🐛 Herbicides — Classification and Selectivity

This lesson builds core elective concepts in BSc Agriculture with practical applications and exam-oriented clarity.

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Herbicides — Classification and Selectivity

Herbicides are agrochemicals used to control weeds that compete with crops for nutrients, water, light, and space. They are the most widely used category of pesticides globally and in India.

Classification by Time of Application

Timing	Application Stage	Examples
Pre-plant incorporated (PPI)	Mixed into soil before planting	Trifluralin, Fluchloralin
Pre-emergence	Applied to soil after sowing but before crop/weed emergence	Pendimethalin, Atrazine, Butachlor
Post-emergence	Applied after crop and weed emergence	2,4-D, Metsulfuron-methyl, Bispyribac-sodium

Classification by Selectivity

• Selective herbicides — kill target weeds without harming the crop (e.g., 2,4-D in wheat kills broadleaf weeds but not the crop)
• Non-selective herbicides — kill all vegetation (e.g., Glyphosate, Paraquat — used for land preparation or directed spray)

Basis of Selectivity

Selectivity depends on several factors:

• Morphological — differences in leaf angle, cuticle thickness, or root depth
• Physiological — differential uptake and translocation rates
• Biochemical — crop plants may detoxify the herbicide faster than weeds
• Positional — herbicide placed in the weed root zone but away from crop roots

Classification by Mode of Action (HRAC)

HRAC Group	Mode of Action	Examples
Group 1	ACCase inhibitors	Clodinafop, Fenoxaprop (grassy weed killers)
Group 2	ALS inhibitors	Metsulfuron-methyl, Bispyribac-sodium, Chlorimuron-ethyl
Group 4	Synthetic auxins	2,4-D, Dicamba (broadleaf weed killers)
Group 5	PS-II inhibitors	Atrazine, Metribuzin
Group 9	EPSP synthase inhibitor	Glyphosate
Group 10	Glutamine synthetase inhibitor	Glufosinate
Group 14	PPO inhibitors	Oxyfluorfen, Fomesafen
Group 15	VLCFA inhibitors	Pendimethalin, Butachlor, Pretilachlor
Group 22	PS-I electron diverter	Paraquat, Diquat

Herbicide Resistance

• Herbicide-resistant weeds have emerged due to repeated use of the same mode of action
• In India, Phalaris minor resistance to isoproturon in wheat is a well-documented case
• Management strategies include:
  • Rotation of herbicide groups
  • Tank mixing herbicides with different modes of action
  • Integrated weed management combining cultural, mechanical, and chemical methods

Key Application Guidelines

• Calibrate sprayers for uniform coverage and correct dose
• Maintain adequate soil moisture for pre-emergence herbicides
• Apply post-emergence herbicides at the 2-4 leaf stage of weeds for best efficacy
• Avoid drift to adjacent crops, especially with volatile herbicides like 2,4-D

Herbicides are indispensable tools in modern agriculture, but their judicious use within an integrated weed management framework is essential for sustainability.

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Summary Cheat Sheet

Topic	Key takeaway
Main focus	Herbicides — classification based on timing, selectivity, mode of action, and HRAC grouping for weed management.
Section context	Revise this lesson with the rest of Synthetic Pesticides for stronger conceptual continuity.