Classification of herbicides

a) Based on Mode of Action; Herbicides differ in how they act on plants. Their mode of action determines how they enter, move, and affect plant tissues.

1. Contact Herbicides

• Definition: Kill only the plant parts they touch; no movement inside the plant.
• Mode: They disrupt cell membranes, leading to leakage of cell contents → rapid death of exposed tissues.
• Examples: Paraquat, Diquat, Propanil, Oxyfluorfen.

2. Systemic (Translocated) Herbicides

• Definition: Absorbed by leaves/roots and transported inside the plant (via xylem/phloem).
• Mode: Inhibit key physiological processes (photosynthesis, amino acid synthesis, enzyme activity).
• Features: Slow but complete kill (including roots, rhizomes, tubers). More effective against perennial weeds.
• Examples: Glyphosate, 2,4-D, Atrazine, Metribuzin, Pendimethalin.
• Note: Atrazine has both systemic + contact action.

b) Based on Time of Application

Timing of herbicide application is critical for weed stage, crop stage, and effectiveness.

Pre-Plant Incorporation (PPI)

• Definition: Herbicide applied to soil before sowing, then incorporated into soil by ploughing/harrowing.
• Purpose: Prevents volatilization & photodecomposition (esp. for volatile herbicides).
• Examples: Fluchloralin, Trifluralin, EPTC.

Pre-Emergence Application

• Definition: Applied after sowing but before weeds/crop emerge (within 1–2 days of sowing).
• Mode: Herbicide remains in soil, affects germinating weed seedlings.
• Examples: Pendimethalin, Atrazine, Alachlor, Butachlor, Metribuzin.

Post-Emergence Application

• Definition: Sprayed after both crop and weeds have emerged (15–30 days after sowing).
• Mode: Usually foliar-absorbed, targeting actively growing weeds.
• Examples: 2,4-D, Isoproturon, Sulfosulfuron, Metsulfuron, Clodinafop-propargyl.
• Uses:
  • Controls already established weeds without damaging crop.
  • Selective herbicides (e.g., 2,4-D → kills broadleaves, safe for wheat).
  • Important for late-emerging weeds.

c) Based on Selectivity Selectivity means herbicides can differentiate between weeds and crops, killing one while sparing the other.

i) Selective Herbicides

• Definition: Kill certain weed species but leave the crop unharmed.
• Examples:
  • 2,4-D → Kills broadleaf weeds, safe for wheat/rice.
  • Butachlor, Pendimethalin → Kill grasses but safe for rice.
• Uses:
  • Widely used in agriculture where crops and weeds coexist.
  • Allow weed control without crop damage.

ii) Non-Selective Herbicides

• Definition: Kill all vegetation regardless of species.
• Mode: Often contact or systemic (destroy both crops & weeds).
• Examples: Glyphosate, Paraquat, Diquat, Glufosinate, Acrolein.

d) Based on Spectrum of Weed Control

• Narrow Spectrum Herbicides; Control specific groups of weeds. Examples:2,4-D → Broadleaf weeds and sedges (not grasses). Metsulfuron-methyl → Broadleaf and sedge species.
• Broad Spectrum Herbicides; Control grasses, sedges, and broadleaves together. Examples: Atrazine → Corn weed control. Pendimethalin → Grasses + broadleaves. Imazethapyr, Alachlor, Butachlor → Multi-weed control.

e) Based on Site of Application

• Soil-Applied / Soil-Active Herbicides; Applied to soil; act on germinating seeds/seedlings. Examples: Pre-plant incorporation: Fluchloralin, Trifluralin. Pre-emergence: Pendimethalin, Alachlor.
• Foliar-Applied Herbicides; Applied on foliage, absorbed through leaves. Examples: Post-emergence: 2,4-D, Isoproturon, Sulfosulfuron. Non-cropped: Paraquat, Glyphosate.
• Both Soil and Foliar Active; Act in both soil and foliage. Examples: Atrazine, Metribuzin.

f) Based on Residual Action in Soil

• Non-Residual (Zero Persistence) Herbicides; Degrade quickly; no long-term soil effect. Examples: Paraquat, Diquat, Glyphosate.
• Residual Herbicides; Persist 15–16 weeks; prevent future weed germination. Examples: Triazines → Atrazine. Phenyl Ureas → Diuron.

g) Based on Chemical Structure

• Inorganic Herbicides; No carbon in structure. Examples:
  • Acids → Arsenic Acid, Arsenious Acid, Arsenic Trioxide, Sulfuric Acid.
  • Salts → Borax, Copper Sulfate, Ammonium Sulfate, Sodium Chlorate, Sodium Arsenite, Copper Nitrate.
• Organic Herbicides; Carbon-containing compounds (widely used).

•  Categories:
  • Oils → Diesel oil, Aromatic oils, Xylene oils.
  • Aliphatics → Dalapon, TCA, Acrolein, Glyphosate, Methyl Bromide.
  • Amides → Propanil, Butachlor, Alachlor, CDAA, Diphenamide, Naphthalam, Propachlor.
  • Benzoics → Dicamba, Triclopyr, Chloramben, TBA.
  • Bipyridyliums → Paraquat, Diquat.
  • Carbamates → Barban, Chloropropam, Propamocarb.
  • Thiocarbamates → EPTC, Butylate, Molinate, Triallate, Pebulate, Vernolate.
  • Dithiocarbamates → Metham, CDEC.
  • Nitralins (Benzonitrates) → Dichlobenil, Bromoxynil, Ioxynil.
  • Dinitroanilines → Trifluralin, Butralin, Fluchloralin, Oxyfluorfen, Nitralin.
  • Phenoxy Herbicides → 2,4-D, 2,4-DB, MCPB, 2,4,5-T, 2,4-DP.
  • Triazines → Atrazine, Simazine, Metribuzin, Ametryne, Prometryn.
  • Ureas → Diuron, Monuron, Fenuron, Neburon.
  • Uracils → Bromacil, Terbacil, Lenacil.
  • Diphenyl Ethers → Nitrofen, Flurodifen.
  • Organic Arsenicals → MSMA, DSMA, Cacodylic Acid.
  • Other Classes → Picloram, Bentazon, Pyrazon, Bensulfuron, Tembotrione, Endothall.

Based on Formulations

• Wettable Powders (WP) → Simazine, 2,4-D Na Salt, Diuron, Linuron.
• Water-Soluble Concentrates (WSC) → 2,4-D Amine, Dicamba, Diquat, Paraquat.
• Emulsifiable Concentrates (EC) → 2,4-D Ester, Alachlor, Nitrofen, Diallate.
• Liquid Suspensions (LS) → Atrazine, Cyprazine, Nitralin.
• Soluble Powders (SP) → 2,4-D Na Salt, TCA, Dalapon, Endothall.
• Granules → Butachlor granules, 2,4-D granules.