Herbicide resistance

Page last updated: Thursday, 13 February 2020 - 4:44pm

Commonly used herbicide resistance terms

Herbicide mode of action groups

Herbicides act by targeting specific plant processes. This process-specific activity is termed 'mode of action' (MOA). In Australia all herbicides are classified into groups based on their MOA. MOA group classifications can be found on all herbicide labels, to identify the group to which a herbicide belongs.

MOA groups are ranked according to the risk of weed populations becoming resistant to those herbicides. Groups A and B are high risk while the remainder are moderate risk.

MOA subgroup chemical classes

Within a herbicide MOA group there may be two or more subgroups. Subgroups are usually based on the different chemical classes that inhibit the same enzyme. Group Z contains those herbicides that do not fit elsewhere.

Selection pressure

Selection pressure is a term used to describe the amount of selection for resistance applied by the herbicide application. Every time herbicide is used, susceptible individuals are killed and resistant individuals survive. The greater the number of susceptible individuals killed by the herbicide, the higher the selection pressure.

Resistance mechanisms

This term is used to describe the specific processes that enable the plant to survive an application of herbicide. Resistant populations of weeds may have either 'target site' or 'non-target site' resistance.

Target site resistance

Target site resistance occurs when there is an alteration at the target site. The alteration occurs at the normal herbicide site of action within the plant and is in the form of either a structural or biochemical change. This means that the herbicide will no longer be able to bind to its site of action, allowing the plant to survive the herbicide treatments.

Non-target site resistance

Non-target site resistance (also referred to as metabolic resistance) is used to describe mechanisms other than changes at the target site which enable an individual plant to survive a herbicide application. The potential mechanisms include reduced herbicide uptake, reduced translocation, reduced herbicide activation, enhanced herbicide detoxification, changes in intra or inter-cellular compartmentalisation and enhanced repair of herbicide-induced damage.


Cross-resistance is defined as the ability of a weed population to express resistance to more than one herbicide. It may arise without the weed population ever being exposed to one of the herbicides. There are two types of cross-resistance:

  1. Across herbicide subgroups. This occurs when a weed population is resistant to more than one herbicide subgroup within a specific MOA group. For example, populations of wild oats that are resistant to group A 'fops' may also be resistant to group A ‘dims’, even though they have not been exposed to a herbicide from the ‘dim’ subgroup. This is usually target-site resistance.
  2. Across herbicide mode of action groups. This occurs when a weed population is resistant to herbicides from within more than one MOA group. For example, a population of annual ryegrass selected only by group A herbicides may become resistant to both group A and group B herbicides. This is usually non-target site resistance.

Multiple resistance

Multiple resistance is a term used to describe weed populations that exhibit more than one resistance mechanism, allowing the plant to withstand herbicides from different subgroups. Some populations of resistant annual ryegrass possess both target and non-target site resistance. For example, one population of annual ryegrass present in Australia exhibits confirmed resistance to five different herbicide MOA groups.

Partial resistance or developing resistance

Partial resistance and developing resistance are terms used to describe a situation where only a small proportion (often less than 20%) of the population survives the standard application rate of the herbicide in question. Weed populations are normally classified by testing services as resistant when more than 20% of the population survives the standard application rate of herbicide.

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