Part
5. Management of Resistant Populations
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Contents 5.1. Guideline
to the Management of Herbicide Resistance 5.2. Herbicide Resistance and Herbicide - Crop
– Weed Management |
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5.1.
Guideline to the Management of Herbicide Resistance
1. Background
In recent years, there has been
an increasing reliance on modern herbicides leading to a reduction in the need
for ‘traditional’ techniques of weed control. Cropping patterns have adapted,
driven by the possibility to further increase crop output, to rely more and
more on these products. Whilst economically this shift has been rewarding to
farmers, some negative consequences have emerged which now need to be addressed
in the interest of longer-term sustainability.
One result of modern agriculture
and the reliance on herbicides is the emergence of populations of weeds that
are resistant to products designed to control them.
All natural weed populations
regardless of the application of any weed killer probably contain individual
plants (biotypes) that are resistant to herbicides. 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 a consequence, the resistant weed population may increase to the
point that adequate weed control cannot be achieved by the application of that
herbicide.
The first case of herbicide
resistance in weeds was identified in 1964. By 1997, more than 150 resistant
grass and broadleaf weed biotypes are recorded in about 50 countries worldwide
(Heap, 1997). In spite of this seemingly dramatic development, no herbicides
have been lost to agriculture; they are today, and will remain, an integral
part of food production through their effective use in combination with other
weed control practices.
2.
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 of
resistant biotypes may result in control failures.
Cross Resistance: Cross
resistance exists when a weed population is resistant to two or more
herbicides.
The
presence of such mechanism can complicate the selection of alternate herbicides
as tools to control a resistance situation. It is for this reason that
management strategies must incorporate more than simply a switch of product.
Resistance Mechanisms: The
resistance mechanism refers to the method by which a resistant plant overcomes
the effect of a herbicide. The mechanism present will influence the pattern of
resistance, particularly to the cross-resistance profile and the dose response.
The most common
The
mechanisms of resistance are explained briefly below.
An altered target site within
a plant may mean that a herbicide no longer binds to its normal site of action
due to a change in the structure of the target site, thereby allowing the plant
to survive the herbicide treatment which relies on this site for its activity.
Enhanced metabolism means
that the resistant plant can degrade a herbicide to non-phytotoxic substances
faster than a normal sensitive plant, thereby surviving a herbicide treatment
in much the same manner as many crop plants.
Compartmentalism sequestration
means that the herbicide is removed from sensitive parts of the plant cell to a
tolerant site, such as a vacuole, where it is effectively harmless to plant
growth.
Herbicide Mode of Action:
Refers to the biochemical mechanism by which a herbicide causes growth to cease
in target weeds. Herbicides can be classified into groups according to their
site of activity within the plant
3. The process of selection for herbicide resistance
The development of resistance in
a field is a process of selection.
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 that herbicide will allow these plants to
survive and set seed. Over a period of several such ‘selections’ the resistant
biotype can dominate the weed population.
This process is shown
diagrammatically below:
4. 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 MAKE UP 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, the higher 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 a longer 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.
CROP
MANAGEMENT PRACTICES WHICH MAY ENHANCE RESISTANCE DEVELOPMENT
Frequent use of herbicides with a similar mode of action: The combination of ‘frequent use’ and ‘similar mode
of action’ is the single most important factor in the development of herbicide
resistance.
Cropping rotations with reliance primarily on herbicides for weed
control: The crop rotation is important
in that it will determine the frequency and type of herbicide able to be
applied. It is also the major factor in the selection of non-chemical weed
control options. Additionally, the cropping period for the various crops will
have a strong impact on the weed flora present.
Lack of non-chemical weed control practices: Cultural or 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
*
Cultural control can be by using cultivation, stubble burning, competitive
crops, stale seedbeds etc.
See
HRAC
guidelines for more details
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 resistance development.
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.
5. Resistance confirmation
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 still suspected, then consideration of historical
information may point to factors leading to resistance development. The following
questions are recommended:
• Has the same herbicide or herbicides
with the same mode of action been used in the same field or in the general area
for several years?
• Has the uncontrolled species been
successfully controlled in the past by the herbicide in question or by the
current treatment?
• Has a decline in the control been
noticed in recent years?
• Are there known cases of resistant
weeds in adjacent fields, farms, roadsides, etc.?
• Is the level of weed control generally
good on the other susceptible species except the ones not controlled?
If the answer to any of these
questions is 'yes' and all other factors have been ruled out, then resistance
should be strongly suspected. Steps should then be taken to leave a small area
in order to collect a sample of whole plant or seed from the suspected
resistant weed population for a resistance confirmation test.
Seed sample collection
guide:
6. Guidelines for the prevention and management of herbicide resistance
The prevention of resistance
occurring is an easier and cheaper option than managing a confirmed resistance
situation.
Experience has shown that simply
changing herbicides is not enough to overcome resistance in the mid to long
term and that a sustainable, integrated system needs to be developed which is
appropriate for the farm in question.
Integrated Weed Management is
defined as the use of 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 areas of weed management; Crop management, Cultural
techniques and Chemical tools which, when employed in a rotational and integrated
approach will help to reduce the selection pressure on any weed species – hence
significantly reducing the chance of survival of resistant weeds.
ROTATION
OF CROPS
The principle of crop rotation as
a resistance management tool is: to avoid successive crops in the same field
that require herbicides with the same mode of action for control of the same
weed species.
Crop
rotation allows the following options:
•
Different crops will allow rotation of herbicides having a different mode of
action
• Growth season of the weed can be
avoided or disrupted
• Crops with differing sowing times and
different seedbed preparation can lead to a variety of cultural techniques
being employed to manage a particular weed problem.
• Crops also differ in their inherent
competitiveness against weeds. A strongly competitive crop will have a better
chance to restrict weed seed production.
CULTURAL
TECHNIQUES
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
general agronomy 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:
• Cultivation or ploughing prior to
sowing to control emerged plants and to bury non-germinated seed .
• Delaying planting so that initial weed
flushes can be controlled with a non-selective herbicide.
• Using certified crop seed free of weed
.
• Post harvest grazing, where practical.
• Stubble burning, where allowed, can
limit weed seed fertility.
• In extreme cases of confirmed
resistance, fields can be cut for hay or silage to prevent weed seed set.
HERBICIDE
ROTATION AND HERBICIDE MIXTURES
Herbicide rotation or mixtures
refers to the rotation or mixtures of Herbicide Mode of Action against any
identified weed species. HRAC has recently prepared a classification of
herbicides according to mode of action (see Part
7). When planning a weed control program, products should be chosen from
different mode 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:
• Avoid continued use of the same
herbicide or herbicides having the same mode of action in the same field unless
it is integrated with other weed control practices
• Limit the number of applications of a
single herbicide or herbicides having the same mode of action in a single
growing season
• Where possible, use mixtures or
sequential treatments of herbicides having a different mode of action but which
are active on the same 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 of herbicides alone is not enough to prevent the development of
resistance. To retain these valuable tools, the chemical rotation explained
must be employed in association with at least some of the other weed control
measures outlined.
In cases where metabolic
resistance is already present, the mode of action of the herbicide is not
always the key criterion. In these cases, the mechanism of degradation can be
very important and cross mode of action groups and chemistries. No
classification of herbicides relating to degradation is available and such
examples need to be handled on a case-by-case basis.
THE
USE OF CHEMICAL MIXTURES TO PREVENT RESISTANCE
Mixtures can be a useful tool in
managing or preventing the establishment of resistant weeds.
For
chemical mixtures to be effective, they should:
•
Include active ingredients which both give high levels of control of the target
weed, and
•
Include active ingredients from different mode of action groupings.
The HRAC classification of
herbicides according to mode of action is in itself not a recommendation of
which herbicide to use. The system is not based on resistance risk assessment
but solely chemical mode of action.
The guide is designed to be used
as a tool to select herbicides from different mode of action groups so that
appropriate mixtures or rotations can be planned within an integrated weed
management system.
Additional
to the above guideline, the grower should:
• Know which weeds infest his field or
non-crop area and where possible, tailor his weed control program according to
weed densities and/or economic thresholds
• Follow label use instructions
carefully. This especially includes recommended use rates and application
timing for the weeds to be controlled
• Routinely monitor results of herbicide
applications, being aware of any trends or changes in the weed populations
present
• Maintain detailed field records so
that cropping and herbicide history is known
7. HRAC mode of action classification
(See
also the full mode of action classification table in Part 7)
Group
A Inhibition of acetyl CoA carboxylase (ACCase)
Aryloxyphenoxy-propionates, cyclohexanediones
Group
B Inhibition of acetolactate synthase ALS (acetohydroxyacidsynthase AHAS)
Sulfonylureas, imidazolinones, triazolopyrimidines, pyrimidinylthiobenzoates
Group
C1 Inhibition of photosynthesis at photosystem II
Triazines, triazinones, uracils, pyridazinone, phenyl-carbamates
Group
C2 Inhibition of photosynthesis at phostosystem II
Ureas, amides
Group
C3 Inhibition of photosynthesis at phostosystem II
Nitriles, benzothiadiazole, phenyl-pyridazines
Group
D Photosystem-l-electron diversion
Bipyridyliums
Group
E Inhibition of protoporphyrinogen oxidase (PPO)
Diphenylethers, n-phenylphthalimides, thiadiazoles, oxadiazoles, triazolinones
Group F1 Bleaching: Inhibition of
carotenoid biosynthesis at the phytoene desaturase step (PDS)
Pyridazinones, nicotinanilides, others
Group
F2 Bleaching: Inhibition of 4-hydroxyphenyl-pyruvate-dioxygenase (4-HPPD)
Triketones, isoxazoles, pyrazoles
Group
F3 Bleaching: Inhibition of carotenoid biosynthesis (unknown target)
Triazole, isoxyzolidinones, urea
Group
G Inhibition of EPSP synthase
Glycines
Group
H Inhibition of glutamine synthetase
Phosphinic acids
Group
I Inhibition of DHP (dihydropteroate) synthase
Carbamates
Group
K1 Microtubule assembly inhibition
Dinitroanilines, phosphoroamidates, pyridazines, benzoicacids
Group
K2 Inhibition of mitosis/microtubule organisation
Carbamates
Group
K3 Inhibition of cell division
Chloroacetamides, carbamates, acetamides, benzamides, oxyacetamides
Group
L Inhibition of cell wall (cellulose) synthesis
Nitriles, benzamides
Group
M Uncoupling (membrane disruption)
Dinitrophenols
Group
N Inhibition of lipid synthesis – not ACCase inhibition
Thiocarbamates, phosphorodithioates, benzofurans, chloro-carbonic-acids
Group
O Action like indoleacetic acid (synthetic auxins)
Phenoxy-carboxylic-acids, benzoic acids, pyridine carboxlyic acids, quinoline
carboxylic acids
Group
P Inhibition of indoleacetic acid action
Phthalamate, semicarbazones
Group
R/S/T –
Group
Z Unknown
Arylaminopropionic acids, organoarsenicals, others, benzylethers
8. What to do
in cases of confirmed herbicide resistance
In 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 of the crop in the field and the extent of the problem.
Some
options to consider:
• Eradicate the remaining weed
population, if growing in patches, in order to limit build-up and spread of
seed in the soil.
• Limit the field to field movement of
resistant populations by cleaning planting, cultivation and harvesting
equipment to avoid transfer of resistant weed seed.
• Avoid using the herbicide to which
resistance has been confirmed unless used in conjunction with herbicides having
a different mode of action, active on the resistant weed population.
• If the resistant population is
widespread consider grazing the crop or cut for feed being careful not to
transfer resistant seed via manure.
• Select these fields for rotation or
set aside for the following cropping season.
• Seek advice to assist in the long term
planning of weed control in these fields.
Once resistant weed numbers are
at a controllable level, implementation of an integrated weed management system
as outlined herein will ensure that crops can continue to reach high levels of productivity
in the fields in question.
9. The cost of herbicide resistance
A recent case study analysis
carried out in England (Orson and Harris, 1997) (see also Part 4) has identified that the
development of resistance can be categorized into stages, each stage requiring
a new intensity of management. 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, the rapid increase of uncontrollable
weeds will also severely affect crop yields and may eventually impact on land
value itself.
Key to the measurement of the
cost of 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 other
situations can offer principles but not the specific detail.
10. Conclusions
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).
It is only through the
development and implementation 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.
Steps towards the management of herbicide
resistance
1. Assessment of risk through a cropping system
checklist
2. Evaluation of options (including costs) to be
adapted to local conditions
3. Implementation of a sustainable weed control
program
4. Rotation of crops to enable a variety of weed
control options
5. Rotation of cultural practices to lower the
reliance on herbicides
6. Rotation of herbicide mode of action to reduce the likelihood of resistance to a specific product group
Source
GCPF Headquarters
143 Avenue Louise
1050 Brussels, Belgium
http://www.plantprotection.org/HRAC/Guideline.html
Further information
Dr.
David Vitolo
Syngenta R-1004, Basel, CH-4002, Suiza
http://www.plantprotection.org/HRAC/
5.2.
Herbicide Resistance and Herbicide - Crop – Weed Management
From the above mentioned
presentation it is clear that there are two potent factors for the development
of herbicide resistance in some cropping system: the frequent use of a limited
range of effective herbicides and reliance upon these herbicides to the
exclusion of other forms of weed control. Under crop regimes where these
conditions prevail, resistance will occur if there is heritable variability in
response to herbicide application in weed populations and selective mortality
from the herbicides.
Cases of resistance have
developed to nearly all of the selective herbicides used in cereal and small
grain production. When weed populations become sufficiently enriched with
resistant biotypes such that they cannot be controlled by the usual rate of
herbicide and the weed burden causes loss of crop production, changes in weed
control techniques must be implemented (Matthews, 1994) and alternative
herbicide options are usually the first management consideration. The success
and range of alternative herbicides depends upon the resistance spectrum or the
extent of multiple or cross- resistance of the resistant population.
With non-target site resistant
biotypes, resistance due to the use of one herbicide or class of herbicide has
caused resistance to other unrelated chemical groups and modes of action. Some
biotypes of the Avena spp. display this type of resistance (Morrison et
al, 1992). Cross-resistance and multiple-resistance are the most difficult
types of resistance from the field management viewpoint.
Resistant Avena
populations from both North America and Australia selected with ACCase
inhibiting herbicides display target site cross-resistance to CHD herbicides
(Seefeldt et al, 1993). A non-target site cross-resistance mechanism
occurred in an APP- and CHD-resistant A. fatua biotype (Devine, et
al, 1993).
There have been considerable
advances in the understanding of the causes, nature, genetics, mechanisms and
solutions for herbicide-resistant weeds since the first triazine-resistant Senecio vulgaris
was documented 30 years ago. Understanding these factors is a necessary step in
devising effective herbicide-resistance management strategies. However,
implementing these resistance management strategies has proven to be the most
difficult step. Most growers still consider herbicide-resistance avoidance a
low priority and do not change their weed control programs to avid resistance
because of financial or logistic constraints (Heap and LeBaron, 2001).
Modeling is recently applied to
help managing the risks of herbicide resistance. In a modeling research on wild
oat (Cavan et al, 2001), plough cultivation could delay the development
of resistance relative to tine cultivation; herbicide rotation can dramatically
increase the times required for field resistance to develop in a tine
cultivation system; with annual use of APP/CHD herbicides, field resistance
develops in 15 years, whereas using alternative modes of action ones in 2 years
delays field resistance to 28 years and the resistance can be delayed for at
least 66 years if three herbicides, each with a different mode of action, are
rotated; the number of years required for field resistance to develop were not
highly sensitive to the initial density of seed bank (10‑2 - 10‑4), the
mutation rate for resistance (10‑4 - 10‑7 per generation), the rate of
out-crossing (0.1 to 100%) or the herbicide kill rate (80-95%).
In many intensive cropping
systems the simplest management option may involve techniques to reduce weed
populations such that expected weed numbers are below the economic threshold.
This can be achieved by introducing periods of land use in the crop rotation
where resistant weeds are prevented from producing seeds. In addition, many
studies indicate that viable population in the seed bank can be substantially
reduces by preventing seed set for 3 to 5 years (Zorner et al, 1984).
The introduction of alternative crops into a crop rotation may give the
opportunity to change herbicides, to alter the herbicide application rate or
change other weed control techniques.
Harvesting and grain handling
equipment are the usual sources of introduced weed seeds. The introduction of
resistant weed seeds at planting time from contaminated seed reserves retained
from the previous harvest is also a high risk. As the number of sites and the
proportion of resistant individuals increase, the possibility that mature weeds
present at harvest are resistant increases.
In most modern harvesting
machinery, the weed seeds are immediately dispersed back onto the field during
the harvesting process and continue to reinfest the site (Petzold, 1956).
Collection and removal from the field of all weed seeds processed through the
harvester during harvesting can dramatically reduce reinfestation. However,
this method of reducing weed seed infestation may not be successful for
resistant grass weeds like Avena
spp, as seed of A. fatua
and A. sterilis are
shed at maturity so many of the earlier maturing seeds fall to the ground
before harvest, that reflects the importance of implementing harvesting process
as soon as possible to reduce the weed shed seeds.
Burning dry undisturbed pastures
or crop residues after harvest has been shown to be an effective method of
destroying weed seed. The density of recently shed Avena spp. Seed for
example can be reduced by stubble burning, although the results of trials have
been variable (Wilson and Cussans, 1975). Legislative restrictions to burning
and concerns about soil degradation have limited the use of stubble burning in
many areas.
Prompt identification of the
resistant status of surviving weeds before the seed bank becomes enriched with
resistant seed is an important aspect of resistant management. The matter
should be taken seriously especially if the herbicide efficacy is declining, as
the resistance may be the cause.
It should be noted that resistant
weeds are not usually spread evenly across the whole field in the early stages
of resistance development. Patches of resistant weeds around a site of origin
of resistance can be expected and if the resistance is detected early, the size
of the infested area can be restricted. This is especially so with autogamous
weed species “like wild oat”. Prevention of weed seed set in the resistant
patches by nonselective herbicides or by cutting, if possible, is preferable.
Herbicide resistance testing to
establish the extent and spectrum of resistance is important as an aid for
management of field populations. This can be achieved with the use of herbicide
test plots in the field or from laboratory testing. Collection of mature seed
allows herbicide resistance testing to assess the effectiveness of herbicides
applied to seedlings grown from populations suspected of resistance. Laboratory
tests can be performed to identify the resistance spectrum and to identify
effective substitute herbicides.
Users are often not familiar with
the grouping of herbicides into chemical mode of action and container labels
and product information may not show the mode of action, chemical group, or
resistance risk. This increases the difficulty of appropriate herbicide
selection.
All aspects mentioned above
reflect the necessity of adopting more diverse weed control techniques (IWM) to
cope with main problems encounter in weed infestation.
5.3. The Situation in Egypt
A rare literature is available on
herbicide resistance, especially on grass weeds in Egypt, despite the use of several
herbicides including APP’s for many years to control grasses such as wild oat (A.
fatua) and Italian
ryegrass (Lolium multiflorum) in wheat. Malik (1996) reported
resistant barnyardgrass (Echinochloa
crus-galli) biotype against butachlor
in Egypt. Research is being conducted in Assiut area to investigate herbicide
resistance in wild oat infesting wheat crop (Ahmed and Abdel-Wahab, 2004).
Table 2 indicates the local consumption of herbicides used
for the control of grass weeds in Assiut area only, in the last 3 years
(Ministry of Agriculture, 2003). Thus,
research must be directed toward the study of resistance development in such
weeds, to clarify the possible occurrence of resistance in weeds invading the
main economic crops in the country.
Table 2. Rate of
application (per feddan) and local consumption of herbicides used for the
control of grass weeds in Assiut area (in the last 3 years) (Ministry of
Agriculture, Egypt, 2003).
____________________________________________________________________________________________________________________________
Herbicide Weed targeted Rate of application Amount consumed
Fenoxaprop (Puma) (APP) Italian
ryegrass 0.5
lit. 965
lit.
Clodinafop (Topik) (APP) Italian
ryegrass 0.14
kg. 5472
kg.
Benzoylprop (Suffix) (APP) wild
oat 1.25
lit. 340
lit.
____________________________________________________________________________________________________________________________
Biotypes of other weed
species, i.e. hairy fleabane
(Conyza bonariensis) (a dicot weed of Asteraceae) were first detected
resistant to herbicides in Egypt. According to WSSA, hairy fleabane was first evolved resistance to Group D/22
herbicides in 1989 and infests Unspecified. Group D/22 herbicides are known as
Bipyridiliums (Photosystem-I-ctron electron diversion). Research has shown that
these particular biotypes are resistant to paraquat and they may be
cross-resistant to other Group D/22 herbicides. Table 3 represents WSSA
citation about herbicide resistant weeds in Egypt with brief stats. No
information are recorded in the database of WSSA on the distribution or level
of infestation of Group D/22 resistant hairy fleabane in Egypt.
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Table
3. Brief statistics about the resistant weed species in Egypt.
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* The Group letters/numbers refer to the classification of
herbicides by their mode of action. To see a full list of herbicides and HRAC
herbicide classifications see Part 7.