Principles of Herbicide Resistance

Herbicide resistance can have severe economic consequences for turfgrass managers and producers by affecting aesthetics, playability, surface stability, and yield. In order to control resistant weeds, turf managers may choose to increase their input costs by purchasing alternative herbicide chemistries and investing in additional labor to culturally and mechanically control resistant weed populations.

Repeated use of herbicides with the same site of action will select for resistant plants. These uncontrolled plants will reproduce, leading to an increased prevalence of resistance within a local population. That population may move off-site as seed on equipment or athletic attire.

Controlling resistant populations can be aided by understanding what herbicides they may be resistant or susceptible to. But ultimately, chemical weed control with multiple sites of action will be more effective than repeatedly relying on the same failed treatment.

It is also well known that larger plants are much less susceptible to postemergence herbicides. Treatments prior to tillering are most effective, and when possible, treatment when temperatures and light conditions are conducive to active growth and herbicide uptake may also aid in control.

    Mechanisms of resistance are simply how plants are able to tolerate or recover from herbicide application. Common mechanisms include:

  • Target site resistance occurs when the intended herbicide binding location undergoes a conformational change in structure. This change prevents or reduces the binding of a herbicide. Most populations have very low occurrence of this structural “defect”. In fact, these binding sites that are structurally different from that normally present in the natural population may have a negative consequence on the plant’s health, referred to as “fitness penalty” or “fitness cost.”
    It is the process of repeated selection using the same herbicide site of action that expands this trait across an entire population, leading to full blown resistance.
  • Non-target site resistance may occur as a result of decreased herbicide absorption or translocation, as well as changes in plant metabolism.