blackgrass

Herbicide resistance is the inherited ability of a weed to survive a rate of herbicide which would be lethal to a member of the normal population. It can develop over time based on repeated selection pressure imposed on the weed. This selection pressure changes the population from susceptible to resistant.

Herbicide resistance is becoming an increasing problem in UK agriculture, in both grass weeds and broad-leaf weeds. See also the general topic on herbicides.

Herbicide resistance is genetic within the plant and should not be confused with poor herbicide efficacy (e.g. poor weather, timing of application, spray quality, spray conditions etc.)

Symptoms of herbicide resistance to look out for include: healthy plants beside dead plants of the same species, a susceptible species having poor control when other susceptible species are well controlled, discrete weed patches and a general decline over a number of years.

 

The two main types of resistance mechanisms are:

Target site resistance : occurs due to a mutation in a specific gene which changes the structure of the encoding protein that the herbicide mode of action specifically targets. This means the herbicide can no longer bind the protein and exert its phytotoxic effect. 

Enhanced metabolism : ability of the weed to detoxify (metabolise) the herbicide in question. Often involves multiple genes and mechanisms and can include things such as reduce uptake, reduced translocation or comparmentalisation of the herbicide. 

 

Some other key terms used in herbicide resistance

MOA : mode of action. Refers to the specific cellular process targeted by the herbicide (e.g inhibition of microtubule assembly, aceto-lactate synthase (ALS) inhibition)

Cross-resistance: a single mechanism in the weed that confers resistance to more than one herbicide active ingredient. For example plants with an enhanced metabolism resistance mechanism may detoxify both Group 1 (ACCase inhibitors) and Group 2 (ALS inhibitors) herbicides,  or a weed with a target site resistance mechanism may cause resistance to other herbicides that have the same MOA, even if that herbicide has not been used on the weed.

Multiple resistance : when plants exhibit 2 or more resistance mechanisms to different herbicide active ingredients. For example the weed may display both target site and enhanced metabolism mechanisms conferring resistance to more than one herbicide MOA.

Partial resistance : only a small proportion of the weed population (typically less than 20%) survive the specific herbicide treatment at the standard application rate

 

Herbicide resistance in the UK

So far in the UK resistant populations of weeds have been found in:

Grasses

  • Black-grass
  • Italian-rye grass
  • Wild oats
  • Bromes

Broad leaved weeds

  • Poppy
  • Chickweed
  • Mayweed

Resistance has also been found by ADAS in an ALS resistant prickly sow thistle

The majority of herbicide resistance cases are to ALS and ACCase inhibitors.  Rotating herbicide modes of action can reduce risk of resistance developing to these widely used herbicide groups.

Testing for herbicide resistance

Herbicide resistance tests are key to confirm resistance and help future management of resistant weeds by choosing the correct chemistry. These can be done by collecting seed from suspected resistant weeds and sending to ADAS who will test the seed for its resistance status. Either pot tests or petri dish tests can be done depending on the weed species and types of herbicide to be tested. 

Genetic testing (detecting mutations at the molecular level) has also been widely developed over the last few years and this is something ADAS may look to expand its services to.

 

Reporting herbicide resistance

 Herbicide resistant weeds can be reported to the International Herbicide-Resistant Weed Database. This resource provides details of all reported cases, the weed species, in which crops and to which herbicide group.

Other useful sites

The Herbicide Resistance Action Committee (HRAC) provides comprehensive information on herbicide resistance and weed management and has useful tools such as the herbicide classification look up

The Weed Resistance Action Group (WRAG) consists of experts working to produce guidance on weed resistance issues.

Related Organisations

Content below is from across the PEP community and is not necessarily endorsed by Stewards or by PEP

Connected Content

Integrated Pest Management (IPM)

Integrated Pest Management highlights the growth of a healthy crop with the least possible disruption to agro-ecosystems and encourages natural pest control mechanisms. IPM is one of the tools for low-pesticide-input pest management, and IPM must now be implemented by all professional agchem users.

Weeds

A weed can have many definitions, one being, any plant growing in a place it is not wanted. This topic focuses mainly on weeds affecting agriculture. Generally weeds are classified into two groups: grass-weeds (e.g Black-grass, Italian ryegrass, Bromes) and broad-leaved weeds (e.g Mayweed, Chickweed, Poppy, Cleavers). 

Black-grass

Black-grass (Alopecurus myosuroides) is a major weed in winter sown cereals.

WeedManager

Online guide to identifying arable weeds from ADAS, AHDB and BASF

Managing weeds in arable rotations

AHDB Guide from 2021 incorporating WRAG guidelines. Weed control is vital for high yields of good-quality crops and to prevent the spread of pests and diseases, e.g. ergot. Yet with fewer active ingredients, a need to protect water and manage herbicide resistance, the weed challenge must be managed across the rotation.

European Weed Research Society Workshop - Cambridge May 2023

'Integrated Weed Management (IWM) systems in the agroecology context; new challenges.' NIAB, Cambridge, UK 24th - 26th of May 2023

Weed Resistance Action Group - WRAG

The Weed Resistance Action Group (WRAG) produces guidance on pesticide resistance issues. Hosted by AHDB, this information can be used to help protect crops and the long-term efficacy of herbicides.

Agronomy

This Topic doesn't yet have a Stewarded summary, but connected groups, content and organisations show below. Click the 'Ask to Join' button if you would like to be a Steward for this Topic and provide a summary of current knowledge and recommend useful resources, organisations, networks and projects. "Like" this Topic if you would like to see it prioritised for providing a wikipedia style summary.

Agrochemicals

Agrochemicals are chemical products used for agricultural purposes. Although agrochemicals have high input costs, they are widely used in the farming industry for their beneficial effects on crop yields and quality and associated reduced labour costs. Together with advances in agricultural machinery and infrastructure, the use of agrochemicals played a large role in the Green Revolution. This was a period in the 21st century in which the spread of various agricultural technologies led to greatly increased yields and production globally. However, a number of concerns around agrochemicals exist including negative effects on human health and the environment and the development of pest populations that are becoming resistant to them. A major challenge of modern agriculture is to try and reduce reliance on agrochemicals whilst continuing to increase yields and feed a growing global population.

Herbicides

Herbicides are substances (usually chemical) used to control weeds in a variety of situations including agriculture, horticulture and managed landscapes. Herbicides are classified according to their mode of action (MOA) - this is the precise biochemical mechanism in which the herbicide targets and kills the weed. The 'active ingredient' of the herbicide is the specific herbicidal compound that has the phytotoxic effect and this is formulated with a variety of other ingredients (including other active substances, surfactants, buffers, adjuvants e.t.c) to make a final product which is given a trade name by the herbicide manufacturer. With any herbicide product you will find an associated product label which explains how to use the product safely and legally. This page provides an overview of herbicides including how they are classified and used. Please link any pages or projects relating to herbicide use to this topic page.  Some widely used herbicides (e.g glyphosate) also have their own topic page. Other related topics on FarmPEP include herbicide resistance,  bioherbicides, broad leaved weeds and grass weeds.  

Broad-Leaved Weeds

Broad-leaved weeds are a varied group of weeds that can grow and cause significant problems in arable fields in the UK. Some of the most common broad-leaved weeds in the UK include: Common Chickweed (Stellaria media) , Scentless Mayweed (Tripleurospermum inodorum), Common poppy (Papaver rhoeas), Charlock (Sinapis arvensis L.), Fat-hen (Chenopodium album L.), Common groundsel (Senecio vulgaris), Docks (Rumex spp) and Creeping thistle (Cirsium arvense). See also related topics of: grass-weeds, herbicides, herbicide resistance and integrated weed management.

Grass Weeds

Grass weeds are a major challenge in UK agriculture and are often highly competitive in arable crops. Some of the most common grass weeds in the UK include: Black-grass (Alopecurus myosuroides), Italian rye-grass (Lolium multiflorum), Brome (Bromus sp.), wild-oats (Avena sp.), Couch grass (Elytrigia repens), annual meadow grass (Poa annua) and Rat's-tail Fescue (Vulpia myuros). You can find specific topic pages for black-grass and brome. See also related topics of: broad-leaved weeds, herbicides, herbicide resistance and integrated weed management

Glyphosate

The herbicide glyphosate was first registered in 1974 under the original trade name RoundUp (Monsanto) and has since become one of the most commonly used herbicides globally. It is a non-selective, post-emergent herbicide with a wide range of uses including arable, forestry, and horticulture.

Integrated Weed Management (IWM)

IWM involves using numerous weed control methods to try and manage a weed problem sustainably. Whilst herbicides can still be used as part of an IWM approach, a major aim is to reduce reliance on them by also incorporating methods including, cultural, mechanical, biological, thermal and genetic control. A combination of such approaches can allow for optimal control of a specific weed problem. Related topics on FarmPEP include herbicides and herbicide resistance. You can also view the topic pages on grass weeds and broad-leaved weeds which provide examples of suitable control methods.

Agricultural modelling for weed management

Crop modelling is a useful tool in agriculture to improve our understanding how a crop grows in interaction with all external factors, including environmental interactions and the crop management practices in place. The idea is that by modelling cropping system factors as a mathematical representation, and incorporating real past data to 'calibrate' the model,  you can then simulate various scenarios and predict the impacts of certain changes on crop growth. One example of this would be a simulation of predicted environmental conditions under various climate scenarios to predict the impacts of climate change on crop growth and yields. Whilst models are oversimplifications of reality and can never capture all the complexity of agricultural systems, modelling has been a vital tool underpinning key agricultural developments and models are constantly being developed to be more dynamic, complex and sensitive. One particular application of modelling is in weed control. This page highlights the application of modelling in weed management and introduces a case study of a weed model from a DEFRA report.       

Assessing pesticide risks to non-target terrestrial plants

A 1998 study commissioned by the Pesticides Safety Directorate to assess the non-target impacts of pesticides on non-target terrestrial plants (NTTP's).  'To conserve and enhance biological diversity within the UK' was a stated aim of of the UK Biodiversity Action Plan (1994). Over 25 years later this is still a vital issue and the UK has stated its commitment to secure a post-2020 Biodiversity Framework as well as developing a 10 point plan for financing biodiversity (UK Government, 2022). However, a threat to plant biodiversity is non-target effects of pesticides - this is when pesticide materials reach areas beyond the target application area and affect the species there. Effects of pesticides on non-target plants can range from lethal (death of the plant) all the way to enhanced growth of these plants. Such varied effects depend on a range of things including the type of pesticide used, the concentration of its active ingredient and the sensitivity and spatial range of affected plants. Importantly, plants are part of complex food and pollination webs and so pesticide effects on plants can have cascading effects on the fauna that rely on them, and vice versa.

Developing strategies for reducing the risk from herbicide-resistant wild oats

Wild-oats are a highly competitive grass-weed in the UK of which there are two important species: common wild-oats (Avena fatua) and winter wild-oats (Avena sterilis ssp. ludoviciana). Failure to control wild oats in arable fields can lead to high penalties to yield, seed crop contamination, income loss for farmers and reduced competitiveness of UK agriculture. Whilst herbicide resistance selection has been slower in wild-oats compared to other grass weeds such as Black-grass (Alopecurus myosuroides), it is still a growing problem and since this report was published in 2001, resistance has been reported across hundreds of farms across many UK counties.  Continuing development of herbicide resistance in wild-oats will limit pesticide choice and possibly lead to a higher dependence on environmentally 'risky' herbicides that are more likely to reach water. The objective of this project was to conduct and collate research on the characterisation of herbicide resistance in wild-oats with the aim of informing strategies for the prevention, containment and control of herbicide resistant populations. Links to the full project are provided at the bottom of the page.

ALS herbicide resistance found in Prickly sow thistle (Sonchus asper)

In 2019, for the first time in the UK, an ALS-resistant prickly sow thistle (Sonchus asper) population was identified by ADAS researchers.   

Wild oats Identification

Dr Stephen Moss has kindly given permission for his useful Wild-oat identification posters to be shared on FarmPEP. 

The rise of herbicide resistance in black-grass – a 35-year story

A new paper published by ADAS summarises evolving black-grass herbicide resistance over the last 35 years, including the effectiveness of the dwindling actives left, and the vital role of routine resistance testing.

Lessons from [over] 60 years of weed management experience

Written By James Clarke - ADAS Research Director

Across the Straits: A Unified Approach to Tackle Grassweed Herbicide Resistance

The Straits committee is a voluntary partnership between Kent County Council and neighbouring local authorities in France (Département du Pas-de-Calais, Département du Nord), Belgium (Provincie Oost-Vlaanderen, Provincie West-Vlaanderen) and the Netherlands (Provincie Zuid-Holland, Provincie Zeeland). This partnership helps to support organisations on either side of the channel to work together where it is useful to do so.  As part of the The Straits Committee Small Project Initiative, ADAS have teamed up with agricultural organisations in France (Chambre d'agriculture), Belgium (Inagro & UGent/HOGENT) and the Netherlands (AIKC Rusthoeve) following discussions that grassweeds and herbicide resistance were common problems in all of these regions. An escalating issue across arable farming in  Europe is herbicide resistance. This is defined as 'the inherited ability of a weed to survive a rate of herbicide that would normally kill it’. It impacts crop yield, increases costs for farmers, and limits crop rotation options. In particular, Black-grass (Alopecurus myosuroides) and other grassweeds are highly problematic in the South-East UK and cereal growing coastal regions of Belgium, France, and the Netherlands. The project aims to Assess resistance to determine the extent of the problem in the Straits regions Foster ongoing collaboration among agricultural organisations working in these areas to help support growers Provide joint and cohesive guidelines to help growers manage resistance effectively, reduce ineffective use of herbicides and promote best use of non-chemical control as part of an integrated weed management (IWM) approach. Updates will be made to this page as the project progresses, including survey links and results, guidelines, advice sheets, fact-files and more, so please do follow, like and comment on this page to keep updated!    This is the Grassweeds project, co-funded under the Straits Committee small project initiative by Kent County Council (UK), Provincie West-Vlaanderen (BE), Provincie-Oost-Vlaanderen (BE), Provincie Zeeland (NL), & Département du Pas-de-Calais (FR).

Managing the resistance risk to retain long-term effectiveness of glyphosate for grass weed control in UK crop rotations

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