Are soil-applied herbicides immune to resistance?
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?