiple herbicides/multiple, effective herbicides/herbicide mixtures 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 you 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 problems 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 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 is known 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 done demonstrating herbicide mixing as 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 undertaken in the past five to eight years has not involved target site-based resistance mechanisms, but rather non-target
site-based resistance mechanisms, most notably the ability of resistant waterhemp to rapidly metabolize a herbicide before it has a lethal effect. To our knowledge, there is no similar data that describe whether herbicide mixtures are the most effective way to mitigate the evolution of nontarget site-based resistance mechanisms.
Waterhemp populations have been identified in which the plants 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 non-target site mechanism (usually enhanced herbicide metabolism). So then, how do we know which herbicides remain effective against any given 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 tank mixing 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 is 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 plants 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.
There is 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.