Herbicides are chemicals used to kill weeds (unwanted plants) that tend to compete with the grown crops. Herbicides can be selective or non-selective. Each of these groups can be sub-divided into three namely:

  • Contact
  • Systemic
  • Residual

Selective Herbicides

These are weed killers which by their chemical composition are capable of acting bio-chemically on either broad-leaved weeds or grasses. They are used predominately in monocropping situations.

However, when these herbicides are used in high concentrations, they tend to have a phytotoxic effect (harmful effects caused by chemicals) on cultivated crops. Selective herbicides that kill grasses only are Alachlor, Pendimenthalin etc., and those that kill broad leaves weeds only are Atrazine, Imasaquine etc.

Examples of selective commercial herbicides are:

  • Basamaize for maize.
  • Ronster for rice.
  • Galex for cowpea, soyabeans, groundnut.
  • Basegram for rice and maize.
  • Cotoran for cassava.
  • Primextra for maize, cassava and yam.
  • Diuron for yam and cassava.

 

Non-Selective Herbicides

These are otherwise known as total weed killers. They are used for weed control in any weed combination in the field, and will kill any kind of vegetation. An example is Glyphosate (Round-up).

Classification of Herbicides by Mode of Action

  • CONTACT HERBICIDES: These are types of herbicides that kill any vegetation or weeds immediately after application. They stop all biological activities. E.g paraquat, gramoxone, basta etc.
  • SYSTEMIC HERBICIDES: These are slow action types whose effects are not noticed until a few days or weeks after application. Their effect is a thorough but gradual killing of the most stubborn and perennial weeds. They have the ability to move around the transportation and translocation systems to achieve a total kill e.g Glyphosate.
  • RESIDUAL HERBICIDES: These are soil-acting weed killers whose effects on weed control are primarily as photosynthetic inhibitors. They can have both pre and post-emergence activity; they are applied to the soil after cultivation and effectively control germinating weed seeds. Examples are simazine, Diuron etc.
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Classification of Herbicides Based on Time of Usage

  • PRE-PLANTING HERBICIDES: This can be a foliar; selective or non-selective herbicide, applied before the crop is planted. Examples are Glyphosate, Paraquat.
  • PRE-EMERGENCE HERBICIDES: These are types of herbicides applied in the soil before the emergence of weeds, immediately after cultivation e.g. Galex  + Metolachar used for cowpea, Corcoran used for cassava etc.
  • POST-EMERGENCE HERBICIDES: These are applied after the emergence of weeds and can have both contact and systemic actions, but are usually selective so as not to have a phytotoxic effect on cultivated crops. Examples are Laddox in maize, Paraquat, Propanil, Glyphosate.

Source: agric.com