Herbicide Resistance

A comprehensive source of Herbicide Resistance information from our experts and partners.

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Select your grassweed to find out more


Black-grass

First found in the UK in 1982, herbicide-resistant populations now occur on virtually all of the 20,000 farms where black-grass occurs.  Populations show variable degrees of resistance to a wide range of different modes of action, with both ACCase and ALS target site resistance (TSR) and non-target site resistance (NTSR), especially enhanced metabolism, now widespread.

 

Type of resistance

Mechanism

Non-target site – principally enhanced metabolism

Results in herbicide detoxification and is the commonest resistance mechanism in black-grass It affects most herbicides to varying degrees, but only in severe cases results in complete loss of control.  Tends to increase slowly.

ACCase target site

Blocks the site of activity specific to ‘fop’, ‘dim’ and ‘den’ herbicides.  It only affects these groups of herbicides, but often results in very poor control and can increase rapidly.

ALS target site

Blocks the site of activity of sulfonylurea and related herbicides.  It only affects these groups of herbicides, but often results in poor control and can increase rapidly.

 

> Find out more about Black-grass Management Control methods 

 

Brome

Herbicide resistance has not been detected so far in any UK brome species although two populations of sterile brome showing higher levels of tolerance to glyphosate have been found in Leicestershire and Oxfordshire.  Populations of sterile brome resistant to ACCase and ALS inhibitors have been found in France and Germany and resistance in great brome and rye brome has been recorded in other parts of the world.  Hence, herbicide resistance in brome species is a potential threat in the UK.  Seed or plant samples from fields where resistance is suspected should be collected and tested in standard assays.  

> Find out more about Brome Management Control methods

 

Couch

Only one case of evolved resistance to herbicides has been recorded in any species of couch worldwide - resistance to amitrole in Creeping Bent in pears in Belgium in 1986.  Couch appears to have low risk of evolving herbicide-resistance – almost certainly because it is largely propagated vegetatively, rather than by seed.

> Find out more about Couch Management Control methods

 

Italian Rye-grass

First found in the UK in 1990, over 475 herbicide-resistant populations have now been identified in 33 counties of England.  Populations show variable degrees of resistance to a wide range of different modes of action.  ACCase and ALS target site resistance (TSR) occur much less commonly than in black-grass, although this type of resistance is likely to increase.  Non-target site resistance (NTSR), especially enhanced metabolism, is the most widespread type of resistance – as it is in black-grass.

 

Type of resistance

Mechanism

Non-target site – principally enhanced metabolism

Results in herbicide detoxification and is the commonest resistance mechanism in rye-grass It affects most herbicides to varying degrees, but only in severe cases results in complete loss of control.  Tends to increase slowly.

ACCase target site

Blocks the site of activity specific to ‘fop’, ‘dim’ and ‘den’ herbicides.  It only affects these groups of herbicides, but often results in very poor control and can increase rapidly.  Detected much less frequently than in black-grass but likely to be increasing rapidly.

ALS target site

Blocks the site of activity of sulfonylurea and related herbicides.  It only affects these groups of herbicides, but often results in poor control and can increase rapidly.  Found for the first time in 2012 in UK populations of rye-grass - much more recently than in black-grass.  Appears to be much less common than in black-grass but likely to be increasing rapidly.

 

Although resistance is currently less of an issue with rye-grass compared with black-grass, there is no room for complacency.  It should be borne in mind that:

  •          Rye-grass species globally have shown cross-resistance to 11 different herbicide modes of action – more than for any other weed.
  •          The first case of a glyphosate-resistant weed in a European annual arable cropping system has recently been reported – it was rye-grass in Italy. 

> Find out more about Italian Rye-Grass Management Control methods 

 

Wild Oats

Herbicide resistant wild-oats were first found in the UK in 1994 and is now widespread in 28 counties.  Resistance has been identified in both species.  ACCase (‘fop’/’dim’/’den’) target site resistance has been found but, in contrast to black-grass, this tends to affect ‘fops’ only and not ‘dims’ (e.g. cycloxydim) or ‘dens’ (pinoxaden), which often remain effective.  However, enhanced metabolic resistance is the commonest mechanism and this can reduce the efficacy of most ACCase and ALS inhibiting herbicides.

Although resistance in wild-oats is currently much less of a problem compared with black-grass, the risk should not be ignored because:

  •          All the most effective wild-oat herbicides are either ACCase or ALS inhibitors which are ‘high risk’ from a resistance perspective.
  •          There is high dependency on herbicides as, in contrast to other grass-weeds, fewer effective non-chemical methods of control are available.

If resistance is suspected, seed samples should be collected in late July and tested in standard assays.

> Find out more about Wild Oats Management Control methods

 

Further Reading

Herbicide Resistance Action Committee        AHDB   Weed Resistance Action Group    Herbicide Resistance - AHDB Video