Classification by Selectivity and Timing
Agricultural weedicides are categorized based on their selectivity toward cultivated crops and their application timing relative to crop emergence.
Selecting the correct herbicide class ensures weed suppression without causing vegetative injury to desirable crop plants.
- Pre-emergence selective herbicides: Applied to soil before crop or weed emergence (e.g. Pendimethalin, Atrazine, Pretilachlor)
- Post-emergence selective herbicides: Applied over standing crops to control emerged weeds (e.g. 2,4-D, Fenoxaprop, Bispyribac-sodium)
- Non-selective systemic herbicides: Destroy all green vegetation and underground root systems (e.g. Glyphosate)
- Non-selective contact herbicides: Rapidly burn all exposed green plant foliage on contact (e.g. Paraquat, Glufosinate)
Common Chemical Classes and Modes of Action
Herbicides interfere with vital plant biochemical pathways such as amino acid synthesis, photosynthesis, or cell division.
Rotating between differing herbicide modes of action is vital to prevent weeds from developing herbicide resistance.
- ALS Inhibitors: Sulfonylureas (e.g. Halosulfuron, Metsulfuron) stopping branched-chain amino acid synthesis
- ACCase Inhibitors: Selective graminicides (e.g. Quizalofop, Fenoxaprop) halting fatty acid synthesis in grasses
- Photosystem II Inhibitors: Triazines and ureas (e.g. Atrazine, Diuron) shutting down photosynthetic electron flow
- Synthetic Auxins: Growth regulators (e.g. 2,4-D amine/ester) causing uncontrolled twisted growth in broadleaf weeds
Safe Application Practices and Drift Prevention
Applying weedicides requires calibrated equipment, low-pressure flat fan nozzles, and adequate soil moisture for root-absorbed compounds.
Applicators must avoid spraying on windy days to prevent spray drift from damaging neighboring sensitive crops.
- Nozzle selection: Flat fan or floodjet nozzles deliver uniform coarse droplets that minimize drift
- Soil moisture: Pre-emergence herbicides require moist soil to create a stable chemical barrier
- Personal safety: Wear protective rubber gloves, mask, and goggles during preparation and application
Official source
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Central Insecticides Board & Registration Committee ↗Related on KisanPe
Common questions
What is an example of a pre-emergence herbicide?
Pendimethalin 30% EC is a common pre-emergence herbicide applied to moist soil after sowing to prevent weed emergence.
What is the difference between selective and non-selective weedicides?
Selective weedicides kill specific weeds without harming the crop, whereas non-selective weedicides kill all green plant tissue they touch.
Why is 2,4-D used in cereal crops?
2,4-D is a selective post-emergence herbicide that controls broadleaf weeds in narrow-leaf cereal crops like wheat, rice, and maize.
General guidance, not personal advice. For your farm, ask your local Krishi Vigyan Kendra or agriculture officer.
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