Standard chemical background

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.


Classification of herbicides

Herbicides are classified according to the Herbicide Resistance Action Committee (HRAC) system. This is a system of numbered groups, each number referring to a specific mode of action. Within each group active ingredients are further grouped according to their chemical families.

The 2 most common herbicide groups are:

Group 1: Acetyl-CoA carboxylase (ACCase) inhibitors

  • ACCase is an enzyme involved in fatty acid synthesis. Herbicides in this group inhibit this important metabolic process in plants.
  • Their are 3 main chemical families in this group, referred to as FOPs (e.g clodinafop) , DIMs (e.g cycloxydim)  and DENs (e.g pinoxaden).

Group 2: Acetolactate synthase (ALS)  inhibitors

  • ALS is an enzyme involved in the biosynthesis of branch-chain amino acids (valine, leucine and isoleucine). ALS inhibiting herbicides block this process meaning the plant cannot make vital proteins it needs to survive.
  • There are several chemical families in this group including the sulfonylureas (e.g mesosulfuron-methyl),  Imidazolinones (e.g imazamox) and Triazolopyrimidines (e.g florasulam).

You can use the HRAC Global herbicide classification look-up (link provided at the bottom of this topic) to find all the different groups of herbicides, chemical families and active ingredients of herbicides.


Types of herbicides

Selective vs non-selective (broad-spectrum)

  • Selective herbicides only work on a specific range of weeds whereas non-selective herbicides can control a wider variety of weeds.
  • For example ACCase inhibitors (HRAC group 1) selectively control grass weed species (as many broadleaf species have a naturally tolerant ACCase enzyme) whereas the herbicide glyphosate (HRAC Group 9) can target both broadleaf and grass species.

Contact vs Systemic (translocated)

  • Contact herbicides kill only the part of the weed that the herbicide contacts directly whereas systemic herbicides are absorbed by the weed at the point of application and are then circulated by the vascular system, thus reaching other tissues.
  • Contact herbicides tend to be more rapidly acting but may allow re-growth if certain tissues such as the roots are unaffected by the herbicide. Systemic herbicides tend to take longer to exert their effects on the weed but will kill the entire plant. 
  • For example glufosinate (HRAC Group 10) is only effective where it comes into contact with the weed whereas glyphosate (HRAC Group 9) is absorbed by the leaves and then moves and accumulates in the plant roots.

Residual vs non-residual

  • Residual herbicides will remain in the soil in an active form for longer periods of time, allowing longer lasting control of weeds whereas non-residual herbicides are rapidly broken down in the soil and do not have lasting effects.
  • For example flazasulfuron (HRAC Group 2 -sulfonylurea) may provide up to half a year residual activity whereas glyphosate (HRAC Group 9) has either no or very little soil residual activity as it loses herbicidal properties upon binding with soil particles. 

Pre-emergent vs post-emergent

  • Pre-emergent herbicides are applied to soil to prevent any weeds from germinating and have residual effects whereas post-emergent herbicides are applied to kill weeds once they have already germinated and emerged and tend not to have residual effects.
  • For example the pre-emergence herbicide  triallate (Group 15) is applied to soil where it prevents grass weeds from germinating whereas fluazifop (HRAC Group 1) is applied to grass weeds after they have emerged where it is absorbed through the leaves.



Useful resources:

Herbicide products authorised for use in the UK: Health and safety executive pesticides register 

Herbicide classification: HRAC Global Herbicide Classification look-up

Related Organisations

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Connected Content

Crop protection refers to practices and measures employed in agriculture to safeguard crops from both biotic (pests, diseases and weeds) and abiotic (environmental factors) stresses. They key goal of crop production is to maintain crop productivity, health and quality whilst minimising yield losses.

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.

Insecticides are a type of agrochemical used to kill, harm or deter insects that either directly infect cultivated plants/animals or that are carriers of disease. In agricultural settings, insecticides may be used in both arable and livestock husbandry situations. The classification of insecticides can occur in various ways: via their biochemical mode of action, their mode of penetration or on the basis of their chemistry. They can also come in various formulations and delivery-systems such as sprays, gels or baits.

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.

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

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

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.

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.

Written By James Clarke - ADAS Research Director

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