Guideline to the Management of Herbicide Resistance
The Herbicide Resistance Action Committee (HRAC) is an industry initiative which fosters cooperation between plant protection
manufacturers, government, researchers, advisors and farmers. The objective of the working group is to facilitate the effective
management of herbicide resistance.
AgrEvo, American Cyanamid, BASF, Bayer, DowElanco, DuPont, F.M.C., Monsanto, Novartis, Rhône-Poulenc, Rohm and Haas,
Tomen, Zeneca.
HRAC is regionally represented by the following working groups:
European Herbicide ResistanceWorking Group (EHRWG)
North American Herbicide Resistance Working Group (NAHRWG)
National resistance working groups in Australia, South Africa, and others.

Table of Contents
I. BackgroundII. Definitions
III. The process of selection for herbicide resistanceIV. Resistance risk assessment
V. Guidelines for the prevention and management of
herbicide resistanceVI. Herbicide rotation and herbicide mixtures
VII. HRAC site of action classification
VIII. What to do in cases of confirmed herbicide
resistance
IX. Conclusions and references

I. BackgroundIn recent years, there has been an increasing relianceon modern herbicides leading to a reduction in theneed for ‘traditional’ techniques of weed control.
Cropping patterns have adapted, driven by thepossibility to further increase crop output, to rely more
and more on these products. Whilst economically thisshift has been rewarding to farmers, some negative
consequences have emerged which now need to beaddressed in the interest of longer term sustainability.
One result of modern agriculture and the reliance on
herbicides is the emergence of populations of weeds
which are resistant to products designed to control
them. All natural weed populations regardless of theapplication of any weed killer probably containindividual plants (biotypes) which are resistant toherbicides. Repeated use of a herbicide will expose
the weed population to a "selection pressure" which
may lead to an increase in the number of surviving

resistant individuals in the population. As aconsequence, the resistant weed population may
increase to the point that adequate weed control
cannot be achieved by the application of thatherbicide.
The first case of herbicide resistance in weeds was
identified in 1964. Currently, there are recorded morethan 150 resistant grass and broadleaf weed biotypes
in about 50 countries worldwide (Heap 1997). In spiteof this seemingly dramatic development, no
herbicides have been lost to agriculture; they aretoday, and will remain, an integral part of foodproduction through their effective use in combination
with other weed control practices.

II. Definitions
Weed Resistance – Resistance is the naturally
occurring inheritable ability of some weed biotypes
within a given weed population to survive a herbicide
treatment that would, under normal use conditions,effectively control that weed population. Selection ofresistant biotypes may result in control failures.
Cross Resistance – Cross resistance exists when aweed population is resistant to two or moreherbicides. The presence of a such a mechanism cancomplicate the selection of alternate herbicides astools to control a resistance situation. It is for thisreason that management strategies must incorporate
more than simply a switch of product.
Resistance Mechanisms – The resistancemechanism refers to the method by which a resistant
plant overcomes the effect of a herbicide. The
mechanism present will influence the pattern ofresistance, particularly to the cross resistance profileand the dose response. The most common
mechanisms of resistance are explained briefly below.
Compartmentalism Sequestration – This meansthat the herbicide is removed from sensitive parts ofthe plant cell to a tolerant site, such as a vacuole,where it is effectively harmless to plant growth.

	  Herbicide Site of Action – Refers to the biochemicalmechanism by which a herbicide causes growth tocease in target weeds. Herbicides can be classified
into groups according to their site of activity within theplant (see table 2).

III. The process of selection for herbicide
resistance
It is assumed that a small number of plants in any
weed population is naturally resistant to a given
herbicide and that repeated application of thatherbicide will allow these plants to survive and set
seed. Over a period of several such ‘selections’ theresistant biotype can dominate the weed population.
This process is shown diagrammatically below:

Crop management practices which may enhance
resistance development
Frequent use of herbicides with a similar site ofaction: The combination of ‘frequent use’ and ‘similarsite of action’ is the single most important factor in thedevelopment of herbicide resistance.
Cropping rotations with reliance primarily on
herbicides for weed control: The crop rotation isimportant in that it will determine the frequency andtype of herbicide able to be applied. It is also themajor factor in the selection of non-chemical weed
control options. Additionally, the cropping period forthe various crops will have a strong impact on theweed flora present.
Lack of non-chemical weed control practices: Culturalor non-chemical weed control techniques,
incorporated into an integrated approach is essential
to the development of a sustainable crop
management system.
Table 1: Assessment of the Risk of Resistance
Development per Target Species
Cropping System Evaluation

IV. Resistance risk assessment
How does a farmer establish that a herbicide
resistance problem is developing or if his farming
practices may lead to resistance appearing?
There are several factors to consider when evaluating
herbicide resistance risk. Some of these relate to the
biology of the weed species in question, others relate
to particular farming practices. Some examples are
given below:
Biology and genetic makeup of the weed species
in question
Number or density of weeds: As resistant plants are
assumed to be present in all natural weed
populations, the higher the density of weeds, thehigher the chance that some resistant individuals will
be present.
Natural frequency of resistant plants in the population:
Some weed species have a higher propensity toward
resistance development; this relates to genetic
diversity within the species and, in practical terms,
refers to the frequency of resistant individuals within
the natural population.
Seed soil dormancy potential: Plant species with alonger soil dormancy will tend to exhibit a slower
resistance development under a selection pressure as
the germination of new, susceptible, plants will tend to
dilute the resistant population.

*Cultural control can be by using cultivation, stubble burning, competitive crops,
stale seedbeds, etc.

Risk of ResistanceManagement
option:Low

ModerateHigh

Herbicide mix
or rotation in
cropping system> 2 modes ofaction2 modes ofaction1 mode ofactionWeed control in
cropping system

Cultural*,mechanicaland chemical

Cultural andchemicalChemical
only

Once

More thanonce

Many times

Full rotation

Limitedrotation

No rotation

Unknown

LimitedCommonWeedinfestation

Low

Moderate

High

Control in last 3
years

Good

Declining

Poor

Use of samemode of action
per season
Cropping
systemResistancestatus to modeof action

Table 1 (above): Provides a checklist of the major risk
factors within a cropping system and ranks these as'low', ‘medium’, or ‘high’ risk of resistancedevelopment.
The checklist is to be used per weed species where a'Cropping System' in its simplest form is the
management of crop production in an individual field.

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Failure to achieve expected weed control levels does
not in most cases mean that a farmer has resistance.
A full analysis of the herbicide application, rate of use,
weed type and stage of growth, climatic conditions
and agronomic practice should be reviewed.
If, after the initial investigation, resistance is stillsuspected, then consideration of historical information
may point to factors leading to resistancedevelopment. The following questions arerecommended:1)

Has the same herbicide or herbicides withthe same site of action been used in thesame field or in the general area forseveral years?

2)

Has the uncontrolled species beensuccessfully controlled in the past by theherbicide in question or by the current
treatment?

system needs to be developed which is appropriate
for the farm in question.
Integrated Weed Management is defined as the useof a range of control techniques, embracing physical,
chemical and biological methods in an integrated
fashion without excessive reliance on any one method(Powles and Matthews, 1992).
The following information outlines the three key areasof weed management: crop management, cultural
techniques and chemical tools which, when employed
in a rotational and integrated approach will help toreduce the selection pressure on any weed species –hence significantly reducing the chance of survival ofresistant weeds.
Rotation of Crops
The principle of crop rotation as a resistancemanagement tool is: to avoid successive crops in thesame field which require herbicides with the same siteof action for control of the same weed species.

3)

Has a decline in the control been noticed
in recent years?

4)

Are there known cases of resistant weeds
in adjacent fields, farms, roadsides, etc.?

1)5)

Is the level of weed control generally goodon the other susceptible species except
the ones not controlled?

Different crops will allow rotation ofherbicides having a different site of action.

2)

The growth season of the weed can be
avoided or disrupted.

3)

Crops with differing sowing times anddifferent seedbed preparation can lead toa variety of cultural techniques being
employed to manage a particular weed
problem.

4)

Crops also differ in their inherent
competitiveness against weeds. A strongly
competitive crop will have a better chance
to restrict weed seed production.

If the answer to any of these questions is 'yes' and allother factors have been ruled out, then resistanceshould be strongly suspected. Steps should then be
taken to leave a small area in order to collect asample of whole plant or seed from the suspected
resistant weed population for a resistanceconfirmation test.

Crop rotation allows the following options:

Seed Sample from Suspect Plant:

Cultural Techniques

V. Guidelines for the prevention andmanagement of herbicide resistance
The prevention of resistance occurring is an easierand cheaper option than managing a confirmed
resistance situation.
Experience has shown that simply changing
herbicides is not enough to overcome resistance inthe mid to long term and that a sustainable, integrated

Cultural (or non-chemical) weed control methods do
not exert a chemical selection pressure and assist
greatly in reducing the soil seed bank. Cultural
techniques must be incorporated into the generalagronomy of the crop and other weed control
strategies. Not all of the examples given are adequate
in all situations.
Some of the cultural measures for weed control could
include:1)

cultivation or ploughing prior to sowing tocontrol emerged plants and to bury nongerminated seed2)

delaying planting so that initial weed

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flushes can be controlled with a nonselective herbicide

The Use of Chemical Mixtures to Prevent
Resistance3)

using certified crop seed free of weed

4)

post-harvest grazing, where practical5)

stubble burning, where allowed, can limit
weed seed fertility

Mixtures can be a useful tool in managing or
preventing the establishment of resistant weeds.
For chemical mixtures to be effective, they should:

6)in extreme cases of confirmed resistance,
fields can be cut for hay or silage toprevent weed seed set

VI. Herbicide rotation and herbicide
mixtures
Herbicide rotation or mixtures refers to the rotation or
mixtures of Herbicide Site of Action against any
identified weed species. HRAC has recently prepareda classification of herbicides according to site ofaction (summary shown as table 2). When planning aweed control program, products should be chosenfrom different site of action groups to control the same
weed either in successive applications or in mixtures.
A general guideline for the rotation of chemical groups
should consider:
1)

2)

3)

4)

avoid continued use of the same herbicide
or herbicides having the same site ofaction in the same field, unless it isintegrated with other weed control
practiceslimit the number of applications of a single
herbicide or herbicides having the samesite of action in a single growing season
where possible, use mixtures or sequential
treatments of herbicides having a different
site of action but which are active on thesame target weeds
use non-selective herbicides to control
early flushes of weeds (prior to crop
emergence) and/or weed escapes

From experience, we can conclude that rotation ofherbicides alone is not enough to prevent thedevelopment of resistance. To retain these valuable
tools, the chemical rotation explained must be
employed in association with at least some of theother weed control measures outlined. In cases where
metabolic resistance is already present, the site ofaction of the herbicide is not always the key criterion.
In these cases, the mechanism of degradation can bevery important and cross site of action groups andchemistries. No classification of herbicides relating todegradation is available and such examples need tobe handled on a case-by-case basis.

1)

include active ingredients which both givehigh levels of control of the target weed;
and,

2)

include active ingredients from different
site of action groupings

The HRAC classification of herbicides according tosite of action is in itself not a recommendation ofwhich herbicide to use. The system is not based onresistance risk assessment but solely chemical site ofaction. The guide is designed to be used as a tool toselect herbicides from different site of action groups
so that appropriate mixtures or rotations can beplanned within an integrated weed management
system.
Additional to the above guideline, the grower should:
1)know which weeds infest his field or noncrop area and where possible, tailor his
weed control program according to weed
densities and/or economic thresholds

2)

follow label use instructions carefully; thisespecially includes recommended use
rates and application timing for the weeds
to be controlled3)

routinely monitor results of herbicide
applications, being aware of any trends orchanges in the weed populations present

4)

maintain detailed field records so thatcropping and herbicide history is known

VII. HRAC site of action classification
Classification of Herbicides According to Site of
ActionVisit hracglobal.com to view the Global Classification
Lookup tool.

VIII. What to do in cases of confirmed
herbicide resistanceIn cases where a control failure has been confirmed
as resistant, immediate action is required to limit
further seed production of the resistant plants.
The degree of the action will depend on the stage ofthe crop in the field and the extent of the problem.

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Some options to consider:

IX. Conclusions and references1)

Eradicate the remaining weed population if
growing in patches in order to limit
build-up and spread of seed in the soil

2)

Limit the field to field movement ofresistant populations by cleaning planting,cultivation and harvesting equipment toavoid transfer of resistant weed seed3)

Avoid using the herbicide to which
resistance has been confirmed unlessused in conjunction with herbicides having
a different site of action, active on theresistant weed population

It is only through the development andimplementation of an integrated weed management
program utilizing as wide a variety of weed control
practices as are economically feasible that the
problem can be effectively managed or prevented.

4)

If the resistant population is widespread,consider grazing the crop or cut for feed
being careful not to transfer resistant seed
via manure

Steps towards the management of herbicide
resistance

How quickly the resistant weed species will revert to"natural levels" within the population, if ever, will
depend on a number of factors such as the relative
fitness of the resistant versus susceptible biotypes,
the weed's germination pattern and the weed's
reproductive capabilities (genetics of resistance,
pollination system, number of seeds produced per
season, seed bank longevity, etc.).

1)

Assessment of risk through a cropping
system checklist

5)

Select these fields for rotation or set aside
for the following cropping season

2)

Evaluation of options (including costs) tobe adapted to local conditions

6)

Seek advice to assist in the long termplanning of weed control in these fields

3)

Implementation of a sustainable weed
control program

4)

Rotation of crops to enable a variety ofweed control options

5)

Rotation of cultural practices to lower the
reliance on herbicides

6)

Rotation of herbicide site of action to
reduce the likelihood of resistance to a
specific product group

Once resistant weed numbers are at a controllable
level, implementation of an integrated weed
management system as outlined in this document will
ensure that crops can continue to reach high levels of
productivity in the fields in question.
A recent case study analysis carried out in England(ref. Orson and Harris, 1997) has identified that thedevelopment of resistance can be categorized into
stages, with each stage requiring a new intensity ofmanagement. These management levels naturally
carry a cost over what is considered as the standard
farming practice. An example is the option of delayed
sowing.
Whilst this is a very effective tool for managing weed
numbers, the cost of doing so – if yield is reduced –can be significant.
The possible increased costs incurred to manage
resistance must be measured against the impact of
not applying these measures. In extreme cases, therapid increase of uncontrollable weeds will also
severely affect crop yields and may eventually impact
land value itself. Key to the measurement of the costof resistance management is the inclusion of several
variables such as crop yield potential, commodity
prices, local costs of various techniques such as
ploughing, the weed species, the soil type and so on.
This means that a cost evaluation can only be
accurate on a local level and extrapolation from othersituations can offer principles but not the specificdetail.

Further Information
Internet home page: http://www.hracglobal.com/
Crop Life International
143 Avenue Louise
1050 Brussels, Belgium
croplife@croplife.org
Internet home page: http://www.croplife.org/
The development of resistance in a field is a process of
selection. It means that the resistant plant can degrade aherbicide to non-phytotoxic substances faster than a normal
sensitive plant, thereby surviving a herbicide treatment inmuch the same manner as many crop plants. “Within aplant” may mean that a herbicide no longer binds to its
normal site of action due to a change in the structure of thetarget site, thereby allowing the plant to survive the herbicide
treatment which relies on this site for its activity.

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