Table 1. Herbicide MOA classification system. HRAC WSSA group

Table 1. Herbicide MOA classification system.

HRAC WSSA

group group Mode of action Chemical family Active ingredients

E 14 Inhibition of Diphenylethers acifluorfen

proto- fomesafen

porphyrinogen lactofen

oxidase oxyfluorfen

(PPO, or

Protox)

O NO2

Basic structure of a diphenylether

Two phenyl rings joined by an ether bridge,

and a nitro group on one of the phenyl rings.

Mode of Action of Diphenylethers

• Inhibit porphyrin synthesis pathway, the

pathway for formation of chlorophyll and

cytochromes

• Specifically, they inhibit protoporphyrinogen

IX oxidase (PPO or PROTOX), the enzyme

that converts protoporphyrinogen IX to

protoporphyrin IX.

• Proto accumulates, free radicals, oxidative

breakdown of membranes

Diphenylether Herbicides

• Behave as contacts.

• Typically applied POST, primarily for

broadleaf weeds.

• Soil persistence varies.

• Very little soil activity with acifluorfen

and lactofen.

• Fomesafen and oxyfluorfen have soil

activity.

Table 1. Herbicide MOA classification system.

HRAC WSSA

group group Mode of action Chemical family Active ingredients

E 14 Inhibition of N-phenyl- flumioxazin

proto- phthalimides flumiclorac-pentyl ester

porphyrinogen

oxidase

(PPO, Protox)

Flumiclorac-pentyl ester

Flumioxazin

Flumioxazin and Flumiclorac-pentyl Ester

• Foliar absorbed; very limited translocation.

• Act as contacts.

• No soil activity with flumiclorac-pentyl

ester; flumioxazin controls weeds PRE for

30 days or more (depending upon rate).

Table 1. Herbicide MOA classification system.

HRAC WSSA

group group Mode of action Chemical family Active ingredients

E 14 Inhibition of Oxadiazoles oxadiazon

proto-

porphyrinogen

oxidase

(PPO, Protox)

oxadiazon

Table 1. Herbicide MOA herbicide classification system.

HRAC WSSA

group group Mode of action Chemical family Active ingredients

E 14 Inhibition of Phenylpyrazoles Fluthiacet-methyl

proto-

porphyrinogen

oxidase

(PPO, Protox)

Table 1. Herbicide MOA herbicide classification system.

HRAC WSSA

group group Mode of action Chemical family Active ingredients

E 14 Inhibition of Triazolinones carfentrazone-ethyl

proto- sulfentrazone

porphyrinogen

oxidase

(PPO, Protox)

N N

N

R

R

R

O

Basic structure of a triazolinone

Five-membered ring containing three nitrogens and an

oxygen double-bonded to the ring.

Triazolinone Herbicides

1. Carfentrazone-ethyl is applied POST or POST-directed only;

behaves as contact; no soil activity.

2. Sulfentrazone applied PPI or PRE. Apoplastic translocation.

Currently no POST labels, but it can also be absorbed by

foliage. Relatively long activity in soil; some rotational

restrictions apply to NC crops.

3. Triazolinone herbicides primarily control broadleaf weeds, but

sulfentrazone is extremely good on nutsedge species.

Uses of Sulfentrazone

2. Packaged mixtures

c. Surge

sulfentrazone + 2,4-D + MCPP + dicamba

POST control of broadleaf weeds in turf

PPO inhibitors

Lactofen, A. York, NCSU Acifluorfen, A. York, NCSU

Fomesafen, A. York, NCSU Flumiclorac, A. York, NCSU

Carfentrazone, A. York, NCSU

Carfentrazone, A. York, NCSU

Acifluorfen, A. York, NCSU

PPO inhibitors

Aim post-directed

A. York, NCSU

PPO inhibitors

Flumioxazin preemergence

Table 1. Herbicide MOA classification system.

HRAC WSSA

group group Mode of action Chemical family Active ingredients

F1 12 Inhibition of Pyridazinones norflurazon

carotenoid

biosynthesis

at PDS

F2 28 at 4-HPPD Triketones mesotrione, tembotrione

Benzoyl pyrazole topramezone

F3 13 unknown Isoxazolidinones clomazone

Bleachers

Pyridazinones, triketones, and isoxazolidinones (and

others not used in NC) are bleachers.

These herbicides inhibit biosynthesis of carotenoids

(yellow, orange pigments). In absence of carotenoids,

chlorophyll is photodegraded, resulting in bleached

plant. White or pale yellow plants.

Table 1. Herbicide MOA classification system.

HRAC WSSA

group group Mode of action Chemical family Active ingredients

F1 12 Inhibition of Pyridazinones norflurazon

carotenoid

biosynthesis

at PDS

F2 28 at 4-HPPD Triketones mesotrione, tembotrione

Benzoyl pyrazole topramezone

F3 13 unknown Isoxazolidinones clomazone

R1 R2

C

O

Ketone: a carbonyl carbon bonded to two

carbon atoms.

Table 1. Herbicide MOA classification system.

HRAC WSSA

group group Mode of action Chemical family Active ingredients

F1 12 Inhibition of Pyridazinones norflurazon

carotenoid

biosynthesis

at PDS

F2 28 at 4-HPPD Triketones mesotrione, tembotrione

Benzoyl pyrazole topramezone

F3 13 unknown Isoxazolidinones clomazone

Clomazone -- Isoxazolidinone family

Clomazone

• Absorbed by roots of dicots; by emerging coleoptiles of grasses

• Apoplastically translocated

• Relatively long life in soil; gives season-long control;

may carryover to susceptible crops

• Somewhat volatile. Do not lose enough via volatilization

to adversely affect weed control (i.e., it works well PRE).However, bleaching on off-target vegetation has been a serious concern. Numerous precautions on label, plusbuffer zones.

• Controls annual grasses (except Texas panicum). Also

very effective on selected broadleaf weeds.

Uses of Clomazone

• PPI or PRE (before or after transplanting) on

tobacco.

• PRE on cotton. Must be used in conjunction

with in-furrow application of phorate or

disulfoton. No longer used in NC.

• PRE on soybeans. No longer used in NC.

• PRE on several vegetable crops. Lot used in

sweet potatoes.

Culpepper, Univ. GA

Norflurazon carryover on cabbage

York, NCSU

Norflurazon

Clomazone

A. York, NCSU

Clomazone

+

phorate

Clomazone

+

aldicarb

Clomazone

A. York, NCSU

Clomazone drift

Mesotrione

A. York, NCSU

Mesotrione

A. York, NCSU

Table 1. Herbicide MOA herbicide classification system.

HRAC WSSA

group group Mode of action Chemical family Active ingredients

G 9 Inhibition of Glycines glyphosate

EPSP

synthase

Glyphosate Mechanism of Action

• Inhibits 5-enolpyruvyl shikimate-3-phosphate

(EPSP) synthase

• EPSP synthase is an enzyme in the shikimate

pathway leading to biosynthesis of the aromatic

amino acids phenylalanine, tryptophan, and tyrosine.

• Plants die slowly (7 to 14 days). Terminal

becomes chlorotic, plants cease growth, and

slowly die.

Characteristics of Glyphosate

• Very low mammalian toxicity

• Very tightly bound to soil; no soil activity; no leaching.

• Very good environmental profile.

• Applied POST to weeds, foliar absorption, symplastic

translocation

• Very limited metabolism in plants

• Broad spectrum control of grasses and broadleaf weeds;

but some species only marginally controlled or not

controlled

• Resistant biotypes of weeds are known to occur

HO -

O

C -

H

C -

H

H

N -

H

C -

H

O

P -

OH

OH

Glyphosate acid

Glyphosate Formulations

Glyphosate is formulated as one of three salts:

• diammonium salt (currently not on market)

• potassium salt

• isopropylamine salt

HO -

O

C -

H

C -

H

H

N -

H

C -

H

O

P -

OH

OH

H3N - O -

O

C -

H

C -

H

H

N -

H

C -

H

O

P -

OH

O - NH3

Diammonium salt of glyphosate

Glyphosate acid

HO -

O

C -

H

C -

H

H

N -

H

C -

H

O

P -

OH

OH

HO -

O

C -

H

C -

H

H

N -

H

C -

H

O

P -

OH

CH3

O - NH2 - CH

CH3

Isopropylamine salt of glyphosate

Glyphosate acid

HO -

O

C -

H

C -

H

H

N -

H

C -

H

O

P -

OH

OH

HO -

O

C -

H

C -

H

H

N -

H

C -

H

O

P -

OH

O - K

Potassium salt of glyphosate

Glyphosate acid

Comparing Glyphosate Formulations

• At equivalent rates, research has shown no

practical differences in crop tolerance or weed control

among the three basic formulations or among

the many trade names available.

• Exception has been two brands containing

isopropylamine salt of glyphosate. Contact burn

on cotton foliage has been observed. Thought to

be due to adjuvants or contaminants in the formulated

product, and not due to glyphosate per se.

Untreated

Clearout 41 Plus

Grower field, Clearout 41 Plus, 5 DAT

Glyphosate drift on non-RR corn

Glyphosate drift on non-RR corn

Glyphosate on non-RR corn; sprayer contamination

Glyphosate on non-RR corn; sprayer contamination

Glyphosate applied to non-RR corn

Glyphosate drift

Univ. Wisconsin

Univ. Wisconsin

Glyphosate drift

Double rate of glyphosate on RR soybean

Glyphosate sprayer contamination

N. Porter, UniroyalN. Porter, Uniroyal

Glyphosate on RR cotton

Glyphosate on RR cotton

Glyphosate on RR cotton

Glyphosate overtop RR cotton after 4-leaf stage

Table 1. HRAC herbicide classification system.

HRAC WSSA

group group Mode of action Chemical family Active ingredients

H 10 Inhibition of Phosphinic acids glufosinate-ammonium

glutamine

synthase

Glufosinate acid

Glufosinate-ammonium

Glufosinate Mode of Action

Inhibits enzyme glutamine synthetase.

NH3 accumulates, which is toxic.

Glutamic acid + glutamine

NH3

amino acids

Glufosinate

• Weakly adsorbed to soil; very rapidly degraded; no soil activity.

• Applied postemergence.

• Limited (localized) translocation. Acts much like a contact.

• Controls most annual broadleaf weeds; marginal onAmaranthus.

• Controls annual grasses if treated when small; exception is goosegrass.

• Does not control nutsedge, perennial broadleaf weeds, or bermudagrass. Controls johnsongrass with multiple applications.

• Liberty Link crops (transgenic, resistant to glufosinate) very tolerant of glufosinate

Glufosinate

Univ. Wisconsin

A. York, NCSU

A. York, NCSU

Glufosinate

A. York, NCSU

A. York, NCSU

Univ. Wisconsin

Table 1. HRAC herbicide classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

K1 3 Microtubule Dinitroanilines benefin

assembly ethalfluralin

inhibition oryzalin

pendimethalin

prodiamine

trifluralin

Pyridines dithiopyr

Benzamides pronamide

Benzene- DCPA

dicarboxylic

acids

H-N-H

NO2NO2

Basic structure of a dinitroaniline

Aniline ring with nitro group at 2- and 6-position of ring

Dinitroanilines -- Mode of Action

• Seedling growth inhibitors.

• Inhibit root or shoot development (depending upon site of absorption) by interfering with cell division in meristematic areas.

• Specifically, dinitroanilines interfere with microtubule assembly. Prevent normal function of spindle fibers during mitosis.

Characteristics of Dinitroanilines

• Commonly referred to as DNAs.

• They are yellow. Some DNA compounds

used in dye industry. Commonly referred

to as “yellow herbicides”.

• Nearly immobile in soil. Low water

solubility and tightly adsorbed.

• DNAs, except oryzalin, are relatively

volatile. Also subject to photodegradation.

Characteristics of Dinitroanilines

• Absorbed by roots of dicots, by

emerging coleoptile of grasses.

Characteristics of Dinitroanilines

• Absorbed by roots of dicots, by

emerging coleoptile of grasses.

• Non-mobile in plants.

• No postemergence activity.

• Symptoms are swollen root tips, or

club-shaped roots. Lateral root growth

inhibited more than tap root growth.

Univ. Wisconsin

Root pruning with DNA’s

Root pruning

with DNA’s

Stunting by DNA herbicide

A. York, NCSU

Characteristics of Dinitroanilines

• Relatively long soil residual. Season-long

control of susceptible species if applied

properly.

• Generally do not have carryover concerns,

but there have been exceptions. Hard pan

tends to complicate carryover problems.

• Control annual grasses and certain small-

seeded broadleaf weeds.

Table 1. HRAC herbicide classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

K1 3 Microtubule Dinitroanilines benefin

assembly ethalfluralin

inhibition oryzalin

pendimethalin

prodiamine

trifluralin

Pyridines dithiopyr

Benzamides pronamide

Benzene- DCPA

dicarboxylic

acids

N

Pyridine

Pyridine herbicides consist of pyridine ring with various

substituent groups off the ring

Table 1. HRAC herbicide classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

K1 3 Microtubule Dinitroanilines benefin

assembly ethalfluralin

inhibition oryzalin

pendimethalin

prodiamine

trifluralin

Pyridines dithiopyr

Benzamides pronamide

Benzene- DCPA

dicarboxylic

acids

pronamide

Table 1. HRAC herbicide classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

K1 3 Microtubule Dinitroanilines benefin

assembly ethalfluralin

inhibition oryzalin

pendimethalin

prodiamine

trifluralin

Pyridines dithiopyr

Benzamides pronamide

Benzene- DCPA

dicarboxylic

acids

DCPA

Table 1. Herbicide MOA classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

K3 15 Inhibition of Chloroacetamides acetochlor

cell division alachlor

(inhibition of dimethenamid-P

VLCFA) metolachlor

Acetamides napropamide

Oxyacetamides flufenacet

Mode of Action

Acetamides

Chloroacetamides

Oxyacetamides

Seedling shoot growth

inhibitors. Impact several

processes in plants. Proposed

mechanism of action is

inhibition of VLCFAs.

Characteristics of Chloroacetamides

• Applied PPI or PRE. Occasionally applied POST,

but activity is preemergence on weeds.

• Control annual grasses and some small-seeded

broadleaf weeds; most suppress yellow nutsedge.

• Absorbed by roots of dicots, emerging shoots of grasses.

• Relatively mobile in soil; sometimes found in

ground water.

• Relatively short persistence; 4 to 8 weeks of

control; no carryover problems.

Metolachlor

O

C CH2 Cl

N

CH CH2 O CH3

CH3

*

*

Metolachlor has a chiral center and a chiral axis.

CH2 CH3

CH3

Table 1. Herbicide MOA classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

K3 15 Inhibition of Chloroacetamides acetochlor

cell division alachlor

(inhibition of dimethenamid-P

VLCFA) metolachlor

Acetamides napropamide

Oxyacetamides flufenacet

napropamide

Table 1. Herbicide MOA classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

K3 15 Inhibition of Chloroacetamides acetochlor

cell division alachlor

(inhibition of dimethenamid-P

VLCFA) metolachlor

Acetamides napropamide

Oxyacetamides flufenacet

Chloroacetamides

Chloroacetamides

Dewey Lee, UGA

Purdue Univ.

Chloroacetamides

Leafing out underground

A. York, NCSU

Chloroacetamides

Metolachlor preemergence

Chloroacetamides

A. York, NCSU

Table 1. Herbicide MOA classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

L 20 Inhibition of Nitriles dichlobenil

cell wall

(cellulose)

synthesis

21 Benzamides isoxaben

Cl Cl

C N

dichlobenil

Table 1. Herbicide MOA classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

L 20 Inhibition of Nitriles dichlobenil

cell wall

(cellulose)

synthesis

21 Benzamides isoxaben

Table 1. Herbicide MOA classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

N 8 Inhibition of Thiocarbamates butylate

lipid synthesis EPTC

(not ACCase pebulate*

inhibition) vernolate*

Phosphoro- bensulide

dithioates

Benzofurans ethofumesate

* No longer marketed

R1 O

N - C - S - R3

R2

Basic structure of

a thiocarbamate

Thiocarbamates

1. Exact site of action unknown. Inhibit lipid synthesis, but not at ACCase

2. Control annual grasses, certain small-seeded broadleaf weeds, and nutsedge

3. Primarily absorbed by coleoptile of grasses

4. Highly volatile; incorporate immediately

5. Short persistence. Control for 3-6 wk or less.

6. May have accelerated degradation

7. Easily leached

Thiocarbamates

Table 1. HRAC herbicide classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

N 8 Inhibition of Thiocarbamates butylate

lipid synthesis EPTC

(not ACCase pebulate

inhibition) vernolate

Phosphoro- bensulide

dithioates

Benzofurans ethofumesate

bensulide

Table 1. HRAC herbicide classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

N 8 Inhibition of Thiocarbamates butylate

lipid synthesis EPTC

(not ACCase pebulate

inhibition) vernolate

Phosphoro- bensulide

dithioates

Benzofurans ethofumesate

Table 1. HRAC herbicide classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

O 4 Act as Phenoxy- 2,4-D

synthetic carboxylic-acids 2,4-DB

auxins 2,4-DP (dichlorprop)

MCPA

MCPP (mecoprop)Benzoic acids dicamba

Pyridine clopyralid

carboxylic acids fluroxypyrpicloramtriclopyraminopyralid

Quinoline quinclorac

carboxylic acids

Mode of Action

Phenoxy-carboxylic-acids Benzoic acids

Pyridine carboxylic acids Quinoline carboxylic acids

Act as synthetic auxins. Exact mode of action unknown.

Involves interference with nucleic acid metabolism and

disruption of normal transport systems through induced

massive cell proliferation.

O

O - (CH2)i - C - OH

Basic structure of a phenoxy-carboxylic acid

Phenoxy-carboxylic acid herbicides

• Typically applied POST to weeds.

• Absorbed by foliage. Can also be absorbed by roots

from soil.

• Systemic in plants, moving in both apoplast and symplast.

• Selective on grasses; most dicots are sensitive.

• Very weakly bound by soil. Readily leached.

• Short soil residual; average half-life 5 to 10 days.

• Formulated as salts or esters.

O

O - CH2 - C - OH

O CH3

O - CH2 - C - O - NH

CH3

O CH2 - CH3

O - CH2 - C - O - CH2 - CH - CH2 - CH2 - CH2 -CH3

isooctyl (2-ethylhexyl) ester of 2,4-D

2,4-D acid dimethylamine salt of 2,4-D

Practical differences in esters and amines salts of 2,4-D

1. Amines form solutions with water; esters form emulsions.

2. Esters mix well with liquid N, amines do not. Must

premix amines in water, maintain good agitation.

3. Esters more effective in cool weather; may be more

effective on perennials.

4. Esters absorbed better by waxy leaves.

5. Esters less subject to washoff; less water soluble.

6. Amines easier to clean from sprayer.

7. Esters subject to vapor drift.

Spray Drift vs. Vapor Drift

• Spray drift can occur with any pesticide sprayed

• Vapor drift occurs with volatile herbicides

• Spray drift is typically short distance

• Vapor drift can be long distance

• Some crops, especially cotton and tomatoes,

are very sensitive to 2,4-D

• Ester formulations of 2,4-D, even though they are

“low volatile” esters, are still volatile

• Ester formulations should not be used with 1 mile

of cotton and other sensitive crops

Univ. Wisconsin

2,4-D Postemergence

2,4-D

A. York, NCSU

Purdue Univ.

Univ. Wisconsin

Epinastic Response

to 2,4-D and 2,4-DB

A. York, NCSU

Soybean leaf distortion by 2,4-D

A. York, NCSU Purdue Univ.

Cotton leaf distortion by 2,4-D

A. York, NCSU

A. York, NCSU

2,4-D sprayer

contamination

on tobacco

A. York, NCSU

Malformed wheat heads

2,4-D

Table 1. HRAC herbicide classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

O 4 Act as Phenoxy- 2,4-D

synthetic carboxylic-acids 2,4-DB

auxins 2,4-DP (dichlorprop)

MCPA

MCPP (mecoprop)Benzoic acids dicamba

Pyridine clopyralid

carboxylic acids fluroxypyrpicloramtriclopyraminopyralid

Quinoline quinclorac

carboxylic acids

Dicamba

1. Little binding to soil, readily leached

2. Short soil half-life, usually less than 14 days

3. Can be absorbed by roots, apoplastically translocated

4. Usually applied POST; readily absorbed by foliage,

symplastically translocated

5. Controls dicots, activity on some monocots (such as

wild garlic), little to no activity on grasses

6. Tobacco, soybeans, and most other broad leaf crops

very sensitive

A. York, NCSU

Dicamba

Leaf cupping by dicamba

University of Missouri

Kansas State Univ.

A. York, NCSU

“Cobra-hood” caused by

dicamba or picloram

Univ. Missouri

dicamba

Table 1. HRAC herbicide classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

O 4 Act as Phenoxy- 2,4-D

synthetic carboxylic-acids 2,4-DB

auxins 2,4-DP (dichlorprop)

MCPA

MCPP (mecoprop)Benzoic acids dicamba

Pyridine clopyralid

carboxylic acids fluroxypyrpicloramtriclopyraminopyralid

Quinoline quinclorac

carboxylic acids

clopyralid, acid picloram, acid

triclopyr, acid

Table 1. HRAC herbicide classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

O 4 Act as Phenoxy- 2,4-D

synthetic carboxylic-acids 2,4-DB

auxins 2,4-DP (dichlorprop)

MCPA

MCPP (mecoprop)

Benzoic acids dicamba

Pyridine clopyralid

carboxylic acids fluroxypyrpicloramtriclopyr

Quinoline quinclorac

carboxylic acids

quinclorac

Table 1. HRAC herbicide classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

P 19 Inhibition of Phthalamates naptalam

auxin trans-

port

Naptalam

1. Limited mobility in plants

2. Weakly bound to soil

3. Leaches readily

4. Average soil half-life of 14 days

5. Applied PRE to weeds

6. Controls selected broadleaf weeds

Table 1. HRAC herbicide classification system.

HRAC WSSAgroup group Mode of action Chemical family Active ingredients

Z 17 Unknown Organoarsenicals DSMA

MSMA

DSMA

MSMA

Organoarsenicals

1. Contain pentavalent As.

2. Pentavalent As is low toxicity to mammals, as

opposed to trivalent As in rat poison

3. Have no soil activity; bound tightly

4. Applied POST; limited apoplastic translocation;

act like contacts

5. Long soil persistence, but no carryover problems

because tightly bound to soil

6. Mode of action unknown.