🚫 Herbicide Classification and Mode of Action

This lesson explains key concepts in a structured way and connects them to practical agricultural applications and exam-oriented understanding.

---

HERBICIDES

HERBICIDES CHARACTERISTICS –

Introduction

A herbicide in the broadest sense is any compound that is capable of killing or

severely injuring plants and may be used for elimination of plant growth.

A weed is an plant, wild or cultivated that is undesired in that particular place. In

Agriculture and horticulture weeds are thus any ant other than the specific crop being

grown. On railway tracks, industrial sites, air port paths, open spaces and the like the

entire vegetation can be regarded as weeds.

Weeds are conveniently divided into dicotyledonous plants, termed broad leaf

weeds and monocotyledonous plants, termed grass weeds. Weeds completed with

plants for water, light, food and above and below the surface. The yield loss due to

weeds is estimated to be 9-10%. Also herbicide economies or helps in crop production

by reducing the cultural operations.

Herbicides may be classified based on one or more common characteristics such

as chemical composition or mode of action or time of application etc.

----------------------------- Organic

Chemical

Contact

Mode Of Action

Pre-emergence or post-emergence is with respect to the

emergence of crop and not of the weed

Soil herbicides - root absorption systemic

Manner of absorption

Range of application

Aromatic-Carboxylic

Foliage herbicide - contact

Total - unspecified vegetation

Selective

a) 2, 4-D : 2,4-Dichlorphenoxy acetic acid

LD50: 375

2, 4-D can be used for destruction of dictotyledonous plants and it is well tolerated

by many monocotyledonous crop plants.

2, 4-D is used as a selective herbicide particularly in cereals. The great advantages

are cheap manufacture and relatively low mammalian toxicity. It is a systemic herbicide.

b) 2,4,5, T : 2,4,5, Trichlorophenoxy acetic acid

LD50: 500

2,4,5, T has particularly high activity against woody plants and usually used in

combination with other herbicides for control of trees, shrubs and control of otherwise

intractable broad leaf weeds.

Structural analogues of IAA such as NAA or 2,4-0 promote growth in lower doses

but have a herbeidal effect at higher concentrations. 2,4,5, T is more persistent in soil

than 2,4-D or MCP A (2-methyl-4-chlorophenoxy acetic acid).

Mode of Action

• Interference with nucleic acid metabolism

• Disruption of translocation system

Anilides

The type of activity and the range of weed control vary greatly within

this group some being used post-emergence while others are active

through the soil.

In 1965, 1966 and 1969 Monsanto introduced three anilides for pre

emergence control of annual weeds.

(1) (α-chloro-N-isopropyl acetanilide) as 'Ramrod' which shows a

high degree of specificity for annual grass weeds and certain

broad- leaved weeds in maize, soybeans, sugarcane, peanuts and

certain vegetables. (ii) Alachlor (α-chloro-2'6'-diethyl

Nimethoxy methyl acetamide) as 'Lasso’ for use in maize, cotton,

soybeans, sugarcane, peanuts and certain vegetable crops where it

shows very good activity against annual grasses, particularly

Echinochloa crus-galli, Setaria Spp. and Digitaria Spp. (iii)

Butachlor (N-[butoxymethyl]-chloro-2',6'-diethyl acetanilide) as

'Machete' for the control of most annual grasses, certain broad

leaved species in transplanted rice. In 1974, Ciba Geigy

introduced metolachlor (2-chloro-6'-ethyl-N- [2-methoxy-l

methylethyl] acet-o-toluidide) as 'Dual', a pre- emergence

germination inhibitor, active mainly on grasses for use in maize,

soybeans and groundnut.

Butachlor

oxyfluorfen

Thiocarbamates

EPTC (S-ethyl-N.N-dipropylthiocarbamate).'Eptam', was introduced

by Stauffer in 1954. It kills germinating seeds, a number of annuals and

inhibits bud development in the underground organs of perennial weeds

such as couch grass (Agropyron repens) and sedges ( Cyperus Sp.). It may

be used soil incorporated 3 weeks before planting potatoes, field beans,

sugar beet and others. Monsanto introduced di-allate as 'Avadex’ (S

2,3-dichloroallyl-N,N-di-isopropyl (thiocarbamate)), a volatile herbicide

for pre-plant control of Avena fatua and Alopecurus myosuroides in

brassica and beet crops in 1960; and tri-allate (S-[2,3,3’- trichloroallyl]-di

isopropyl (thiocarbamate)) in for the control of these grasses and others in

cereals and peas.

In 1970 thiobencarb (S-4-chlorobenzyl diethyl thiocarbamate) was

introduced It is an important herbicide for the control of weeds in rice

showing very high selectivity between rice and barnyard grass

( Echinochloa crus-galli). In addition it controls many other grass

cyperaceous and broadleaved weeds.

Thiobencarb

Substituted Ureas

Diuron (3-[3,4-dichlorophenyl]-1,1-dimethylurea) as 'Karmex’, was

introduced by Du Pont in 1954 and Fenuron (I,I-dimethyl-3- phenyl urea)

as 'Dybar' was introduced in 1957 and is used for the control of woody

plants by basal application.

Fluometuron (1, I –dimethyl-3-[α-trifluoro-m-tolyllurea) as 'Cotoran'. was

introduced by Ciba Geigy and is used for the control of weeds in cotton.

Another very important herbicide for the control of annual, grasses

including Alopecurus myosuroldes, Avena fatua and Poa annua and many

annual broadleaved weeds in cereals was isoproturon (3-[4-isopropyl

phenyl]- 1, I –dimethyl urea) marketed by three companies - Hoechst (as

'Arelon'), Ciba Geigy (as 'Graminon') and Rhone-Poulenc (as 'Tolkan') in

Diuron lsoproturon

Heterocyclic Nitrogen Compounds Triazines

In general triazines have little effect on germination and they are

taken up by the roots or leaves. Due to an inhibition of the Hill reaction of

photosynthesis, affected plants turn yellow and necrotic symptoms

develop. With a few exceptions, the symmetrical triazines have got

substituted amino groups at two of the carbon atoms while the third carbon

has a chloro, a thioether or a methoxy function. The chloro compounds

(CI) end in azine, the thioethers (-S-) end in tryne (e) and the methoxy

ones (CH30) in ton. In the list of herbicides which follows all were

introduced by Ciba Geigy unless stated otherwise. The first

commercial triazine, simazine (2 – chloro - 4, 6 - bis [ethyl -amino] - I,3,5

-triazine) was introduced as 'Gesatop' in 1956 for the selective residual

pre-emergence control of a great many annual grass and broadleaved

weeds in a variety of deep-rooted crops (including citrus fruits, coffee, tea

and cocoa), due to its low solubility in water (3.5 mg I [-1] at 20°C). It is also

used for the control of most annual and perennial weeds in non-crop areas.

It is remarkably selective for use on maize because of the ability of this

crop to degrade it non-enzymically to the non-active hydroxy derivative.

Simazine

The second introduction atrazine (2-chloro-4-ethylamino-6

isopropylamino-1,3,5-triazine), introduced as 'Gesaprin’ and 'Primatol in

1958, is both foliar and soil acting being taken up both by leaves of

emerged weeds and by the roots of weed seedlings emerging after

spraying. In maize, where it is also degraded in, a manner similar to

simazine, it is preferred to the latter especially in dry years. Being more

water soluble (30mg I [-1] at 20°C) it is more suitable for the dry soils on

which this crop is grown where it will effectively control couch grass

(Agropyron repens) and other perennial grasses. It is also used in roses,

for selective use in coniferous forests and for non- selective use on non

crop land and industrial sites.

Atrazine

Pyridines

In 1957 and 1958 I.C.I. introduced two very important bipyridillium

quaternary herbicides. Both are broad spectrum, rapidly acting causing

wilt and desiccation, and are translocated to a certain extent. Diquat (1,

I'-ethylene-2,2'-bipyridillium ion formulated as dibromide) under various

trade names 'Reglone', Weedol', 'Path clear' is used for potato haulm

desiccation, for seed crop desiccation and for aquatic weed control. The

Chapman Chemical markets it as 'Aquacide’. Paraquat (1,1'-dimethyl

4,4'-bipyridilium ion formulated as dichloride) as 'Gramoxone' destroys

photosynthetic tissues and Is used for a variety of purposes Including

stubble cleaning, inter-row weed control, desiccation of various crops and

killing out of old pastures which can then be resown without ploughing.

It is very fast acting, the first effects being noticeable after a few hours

and kill is usually completed in 3-4 days. It is quickly absorbed on to soil

(particularly clay) particles so that sowing can follow soon after

application.

Diquat Paraquat

The Dow chemical has Introduced three foliar-applied, selective,

growth-regulatory herbicides which produce symptoms on susceptible

plants very similar to those produced by the auxin type herbicides, namely

tissue proliferation, epinasty, leaf curling and production of adventitious

roots.

Organophosphorus Compounds

The organophosphorus compounds include: (1) bensulide (0,0

diisopropyl-S-2-phenyl-sulphanylaminoethyl phosphorodithioate) which

was introduced by Stauffer in 1964 as 'Prefair' for pre-plant pre-emergence

use on cucurbits, brassicas, lettuce and cotton, and as 'Betasan' for pre

emergence control of annual grasses and broad-leaved weeds in lawns. (2)

Piperophos (S-2-methylpiperidino-carbonyl methyl-0,0-dipropyl

phosphorodithioate) was introduced by Ciba Geigy in 1969. It can be used

pre-emergence in rice, maize, cotton, soybeans and groundnuts for the

control of many monocotyledonous weeds including Cyperus Sp.,

Echinochloa Sp., Trianthema portulacastrum and Monochoria

vaginallis. Glyphosate (N-[phosphonomethyl] glycine) a derivative of the

amino acid, glycine, was introduced by Monsanto as 'roundup' in 1971. It

is used post-emergence and is rapidly absorbed by the leaves and

translocated from vegetative parts to underground parts, rhizomes or

stolens of perennial grass and broad-leaved weed species giving good

control of both above-ground and underground organs Agropyron repens

is very sensitive. Glyphosate is inactivated on contact with the soil. It

provides excellent weed control in pre-tillage of post-harvest treatments of

annual crops or when applied as a direct spray in woody crops such as

vineyards, deciduous fruit, rubber, coffee, citrus, tea and oil palm.It can

also be used in non-agricultural areas and for bush control in forestry.

Piperophos Glyphosate

---

Summary Cheat Sheet

Topic	Key exam point
Main class	Herbicides
Common classification bases	Pre- vs post-emergence; selective vs non-selective
Additional classification	Grouping by chemistry and mode of action
Chemical groups named	Anilides, thiocarbamates, substituted ureas, triazines, pyridines
Main use	Weed control in crop and non-crop situations
Mode-of-action focus	Herbicides are often classified by site of action in modern weed science
Agronomic importance	Correct herbicide selection depends on weed flora, crop tolerance, and application timing
Exam distinction	Application-timing classification and chemical-group classification are separate
Management caution	Dose and timing errors can injure crop or fail to control weeds
Trap	Do not treat all pre-emergence herbicides as selective or all post-emergence herbicides as non-selective

References

3 sources • [1] [2] [3]

[1]

Indian Council of Agricultural Research (ICAR)

Official

[2]

Principles of Soil Science and Agricultural Chemistry — Standard BSc Agriculture Textbook

Book

[3]

Department of Agriculture & Farmers Welfare

Website