How Do Herbicides Work?

What is a herbicide?
            Herbicides are substances that are used for killing all types of plants, especially noxious weeds.  Herbicides can be put into eight different groups including one group that is an ungrouped or, catch-all catagory.

How herbicides work?
            Herbicides work in many different ways.  In order to be effective, herbicides must first be applied to a plant.  After the herbicide is applied, it must then be retained on the leaf and penetrate the waxy cuticular layer that is on the leaf’s surface.  Then it must move through the water-filled space that is around the cell.  Once the herbicide has moved through the water-filled cell it must then enter a new cell while passing through a lipid-like membrane.  When it has passed through that cell, the herbicide will usually reach its target.  The target is usually and most likely an enzyme (a protein catalyst that helps a chemical reaction within the cell).  Finally when the herbicide reaches the target, it will bind to and inhibit the target enzyme.
            To be selective, the herbicide must take a different approach.  It must first affect the target weed, not the crop.  Then it must be metabolized, or broken down, by the crop plant, and not by the weed itself.

When must herbicides work reliably?
            Herbicides work reliably when the crop plants and weeds are growing rapidly.  They also work reliably when the crop plants and weeds are under stress.

What are the target sites for herbicides?
             Herbicides have certain target sites that they effect.  The most common target site for an herbicide is an enzyme, usually only one.  When herbicides bind to the target they are stopping and slowing down the enzyme's function.  If herbicides are in the same group, they can vary in the types of weeds that they have affects on.  Plus they can also wary in the crops in which they can be used.  For more information on the types of herbicides and their target sites, refer to the chart below.                               

 

Target site

Active Ingredient

Product Names

Group 1

ACCase
(grass herbicides)

clethodim

Select

 

 

clodinofop

Horizon

 

 

diclofop

Hoegrass, Hoegrass II*

 

 

fenoxaprop

Fusion, Laser, Laser DF*, Puma,
Triumph Plus*

 

 

fluazifop

Fusion, Venture

 

 

quizalofop

Assure

 

 

sethoxydim

Achieve, Achieve Extra*

 

 

tralkoxydim

Poast, Poast Flaxmax*

Group 2

ALS

ethametsulfuron

Muster

 

 

imazamethoabenz

Assert

 

 

imazamethapyr

Pursuit

 

 

metsulfuron

Ally

 

 

thifensulfuron

Laser DF, Refine Extra*,
Triumph Plus*

 

 

triasulfuron

Amber, Unity*

 

 

tribenuron

Express, Refine Extra*

Group 3

Cell division

ethalfurlain

Edge

 

 

trifluralin

Advance, Fortress,
 Rival, Treflan

Group 4

Auxin mimics

2,4-D

2,4-D, Attain*, Champion Plus*, Dycleer*,
 Estaprop*, Thumper*, Tordon 202C*,
Turboprop*

 

 

2,4-DB

Caliber, Cobutox, Embutox

 

 

2,4-DP

Diphenoprop, Estaprop

 

 

clopyralid

Lontrel, Prevail*, Curtail*,
Poast Flaxmax*

 

 

dicamba

Banvel, DyVel, DyVel DS,
Target

 

 

fluroxpyr

Attain*

 

 

MCPA

MCPA, Achieve Extra*, Buctril M*,
Cahmpion Plus*, Dyvel*, Laser*,
Laser DF*, Mirage*, Poast Flaxmax*,
Stampede CM*, Target*, Triumph Plus*,
Tropotox*

 

 

MCPB

Tropotox*

 

 

mecoprop

Mecoprop, Compitox

 

 

picloram

Tordon

Group 5

PSII inhibitor

cyanazine

Bladex, Blagal

 

 

metribuzin

Crossfire*, Lexone, Sencor

Group 6

PSII inhibitor

bromoxynil

Achieve Extra*, Buctril M*, Hoe-Grass II*,
Laser, Torch/Pardner, Unity*

Group 7

PSII inhibitor

linuron

Afolan, Lorex

Group 8

More than one target

triallate

Avadex BW, Avenge Fortress

Ungrouped

Each are ungrouped
and unrelated

Glyphosate

Laredo, Renegade, Roundup, Rustler,
Touchdown, Wrangler

 

 

bentazon

Basagran

 

 

EPTC

Eptam

* contained in mixtures

Why herbicides kill plants?
            Even though there is much known about the target sites, it is not always clearly understood as to why plants die after an herbicide application.  We do know how herbicides kill plants, even if we do not know a whole lot about why they die.  
            Herbicides kill plants by causing a build up of a toxic substance, where the toxic compounds stay at reasonably low levels.  By inhabiting the target site (enzyme), herbicides cause substances to build up and damage the plant.  This is how the herbicide glyphosate works.  In some other cases, the death of the target plants seems to occur from de-regulation of the vary carefully controlled process of cell growth.  This is how herbicides such as 2,4-D are effective.  The plant essentially grows it self to death.

What are some selective and non-selective types of herbicides?
            There are many types of herbicides, but some are selective and others are non-selective.  The selective ones include metsulfuron methyl and it does not affect cereal crops.  They usually  kill mainly broad-leaveled weeds but, not the grasses.  The non-selective ones include glyphosate and they affect most plants both grasses and broad-leaved.

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Literature Cited
Alberta agriculture, Food and Rural Development, How Herbicides Work, Mechanisms of Action, copywrite 1996, 56pgs.

By:  Cliff Martin, 3/24/04