Classification of Herbicides

Herbicide: It is a chemical used to kill some targeted plants.

Principles of chemical weed control

he selectivity exhibited by certain chemicals to cultivated crops in controlling its associated weeds without affecting the crops forms basis for the chemical weed control. Such selectivity may be due to differences in the morphology, differential absorption, differential translocation, differential deactivation etc.

Classification of Herbicides

1. Based on Method of application

    1. Soil applied herbicides: Herbicide act through root and other underground parts of weeds. Eg. Fluchloralin
    2. Foliage applied herbicides: Herbicide primarily active on the plant foliage Eg.Glyphosate, Paraquat

2. Based on Mode of action

  1. Selective herbicide: A herbicide is considered as selective when in a mixed growth of plant species, it kills some species without injuring the others. Eg. Atrazine
  2. Non-selective herbicide: It destroys majority of treated vegetation Eg. Paraquat

3. Based on mobility

  1. Contact herbicide: A contact herbicide kills those plant parts with which it comes indirect contact Eg. Paraquat
  2. Translocated herbicide: Herbicide which tends to move from treated part to untreated areas through xylem/phloem depending on the nature of its molecule. Eg. Glyphosate

4. Based on Time of application

1. Pre-plant application (PPI)

Application of herbicides before the crop is planted or sown. Soil application as well as foliar application is done here.For example, fluchloralin can be applied to soil and incorporated before sowing rainfed groundnut while glyphosate can be applied on the foliage of perennial weeds like Cyperus rotundus before planting of any crop.

2. Pre-emergence

The application of herbicides before a crop or weed has emerged. In case of annual crops application is done after the sowing of the crop but before the emergence of weeds and this is referred as pre-emergence to the crop while in the case perennial crops it can be said as pre-emergence to weeds.

For example soil application by spraying of atrazine on 3rd DAT to sugarcane can be termed as pre-emergence to cane crop while soil application by spraying the same immediately after a rain to control a new flush of weeds in a inter-cultivated orchard can be specified as pre-emergence to weed. Eg. Atrazine, Pendimethalin, Butachlor, Thiobencarb, Pretilachlor

3. Post-emergence

Herbicide application after the emergence of crop or weed is referred as postemergence application. When the weeds grow before the crop plants have emerged through the soil and are killed with a herbicide then it is called as early postemergence.

For example spraying 2,4-D Na salt to control parasitic weed striga in sugarcane is called as post-emergence while spraying of paraquat to control emerged weeds after 10-15 days after planting potato can be called as early postemergence. Eg. Glyphosate, Paraquat, 2,4-D Na Salt.

4. Early post-emergence:

Another application of herbicide in the slow growing crops like potato, sugarcane, 2-3 week after sowing is classified as early post emergence.

5. Based on molecular structure

  1. Inorganic compounds
  2. Organic compounds

 

Formulations

Herbicides in their natural state may be solid, liquid, volatile, non- volatile, soluble or insoluble. Hence these have to be made in forms suitable and safe for their field use.

An herbicide formulation is prepared by the manufacturer by blending the active ingredient with substances like solvents, inert carriers, surfactants, stickers, stabilizers etc.

Objectives in herbicide formulations are;

  • Ease of handling
  • High controlled activity on the target plants

Need for preparing herbicide formulation

  • To have a product with physical properties suitable for use in a variety of types of application equipment and conditions.
  • To prepare a product which is effective and economically feasible to use
  • To prepare a product which is suitable for storage under local conditions?

 

Types of formulation

1. Emulsifiable concentrates (EC): A concentrated herbicide formulation containing organic solvent and adjuvants to facilitate emulsification with water eg., Butachlor

2. Wettable powders (WP): A herbicide is absorbed by an inert carrier together with an added surface acting agent. The material is finely ground so that it may form a suspension when agitated with a required volume of water eg., Atrazine

3. Granules (G): The inert material (carrier) is given a granular shape and the herbicide (active ingredient) is mixed with sand, clay, vermiculite, finely ground plant parts (ground corn cobs) as carrier material. eg. Alachlor granules.

4. Water-soluble concentrates (WSC): eg. paraquat

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