1. Herbicide Modes of Action
Eric P. Prostko
Extension Weed Specialist
Department of Crop & Soil Sciences
The University of Georgia
April 2008

2. Some things to remember….
photosynthesis (food)
respiration (energy)
amino acids (proteins/growth)
lipids (cell membranes)
pigments (energy/light capture)
chlorophyll, carotenoids
mitosis (cell division)

3. 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

4. Mechanism of Action (Site)
Location at which a herbicide exerts its toxicity at the cellular level
more specific

5. 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

6. “rotted” meristem
Source: W. K. Vencill, UGA

7. ALS Inhibitors Inhibit acetolactate synthase (ALS) enzyme
enzyme needed to produce certain amino acids (isoleucine, leucine, valine)

8. Commonly Used ALS Herbicides
Accent
Ally/Cimarron
Beacon
Beyond
Cadre
Classic
Envoke
Exceed
Express
Finesse
FirstRate
Glean
Harmony Extra
Maverick Pro
Osprey
Peak
Permit
Pursuit
Python
Scepter
Staple
Strongarm

9. Cadre “yellow flash”
Staple+ on Sorghum

10. Resolve 25WG (rimsulfuron) Injury to Field Corn
18 DAT
1.0 oz/A Applied PRE

11. Photosystem II Inhibitors
Inhibit photosynthesis by binding to QB-binding niche on D1 protein
Evik, Caparol, Atrazine, Basagran, Buctril, Direx, Cotoron, Lorox, Sencor, Simazine

12. Bladex
Cotoran
Source: W. K. Vencill, UGA

13. Photosystem I Inhibitors
Accept electrons from PSI to form radicals
Super radicals formed to create peroxides
Diquat, paraquat

14. Paraquat
Source: Purdue University

15. 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

16. Blazer Injury on Peanut
Valor Injury on Peanut
Blazer Injury on Peanut
Cobra on Soybeans

17. Carotenoid Biosynthesis Inhibitors
Inhibit phytoene desaturase (PDS) or hydroxyphenyl pyruvate dioxygenase (HPPD)
Command, Callisto, Laudis, Solicam/Zorial, Impact

18. Command
Zorial
Laudis

19. EPSP Synthase Inhibitor
EPSP synthase needed for production of certain amino acids (tryptophan, tyrosine, and phenylalanine)
glyphosate

20. Source: Purdue University Weed Science
Source: MSU Weed Science

21. Glutamine Synthetase Inhibitors
Glutamine synthetase converts glutamate and ammonia to glutamine
Causes ammonia accumulation in plants
Liberty, Ignite

22. Source: University of Missouri Weed Science

23. Mitosis Inhibitors Microtubule inhibition
Treflan, Prowl, Sonalan, Surflan, Dacthal
Inhibition of very long-chain fatty acids
Dual Magnum, Outlook, Intrro

24. Prowl Injury on Corn

25. Dual Injury
Source: University of Wisconsin Weed Science

26. Fatty Acid and Lipid Biosynthesis Inhibitors
Inhibition of lipid synthesis (not ACCase)
Prefar, Sutan, Eptam, Eradicane, Ordram, Tillam, Bolero

27. Source: Purdue University Weed Science
Source: UC-Davis Weed Science

28. Synthetic Auxins Disrupt hormonal balance
2,4-D, Banvel, Clarity, Status, Stinger, MCPA, Facet, Remedy, Grazon P+D

29. Source: Purdue University Weed Science

30. Unknown MOA
DSMA, MSMA
Cell membrane destruction???

31. MSMA on Peanut
MSMA on Zoysia

32. UGA Weed Science