Weed is a plant considered undesirable, unattractive or troublesome, especially are growing where it is not wanted. These unwanted plants grow in the fields where they compete with the crop for water, soil nutrients, light and space, interfere with irrigation and harvesting operations and finally reduce the crop yields. Weeds also cause several diseases in human beings and animals. They also harbour insect pests and diseases. A recent estimate by the Directorate of Weed Science Research, Jabalpur showed that weeds account for 37% of the total- annual loss in productivity, with a range from 20 to 100% in different crops /situations.

Weed management through use of herbicides is gaining popularity due to several reasons, and one important of which is non-availability of agriculture labour. Any chemical that kill the plants or inhibit their growth is known as herbicide. The herbicides are grouped by their Mode of action, Selectivity, Time of application, Chemical, Mechanism of action etc.

By Mode of Action

(a) Contact Herbicides

  • They destroy only the plant tissues which come in contact with the chemical.
  • Generally, these are the fast acting herbicides.
  • They are not active in soil.
  • They are less effective in perennial plants and are generally used to control annual weeds.
  • They are non-selective herbicides.
  • Uniform spray coverage and spray particle size are essential for adequate weed control.
  • Example: Ozone (Paraquat dichloride)

    (b) Systemic Herbicides

  • They are absorbed and translocated through the plant.
  • They may be active or non-active in the soil.
  • They control both annual and perennial weeds.
  • They may be selective or non-selective.
  • They may be slower in action but ultimately more effective than contact herbicides.
  • Example: Targa Super (Quizalofop ethyl)

By Selectivity

(a) Selective Herbicides

  • They control or suppress only certain weeds without negative effect on selective crops.
  • They can be used only on specific crops.

   Examples

  • Targa Super (Quizalofop ethyl) – used to control narrow leaf weeds in broad leaf crops.
  • Dynofop (Piroxofop-propanyl (Clodinafop-propargyl)  – to control Phalaris minor and Wild oat in wheat
  • Qurin (Chlorimuron ethyl) – to control broad leaf weeds in soybean.



(b) Non-selective Herbicides

  • They control plants regardless of plant species.
  • They kill all plants which contain Chlorophyll.
  • They are not used on standing crops.

Example: No Weed (Glyphosate) ; Ozone (Paraquat)

By Time of Application

(a) Pre-plant Incorporation

  • They are mechanically incorporated in soil prior to planting of crops. The objective for incorporation is to prevent their dissipation through photo-decomposition and /or volatility.

        Example: Dhanutop (Pendimethalin)
(b) Pre-emergent Herbicides

  • They are applied in soil after sowing but before the crop and weed emergence. They form a thin barrier at or right below the soil surface and prevent germination or growth of the weed seeds.



(c) Post-emergent Herbicides

  • They can be foliar or root absorbed, selective or non-selective, and contact or systemic.
  • They are applied directly on to the emerged weeds & crops, and are usually more effective on young weeds.

        Example:Targa Super (Quizalofop ethyl)

By Chemical

(1). Aryloxy phenoxy-propionate – Targa Super (Quizalofop ethyl)

(2). Cyclohexanediones – Alloxydim

(3). Sulfonylureas – Qurin (Chlorimuron ethyl), Sultop (Sulfosulfuron)

(4). Imidazolinones – Imazethapyr

(5). Triazolopyrimidines – Diclosulam, Penoxsulam

(6). Pyrimidinyl (thio) benzoates – Pyribenzoxim

(7). Sulfonylaminocarbonyl – Triazolinones, Flucarbazone – Na, Propoxycarbazone –Na

By Mechanism of Action

The main mechanisms of action are:

(a) ACCase inhibitors: ACCase inhibitors are compounds that kill grasses. Acetyl coenzyme, A carboxyl ase (ACCase) is part of the first step of lipid synthesis. Thus, ACCase inhibitors affect cell membrane production in the meristems of the grass plant. The ACCase of grasses are sensitive to these herbicides, whereas the ACCase of dicot plants are not. eg. Clodinofop – propargyl.

(b) ALS inhibitors: The acetolactate synthase (ALS) enzyme (also known as acetohydroxyacid synthase, or AHAS) is the first step in the synthesis of the branched-chain amino acids (valine, leucine, and isoleucine). These herbicides slowly starve affected plants of these amino acids which eventually lead to inhibition of DNA synthesis. They affect grasses and dicots alike. The ALS inhibitor family includes sulfonylureas (SUs), imidazolinones (IMIs), triazolopyrimidines (TPs), pyrimidinyl oxybenzoates (POBs), and sulfonylamino carbonyl triazolinones (SCTs). ALS is a biological pathway that exists only in plants and not in animals thus making the ALS-inhibitors among the safest herbicides.

(c) EPSPS inhibitors: The enolpyruvylshikimate 3-phosphate synthase enzyme (EPSPS) is used in the synthesis of the amino acids tryptophan, phenylalanine and tyrosine. They affect grasses and dicots alike. Glyphosate (Roundup) is a system EPSPS inhibitor but inactivated by soil contact.

(d) Synthetic auxin: The era of organic herbicides began with it. They were discovered in the 1940s after a long study of the plant growth regulator auxin. Synthetic auxins mimic this plant hormone. They have several points of action on the cell membrane, and are effective in the control of dicot plants. 2, 4-D is a synthetic auxin herbicide.

(e) Photosystem II inhibitors: They reduce electron flow from water to NADPH2 + at the photochemical step in photosynthesis. They bind to the Qb site on the D1 protein, and prevent quinone from binding to this site. Therefore, this group of compounds causes electrons to accumulate on chlorophyll molecules. As a consequence, oxidation reactions in excess of those normally tolerated by the cell occur, and the plant dies. The triazine herbicides (including atrazine) and urea derivatives (diuron) are photosystem II inhibitors.

(f) Photosystem I inhibitors: They steal electrons from the normal pathway through FeS – Fdx – NADP leading to direct discharge of electrons on Oxygen. As result ROS (reactive oxygen species) are produced and oxidation reactions in excess of those normally tolerated by the cell occur leading to plant death.

Bipyridyliums herbicides (like Diquat and Paraquat) hit “Fe-S – Fdx step”. Diphenilethers herbicides (like Nitrofen, Nitrofluorfen, Acifluoren) hit “Fdx – NADP step”.