Introduction to herbicide resistance in arable weeds

Find out about the causes of herbicide resistance and how to detect it in the field. Learn about the main UK weed species affected and how to manage resistance risks.

Herbicide resistance occurs when a weed survives a rate of herbicide that would normally kill it. The surviving plant then passes the genetic advantage on to its offspring. With repeated selection, resistant plants multiply until they dominate the population. It is critical that herbicide programmes take account of resistance already present in field populations, as well as all resistance risks.

Prevention of herbicide resistance is best, followed by early detection. By spotting problems early, more options are available. The following may be signs of herbicide resistance in the field:

• A gradual decline in control (over several years)
• Healthy plants (patches) beside dead plants of the same species
• Poor control of one susceptible species when other susceptible species are well controlled

The results of regular herbicide resistance tests (on seeds or plants) provide a strong indication of the unique resistance profiles present across each field.

Our web page includes information on how to sample and interpret test results to inform your long-term weed management strategies.

Herbicide resistance is an irreversible process – once present, it does not disappear or decline, even if the herbicide is no longer used. However, you can slow the development of herbicide resistance, by following these simple steps.

Herbicide active ingredients are grouped according to the way they control susceptible plant populations – this is their mode of action (MoA). The use of a variety of MoA – in mixtures or sequences – is the foundation of resistance management. As a result, it is important to understand the types of MoA available to optimise any herbicide programmes.

To assist resistance management, by 2023, all UK plant protection product labels will include MoA information.

Many of the most active herbicides pose a very high resistance risk. Resistance mechanisms can occur independently, in different plants within a single field, or even within the same plant.

Non-target site resistance (NTSR)

Weeds become more tolerant of a broad range of herbicides, irrespective of their chemistry or mode of action. Generally, this is due to the weed being better able to detoxify crop protection agents. It affects most herbicides to varying degrees, but only results in complete loss of control in severe cases. It tends to increase slowly and is the most common resistance mechanism in UK grass weeds. It is also termed metabolic or multiple herbicide resistance (MHR).

Target site resistance (TSR)

This form of resistance, which poses a very high risk of resistance development, is relatively well understood. Examples of TSR:

ACCase: Blocks the site of action specific to ‘fop’, ‘dim’ and ‘den’ herbicides in grass weeds. It can increase rapidly and result in poor control.

ALS: Blocks the site of action of sulfonylurea and related herbicides in grass and broad-leaved weeds. It can result in poor control. Currently less common than ACCase TSR, but is increasing.

Grass weeds

First identified in black-grass in 1982, herbicide resistance also affects wild-oat and Italian rye-grass. Recent studies have also confirmed the evolution of resistance in brome. Target site and non-target site resistance mechanisms are present in UK grass weeds, to various degrees.

Broad-leaved weeds

Most cases of broad-leaved weed (BLW) resistance are to the ALS inhibitor group of herbicides. First identified in the UK at the turn of the millennium, resistant poppy is the most common, followed by chickweed and then mayweed. Resistant plants are almost completely unaffected by herbicides applied at normal field rates. In addition, groundsel populations resistant to triazinone herbicides have been recorded in UK asparagus fields.

The encyclopaedia of arable weeds is the definitive guide to arable weeds. For each weed, the guidance contains information on its life cycle, distribution and management. Where known, the herbicide resistance status is provided.

The AHDB-supported resistance action groups (RAGs) produce guidance on pesticide resistance issues. As part of its support to the resistance action groups, AHDB publishes their guidance.