Definition and Broad Scientific Classification
Under agricultural science and national pesticide legislation, herbicides encompass all compounds designed to manage unwanted vegetation.
They are systematically classified based on their crop selectivity, application timing, and translocation behavior within plant tissues.
- Selective herbicides: Eliminate target weeds without harming the standing crop (e.g. Atrazine in maize, Pendimethalin in pulses)
- Non-selective herbicides: Destroy all green vegetation they contact (e.g. Glyphosate, Paraquat)
- Contact herbicides: Kill only the green foliage directly exposed to spray droplets (e.g. Oxyfluorfen, Paraquat)
- Systemic herbicides: Translocate through vascular xylem or phloem to kill root crowns and tubers (e.g. 2,4-D, Halosulfuron)
Application Timing Windows in Farming
Herbicide success depends heavily on deploying the chemical at the precise physiological growth window.
Deploying chemicals at incorrect stages can result in inadequate weed control or severe phytotoxic crop injury.
- Pre-plant incorporation (PPI): Inverted into soil before sowing to create a subterranean barrier
- Pre-emergence (PRE): Sprayed onto moist soil surfaces after sowing but before crop or weed seedlings emerge
- Post-emergence (POST): Sprayed over standing crops when weeds are actively photosynthesizing at the two to four leaf stage
- Directed post-emergence: Sprayed between crop rows using drift-shielded nozzles to avoid contact with crop foliage
Herbicide Resistance Management and Safe Use
Repeated reliance on a single herbicide mode of action encourages the survival and proliferation of resistant weed biotypes.
Growers must practice Integrated Weed Management (IWM) by combining crop rotations, mechanical cultivation, and rotating herbicide classes.
- IWM integration: Combine manual rogueing, cover cropping, and chemical rotation to delay resistance
- Spray calibration: Use flat fan nozzles at low operating pressure to maintain uniform coarse spray droplets
- Operator safety: Always wear personal protective gear including chemical gloves, face mask, and eye protection
Official source
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Central Insecticides Board & Registration Committee ↗Related on KisanPe
Common questions
What is the basic definition of an agricultural herbicide?
A herbicide is a chemical or biological substance used to control, suppress, or kill unwanted plants known as weeds.
What is the difference between pre-emergence and post-emergence herbicides?
Pre-emergence herbicides are applied to soil before weeds emerge, while post-emergence herbicides are sprayed directly onto actively growing weed foliage.
Why is rotating herbicide modes of action important?
Rotating modes of action prevents surviving weed populations from developing genetic resistance against specific chemical families.
General guidance, not personal advice. For your farm, ask your local Krishi Vigyan Kendra or agriculture officer.
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