Herbicide Mode of Action

Herbicide Mode of Action

How a herbicide worksUse herbicides more efficiently

Recognize and diagnose injury problems

Herbicide resistance management

Herbicide –Plant Interactions

Absorption

Translocation

Metabolism

Mechanism of Action

Physiological response

Requirements for Herbicide Activity

Contact with the target weed

Absorption into the plant

Accumulation of toxic levels at the site of action

Herbicide Selectivity

Differential response of plant species to a herbicide

Selective

Nonselective

Herbicide SelectivityApplication RateHerbicide PlacementSpray RetentionDifferential AbsorptionDifferential TranslocationMetabolismAltered Site of Action (Herbicide Resistance)

Absorption of Soil-Applied Herbicides

Roots

Shoots

Absorption of PostemergenceHerbicides

Leaf surface coverageLeaf surface propertiesHerbicide-spray propertiesTemperatureHumidityPlant Vigor

Herbicide MetabolismChemical transformations of a herbicide in a plantMetabolism generally results in detoxification of a herbicide, but also can increase the toxicity of a herbicide

Altered Site of ActionInability of a herbicide to bind to the site of action due to a genetic and conformational difference at the binding site compared to susceptible biotypes.

Herbicide Resistance

Modified Site of Action

Triazine Resistant Palmer Amaranth

Herbicide Cross ResistanceWeed resistance to different herbicides that have the same mode of action.ALS herbicides

SulfonylureasImidazolinonesTriazolopyrimidines

ALS Cross Resistant Cocklebur

Herbicide Multiple Resistance

Weed biotypes with resistance to herbicides having different modes of action.Examples: kochia and pigweeds that are resistant to both triazine and ALS inhibiting herbicides.

ALS & Triazine Resistant Waterhemp

Herbicide Translocation

Movement of the herbicide inside the plant

Contact herbicides: not translocated

Systemic herbicides: translocatedPhloem translocated

Xylem translocated

Herbicide TranslocationPhloem Translocated:RoundupPoast

Foliar Contact: ParaquatCobraBuctrilBasagran

Xylem Translocated:

Phloem & XylemTranslocated:

2,4-DBanvelTordonGleanPursuit

XylemPhloem

Atrazine, Command, Balance

Contact vs. Systemic Herbicides

Timing Effects on Annual Weed Control

Emergence Seedling Vegetative Flowering MaturityGrowth Stage

Wee

d C

ontr

ol

Timing Effects on Perennial Weed Control

Dormancy Vegetative Bud Flowering Maturity Fall

Growth Stage

Wee

d C

ontr

ol

Herbicide Mechanisms of ActionPlant Growth RegulatorsSeedling Growth InhibitorsPhotosynthetic InhibitorsCell Membrane DisruptorsAmino Acid Synthesis InhibitorsLipid Synthesis InhibitorsNitrogen Metabolism InhibitorsPigment Inhibitors

Plant Growth RegulatorsPhenoxies: 2,4-D, MCPA,

2,4-DB , etc.Benzoic Acids: Banvel, Clarity, DistinctPyridines: Tordon, Remedy,

Crossbow, Stinger, Starane

Quinolines: Paramount

Puckered Soybean leaves from Tordon

Tordon Runoff into Soybean Field

2,4-D on Grain Sorghum

2,4-D Damage to Corn Brace Roots

Fall applied 2,4-D on Wheat

Late Application of Banvel + 2,4-D on Wheat

Late Application of Paramount on Giant Foxtail

Growth Regulator Use Concerns

Drift and injury to nontarget plants

Carryover: Tordon

Groundwater Contamination: Tordon

Seedling Growth Inhibitors

Thiocarbamates: Sutan, Eradicane, Eptam, Fargo

Acetanilides: Lasso, Partner, Dual Magnum, Outlook,

Harness, Surpass, Axiom, Ramrod

Dinitroanilines: Treflan, Prowl, Balan, Sonalan

Acetamides(Lasso, Dual, Outlook, Harness, Surpass, Define, Ramrod)

Applied preemergence or with shallow incorporationDisrupts cell development in emerging shoot (coleoptile) during germination and emergence

Acetamide Injury to Soybeans

Acetamide Injury to Grain Sorghum

Safened and Unsafened Sorghum treated with Dual

Dinitroanalines(Treflan, Prowl, Balan, Sonalan)

Generally incorporated to mix the herbicide into the soil and reduce photodecomposition and volatility losses

Inhibits cell division (mitosis)

Active on emerging shoot and the root tips

Treflan Damage to Corn Roots

Swollen hypocotyls and Poor Root Development from Treflan

Brittle Stems and Lodging from Prowl Damage

Photosynthetic InhibitorsTriazines: Atrazine, Sencor, Lexone,

Bladex, Princep, VelparPhenylureas: Lorox, Karmex, SpikeUracils: Sinbar, HyvarNitriles: BuctrilPyridazines: ToughBenzothiadiazones: Basagran

Atrazine Carryover on Wheat

Triazine Injury to Soybeans

Cell Membrane Disruptors

Photosystem I Inhibitors:Dipyridiliums - Gramoxone Extra, Diquat

PPO Inhibitors:

Diphenylethers - Blazer, Cobra, Flexstar

Triazolinones - Aim, Authority, Spartan

Phenylthalimides - Resource

Cell Membrane Disruptors(Paraquat, Cobra, Blazer, Authority)

Contact herbicides - thorough spray coverage and small weeds essential for good weed control

Light activated herbicides that quickly destroy cell membranes, resulting in “leaf burn” type symptom

Cobra Burn on Soybeans

Rapid Burn Activity of Cobra

Aim Injury on Sorghum Leaves

Amino Acid Synthesis InhibitorsALS inhibitors

Sulfonylureas: Glean, Amber, Ally, PeakFinesse, Classic, Pinnacle,

Synchrony STS, Accent, Beacon, Exceed, Permit

Imidazolinones: Scepter, Pursuit, Lightning, Raptor

Triazolopyrimidines: Broadstrike, Python, FirstRate

ESPS inhibitors:Roundup, Touchdown

ALS Inhibitors(Glean, Peak, Accent, Pursuit, Python, etc.)

Absorbed through both roots and shoots and readily

translocated in both xylem and phloem

Inhibits the Acetolactate Synthase (ALS) enzyme

responsible for the production of branched chain amino

acids valine, leucine, and isoleucine

Quick development of herbicide resistance

Chlorosis of New Growth from Glean on Tansy Mustard

ALS Injury to Soybeans

Peak Carryover Injury to Soybeans

Basis Injury to Corn Following Cold Weather

Bottle-brushing of Roots from ALS herbicide Carryover on Corn

EPSP Inhibitors(Glyphosate)

Tightly adsorbed and inactive in soil

Phloem translocated

Inhibits EPSP enzyme responsible for production of aromatic amino acids phenylalinine, tyrosine and tryptophan

Very nontoxic

Gradual Death from Roundup Treatment

Genetically Engineered Roundup Ready Corn

Roundup Drift Damage to Corn

Chlorotic Roundup Ready Soybeans from Late Treatment

Nitrogen Metabolism Inhibitor(Liberty, Ignite 280)

Inhibits glutamine synthetase enzyme involved with nitrogen metabolism

Limited translocation

No soil activity

Non Liberty Link Corn Treated with Liberty

Lipid Synthesis Inhibitors

Postemergence grass control herbicides

Cyclohexanediones (Dims):Poast, Poast Plus, Select

Aryloxyphenoxypropionates (Fops):Assure, Fusilade, Fusion, Option

Corn Treated with Poast Plus

“Rotten Neck” symptom from Post Grass Herbicides

Drift Damage on Corn from Post Grass Herbicide

Drought Stress Effect on Poast Herbicide Activity

Pigment Inhibitors(Command, Balance, Callisto,

Impact, Laudis, Huskie)Absorbed through roots and shoots, but translocated only in the xylemPrevents synthesis of pigments that protect chlorophylFoliage turns white and appears bleached

Bleached Crabgrass from Balance Treatment

Balance Injury to Corn with Cold Wet Conditions

Command Carryover Injury to Double Cropped Wheat

Command Carryover Injury to Oats

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