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and 2002, respectively. Resistantplants are almost completelyunaffected by herbicides applied atnormal field rates. In addition,groundsel populations resistant totriazinone herbicides (eg metribuzinand metamitron) have been recordedin UK asparagus fields.
Always read product labels, consider your local conditions and consult a professional agronomist,if necessary.
Herbicide resistanceAcetolactate synthase (ALS)inhibiting herbicides, such as thesulfonylureas (eg metsulfuron) andtriazolopyrimidines (eg florasulam)are fundamental to most broad-leaved weed control strategies in UKcereals. However, increasingoccurrence of resistance means it isvital that the resistance risk is betterrecognised and managed.
Worldwide experience shows thatuse of ALS inhibitor herbicides posesa high resistance risk. There are now158 species resistant to ALSherbicides in 44 countries – morethan any other herbicide class.Resistance to ALS sulfonylureaherbicides in chickweed, poppy andscentless mayweed was firstidentified in the UK in 2000, 2001
Latest information– Populations of herbicide-resistant
broad-leaved weeds are stilluncommon in the UK but areincreasing.
– Resistant poppy is the mostcommon, followed by chickweedand mayweed.
– Most cases of resistance are tothe ALS inhibitor group ofherbicides.
Action– Avoid using ALS inhibitors and
triazinone herbicides as the solemeans of control.
– Investigate cases of poor weedcontrol and keep good sprayrecords.
– If you suspect resistance, collectseed samples for testing.
– Use herbicides with other modesof action to reduce resistance risk.
Herbicide resistance inbroad-leaved weeds
Information Sheet 54Summer 2016
ALS-resistant poppy populationtreated with ALS herbicides only(left) and with non-ALSherbicides (right)
Key pointsTo minimise resistance risk:– Avoid using ALS inhibitors and
triazinone herbicides as the solemeans of broad-leaved weedcontrol in successive years
– Always use ALS inhibitors andtriazinone herbicides in mixture,sequence or rotation withherbicides with different modesof action
– Use cultural control methods,such as non-cereal rotations,competitive crops andcultivations, where possible
– Investigate cases of poor weedcontrol and keep good sprayrecords
– A long history of use of ALSherbicides and decliningperformance against a broad-leaved weed species when othersusceptible weeds are still wellcontrolled, indicates resistancemay have developed
If resistance is suspected:– Collect seed or plant samples for
resistance testing (see backpage) – seek advice about thebest techniques
– Use alternative herbicides withlower inherent resistance risk aspart of a resistance managementstrategy
– The limited efficacy of most non-chemical control measures,together with the long persistenceof broad-leaved weed seeds in thesoil (often >10 years), mean thatdetecting resistance at an earlystage is vital
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PoppyALS-resistant populations of poppyhave been found in 13 counties inEngland.
Studies of UK populations haveshown that resistance is conferred byALS target site resistance mutations(eg Pro-197-Leu or Pro-197-His), withthese mutations conferring high levelsof resistance to the sulfonylureaherbicides, metsulfuron andtribenuron.
Pendimethalin gives good control ofALS-resistant poppy and is a goodpre-emergence option. MCPA,bromoxynil and the more recentlyintroduced halauxifen-methylherbicides are all effective post-emergence alternatives in cereals.
Resistant poppy seeds are likely topersist in the soil seedbank for manyyears (>10 years), so it is unlikelythat sulfonylureas will ever beeffective again for control of poppy inaffected fields.
ChickweedMore than 50 ALS-resistantchickweed populations have beenidentified in 13 counties of England,Scotland and Northern Ireland. Thefrequency of resistant chickweed inScotland and Northern Ireland maybe linked to continuous spring barleycropping and greater dependence onsulfonylureas for broad-leaved weedcontrol.
Two different ALS target sitemutations have been identified in UKpopulations – Pro-197-Gln and Trp-574-Leu. The Pro-197-Gln mutationappears to be much more commonand is associated with resistance tothe sulfonylurea, metsulfuron-methyl,but not to the triazolopyrimidine,
florasulam. In contrast, the Trp-574-Leu mutation is associated withresistance to both herbicide groups.
If resistance is confirmed, usealternative (non-ALS) herbicides, suchas fluroxypyr in cereals, whichremains highly effective.
Resistance to mecoprop has beenfound in chickweed in the UK in thepast (1985) and inadequate controlhas been reported in Scotland andIreland. However, all samples of ALS-resistant chickweed tested so farhave been well controlled bymecoprop in screening assays byRothamsted Research.
* The Proline 197 mutation conferredresistant to the sulfonylureametsulfuron but not to thetriazolopyrimidine florasulam, whichwas fully effective on this populationfrom Perthshire.
+ The Tryprophan 574 mutation conferredresistance to both metsulfuron andflorasulam and both herbicides gavevery poor control of resistant plants onthis population from Aberdeenshire.
Limited evidence suggests that Proline197 resistance is more common thanTryprophan 574 resistance. No resistancewas found to the alternative herbicidestested. Mecoprop-P and fluroxypyr werefully effective on all populations.
Effects of ALS target site mutations on cross-resistance patterns in chickweed
Maps produced by Rothamsted Research
Defra/PSD-funded researchproject PS2709
100
80
60
40
20
0
-20
% c
on
tro
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pla
nts
Population
Susceptible ALS TS Resistant(Tryptophan 574)+
ALS TS Resistant(Proline 197)*
Metsulfuron Florasulam Mecoprop-P Fluroxypyr
GroundselTriazine resistance has beenconfirmed in UK populations ofgroundsel. Although triazineherbicides were banned in the EU in2007, triazinone herbicides (such asmetribuzin and metamitron) have thesame mode of action as thetriazines. Triazinone herbicides arestill widely used for weed control inpotatoes, sugar beet and othercrops.
The first documented cases oftriazinone resistance were identified
in 2010 in four populations ofgroundsel from asparagus fields inWorcestershire and Warwickshire.These populations are highlyresistant to triazines and have partialresistance to the triazinoneherbicides, metribuzin andmetamitron.
Wherever possible, try to avoid usingthe triazinone herbicides as the soleor main means of control of broad-leaved weeds. Preferably, use themin mixture, sequence or rotation with
alternative modes of action. Ifalternative herbicides are unavailable,consider non-chemical methods,such as inter-row hoeing and hand-rogueing, to reduce dependence onherbicides.
Detecting resistance early can helpgreatly. If resistance is confined to arelatively localised area, it may bepossible to destroy this area withglyphosate or cultivations and henceminimise further spread.
Scentless mayweedALS-resistant populations of scentlessmayweed have been identified in twocounties in England and one inScotland but the true number ofcases is probably under recorded.
Studies with the Scottish populationshowed that resistance was conferredby a Pro-197-Gln target site mutation.
If resistance is confirmed, usealternative (non-ALS) herbicides, suchas clopyralid and bromoxynil, whichremain highly effective.
Future issues– In the UK, herbicide-resistant
broad-leaved weeds are currentlyless of a threat than resistantgrass weeds, because resistanceis confined to the ALS andtriazinone herbicide classes, withalternative modes of actionremaining effective; however, thissituation could change
– Control of ALS-resistant broad-leaved weeds depends on thecontinued availability of effectivealternative herbicides, which maybe affected by further loss ofactive ingredients as aconsequence of regulatorydecisions during re-registration
herbicides is also a particular risk– cases already occur in severalother European countries
– Resistance to glyphosate in bothbroad-leaved and grass-weeds isa threat and, while no cases haveso far been confirmed in the UK,there are now 35 glyphosate-resistant species in 27 countriesworldwide
– It should not be assumed thatresistance cannot, or will not,evolve in other species or to otherherbicides and continued vigilanceis required; particular high-risksituations are those where controlof a specific weed is dependent ona single or very limited range ofherbicide modes of action
– No cases of herbicide-resistantcharlock or wild radish have yetbeen detected in the UK but theUK Weed Resistance Action Groupconsiders these to be some of theweeds most likely to evolveresistance here in future;resistance of fat-hen to triazinone
Further informationLynn Tatnell, [email protected]
Stephen [email protected]
Paul Gosling, [email protected]
Managing weeds in arablerotations – a guide (AHDB, 2014)
Minimising the risk of glyphosateresistance (AHDB, 2015)
cereals.ahdb.org.uk/weeds
Publication orders0845 245 [email protected]
Research organisations, majoragrochemical manufacturers andsome commercial companiesmay test seed for resistance onrequest. Speak to your local rep orone of the people named above.
Information Sheet 54Summer 2016
AHDB Cereals & Oilseeds publications are free to levy payers
Electronic version can be downloaded at cereals.ahdb.org.uk
To join the mailing list, contact:[email protected] © Agriculture and Horticulture Development Board 2016. All rights reserved.
While the Agriculture and Horticulture Development Board seeks to ensure that the informationcontained within this document is accurate at the time of printing, no warranty is given in respectthereof and, to the maximum extent permitted by law, the Agriculture and Horticulture DevelopmentBoard accepts no liability for loss, damage or injury howsoever caused (including that caused bynegligence) or suffered directly or indirectly in relation to information and opinions contained in oromitted from this document.
Reference herein to trade names and proprietary products without stating that they are protected doesnot imply that they may be regarded as unprotected and thus free for general use. No endorsement ofnamed products is intended, nor is any criticism implied of other alternative but unnamed products.
AHDB Cereals & Oilseeds is a part of the Agriculture and Horticulture Development Board (AHDB).
Seed samplingIf herbicide resistance is suspected,collect seeds for testing. This isusually more difficult than with grass-weeds and the best technique andtiming varies with species. If indoubt, seek advice – many cases ofresistance go unreported due to lackof adequate seed samples for testing.
For seed collection you will need:– Large container or paper sack
– Paper bag/envelope with thebottom taped up to prevent seedescape (do not use plastic bags forstorage of freshly collected seeds,as this will prevent them drying out)
– Sharp knife or scissors
Once the seeds have been collected,place them in the paper envelope,leaving it open in a dry, airy place,away from sunlight. When the seedsare dry, send them for resistancetesting. Do not send wet seeds.
Poppy seedsCollect poppy seeds in July/Augustwhen the seedpods become agreyish-brown colour.
The brown seed heads will begin toopen just under the ‘lid’ and a row ofholes will be visible.
– Cut the seedpods from the stem
– Place the seedpods in the paperbag or envelope
– Leave the seeds in a well-ventilated area, keeping the bagopen to the air to promote drying
Scentless mayweed and chickweedseedsCollect mayweed in August whenmature; the petals will havedisappeared and the central domewill be yellow/brown in colour.
Collect chickweed in June, July andAugust. Check chickweed plants forflowers and seedpods. Seedpods willalways be green.
For collecting seeds of both mayweedand chickweed:– Pull up/cut at the plant base large
quantities of mayweed/chickweedand place in a large paper sack – donot pack too tightly but allowenough room for air to circulate
– Place the open sack in a well-ventilated area where the plants candry out; as they dry, seeds will fallfrom the heads to the bottom of thesack
– When dry, remove the mayweed/chickweed straw, shaking any looseseeds into the sack
– When the straw has been removed,tip the seeds into an envelope
Groundsel seedsCollect groundsel seeds betweenMay and October. When plants aremature, the petals will havedisappeared and the seed heads willbe white and fluffy.
– Pinch the white top of the seedsand pull them off the plant
– Place the seeds in a paperbag/envelope
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