How do herbicides work?

Knowing how a herbicide works in detail - its 'mode of action' (MOA) - is important to understanding how to use it most effectively. Herbicide MOA is a major factor in weed control spectrum, crop selectivity and weed resistance


The inherited ability of a plant/weed to survive a dose of a herbicide normally lethal to that species.

Authoritative On-line References and Resources The International Survey of Herbicide Resistant Weeds monitors the evolution of resistant species and assesses their impact. All confirmed instances of new cases are listed.
Symptomology can indicate mode of actionHerbicides control weeds by interfering with how they grow. Different MOAs all ultimately either stop seeds from germinating or establishing as seedlings; prevent plants from making essential carbohydrates, proteins or lipids (oils and fats); or desiccate leaves and stems. Paraquat’s MOA involves diverting the flow of energy captured from sunlight in photosynthesis to produce highly reactive free radicals which destroy cell membranes to quickly desiccate leaves. This happens within hours in bright sunlight because of the high levels of energy running out of control. Almost all green plants are affected by paraquat making it a broad-spectrum, non-selective


A chemical product used for eliminating all types of weeds (annual and perennial grasses and broadleaved weeds).

Authoritative On-line References and Resources An invaluable source of contemporary information about herbicides and weeds from Iowa State University.
herbicide. You can read more and watch a video showing how paraquat works by clicking here.

Mode of action fact file

  • 280+ herbicide active ingredients
  • 20+ herbicide MOAs
  • 1980s: last new MOA introduced
  • 1 MOA in one field spells trouble
Other herbicide MOAs often involve preventing enzymes from making essential products, resulting in an accumulation of possibly destructive biochemicals and a shortage of the usual product which slows growth and ultimately destroys the weed. As plants have evolved, slight differences in their biochemical systems for carrying-out physiological processes have arisen. Gene mutations are responsible for one sort of herbicide selectivity based on MOA. Similarly, mutations are also one way in which weed resistance to herbicides can occur. Rare individual plants which have a mutated enzyme may not be susceptible to a herbicide which is generally effective on that species. An outbreak of weed resistance occurs when such an individual is allowed to reproduce and its progeny come to dominate the weed population in a field. Ensuring that herbicides with different modes of action are used helps to avoid weeds becoming resistant. Several other types of herbicide MOA exist. The first selective herbicides


A chemical product used for eliminating certain types of weeds only (ie either grasses or broadleaved weeds) and not affecting specified crops.

Authoritative On-line References and Resources An invaluable source of contemporary information about herbicides and weeds from Iowa State University.
to be discovered mimic plant hormones – the chemical messengers that control growth and development. Another group affect cell division by, for instance, disrupting the mechanism by which chromosomes are arranged in a cell as it prepares to divide in two.
Recent surveys have found that farmers often think new herbicides will be developed to replace those to which weeds have become resistant. However, the last herbicide with a new MOA was introduced in the 1980s. Weed resistance to herbicides like glyphosate threatens food production and means the intelligent use of the small range of MOAs available is vitally important. For a more detailed discussion on herbicide mode of action click here.