Distribution and management of herbicide resistance in broad-leaved weeds

In the UK, herbicide-resistant broad-leaved weeds affect acetolactate synthase (ALS) and triazinone herbicides, with alternative modes of action (MoA) remaining effective. Find out about resistance in UK populations of chickweed, groundsel, common poppy and scentless mayweed, and the actions required to limit its spread.

In UK broad-leaved weed populations:

• Target site resistance has been confirmed (see sections, below)
• Non-target site resistance has not yet been detected

HRAC MOA Classification Group 2

In UK cereals, ALS-inhibiting herbicides, such as the sulfonylureas (e.g. metsulfuron) and triazolopyrimidines (e.g. florasulam), are fundamental to most broad-leaved weed control strategies. However, there are around 150 weed species resistant to ALS herbicides in almost 50 countries – more than any other herbicide class.

Resistance to ALS sulfonylurea herbicides in chickweed, poppy and scentless mayweed was first identified in the UK in 2000, 2001 and 2002, respectively.

Resistant poppy is the most common, followed by chickweed and mayweed.

Resistant plants are almost completely unaffected by herbicides applied at normal field rates.

Part of HRAC MOA Classification Group 5

Groundsel populations partially resistant to triazinone herbicides, such as metribuzin and metamitron, have been recorded in UK asparagus fields. Triazinone herbicides have the same mode of action as the triazines, which were banned in the EU in 2007.

Triazinone herbicides are widely used for weed control in potatoes, sugar beet and other crops.

As herbicide resistance may evolve in other species, vigilance is required. Higher-risk situations arise when control of a specific weed is dependent on a single or very limited number of herbicide modes of action.

Although no cases of herbicide-resistant charlock or wild radish have been detected in the UK, they are most likely to evolve herbicide resistance, according to the UK Weed Resistance Action Group (WRAG).

Resistance of fat-hen to triazinone herbicides is also a particular risk, as cases have been recorded in several other European countries.

While no cases have been confirmed in the UK, resistance to glyphosate – in both broad-leaved and grass-weeds – has been recorded in many countries.

Resistance to mecoprop (part of HRAC MOA Classification Group 4) in chickweed has been confirmed in the past, but the extent of the problem is uncertain.

Resistance to Acetyl CoA Carboxylase (ACCase) Inhibitors (HRAC MOA Classification Group 1) is an issue in some grass-weed species. However, such herbicides are not active against broad-leaved weeds – making resistance to this chemical group irrelevant in this group of weeds.

To minimise risks in broad-leaved weed species:

• Avoid ALS inhibitors and triazinone herbicides as the sole means of control in successive years
• Use ALS inhibitors and triazinone herbicides in mixture, sequence or rotation with herbicides with other modes of action

Alternative herbicides that give good control of ALS-resistant populations include:

• Fluroxypyr on chickweed (part of HRAC MOA Classification Group 4)
• Pendimethalin on poppy (part of HRAC MOA Classification Group 3)
• Clopyralid and bromoxynil on mayweed (part of HRAC MOA Classification Group 4 and 6, respectively)

Irrespective of the target weed species, follow best resistance management practice.

A long history of use of ALS herbicides and declining performance against a broad-leaved weed species, when other susceptible weeds are still well controlled, indicates resistance may have developed.

The limited efficacy of most non-chemical control measures, together with the long persistence of broad-leaved weed seeds in the soil (often >10 years), mean that detecting resistance at an early stage is vital.