Modern agriculture has grown just like any other sector and has witnessed a number of groundbreaking techniques to deploy unimaginable to the ancient farmer. Traditional way of farming was mostly organic and it worked pretty well since the demand of crops was low and for personal usages.. As the society grew the demand for crops increased. The occupation of farmers rose to new heights. This called for finding new ways to grow a huge no. of crops. Farmers and scientists thus had to find new solutions to the existing problems. 

One such problem was the growth of weeds (unwanted plants). Weeds are the plants that grow out on their own since their seeds get travelled through natural agents. Ex, wind or hopping of birds from one place to the other. They grow effortlessly and consume the space, nutrients and sunlight. Hence, they have to be eliminated or controlled by farmers to grow the agricultural crops.

Herbicides are chemical agents that are used to kill the unwanted plants (weeds). They can be used before, after or during the phase of growth of the crops. Different herbicides are used during different phases. Selection of herbicides also depends on the season, type of soil, which crops have to be planted and which weeds have to be eliminated. The toxicity level has to be examined carefully by the farmers in order to use the right herbicides which, if not done, might damage the soil/crops and can also prove harmful to the animals. 

Herbicides are classified on the basis of different factors –


Chemical composition – There are two types: organic and inorganic. 

  • Organic herbicides consists of oils, carbamates, amides etc. 
  • Inorganic herbicides consist of acids and salts. 

Time of application – This is the most important differentiator of the herbicides to be used. The chemical components present in them act appropriately to eliminate the weeds from taking root. 

  • Pre-planting herbicide – This is used 2-4 days before planting the crops. The most common ones are fluchloralin and trifluralin. 
  • Pre-emergence herbicide – This is used 1-4 days after sowing the seeds and right before the first leaf is about to emerge. Widely used pre-emergence herbicides are atrazine and simazine.
  • Post-emergence – This is used after approximately 30-40 days after the crop has grown. Dalapan and paraquat are the best herbicides in this category. 

Mode of action/physiological – Herbicides in this category work on the different areas of the plants like leaves (foliage application) and soil (root application). There are mainly two types – selective and nonselective. 

Let’s look at some terminologies before going into the classification and further examples. 

  1. Selective Herbicides: The chemicals which kill or retard the growth of weeds with little or no injury to other plants are selective herbicides. 


  1. Non-Selective Herbicides: These chemicals are toxic to all the plants or kill all kinds of vegetation. 


  1. Contact Herbicides: The herbicides which kills only those plants or retards the growth of those plants which comes in direct contact. 


  1. Translocated Herbicides: The herbicides which are absorbed by the one part of the plants and exert a toxic action to other parts.These are also known as systemic herbicides. These absorbed chemicals upset the plant growth and metabolic processes. 


  1. Soil Fumigants:They usually function as a vapour or gas that diffuse through the soil and have relatively short life in the soil. 


  1. Soil Sterilants:Any chemical which prevents the growth of green plants when present in the soil is considered as soil Sterilants. 

The selective and nonselective herbicides are classified further as follows – 

I) Selective: 

A.Foliage Application 

  1. Contact: DNBP, Propanil, EPTC, Nitroten.
  2. Translocated: 2,4-D, 2,4,5-T, MCPB, MCPA, Silvex( 2,4 ,5TP), Propanil, Monuron, MSMA 

D.Soil-(Root) Application: MCPA, TCA, Nitrofen, Dinitrophenols, Butachlor, Simazine, Atrazine 


  1. II) Non-Selective: 


  1. a) Foliage Application: 


  1. Contact:Paraquat, Sulphuric acid, Sodium arsenite, Ammate 
  2. Translocated: Dalapon, Acid Arsenical, Sodium chloride, Glyphosate 

  1. b) Soil Application (Root): 


  • Soil Fumigants: Cyanamide, Methyl bromide, Carbon disulphide, Trifluralin 
  • Soil Sterilant: TCA, Sodium Chloride, Boron, Diuron, Monuron, Atrazine, Fenac

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