Types of herbicides: Classification for everyone’s understanding

Maintaining an agricultural culture without any weeds and controlling unwanted species very easily is the dream of every farmer, isn’t it?

Weed infestations are very harmful and can easily destroy both a small vegetable garden and a huge agricultural production. If left unchecked, they can cause monetary damage by halting the development and yield of crops.

Types of herbicides
Types of herbicides

Maybe in the case of a smaller crop, you can resort to weeding, digging, and even pulling the weeds by hand as often as they appear, but even so, quite a lot of work is required. Needless to say, these options cannot be applied in any case in large-scale agricultural production.

That is precisely why, nowadays, more and more farmers are turning to the use of herbicides.

However, in order to use herbicides properly, it is good to know exactly how many types there are and how they are classified.

So, in today’s article, we will break down each criterion and type of herbicide in order to provide you with better information.

Types of herbicides

Herbicides can be classified mainly according to the following classification criteria:

  • mode of action
  • the spectrum of action
  • mode/time of application
  • chemical group

Mode of action

From the point of view of the mode of action, herbicides can be of the following types:

  • Contact herbicides – They are applied by spraying on the plant and mainly act only on the tissues that have been touched by the herbicide. Their efficiency is not influenced by weather conditions. The action of these herbicides can be observed quite quickly, usually within 24 hours. However, this type of herbicide is not suitable for perennial weeds.
  • Systemic herbicides – They are designed to act on the entire plant. The efficiency of these herbicides is good, but the effect is slower. It should also be noted that this type of active substance needs favorable weather conditions such as periods of precipitation shortly after application. In turn, they are grouped into:
    • herbicides that are absorbed through the leaves
    • herbicides that are absorbed through leaves and roots
    • herbicides that are absorbed exclusively through the roots
    • herbicides that are absorbed through the hypocotyl and coleoptile
  • Residual herbicides – They persist in the soil for a longer time and can protect crops from the next wave of unwanted weeds. The effectiveness of these herbicides depends on the spectrum of weeds, how well the land has been worked, and the amount of precipitation. These types of herbicides are more effective in adequate moisture conditions.

The spectrum of action

Taking into account the spectrum of action, herbicides can be grouped into:

  • Total or non-selective herbicides – They destroy all the plants on the land where they are applied. It is important to remember that they do not make any exceptions, they destroy both annual and perennial plants. The use of these herbicides must be done carefully. Usually, they are used in spaces without agricultural use such as alleys, sidewalks, or uncultivated places. They can also be used on agricultural land, but only during the break between crops or before sowing. The result is visible in about 5-7 days after application.
  • Selective herbicides – They destroy certain plant species, without affecting crop plants. They affect monocotyledonous and dicotyledonous weeds. The selectivity of these herbicides is grouped into two types:
    • Selectivity independent of the mode of action/absorption of herbicides:
      • the selectivity generated by the place where the herbicide is applied
      • physiological selectivity, which depends on plant morphology. For example, monocotyledonous plants have narrower leaves and will absorb less herbicide than dicotyledonous plants.
    • Selectivity depending on the mode of action/absorption of herbicides:
      • selectivity according to absorption – the penetration of the herbicide into the plant and its absorption do not depend on the morphology of the plants, but on other factors;
      • selectivity generated by differences in translocation (the path traveled by the herbicide from the site of penetration to the site of action) – the same herbicide can be effective for several plant species, but the speed of translocation can be lower or higher;
      • selectivity due to the mode and speed of herbicide inactivation – the ability of certain crops to inactivate a greater amount of herbicide than the weeds to be eliminated;
      • selectivity according to the sensitivity of the site of action – this type of selectivity can generate, over time, the resistance of weeds to the action of certain herbicides.

Mode/time of application

This type of herbicide can be divided into:

  • Pre-emergent herbicides – Pre-emergent herbicides are applied to the soil to prevent the germinated weeds from growing and sprouting on the surface of agricultural crops. The soil must be broken up and leveled to ensure the even distribution of the herbicide. The time of application must be made according to the weather forecast. Ideally, the application should be made a few days before the rain. The absorption takes place through the roots of the weeds and prevents their sprouting.
  • Post-emergent herbicides – They are applied after the emergence of weeds and plants. In this situation, it is very important that the treatment solution covers the weed leaf as much as possible. These herbicides are absorbed through the leaves or roots.

By chemical group

WSSA or Weed Science Society of America and HRAC or Global Herbicide Resistance Action Committee each made a classification according to the biochemical mode of action.

In the WSSA classification, herbicides received numbers from 1 to 34, while in the HRAC system, they are grouped from A to Z.

Below, you can find some examples of herbicides classified according to the two systems

WSSA & HRAC herbicides classification
WSSA number: HRAC letter: Place of action: Action mode:
1 A Acc-ase inhibitor blocks the synthesis of lipids, which leads to the impairment of the production of the cell membrane in the mechanisms of gramineae plants;
2 B ALS inhibitor prevents the synthesis of branched-chain amino acids, such as leucine or valine; affects DNA formation;
9 G EPSP inhibitor inhibits EPSPs in annual and perennial grasses and dicotyledons;
4 O Auxin synthetase mimics the action of natural plant hormones; they act on the cell membrane and are effective herbicides against dicotyledonous weeds.
About the author: Dani Martelli
Dani FarmerDB

My name is Dani and I am a farmer with 10 years of experience. I will share with you everything about farming from processes, tips, machines, and more. Farming for me is not just a job but a way of life. Keep reading ...

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