The continual evolution of weed species and populations resistant to herbicides from one or more mechanism-of-action families represents one of the most daunting challenges faced by weed management practitioners. Currently in Illinois, biotypes of 12 weed species have been confirmed resistant to one or more herbicide mechanisms of action. Resistance to herbicides that inhibit the ALS enzyme is the most common type of resistance in Illinois. Waterhemp has evolved resistance to more herbicide mechanisms of action 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) and hydroxyphenyl pyruvate dioxygenase (HPPD).  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 multiple herbicide resistance are becoming increasingly common and greatly reduce the number of herbicide options that remain effective for their control.

Integrated weed management programs offer the greatest potential for long-term, sustainable solutions for weed populations demonstrating resistance to herbicides from multiple families. Frequently, soil-residual herbicides are proposed as components of an integrated weed management program that provide several benefits, including reducing the intensity of selection for resistance to foliar-applied herbicides. However, there appears to be somewhat of a misconception that weeds demonstrate resistance only to foliar-applied herbicides. In some instances this is true, but in many other instances weeds do in fact demonstrate resistance to soil-applied herbicides. The following text describes examples in which resistance occurs to foliar-applied herbicides only, and other examples in which resistance occurs to soil- and foliar-applied herbicides.

Some herbicides are applied directly to plant foliage because they lack any appreciable activity in the soil. These herbicides are adsorbed so tightly to soil colloids that they are unavailable for plant uptake. Glyphosate and paraquat are examples of foliar-applied herbicides that provide no residual weed control because they are rapidly and tightly adsorbed to soil colloids. Worldwide, biotypes of 24 weed species demonstrate resistance to glyphosate, while biotypes of 28 weed species demonstrate resistance to bipyridilium herbicides such as paraquat. Weed resistance to glyphosate or paraquat are examples of resistance to herbicides applied only to the plant foliage since these herbicides possess no appreciable soil residual weed control. But, what about resistance to herbicides that have both foliar and soil-residual activity?

Imazethapyr (the active ingredient in Pursuit and some herbicide premixes) is an ALS-inhibiting herbicide that can be applied to the soil or plant foliage. Worldwide, biotypes of 128 weed species have evolved resistance to ALS-inhibiting herbicides.  As mentioned previously, many Illinois waterhemp populations contain plants resistant to ALS-inhibiting herbicides (including imazethapyr).  These plants demonstrate a high magnitude of resistance to imazethapyr regardless of whether it is applied to the soil or plant foliage.

What about weeds resistant to herbicides that share a common mechanism of action but that are usually applied to either the soil or foliage, such as herbicides that inhibit the PPO enzyme? Table 1 lists examples of PPO-inhibiting herbicides commonly used in Illinois, and indicates whether they are most commonly applied to the soil or foliage. Waterhemp resistant to PPO-inhibiting herbicides is becoming increasingly common across Illinois, but some incorrectly believe this type of resistance exists only to foliar-applied PPO inhibitors. Biotypes of waterhemp resistant to PPO-inhibiting herbicides are resistant to those herbicides regardless of whether the herbicide is applied to the soil or foliage.

Table 1.  Examples of PPO-inhibiting herbicides commonly used in Illinois.

¹Flumioxazin, sulfentrazone and saflufenacil possess some foliar activity, but greater weed control is achieved when these herbicides are taken up from the soil.

²Fomesafen and lactofen are labeled for soil application.

Suspicions of waterhemp resistance to PPO-inhibiting herbicides generally begin after a foliar-applied PPO inhibitor failed to control plants that were treated within label guidelines. PPO-resistant waterhemp plants treated with a foliar-applied PPO-inhibiting herbicide typically demonstrate injury symptoms, such as leaf necrosis, characteristic of this herbicide family. However, unlike susceptible plants, the leaf necrosis is generally much less and the resistant plants begin to recover within 7 to 10 days after the application (Figure 1). But, when a soil-applied PPO-inhibiting herbicide is applied to this same field, the level of waterhemp control often appears comparable to that of a susceptible population. So, why do soil-applied PPO-inhibiting herbicides seem to control a PPO-resistant waterhemp population that foliar-applied PPO-inhibiting herbicides do not control? The answer is because the dose of the soil-applied herbicide is sufficiently high enough to overcome the mechanism of resistance, at least for a while.

Figure 1. Waterhemp plants resistant (back row) or sensitive (front row) to lactofen (Cobra). Numbers indicate the application rate of Cobra (fl oz/acre) applied when plants were 4 inches tall.

Foliar-applied PPO-inhibiting herbicides are applied at rates to control the weeds present when the application is made. In contrast, application rates of soil-applied PPO-inhibiting herbicides are selected to provide several weeks of residual weed control. In other words, the application rate of a soil-applied herbicide is much higher than the rate needed to control weeds present at the time of application. These application rates overcome the mechanism of resistance to PPO-inhibiting herbicides since the magnitude of resistance is relatively low.  So, how is the magnitude of resistance determined and does it vary among populations and herbicides?

Weed scientists characterize the magnitude of resistance (i.e., how resistant the plants are to the herbicide of interest) 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 plants from the suspect-resistant population and to plants from a population known to be sensitive to the herbicide. 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 using 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 an Illinois waterhemp population that demonstrates resistance to herbicides from three mechanism-of-action families (triazine, ALS and PPO). The R/S ratios indicate the population demonstrates a low magnitude of resistance to atrazine, lactofen, and flumioxazin but a very high level of resistance to imazamox.

Table 2. Resistance ratios for an Illinois waterhemp population resistant to atrazine, imazamox, lactofen and flumioxazin. GR₅₀ values represent the rates required to reduce waterhemp biomass (foliar applications) or emergence (soil applications) by 50 percent.

^aHerbicides were applied to the foliage.

^bHerbicides were applied to the soil.

Converting the GR₅₀ values to rates of formulated herbicide illustrates that only 0.06 fluid ounce of Raptor was required for 50% control of the sensitive population compared with 7.58 gallons of Raptor for the resistant population.  Clearly, biotypes demonstrating this magnitude of resistance cannot be controlled by simply increasing the application rate of Raptor. However, the same conversions for the soil-applied PPO inhibitor flumioxazin illustrate that only 0.042 and 0.117 ounce of Valor were required for 50% control of the sensitive and resistant populations, respectively. These rates are much lower than the typical field application rate range of 1–3 ounces per acre.  Figure 2 illustrates this response by depicting emergence of PPO-resistant (R, back row) and sensitive (S, front row) waterhemp as influenced by Valor application rate (white numbers indicate the rate applied to both pots). The two pots on the far left were untreated controls while the two pots on the right were treated with 3 ounces of Valor.  Both resistant and sensitive populations were controlled by Valor at rates of 3 and 0.3 ounces per acre.  However, only the resistant population emerged when the Valor rate was further reduced to 0.03 ounce per acre; the sensitive population was still controlled at that rate.  Both populations emerged when the application rate was reduced another order of magnitude.

Figure 2. Emergence of PPO-resistant (back row) and sensitive (front row) waterhemp as influenced by Valor (flumioxazin) application rate (white numbers).

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.).

So, what might be some practical implications of this?  Consider a waterhemp population that contains a mix of PPO-resistant and sensitive individuals. All plants in this population (resistant and sensitive) will be controlled by a soil-applied PPO-inhibiting herbicide for a period of time after its application. As herbicide degradation proceeds, a threshold concentration is reached at which the sensitive individuals are still controlled but the resistant individuals survive. This provides a selective advantage for the resistant individuals and may partially explain the increasing frequency of PPO-resistant waterhemp populations in Illinois.

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 mechanism-of-action family is reduced when multiple and different tactics are used to control the weed population. As mentioned previously, not every individual waterhemp plant is resistant to a herbicide. An integrated weed management approach which utilizes soil- and foliar-applied herbicides applied at labeled rates in combination with other management tactics can help slow the selection for additional resistances in individual waterhemp plants and populations.