The continual evolution of herbicide-resistant weeds is one of the most daunting problems faced by weed management practitioners. According to Aaron Hager, associate professor of weed science at the University of Illinois, biotypes of 12 weed species in Illinois are known to be resistant to one or more herbicide action mechanisms.
“The occurrence of multiple herbicide resistances within individual plants and/or fields is a particular challenge,” Hager said. “Waterhemp has evolved resistance to more herbicide mechanisms of action than any other Illinois weed species.”
Frequently, soil-residual herbicides are proposed as part of an integrated weed management program. They provide several benefits, including reducing the intensity of selection for resistance to foliar-applied herbicides.
Many growers believe that weeds demonstrate resistance only to herbicides applied to the foliage. In fact, weeds can also develop resistance to soil-applied herbicides.
For example, imazethapyr (the active ingredient in Pursuit and some herbicide premixes) is an acetolactate synthase (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. Many Illinois waterhemp populations contain plants that are resistant to these herbicides whether they are applied to the soil or plant foliage.
The story is similar for herbicides that share a common mechanism of action but that are usually applied to either the soil or the foliage, such as herbicides that inhibit the protoporphyrinogen oxidase (PPO) enzyme.
“Waterhemp resistance to PPO-inhibiting herbicides is becoming increasingly common across Illinois,” Hager said. “Growers began to suspect it was present after a foliar-applied PPO inhibitor failed to control plants that were treated within label guidelines. The treated plants demonstrated injury symptoms, such as leaf necrosis, that are characteristic of this herbicide family. However, the injury was less than normal, and the resistant plants begin to recover within seven to 10 days after the application.”
A common misconception is that resistance to PPO inhibitors exists only to those that are foliar-applied. In reality, biotypes of waterhemp that are resistant to PPO-inhibiting herbicides are resistant regardless of whether they are applied to the soil or foliage. However, when a PPO-inhibiting herbicide is applied in the soil, the waterhemp control level is often comparable to that of a susceptible population.
Why do soil-applied PPO-inhibiting herbicides seem to control PPO-resistant waterhemp populations that foliar-applied PPO-inhibiting herbicides do not control?
“Because the dose of the soil-applied herbicide is high enough to overcome the resistance mechanism, at least for a while,” Hager explained. “Foliar-applied PPO-inhibiting herbicides are applied at rates to control the weeds that are present when the application is made. Rates for soil-applied PPO-inhibiting herbicides are calculated to provide several weeks of residual weed control and thus are much higher than what is needed to control weeds that are present when they are applied. The higher rates overcome the mechanism of resistance to PPO-inhibiting herbicides because the magnitude of resistance is relatively low.”
If a waterhemp population contains a mix of PPO-resistant and sensitive individuals, a soil-applied PPO-inhibiting herbicide will control both resistant and sensitive plants for a period of time after application. As the herbicide degrades, 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 for any herbicide takes place each time it is applied. However, the overall intensity of selection for resistance to a specific herbicide or mechanism-of-action family is reduced when multiple, different tactics are used to control the weed population.
“Not every individual waterhemp plant is herbicide-resistant,” Hager concluded. “An integrated weed management approach that applies soil- and foliar-applied herbicides at labeled rates in combination with other management tactics can help slow the selection for additional resistances in individual plants and populations.”