The continual evolution of weed species and populations resistant to herbicides from one or more site-of-action groups represents one of the most daunting challenges facing weed management practitioners. Waterhemp has evolved resistance to herbicides from more site-of-action groups than any other Illinois weed species, including resistance to inhibitors of acetolactate synthase (ALS), photosystem II (PSII), protoporphyrinogen oxidase (PPO), enolpyruvyl shikimate-3-phosphate synthase (EPSPS), hydroxyphenyl pyruvate dioxygenase (HPPD), and synthetic auxins. The University of Illinois weed science program recently announced confirmation of waterhemp populations resistant to Group 15 herbicides (Table 1), the first such confirmation of resistance in a dicot species to herbicides from this group. Not every individual waterhemp plant is resistant to one or more herbicides, but the majority of field-level waterhemp populations contain one or more types of herbicide resistance. Perhaps even more daunting is the occurrence of multiple herbicide resistances within individual plants and/or fields. Waterhemp plants and populations demonstrating resistance to multiple herbicides are becoming increasingly common and greatly reduce the number of effective herbicide options.
Table 1. Examples of Group 15 herbicides commonly used in Illinois.
Integrated weed management programs offer the greatest potential for long-term, sustainable solutions for weed populations demonstrating resistance to herbicides from multiple groups. Soil-residual herbicides are components of an integrated weed management program that provide several benefits, including reducing the intensity of selection for resistance to foliar-applied herbicides. However, the recent discovery of resistance to Group 15 herbicides is yet another example of how waterhemp continues to challenge herbicide-only management programs. There exists an urgent need for integrated weed management programs that return zero weed seed to the soil seedbank.
When a Group 15 herbicide is applied to the soil in a field with a resistant waterhemp population, the initial level of waterhemp control sometimes appears to be comparable to that of a susceptible population. So, is there actually resistance to Group 15 herbicides? A description of how the magnitude of resistance is determined and application rates of soil-residual herbicides might be helpful to answer this question.
Weed scientists characterize the magnitude of resistance (i.e., how resistant the plants are to the herbicide) by conducting a dose-response experiment in which a range of herbicide rates (often 8 to 10 rates, some more than and some less than a typical field use rate) is applied to the suspect-resistant population and to one or more populations known to be sensitive. Dose-response experiments are most commonly conducted by spraying a foliar-applied herbicide directly onto plant foliage, but these experiments also can be conducted with soil-residual herbicides applied to soil containing seeds of the populations of interest. At some time after application (often 14 or 21 days), a measure of plant response (percent injury, mortality, plant dry weight, etc.) is made for both populations, and a statistical equation is used to determine the herbicide rate that reduced the measured parameter by some value (frequently 50% is used for comparison). The rate derived from the resistant population is divided by the rate derived from the sensitive population, and the quotient is referred to as the resistance ratio; the higher the resistance ratio, the greater the magnitude of resistance to that particular herbicide. Table 2 presents actual resistance ratios for two Illinois waterhemp populations resistant to Group 15 herbicides. The R/S ratios indicate the populations demonstrate the greatest resistance to S-metolachlor, followed by dimethenamid and pyroxasulfone/acetochlor.
What about application rates of soil-residual herbicides? The application rates of most foliar-applied herbicides are usually selected to control only the weeds present when the application is made; in other words, most foliar-applied herbicides do not provide several weeks of residual weed control following application. In contrast, application rates of soil-applied herbicides are selected to provide several weeks of residual weed control. These rates are much greater than the rate needed to control germinating weeds at the time of application. Furthermore, labeled application rates are not determined based on one or two weed species; rather, the labeled rates are those that control a broad spectrum of weed species for several weeks. So then, what rate of S-metolachlor is needed to control a sensitive waterhemp population that germinates the day S-metolachlor is applied? Is this rate greater than or less than the actual field application rate? To answer these questions, we will use an illustration from our greenhouse research with two Group 15-resistant waterhemp populations.
Figure 1 shows the results of a greenhouse dose-response experiment 21 days after S-metolachlor was applied the same day waterhemp seeds were planted. There are four waterhemp populations (CHR-M6 and MCR-NH 40 are resistant to Group 15 herbicides, WUS and ACR are sensitive) aligned in rows that were treated with various rates (0.0078–7.87 pints per acre) of S-metolachlor. The pots in the far left column were not treated, while pots in the far right column were treated with the highest dose (7.87 pints) of S-metolachlor. If we assume 2.5 pints per acre is the label recommend rate for this soil, the actual rate of S-metolachlor needed to control the sensitive populations is only 0.25 pints of S-metolachlor, which in this greenhouse experiment actually controlled these populations for 21 days after application. In contrast, some resistant plants emerged and survived 7.87 pints of S-metolachlor.
One might be tempted to argue this discussion is irrelevant since field-scale applications of soil-residual herbicides are not made at rates low enough to discriminate between resistant and sensitive plants. A portion of that argument is valid, but keep in mind that once a herbicide enters the soil environment it begins the process of degradation. At some point during the course of its degradation, the amount of herbicide remaining in the soil will correspond to these discriminating rates. The amount of time required for a particular herbicide’s degradation process to reach these discriminating rates depends upon many soil- and environmental-related factors (such as soil texture, organic matter content, moisture, pH, etc.). At that discriminating dose, only resistant plants will emerge.
Compared with resistance to foliar-applied herbicides, resistance to soil-applied herbicides generally is more difficult to detect in the field. Resistance to foliar-applied herbicides is manifest as treated plants (assuming appropriate application rate and timing) that are not controlled, whereas resistance to soil-applied herbicides is manifest as a reduced duration of residual control. It’s not always possible to predict if residual control is reduced 2 days, 8 days, 14 days, etc., as populations vary in their response to individual Group 15 herbicides. This does, however, emphasize the necessity of applying full label-recommended rates instead of reduced rates, as reduced rates will further curtail the duration of residual control.
Group 15 herbicides, whether applied at planting or with a postemergence herbicide after crop emergence, will continue to be important weed management tools. The evolution of resistance to this important class of herbicides should serve as another warning that herbicide stewardship is as important as herbicide/trait selection. Selection for herbicide resistance occurs each time a herbicide is applied, regardless of the herbicide or whether it is applied to the soil or plant foliage. However, the overall intensity of selection for resistance to any particular herbicide or site-of-action group is reduced when multiple and different tactics are used to control the weed population.
Table 2. Resistance ratios for two Illinois waterhemp populations resistant to Group 15 herbicides. LD50 values represent the rates required to reduce waterhemp emergence/survival by 50 percent.
Figure 1. Dose-response experiment of four waterhemp populations treated with soil-applied S-metolachlor.
Source: University of Illinois