Herbicide resistance in grassweeds and broad-leaved weeds

Herbicide resistance definition:

‘Herbicide resistance is the inherited ability of a weed population to survive a rate of herbicide that would normally kill it.’

mayweeds and poppies

Herbicide resistance is already widespread in grasses, with resistant blackgrass in particular being increasingly difficult to control, to the extent that it has come to threaten viable cereal production on some farms, predominantly in eastern England. It is now generally accepted that some degree of resistance is found on virtually all farms on which blackgrass herbicides have been regularly used. Herbicide resistance in broad-leaved weeds is also now found in the UK, with three key broad-leaved weeds – common poppy, chickweed and scentless mayweed, having confirmed resistance to the ALS-inhibiting group of herbicides. Although resistance has only been detected in these three species in the UK, worldwide experience shows that resistance could evolve in many other broad-leaved weeds too, so vigilance is needed. Resistance in broad-leaved weeds needs to be taken seriously, to try to avert a situation as severe as in blackgrass.

There are different mechanisms of herbicide resistance:

  • Target-site resistance – due to a modification in the herbicide binding site (usually an enzyme) which precludes a herbicide from effectively binding.
  • Enhanced metabolism resistance – results in herbicide detoxification.

Target site resistanceHerbicide resistance was first identified in the UK in blackgrass in 1982. Resistance in Italian rye-grass is also widespread, though is currently less problematic than with blackgrass. It also affects wild-oat, though currently, resistance tends to be more localised than with blackgrass and rye-grass. Three main types of resistance are present in UK grass-weed populations – enhanced metabolism, ACCase target site resistance and ALS target site resistance.

In poppy, chickweed and mayweed, ALS target site resistance has been confirmed in all three species. Enhanced metabolism has not been found in broad-leaved weeds in the UK.

Herbicide resistance in broad-leaved weed populations is probably underestimated, and farmers should ensure they are mixing modes of action when applying herbicides to prevent the situation worsening. Broad-leaved weed resistance was first confirmed in England in 2000, and the latest data show that almost 100 farms were affected by 2011 (see table). Another survey is being conducted this year, and experts predict that the situation will have worsened.

  Chickweed Poppy Mayweed
Resistance first found 2000 2001 2002
Farms >40 >25 5
Counties 13 9 3
Comment Mainly Scotlandand N. Ireland Limited, but increasing Limited
Source HGCA Managing weeds in arable rotations a guide, updated Spring 2014

Herbicide resistant chickweed is largely confined to Scotland, where a predominantly spring cropping pattern is practised, and ALS-inhibitors have been used for weed control year on year. However, with the increasing blackgrass problem, many growers in England will be switching to spring cropping, particularly spring barley, as an aid to blackgrass control, but this could lead to an increased risk of broad-leaved weed resistance developing. Herbicide resistant poppy populations are also frequently found, especially in eastern England.

Having both herbicide resistant grassweed and broad-leaved weed populations in the same fields could make control options really tricky, resulting in high yield losses from heavy weed burdens.

Early detection of herbicide resistance is important. Symptoms of herbicide resistance are:

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

As with resistant grassweeds, early indications are that there will be no easy answer to resistant broad-leaved weeds. Farmers will have to adopt an integrated approach which combines rotations, cultivations, variety and crop protection choice.

There are a number of factors which lead to the build up of herbicide resistant weeds:

Weed characteristics which favour selection for resistance:

  • annual growth habit
  • high seed production
  • high weed density (higher density = higher chance for resistant individuals)
  • relatively rapid turnover of the seed bank (result of short lived seed dormancy)
  • several reproductive generations/season
  • extreme susceptibility to a particular herbicide
  • lack of susceptibility to various herbicides so that there are few effective options or choices
  • naturally high frequency of resistant gene(s) within the weed population

Herbicide factors that increase intensity of selection for resistance

  • act at a single site of action
  • a common metabolic pathway for detoxification in plants
  • long residual activity in the soil
  • extreme efficacy on a wide range of weed species

Cultural practices that increase selection intensity and resistance

  • Mono-cropping
  • Herbicide only weed control programmes – no cultivation as part of the programme
  • Failure to eliminate weeds which escape the herbicide treatment
  • Continuous or repeated use of the same herbicide or those which have the same mode of action
  • Use of a higher herbicide rate than what is required for consistent weed control

 Advice for dealing with herbicide resistance:

Reliance on herbicides alone is not a sustainable strategy, and as of 1 January 2014, the EU Sustainable Use of Pesticides Directive (2009/128/EC) requires priority to be given to non-chemical methods of plant protection, wherever possible.

Key factors for resistance management

  • Increase use of non-chemical control methods
  • Make greater use of pre-emergence herbicides
  • Place less reliance on high resistance risk post-emergence herbicides
  • Use mixtures and sequences to reduce the threat
  • Monitor herbicide performance in individual fields
  • Carry out regular testing for resistance

It is important not to rely on one mode of action, e.g. ALS-inhibiting chemistry throughout the rotation. Winter oilseed rape in the rotation provides a useful option for employing alternative modes of action. For blackgrass and other grassweeds, the power of KERB® FLO 500 and ASTROKerb® (propyzamide) has stood the test of time. With no reports of blackgrass resistance to date, these products provide a vital weapon to fight grassweeds in the rotation. ASTROKerb also provides enhanced performance against certain broad-leaved weeds such as mayweed and poppy.

By integrating several non-chemical methods, in conjunction with herbicides, overall control should be improved. There is no magic answer for the best non-chemical control strategy; solutions need to be tailored to the weed and resistance problem in each field. Effective weed control depends on understanding specific opportunities in each crop across the whole rotation.