Classification Of Herbicides
1) Base on time of application:
a) Preplant incorporated: Incorporated 1 or 2 days before sowing or planting of crops e.g., Fluchloralin, trifluralin
b) Pre-emergent herbicide: They are applied directly to the soil prior to the emergence of weeds, applied 1 or 2 days after sowing e.g., majority of herbicides Pendimethalin, alachlor, atrazine, diuron, oxyflourfen, metalachlor, butachlor, isoproturon, pyrazosulfuron ethyl
c) Post emergent herbicides: Applied after the emergence of weeds or on grownup weeds. These are sprayed directly on the grownup weeds.
i) Early post emergent: These are sprayed when weeds are young i.e., 1 or 2 weeks after emergence of weeds. Weeds are in 2-3 leaf stage. e.g., 2,4-D, Fenoxoprop-p-ethyl, metamfop, cyhalofop-p-butyl.
ii) Late post emergent: Applied on foliage of fully grown weeds and they are non selective. e.g., Glyphosate, paraquat, diquat
2) Based on selectivity
a) Selective herbicides: Herbicides which kill selectively target plants (weeds) in a mixed population of crops and weeds. Majority of herbicides are selective. e.g., Atrazine, butachlor, fenoxoprop-p-ethyl, metolachlor. Pendimethalin is an excellent herbicide having selectivity to > 60 different crops which includes cereals, pulses, oilseeds etc.
• However selectivity of any herbicide is dose dependent and crop dependent. Therefore herbicide selective to a crop may become non selective if used at higher doses.
b) Non selective herbicides: They kill any group or species of plants irrespective of crop and weeds. So they are not used in cropped area and they are used in industrial area, railway track side, bund side, road side. e.g., Glyphosate, paraquat, diquat
• Even non selective herbicides become selective herbicides when it is sprayed directed to the weeds under controlled conditions.
3) Based on spectrum of weed control
a) Narrow spectrum herbicides: They control a particular group of weed flora. e.g., 2,4-D is basically a broad leaved weed killer Diclofop-methyl and fenoxoprop-p-ethyl are grass killers
b) Broad spectrum herbicides: They control wider weed flora consisting of broad leaved weeds, grasses and sedges. e.g., Glyphosate, paraquat, diquat, atrazine, pendimethalin, isoproturon, oxyfluorfen etc.
4) Based on site of application
a) Soil applied herbicides: They are applied to the soil. They kill germinating and sprouting weed seeds, rhizomes, tubers etc.
• All pre-emergent herbicides are soil applied herbicides
b) Foliage applied herbicides: They are applied on the canopy or the foliage of the weeds. They are applied on the grown up weeds. e.g., paraquat, glyphosate, 2,4-D
5) Based on residue action in the soil
a) Residual herbicides: After application or incorporation in soil, herbicide residues remain in soil for a considerable period of time. e.g., Atrazine, 2,4-D, pendimethalin b) Non residual herbicides: They leave no or less residue in the soil and gets quickly in activated or metabolized falling on to the soil. They do not have extended period of activity in soil. e.g., Glyphosate, Paraquat, Diquat
• Glyphosate is systemic in action or nature.
• Paraquat and diquat can be used in lakes and ponds because of no residue and adverse effects. They are contact herbicides.
• Glyphosate takes 12-15 days for action as it is systematic.
• By spraying contact herbicides, within a day weeds are burned or killed.
6) Based on chemical structure
a) Inorganic herbicides: e.g., Ammonium sulphate, borate, copper sulphate, sodium chlorate
b) Organic herbicides: Most of the herbicides are organic
i) Phenoxy acetic acid group of herbicides: e.g., 2,4-D (Knock weed – 36% EC) 2,4,5-T (Brush killer -48% EC) Fluazfop-butyl (Flurilade- 12.5% EC) MCPB (Methyl Chloro Phenoxy Butyric acid) MCPA (Methyl Chloro Phenoxy Acetic Acid)
• They are called as old generation herbicides but till today they have been widely used for weed control.
• They are hormonal type of herbicides at lower concentration.
• 2,4-D is white crystalline material and it is auxin type herbicide. It is used as preemergent and early post emergent herbicide.
• 2,4-D is effective against broad leaved weeds.
• Pulses are highly sensitive to 2,4-D. Do not use 2,4-D in pulse crops.
• Dicotyledon crops are highly sensitive to 2,4-D.
ii) Triazine group
e.g., Atrazine (Atratap – 50% WP)
Simazine (Gesatop – 50% WP)
Propazine (Gesanil – 50% WP)
Prometryne (Gesaguard – 50% WP)
• These are widely used for selective weed control.
• Applied as pre-emergent spray.
• They are extensively used for weed control in Maize.
• Atrazine is used in sorghum, maize, bajra, sugarcane.
• Atrazine is a broad spectrum herbicide and controls all weeds.
• They are available as wettable powder formulations.
• They are photosynthesis inhibitors.
• They are highly persistent in soil.
• If we go for pulses after maize it is a problem because of atrazine.
iii) Bipyridillium group
e.g., Diquat (Reglone – 20% EC)
Paraquat (Gramaxone – 20% EC)
• They are non selective herbicides or total weed killers.
• In plantation crops we can use paraquat and diquat for weed control.
• They are used in aquatic bodies for weed control.
• They are available in EC formulations.
iv) Pyrimidines (Uracil group)
Bromacil (Hyvar – 80% WP)
Terbacil (Sinbar – 80% WP)
v) Oxadiazone group
Oxadiazone (Ronstar-50% EC) Oxadiazone is used for weed control in vegetables.
vi) Substituted urea group:
e.g., Diuron (Karmex – 80% WP)
Isoproturon (Arelan – 50% WP)
Linuron Diuron is a very good herbicide for cotton.
vii) Sulphonyl urea group:
e.g., Metsulfuron-methyl (10%) + Chlorimuron-ethyl (10%) (Almix-Trade Name) Metsulfuron methyl (Algrip)
Bensulfuron methyl (Londax)
Pyrazosulfuron ethyl (sathi) – used in aerobic rice
• These are called as new generation herbicides.
• They are highly potent herbicides.
• They are applied at a very low dosage (g/ha).
• They are less persistent in the soil.
• They are broad spectrum in nature.
• They have revolutionized the herbicide usage in the world because of their very low rate of application.
viii) Nitroanilines group (Dinitroanilines):
e.g., Fluchloralin (Basalin – 48% EC)
Pendimethalin (Stomp – 30% EC)
Trifluralin (Treflan – 48% EC)
• This group first introduced in 1960’s.
• They are available in EC formulations.
• They are orange yellow in colour.
• They control wide spectrum of grasses.
• Some of the herbicides which are susceptible to photo decomposition are pre-plant incorporated (Fluchloralin).
• Pendimethalin is used in cereals, vegetables, pulses, oilseeds and has selectivity for many crops and is available at low cost.
ix) Nitrophenyl ethers:
e.g., Nitrofen (Tok E- 25% EC) – used in groundnut Oxyfluorfen (Goal – 23.5% EC) (Oxygold – 23.5% EC)
• Oxyfluorfen is used in carrot, onion, garlic and potato.
• For onion, oxyfuorfen is the best herbicide.
e.g., Propham (CPC – 50% EC)
Chloropropham (ClPC – 40% EC)
e.g., EPTC (Eptam- 75% EC)
xii) Anilides / Amides:
e.g., Alachlor (Lasso- 48% EC)- for pulse crops good selectivity Butachlor (Machete – 50% EC) – for rice no.1 herbicide Metolachlor (Dual – 50% EC) Propanil Pretilachlor (Sofit/Refit)- Used in rice
• Sofit is without surfactant and refit is with surfactant
xiii) Organo phosphorus compounds:
e.g., Glyphosate (Roundup – 41% EC) – manufactured by Monsanto (Glycil – 41% EC) – manufactured by Indofil Anilophos (Aniloguard – 30% EC) – used in rice
• Do not use glyphosate in cropped area
7) Based on mode of action
a) Cell division inhibitors: cell division, elongation, tissue differentiation are some of the processes in plants directly affected by some of the herbicides.
i) Microtubule assembly inhibitors:
• During mitosis in metaphase the spindle structures which attach one of the two chromatids of a chromosome and draw them to either of the poles.
• These spindles are composed of filamentous protein called as microtubulin.
• Herbicides in sensitive plants binds to the tubulin in the cytoplasm and further growth of microtubule ceases and eventually complete loss of microtubules. e.g., Dinitroanilines (Pendimethalin, fluchloralin, trifluralin)
ii) Mitosis inhibitors:
• Herbicides block the mitosis in primary meristems. Action relates to mitotic aberrations. e.g., Carbamates, thiocarbamates
b) Photosynthesis inhibitors:
• In photosynthesis many herbicides interfere in hill reaction (light reaction). As a result subsequent CO2 fixation is retarded due to reduced supply of products of light reaction which are essential for dark reaction.
• A large number of herbicides block electron transport in photo system II. e.g., Triazines, uracils, urea group, Bipyridillium group (Diquat and paraquat)
c) Aminoacid/ protein synthesis inhibitors:
1) Blocking or inhibiting EPSP synthase enzyme :
• 5-Enol Pyruvyl Shikimate 3-Phosphate Synthase enzyme is involved in biosynthesis of 3 aromatic aminoacids such as phenyl alanine, tryptophane and tyrosine.
• Glyphosate blocks or inhibits the action of EPSP synthase enzyme.
2) ALS synthase enzyme inhibitors:
• Aceto lactate synthase enzyme is involved in biosynthesis of branched chain aminoacids such as leucine, isoleucine and valine.
• Sulfonyl urea group of herbicides like chlorosulfuron, Bensulfuron methyl, chlorimuron ethyl, metsulfuron methyl and imidazolinones (imazethapyr) inhibit the action of ALS synthase enzyme.
3) Glutamine synthesis inhibitors:
• Glutamine synthetase enzyme is involved in glutamine synthesis. It plays important role in nitrogen metabolism in plant system. By this enzyme ammonia in plant system is converted into aminoacids and glutamic acid.
• Glufosinate inhibits glutamine synthesis
d) Lipid biosynthesis inhibitors
• Lipids constitute major components of cell membranes, tonoplast, cuticular waxes and reserved food in the seeds.
• Acetyl Coenzyme A carboxylase is involved in the biosynthesis of fatty acids. e.g., Clodinafop-propargyl, Diclofop-methyl, fenoxoprop – pethyl, fluazifop-butyl. These herbicides are grass killers and have good selectivity for dicotyledonous plants.
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