Criteria for Confirmation of Herbicide-ResistantWeeds - with specific emphasis on confirming low
level resistance.
Ian Heap, Organizer of the “International Survey of
Herbicide-Resistant Weeds”.
Web: www.weedscience.com,
Email: IanHeap@Weedscience.org
2005.
This paper outlines the criteria used by the“International Survey of Herbicide-Resistant Weeds”
to document new cases of weed resistance on the
www.weedscience.com web site. The intent of the“International Survey of Herbicide-Resistant Weeds”
is to document practical cases of field selected,
genetically inherited resistant weed biotypes thatsurvive a rate of herbicide to which the indigenous
population was controlled. This information assists
farmers and academics in the development ofeffective weed control systems for the field andassists herbicide manufacturers in the developmentof appropriate stewardship programs for theirproducts.
For a weed biotype to be listed on the site it mustmeet all of these criteria.
1. Fulfillment of the WSSA definition ofresistance and the survey’s definition of aherbicide-resistant weed.
2. Data confirmation using acceptable scientificprotocols.
3. The resistance must be heritable
4. Demonstration of practical field impact
5. Identification as a problem weed to species
level, not the result of deliberate/artificial
selection.
Failure to conform to any one of these criteria will
prevent a case from being posted.
Criteria 1. Fulfillment of the WSSA andInternational Survey of Herbicide-ResistantWeeds definition of resistance.
The Weed Science Society of America (WSSA)
defines herbicide resistance as “the inherited abilityof a plant to survive and reproduce following
exposure to a dose of herbicide normally lethal to
the wild type. In a plant, resistance may be naturally
occurring or induced by such techniques as genetic
engineering or selection of variants produced bytissue culture or mutagenesis”.
Note that herbicide-resistant weeds fall within thisdefinition. However not all “herbicide-resistant
plants” are herbicide-resistant weeds; they may beherbicide-resistant crops or laboratory creations.

For the purposes of the survey, herbicide resistanceis defined as “The evolved capacity of a previously
herbicide-susceptible weed population to withstanda herbicide and complete its life cycle when theherbicide is used at its normal rate in an agriculturalsituation” (Source: Heap and Lebaron. 2001 in
Herbicide Resistance and World Grains).
Criteria 2. Data Confirmation of ResistanceResistance must be confirmed by an unbiased
scientist through comparison of resistant andsusceptible plants of the same species in a
replicated and scientifically sound trial. The
Herbicide Resistance Action Committee (HRAC) has
prepared a publication that specifically deals with thedetection of herbicide resistance that can be foundonline at
http://www.plantprotection.org/hrac/detecting.html(HRAC 1999).
Initial Characterization of Resistance vs. Routine
Screening
There is a big difference between testing for the
initial characterization of a putative resistant weed
biotype and the routine testing of hundreds ofsamples that may follow after the initial discovery.
The literature is full of herbicide resistance tests,
such as Petri dish bioassays, shoot assays, target
enzyme assays, and fluorescence tests (Beckie etal. 2000). However for the initial characterization ofa putative resistant weed biotype the most preferred
test is a dose response experiment under controlled
conditions (growth cabinet, glasshouse etc) using
whole plants. Almost all other tests have beendevised as quicker, and/or less expensive tofacilitate routine screening of hundreds of samples.
To confirm a new case of resistance the aim is to
mimic, as closely as possible, the normal field
application conditions in a controlled environment(growth cabinet, greenhouse etc) to determine theGR50 (dose required to reduce shoot weight by 50%
relative to untreated plants) of the resistant andsusceptible populations. A range of herbicide doses
are used that encompass sub-lethal and lethal
doses for both resistant and susceptible populations(Heap, 1994). Resistance in this experiment isconfirmed if there is a statistical difference in
response to the herbicide between the putative
resistant population and the susceptible population(note: this is the scientific definition referred to
below). Non-linear regression models are used tocompare biotypes (Streibig, 1988; Brain andCousens 1989, Seefelt et al. 1995). If theregressions are statistically different then resistanceis usually reported as a ratio based on the GR50’s of
the resistant biotype compared to the susceptible.

Typically it is a relatively straight forward process to
document resistance when the level of resistance
exhibited is clear cut with an R/S resistance ratio(based on GR50’s) of greater than 10 fold. However
the confirmation of low level resistance is much
more difficult and in some cases subjective.
Disputes over the definition of resistance primarily
result from differing view points on what constitutes
natural variation in weed populations and what isclassified as low level resistance. The mostimportant point of this paper is to clarify what we
consider the cut off point for low level resistance andthe requirements needed to test such cases.
Low Level Resistance
At what level do we declare a weed to be resistantand place it on the list of resistant weeds? This
question is not easily answered. It is intriguing tome that many of us that have been involved in
identifying herbicide resistance have an intuitive feelfor true cases of resistance; however it is extremely
difficult to put a clear cut definition down on paper.
There is a scientific definition and an agricultural
field definition of resistance. Neither of them alone
is ideal for the purpose of this survey, which is toaccurately reflect the occurrence of a herbicideresistant weed problem. There are strengths andweaknesses for each definition, and the International
Survey of Herbicide-Resistant Weeds relies upon a
combination of the two. Below is a description ofeach definition, along with the problems that thedefinition presents, and the final combination of thetwo.
1. Scientific definition.
From a scientific view point resistance canbe defined as a genetically inheritedstatistical difference in herbicide response
between two weed populations of the same
species.
The problems with the scientificdefinition.
The scientific definition does not take into
account the recommended rate of aherbicide. Although two populations may
statistically differ in their response to aherbicide it does not necessarily follow thatthe herbicide will not kill the most resistant ofthem at the recommended rate. A problem
arises when a weed population is declaredresistant under the scientific definition, but it
is killed by the recommended rate of theherbicide under field conditions. Weed

populations taken from different regions arelikely to vary in their GR50 values for aherbicide, some of them being clearlystatistically different than others. This isnatural variation and does not warrant listing
as a herbicide-resistant weed. Companies
already know this variation exists and set
their recommended rate for a particular
weed based on widespread trials thatencompass this naturally occurring diverse
response to a herbicide. A better scientificview point would be that a population differs
significantly in response to a herbicide whencompared to the average response fromnumerous populations.
2. Agricultural field definition.
Using this definition a classification of“resistant” requires that the resistantpopulation must survive the recommended
rate of herbicide under normal field
conditions.
The problems with the agricultural fielddefinition.
The problem that scientists have with using
the recommended rate as a yard stick is thatthe recommended rate is a subjective ratethat may vary from region to region
depending on the crop or situation, or even
economics of a herbicide. Thus it would bepossible to define the same weed population
as resistant in one crop and susceptible inanother. (In addition the recommended rateis usually targeted at the most difficult tocontrol weed. Thus some weed species
may evolve a 4 fold level of resistance to aherbicide yet still be killed by the“recommended rate”.
Using the recommended rate as a measureof resistance without consideration for
relative resistance to a susceptible control
can be misleading. Application of therecommended rate on the same biotype will
give varying results depending on theconditions. The environment (weather, soil
type, growing conditions, stress etc) all playa big role in the effect of a herbicide on aweed. For marginal cases of resistance, a
population that typically survives a field
application may succumb to therecommended rate under greenhouseconditions. This result may be because therecommended rate under greenhouse or
growth room conditions is often much more
effective on weeds than when the herbicideis applied under field conditions.
Alternatively, it may indicate that further fieldtesting is necessary to discern whether
environmental conditions were originally atplay. Using the recommended rate alone is
not a sufficient criterion for classification ofresistance.
3. Practical definition of resistance.
If we were to rely upon a scientific definition
alone then the survey would be clogged withmany cases of “resistance” or varied
tolerance that were of no practicalconsequence whatsoever. If we were to rely
solely upon an agricultural field definition
then we would be relying on a relatively
subjective approach that is likely to change
from crop to crop and region to region. A
combination of both a scientific andagricultural perspective is necessary toprovide a practical definition of resistance.
Clearly the scientific definition is the lowest
hurdle, and any population that does notpass this definition cannot be listed. For the
purpose of the survey, the requirement todemonstrate a practical impact of the caseof resistance is that the resistant population
has caused a problem of control in the field
when the herbicide is used at therecommended field rate. When investigating
a case of low level resistance, it isnecessary to provide much moreexperimental evidence than for high-level
resistance (i.e., at least 10 fold). One dose
response experiment will not suffice forcases that exhibit less than 10 foldresistance. It will require both greenhousedose response experiments and field
experiments using susceptible andpotentially resistant plants of similar size andlocation. Ideally field experiments should be
replicated on more than one site and overmore than one year.
Criteria 3: The resistance must be heritable
In some cases scientists have tested for resistanceby removing plants from the field, potting them up,
and then running dose response experiments onthem. This may be a quick test to determine if
further trials should be conducted, but it is not
acceptable for confirmation of a new case ofresistance. The R (potentially resistant) and S(susceptible) populations may be at different growthstages, or may already have been exposed to aherbicide in the field, which can severely affect theoutcome of the experiment.

Fortunately this is not usually an issue, as thetesting procedures usually require the collection ofseed from resistant and susceptible populations.
For the purpose of listing a biotype in the survey,
testing with collected seed is generally required forsexually propagated species. It is also preferred,
but not absolutely required, that second
generation seed from greenhouse grown plants of R
and S populations are collected and tested forresistance.
Criteria 4.
Demonstration of practical field
impactThe survey is intended for practical relevance rather
than to document natural variations in herbicide
response between weed populations. If there is no
detectable difference in control of the weed underfield conditions at the recommended rate then it will
not be added to the survey. Although valid scientificarguments may be made for the occurrence of lowlevel resistance even when the weed is controlled by
the field rate under field conditions, for the purposesof a practical survey, the weed must present a
problem to the farmer when using the herbicide at
the recommended rate. This criterion becomes evenmore critical when low level resistance is involved.
Criteria 5: Be a weed and identified to species
level and not be the result of deliberate/artificial
selection.
To be classified as a herbicide-resistant weed, theplant in question must be a weed and identified
down to species level. Cases of deliberate selection
for herbicide resistance, including herbicide-resistantcrops as volunteers, are not included in the survey.
Conclusion
Listing a weed biotype in the survey must be
conducted with prudence to ensure the claim isaccurate. It should also be timely to allow
appropriate guidance for herbicide manufacturers
and growers. These criteria are intended to facilitatethat end through a combination of objectivity,transparency and consistency that is critical to thescientific method and the practical application ofexpert experience.
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