Herbicides

 Herbicides

 

Herbicide is a chemical substance used to control or manipulate undesirable vegetation, especially weeds. They are extensively used in gardening, farming, and landscape turf management.

 

Classification of herbicides

Herbicides are classified according to their:-

Selectivity

Mode of action

Timing of application

Chemical group

Mechanism of action

 

2.1 Selectivity:

 

A) Non-selective herbicide kills or damages all plant life in a treated area (e.g., Glyphosate).

 

B) Selective herbicide will kill weeds in a germinating or growing crop without   harming the crop beyond the point of recovery (e.g., 2, 4-D used to control broadleaved weeds in a grass pasture; Avadex to control wild oats in cereals)

 

2.2 Mode of action:

 

A) Contact herbicides – Kills plant parts covered by the herbicide and are directly toxic to living cells. There is little or no translocation or movement of the material through the plant. Contact herbicides are effective against annual weeds but they only “burn off” the tops of perennial weeds, chemically mowing them. Contact herbicides may be selective, such as Torch (Bromoxynil) which kills broadleaved weeds in cereals without damaging the crop, or nonselective, such as Gramoxone (Paraquat) which kills any green plant material.

B) Systemic herbicides – Such herbicides transfer or are translocated in the field, consumed either by the roots or above ground portions of the plants. They exhibit a chronic effect; that is, the full effects may not show for a week or more after treatment. An overdose on the leaves may kill the leaf cells more quickly, thus preventing translocation to the site of action in a plant. Systemic herbicides can be selective, as in the case of 2, 4-D, MCPA, Banvel and Tordon or non-selective, as with Glyphosate.

 

C) Soil Sterilants – Chemicals which prevent growth of plant life when present in the soil. These products will prevent plant growth for periods of a few months to a number of years. Example: Bromacil, Tebuthiuron and Atrazine at high rates.

 

2.3 Timing of application:

 

A) Pre-Plant Soil Incorporated – These herbicides are applied to the soil before the crop is sown. They are incorporated in the soil to prevent loss due to vapourization and breakdown by sunlight. Example: Fluchloralin, Pendimethalin etc.

 

B) Pre-Emergence – Applied to soil prior to seeding or after the crop is sown but before crop and weeds emerge. In most cases, the weeds germinate in treated soil while the crop germinates below the herbicide zone. Example: Atrazine, Butachlor, Pretilachlor etc.

 

C) Post-Emergence– Sprayed directly on the weeds after their emergence. Example: Glyphoste, Paraquat, 2, 4-D etc.

 

2.4 Chemical groups:

Sl. No.	Group	Herbicides
1	Aliphatic acid	Glyphosate, Dalapon, TCA, Methyl bromide etc.
2	Amides	Alachlor, Butachlor, Propachlor, Metachlor etc.
3	Benzoics	2,3,6-TBA, Dicamba, Tricamba etc.
4	By Pyridilliums	Diquate, Parquate
5	Carbamates	Propham, Chloropham, Barban etc.
6	Thiocarbamates	Butylate, Diallate, Molinate etc.
7	Dithiocarbamates	CDEC, Metham
8	Nitriles	Bromoxynil, Ioxynil, Dichlobenil etc.
9	Dinitroanilins	Fluchloralin, Pendimethalin, Isoproturon etc.
10	Phenols	Dinoseb, DNOC, PCP etc.
11	Phynoxy acid	2,4-D, 2,4,5-T, 2,4-DB etc.
12	Traizines	Atrazine, Simazine, Metribuzine etc.
13	Ureas	Monuron, Diuron, Linuron etc.
14	Uracils	Bromacil, Terbacil, Lenacil etc.
15	Diphenyl ethers	Nitrofen, Oxyfluorfen etc.
16	Aryloxyphenoxy propionate	Diclopop, Fenoxaprop-p etc.
17	Cyclohexanedione	Cycloxidim, Clethodim
18	Imidazolines	Imazapyr, Imazaquin
19	Isoxazolidionones	Clomazone
20	Oxadiazoles	Oxadiazon
21	Oxadizolides	Methazoles
22	N-phenyl pthalamides	Flumiclorac
23	Phenylpyridazones	Sulfentrazone
24	Phthalamates	Naptalam
25	Pyrazoliums	Difenzoquat
26	Picolinic acids pyridine	Picloram, Thiazopyr etc.
27	Quinolines	Quinclorac
28	Sulfonylureas	Bensufuron, Sulfosulfuron, Metsulfuron etc.
29	Triazolinones	Pryidates
30	Cineoles	Cinmethylin

 

   2.5 Mechanism of action:

 

Mechanism  of action	Chemical family
Inhibitors of Acetyl CoA Carboxylase (ACCase):  These chemicals block an enzyme called ACCase. This enzyme helps the formation of lipids in the roots of grass plants. Without lipids, susceptible weeds die.	-Aryloxyphenoxy propionate (Fop) - Cyclohexanediones (Dim) - Phenylpyrazolin (Den)
ALS/AHAS inhibitors: These chemicals block the normal function of an enzyme called Acetolactate (ALS) Actohydroxy acid (AHAS). This enzyme is essential in amino acid (protein) synthesis. Without proteins, plants starve to death.	- Imidazolinones -Sulfonylamino- -Carbonyltriazolinones - Sulfonylureas -Triazolpyramidines -Triazolones
Microtubule assembly inhibitors: These chemicals inhibit the cell division in roots.	-Dinitroanilines -Benzoic acids
Photosynthetic inhibitors at Photo-system II, Site A: These chemicals interfere with photosynthesis and disrupt plant growth, ultimately leading to death.	-Phenyl-carbamates - Triazines - Triazinones - Uracils
Photosynthetic inhibitors at Photo-system II, Site II:	-Benzthiadiazoles - Nitriles
Photosynthetic inhibitors at Photo-system II, Site B:	-Ureas
Lipid synthesis inhibitors (not ACCase inhibition) : These chemicals inhibit the cell division and elongation in the seedling shoots before they emerge above ground.	-Thiocarbamates, -Pyrazolium
Inhibitors of EPSP synthesis: These chemicals inhibit the amino-acid synthesis.	None
Inhibitors of glutamine synthetase:	None
Carotenoids biosynthesis inhibitors: These chemicals inhibit the carotenoids biosynthesis.	-Triazole
Inhibitors of cell growth and division:	-Chloroacetamides
Cell membrane disrupters. (Inhibitors of Photosystem-I): Chemicals that disrupt the internal cell membrane and prevent the cells from manufacturing food	-Bipyridyliums