Herbicide- Classification, Formulations & Application

Herbicide- Classification, Formulations & Application

What is Herbicide?

Herbicides are chemicals used to manipulate or control undesirable vegetation. They are applied before or during planting to maximize crop productivity by minimizing other undesirable vegetation.

Classification Of Herbicides

Herbicides can be classified based on application method, mode of action, mobility, and time of application.

1. Based on Method of application

• Soil applied herbicides: These herbicide act through root and other underground parts of weeds. E.g., Fluchloralin.
• Foliage applied herbicides: Such herbicides primarily act on the plant foliage E.g., Glyphosate, Paraquat.

2. Based on Mode of action

• Selective herbicide: A herbicide is said to be selective when in a mixed growth of plant species, it kills some species without injury to others. E.g., Atrazine.
• Non-selective herbicide: It destroys majority of treated vegetation, E.g., Paraquat.

3. Based on mobility

• Contact herbicide: A contact herbicide kills those plant parts with which it comes in direct contact, E.g., Paraquat.
• Translocated herbicide: Translocated herbicides move to the site of action via the transport mechanisms within the plant; the xylem and phloem.g., Glyphosate.

4. Based on Time of application

• Pre – plant application (PPI): Application of herbicides before the crop is planted or sown through soil application and foliar application. 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.
• Pre – emergence): Preemergence means – before weeds emerge. Therefore, timing of applications depends on life cycles of the target weeds. Preemergence herbicides prevent germinated weed seedlings from becoming established; either by inhibiting the growth of the root, the shoot, or both. The herbicide must be incorporated into the soil via rainfall or irrigation, and be present when the weed seeds are germinating to be effective. Eg. Atrazine, Pendimethalin, Butachlor, Thiobencarb, Pretilachlor.
• Post – emergence: Herbicide application after the emergence of crop or weed is referred as post-emergence application. When the weeds grow before the crop plants have emerged through the soil and are killed with an herbicide then it is called as early post-emergence. 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 post-emergence. E.g., Glyphosate, Paraquat, 2,4-D Na Salt.

5. Based on molecular structure

• Inorganic compounds
• Organic compounds

What is Formulations

The formulation of an herbicide refers to the material it is carried in or on, and its concentration in that carrier. Every herbicide product is a combination of active ingredients and inert ingredients. The same herbicide can be available in more than one formulation.

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

The main objectives of formulations are: Easy transport and handling, to increase weed killing efficiency, to improve storage properties, to apply small amount of herbicide equally in the field and to increase the spectrum of weed control.

Types of formulation

1. Emulsifiable Concentrates (EC): Formulation in the form of liquid which does not go into solution but form an emulsion when added to water, ai is mostly liquid insoluble in water. Emulsion is a heterogeneous system and emulsifying agent must be added. E.g., 2,4 – D, Bensulphite.
2. Wettable powders (WP): Finely milled powder that form a suspension when added to water. The active ingredient mostly solid and insoluble in water. e.g., Atrazine.

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

1. Water soluble concentrates (WSC): Formulated in the form of liquid which very easily go into solution when added to water. Active ingredient is liquid, must be soluble in water as well as solvent. Solution is homogenous system. e.g., Paraquat.

Methods of Application

The application methods are influenced by many factors like, weed-crop situation, type of herbicides, mode of action and selectivity, environmental factors, and also cost and convenience of application.

The application method is of two types:

• Spraying Method
• Broadcasting Method

Depending on the target site, the herbicides are classified into Soil applied herbicides and Foliage applied herbicides. These herbicides are applied by different methods in soil and on foliage.

Soil Application of Herbicides

1. Surface application: Soil active herbicides are applied uniformly on the surface of the soil either by spraying or by broadcasting. The applied herbicides are either left undisturbed or incorporated in to the soil. Incorporation is done to prevent the volatilization and photo-decomposition of the herbicides. E.g., Fluchoralin – Left undisturbed under irrigated condition. Incorporated under rainfed condition.
2. Subsurface application: It is the application of herbicides in a concentrated band, about 7-10 cm below the soil surface for controlling perennial weeds. For this, special type of nozzle is introduced below the soil under the cover of a sweep hood. E.g., Carbamate herbicides to control Cyperus rotundus and, Nitralin herbicides to control Convolvulus arvensis.

• Band application: Application of herbicide by banding reduces competition between the newly planted seedlings and established herbaceous weeds and grasses. Water, and nutrients within the banded row that would otherwise have been utilized by the herbaceous material is made available to the pines.

1. Fumigation: Application of volatile chemicals in to confined spaces or in to the soil to produce gas that will destroy weed seeds is called fumigation. Herbicides used for fumigation are called as fumigants. These are good for killing perennial weeds and as well for eliminating weed seeds. Eg. Methyl bromide, Metham.
2. Herbigation: It is the application of herbicides with irrigation water both by surface and sprinkler systems. In India farmers apply fluchloralin for chillies and tomato, while in western countries application of EPTC with sprinkler irrigation water is very common in Lucerne.

Foliar Application

1. Blanket spray: It is the uniform application of herbicides over standing crops without considering the location of the crop. Only highly selective herbicides are used here. For example, spraying 2,4-Ethyl Ester to rice three weeks after transplanting.
2. Directed spray: It is the application of herbicides on weeds in between rows of crops by directing the spray only on weeds avoiding the crop. This could be possible by use of protective shield or hood.

• Protected spray: It is a method of applying non-selective herbicides on weeds by covering the crops which are wide spaced with polyethylene covers etc. This is expensive and laborious. However, farmers are using this technique for spraying glyphosate to control weeds in jasmine, cassava, banana.

1. Spot treatment: It is usually done on small areas having serious weed infestation to kill it and to prevent its spread. Rope wick applicator and Herbicide glove are useful here.

List of Herbicides with their Common Name & Chemical Name