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Herbicide Modes of Action
Eric P. Prostko
Extension Weed Specialist
Department of Crop & Soil Sciences
The University of Georgia
April 2008
Some things to remember….
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photosynthesis (food)
respiration (energy)
amino acids (proteins/growth)
lipids (cell membranes)
pigments (energy/light capture)
•
chlorophyll, carotenoids
 mitosis (cell division)
Mode of Action
 the sequence of events that leads to
plant death or growth interruption
 2 phases
* movement to target site
* interaction at target site
Mechanism of Action (Site)
 Location at which a herbicide exerts
its toxicity at the cellular level
 more specific
ACCase Inhibitors
 Inhibit acetyl-CoA-carboxylase
(ACCase) enzyme
 Enzyme needed for fatty acid
synthesis
 Dim’s and Fop’s
 Achieve, Assure, Axial, Fusilade,
Poast, Select
“rotted” meristem
ALS Inhibitors
 Inhibit acetolactate synthase (ALS)
enzyme
 enzyme needed to produce certain
amino acids (isoleucine, leucine,
valine)
Commonly Used ALS Herbicides
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Accent
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Glean
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Ally/Cimarron
Beacon
Beyond
Cadre
Classic
Envoke
Exceed
Express
Finesse
FirstRate
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Harmony Extra
Maverick Pro
Osprey
Peak
Permit
Pursuit
Python
Scepter
Staple
Strongarm
Cadre “yellow flash”
Staple+ on Sorghum
Resolve 25WG (rimsulfuron) Injury
to Field Corn
Pictures @ 18 DAT
1.0 oz/A Applied PRE
Photosystem II Inhibitors
 Inhibit photosynthesis by binding to
QB-binding niche on D1 protein
 Evik, Caparol, Atrazine, Basagran,
Buctril, Direx, Cotoron, Lorox,
Sencor, Simazine
Bladex
Cotoran
Photosystem I Inhibitors
 Accept electrons from PSI to form
radicals
 Super radicals formed to create
peroxides
 Diquat, paraquat
Paraquat
Source: Purdue University
PPO Inhibiting Herbicides
 PPO or Protox
• Protoporphyrinogen
oxidase
• Enzyme found in
chloroplast and
mitochondria needed for
formation of chlorophyll
• Cell membrane
destruction
 Many common herbicides
have this mode of action!!
• Ultra Blazer, Cobra, Goal,
Reflex, Ronstar, Resource,
Aim, Valor, Storm, Spartan,
ET
Blazer Injury on Peanut
Valor Injury on Peanut
Cobra on Soybeans
Carotenoid Biosynthesis Inhibitors
 Inhibit phytoene desaturase (PDS)
or hydroxyphenyl pyruvate
dioxygenase (HPPD)
 Command, Callisto, Laudis,
Solicam/Zorial, Impact
Command
Zorial
Laudis
EPSP Synthase Inhibitor
 EPSP synthase needed for
production of certain amino acids
(tryptophan, tyrosine, and
phenylalanine)
 glyphosate
Source: Purdue University Weed Science
Source: MSU Weed Science
Glutamine Synthetase Inhibitors
 Glutamine synthetase converts
glutamate and ammonia to
glutamine
 Causes ammonia accumulation in
plants
 Liberty, Ignite
Source: University of Missouri Weed Science
Mitosis Inhibitors
 Microtubule inhibition
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Treflan, Prowl, Sonalan, Surflan,
Dacthal
 Inhibition of very long-chain fatty
acids
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Dual Magnum, Outlook, Intrro
Prowl Injury on Corn
Dual Injury
Source: University of Wisconsin Weed Science
Fatty Acid and Lipid Biosynthesis
Inhibitors
 Inhibition of lipid synthesis (not
ACCase)
•
Prefar, Sutan, Eptam, Eradicane,
Ordram, Tillam, Bolero
Source: Purdue University Weed Science
Source: UC-Davis Weed Science
Synthetic Auxins
 Disrupt hormonal balance
 2,4-D, Banvel, Clarity, Status,
Stinger, MCPA, Facet, Remedy,
Grazon P+D
Source: Purdue University Weed Science
Source: Purdue University Weed Science
Unknown MOA
 DSMA, MSMA
 Cell membrane destruction???
MSMA on Peanut
MSMA on Zoysia
UGA Weed Science
www.gaweed.com