Classification of Herbicides

Based on Chemical Nature

Inorganic Herbicides

  • Inorganic herbicides do not contain carbon atoms in their molecules
  • These were the first chemicals used for weed control
  • Ex-Arsenic acid, arsenous acid, arsenic trioxide and sulphuric acid are acid type of inorganic herbicides
  • Whereas sodium arsenate, sodium chlorate, copper sulphate etc are inorganic salts are used as herbicides.

Organic Herbicides

  • These herbicides contain carbon atoms in their molecules
  • Can be oils or non-oils
  • Eg. Diesel oil, Xylene type of aromatic oils
  • Majority of present day herbicides are organic compounds which are non-oils.

Based on Selectivity

Selective Herbicides

  • Selective herbicides kill only targetted plants or weeds while crops are not affected.
  • E.g. Simazine, Atrazine, 2,4-D, MCPA, Butachlor, Alachlor, Fluchloralin and Pendimethalin.

Non-selective Herbicides

  • Non-selective herbicides kill all vegetation that they come in contact with irrespective of whether it is a crop or weed
  • E.g. Paraquat and Diquat.

Based on Translocation

Systemic Herbicides

  • Systemic herbicides move within the plant either through xylem or phloem.
  • Most of the systemic herbicides are selective at recommended rates
  • e.g. Atrazine, Simazine, Propanil, 2,4-D, MCPA, 2,3,6-TBA.

Contact Herbicides

  • Contact herbicides kill plants when they come in contact with plants.
  • They kill the part of the plant that is in contact with herbicides;
  • e.g. Paraquat, Diquat.

Based on Time of Application

Fallow Application

  • Application of herbicides well in advance of sowing i.e. More than 10 days before sowing is known as fallow application.
  • Fallow application of herbicides is resorted to control problematic weeds with higher dose of non- selective herbicides when there is no crop.
  • It is generally done during the fallow period to control weeds like Cyperus and Cynodon with higher doses.

Pre-plant Application

  • Application of soil active herbicides 2 to 10 days before sowing or planting is called pre-plant application.
  • Volatile herbicides like fluchloralin, trifluralin are incorporated into the soil to avoid volatilization losses.
  • As it is not possible to incorporate herbicides into the soil after sowing of the crop, they are applied before planting and this process is known as pre-plant incorporation (PPI).

Pre-emergence Application

  • Application of herbicides after sowing of the crop but before emergence of crop and weeds is called pre-emergence application.
  • Pre -emergence application of herbicides is done from one day to four days after sowing.
  • Only selective herbicides are used for pre-emergence application.

Post-emergence Application

  • Application of herbicides after the emergence of crop is called post-emergence application.
  • Generally, post- emergence application of herbicides is done about 30 to 40 days after sowing.