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Herbicide-resistant weeds are one of the biggest challenges to top-end yield potential. However, there are steps you can take to mitigate herbicide resistance and control weeds in your fields.
This post covers:
Weeds have evolved resistance to 21 of the 31 known herbicide sites of action and 168 different herbicides, according to the Herbicide Resistance Action Committee.
If you struggle to manage tough weeds, it may be time to rethink your herbicide strategy. Relying too heavily on the same herbicides or herbicides with the same sites of action can result in less effective weed control and, over time, lead to herbicide-resistant weed populations.
You’re probably familiar with the terms “mode of action” and “site of action” in relation to weed control. Sometimes, the words are used interchangeably, but they aren’t quite synonymous.
A herbicide’s mode of action is how it works to eliminate the weed. Some common modes of action include cell membrane disruptors, growth regulators, and amino acid synthesis inhibitors.
In contrast, a herbicide’s site of action is where it works or what particular enzyme or metabolic process it disrupts. For example, ACCase inhibitor herbicides target the acetyl CoA carboxylase enzyme, which inhibits fatty acid and lipid production in weeds.
If it’s time to change your herbicide strategy, use the chart below to identify opportunities to diversify your modes and sites of action.
Site of Action 1 | ACCase Inhibitor
Active Ingredient | Product Example |
clodinafop | Discover® NG |
cyhalofop | Clincher® |
fenoxaprop | Ricestar® HT, others |
fluazifop | Fusilade® DX |
quizalofop | Se-Cure EC, Assure II®, Targa® |
clethodim | Select Max®, others |
sethoxydim | Poast®, Poast Plus |
pinoxaden | Axial® XL |
Site of Action 4 | Synthetic Auxins
Active Ingredient | Product Example |
dicamba | |
2,4-D | 2,4-D LV6 Value Pick, Enlist One™ |
MCPA | |
aminopyralid | Milestone® |
clopyralid | Bite, Stinger® HL |
halauxifen | Elevore® |
fluroxypyr | Starane® Ultra |
triclopyr | Garlon®, Remedy® Ultra |
quinclorac | Facet® |
Site of Action 19 | Auxin transport inhibitor
Active Ingredient | Product Example |
diflufenzopyr | component of Status® |
Site of Action 19 | Auxin Transport Inhibitor
Active Ingredient | Product Example |
imazamox | Raptor®, Beyond® |
imazapic | Plateau® |
imazapyr | Arsenal® |
imazaquin | Scepter® |
imazethapyr | Willowood Imazethapyr 2SL*, Pursuit® |
bispyribac | Regiment® |
pyrithiobac | Staple® |
bensulfuron | Londax® |
chlorimuron | Classic® |
chlorsulfuron | Glean® |
halosulfuron | Permit®, Sandea® |
iodosulfuron | Autumn™ |
mesosulfuron | Osprey® |
metsulfuron | Ally® |
nicosulfuron | Accent® Q, Zest™ |
orthosulfamuron | Strada® |
primisulfuron | Beacon® |
prosulfuron | Peak® |
rimsulfuron | Pruvin®, Matrix® |
sulfosulfuron | Outrider® |
thifensulfuron | Harmony® |
triasulfuron | Amber® |
tribenuron | Express® |
trifloxysulfuron | Envoke® |
triflusulfuron | UpBeet® |
flucarbazone | Everest®, Pre-Pare® |
propoxycarbazone | Olympus® |
thiencarbazone | Varro® |
cloransulam | Willowood Cloran DF, FirstRate® |
florasulam | Orion®, Quelex®, Starane® Flex |
flumetsulam | Python® |
penoxsulam | Grasp® |
pyroxsulam | PowerFlex® HL |
Site of Action 9 | EPSP Synthase Inhibitor
Active Ingredient | Product Example |
glyphosate | Willowood Glypho 5, RoundUp® |
Site of Action 6 | Photosystem II Inhibitors
Active Ingredient | Product Example |
bentazon | BashAzon Herbicide, Basagran® |
bromoxynil | Maestro®, Moxy®, others |
pyridate | Tough® |
Site of Action 5 | Photosystem II Inhibitors
Active Ingredient | Product Example |
propanil | SuperWham!® |
atrazine | GCS Atrazine 4L*, AAtrex®, others |
prometon | Pramitol® |
prometryn | Caparol® |
simazine | Princep® |
hexazinone | Velpar® |
metribuzin | |
terbacil | SINBAR® |
diuron | Alligare Diuron 4L, Direx® |
fluometuron | Cotoran® |
linuron | Lorox®, Linex® |
tebuthiuron | Spike® |
Site of Action 19 | Glutamine Synthetase Inhibitors
Active Ingredient | Product Example |
glufosinate | Willowood Glufosinate 280SL, Liberty® |
Site of Action 12 | Phytoene Desaturase (PDS) Inhibitors
Active Ingredient | Product Example |
fluridone | Brake®, Sonar® |
norflurazon | Solicam® |
diflufenican | ––––– |
Site of Action 13 | DOXP Synthase Inhibitor
Active Ingredient | Product Example |
clomazone |
Site of Action 27 | HPPD Inhibitors
Active Ingredient | Product Example |
isoxaflutole | Alite™ 27, Balance® Flexx |
pyrasulfotole | component of Huskie® |
topramezone | Armezon®, Impact® |
tolpyralate | Shieldex® |
bicyclopyrone | Optogen™ |
mesotrione | Willowood Mesotrione 4SC, Callisto® |
tembotrione | Laudis® |
Site of Action 14 | PPO Inhibitors
Active Ingredient | Product Example |
acifluorfen | Ultra Blazer® |
fomesafen | Willowood Fomesafen 1.88SL, Flexstar® |
lactofen | Willowood Lactofen 2EC, Cobra® |
flumiclorac | Resource® |
flumioxazin | Valor®, Chateau®, others |
saflufenacil | |
tiafenacil | Reviton™, Gamma™ |
carfentrazone | |
sulfentrazone | Willowood Sulfen 4SC, Spartan® |
Site of Action 22 | Photosystem I Electron Diverter
Active Ingredient | Product Example |
diquat | Reglone® |
paraquat | Willowood Paraquat 3SL*, Gramoxone SL® |
Site of Action 3 | Microtubule Assembly Inhibitors
Active Ingredient | Product Example |
pronamide | Willowood Pronamide 3.3SC*, Kerb® |
ethalfluralin | Sonalan® |
pendimethalin | Pendimethalin 3.80 Value Pick, Prowl® H2O |
trifluralin | Treflan®, others |
dithiopyr | Dimension® |
Site of Action 15 | Very Long-Chain Fatty Acid Synthesis Inhibitors
Active Ingredient | Product Example |
ethofumesate | Willowood Ethofumesate 4SC*, Nortron® |
acetochlor | Arrest Plus, Harness®, Surpass®, Warrant® |
dimethenamid-P | Outlook® |
s-metolachlor | Dual Magnum®, others |
pyroxasulfone | |
flufenacet | Define™ |
cycloate | Ro-Neet® |
EPTC | Eradicane®, Eptam® |
thiobencarb | Bolero® |
triallate | Far-Go® |
Site of Action 29 | Cellulose Biosynthesis Inhibitors
Active Ingredient | Product Example |
indaziflam | Esplanade®, Rezilon® |
isoxaben | Gallery®, Trelli® |
Site of Action 0 | Unknown
Active Ingredient | Product Example |
napropamide | Devrinol® |
MSMA | MSMA |
For a list of herbicide premixes and their corresponding sites of action, review the Take Action Herbicide Classification guide.
Herbicide-resistant weed populations continue to evolve and expand, but there are steps you can take to ensure effective weed control in your fields. You can reduce your risk for herbicide-resistant weed populations by:
Using multiple modes of action to increase herbicide diversity
Using multiple sites of action to delay the development of herbicide-resistant weeds
Rotating crops to avoid overusing the same herbicides
Using full herbicide rates
Using best herbicide application practices to keep herbicides on target
Adding appropriate adjuvants to the tank mix to improve spray coverage
Incorporating tillage where appropriate to manage hard-to-control weed populations
Developing a season-long weed management strategy
An effective herbicide program can protect your crop’s yield potential throughout the season. FBN provides a range of herbicides to diversify your management program for effective and sustainable weed control.
With 24/7 digital shopping access, direct-to-farm delivery, transparent pricing and savings opportunities, and detailed label information for each product, FBN offers the information and products you need to build an effective weed management strategy this season.
Herbicides 101: Everything Farmers Need to Know About Herbicides
10 Tips for Improving Herbicide Efficacy Under Difficult Conditions
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Discover® is a registered trademark of Syngenta Crop Protection, LLC.
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Linex® is a registered trademark of Tessenderlo Kerley, Inc.
Spike® is a trademark of Nutrichem Company Limited.
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SHIELDEX® is a registered trademark of Ishihara Sangyo Kaisha, Ltd.
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Flexstar® is a registered trademark of Syngenta US.
Cobra® is a registered trademark of Valent U.S.A. Corporation.
Resource® is a registered trademark of Valent U.S.A. Corporation.
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Willowood Imazethapyr 2SL is a restricted use pesticide.
GCS Atrazine 4L is a restricted use pesticide.
Willowood Paraquat 3SL is a restricted use pesticide.
Willowood Pronamide 3.3SC is a restricted use pesticide.
Willowood Ethofumesate 4SC is a restricted use pesticide.
*RESTRICTED USE PESTICIDE Due to acute toxicity. For retail sale to and use by certified applicators only – NOT to be used by uncertified persons working under the supervision of a certified applicator.
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