📈 Chemical Weed Control — Herbicide Classification

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

---

Chemical Weed Control — Herbicide Classification

Herbicides are chemical compounds used to kill or suppress weeds. They account for nearly 50% of global pesticide use and are classified based on timing of application, mode of contact, selectivity, and chemical family.

Classification by Time of Application

Timing	Definition	Examples
Pre-plant incorporated (PPI)	Applied to soil and mixed before sowing	Fluchloralin, trifluralin
Pre-emergence (PE)	Applied after sowing but before crop/weed emergence	Pendimethalin, atrazine, butachlor
Post-emergence (PoE)	Applied after crop and weed emergence	2,4-D, metsulfuron-methyl, fenoxaprop
Directed/shielded	Post-emergence spray directed at weeds, shielded from crop	Paraquat in sugarcane, glyphosate in orchards

Classification by Contact Type

• Contact herbicides — kill only the plant tissue they directly contact; do not translocate
  • Examples: paraquat, diquat
  • Effective against annual weeds; perennials regrow from underground parts
• Systemic (translocated) herbicides — absorbed and moved through the plant via phloem or xylem
  • Examples: glyphosate, 2,4-D, imazethapyr
  • Effective against both annual and perennial weeds, including deep-rooted species

Classification by Selectivity

Selectivity determines whether a herbicide can be used safely in a crop:

• Selective herbicides — kill specific weed types without harming the crop
  • 2,4-D kills broadleaves in wheat/rice
  • Fenoxaprop kills grasses in wheat
  • Imazethapyr controls weeds in soybean/groundnut
• Non-selective herbicides — kill all vegetation indiscriminately
  • Glyphosate and paraquat — used for pre-plant burndown, orchard floors, non-cropped areas

Basis of Selectivity

Herbicide selectivity depends on multiple factors:

• Morphological — waxy leaf surface of grasses causes spray droplets to roll off
• Physiological — crops may metabolise the herbicide faster than weeds
• Biochemical — target enzyme may be absent or less sensitive in the crop
• Positional — herbicide placed in the surface soil zone where weed seeds germinate but below crop seed depth

Herbicide Formulations

Common formulation types include:

• EC (Emulsifiable Concentrate) — mixed with water, forms emulsion
• WP (Wettable Powder) — powder mixed in water for spraying
• SL (Soluble Liquid) — dissolves directly in water
• WG/WDG (Water Dispersible Granules) — granules that disperse in water
• GR (Granules) — applied directly to soil/water surface without spraying

Choosing the correct herbicide class, timing, and formulation is critical for effective and economical weed control.

---

Summary Cheat Sheet

Topic	Key takeaway
Main focus	Chemical Weed Control — Herbicide Classification covering pre/post-emergence timing, contact vs systemic action, and selectivity principles.
Section context	Revise this lesson with the rest of Weed Management for stronger conceptual continuity.