Herbicide Classification, Formulations and Methods 0f Application

Herbicide is a chemical used to kill some targeted plants.

Principles of chemical weed control

The selectivity exhibited by certain chemicals to cultivated crops in controlling their associated weeds without affecting the crops forms basis for the chemical weed control. Such selectivity may be due to differences in the morphology, differential absorption, differential translocation, differential deactivation, etc.

Classification of herbicides

Based on the Method of application

Soil-applied herbicides:

Herbicides act through roots and other underground parts of weeds.
g. Fluchloralin

Foliage-applied herbicides:

Herbicides primarily active on the plant foliage
g. Glyphosate, Paraquat

Based on the Mode of action

Selective herbicide:

A herbicide is considered selective when in a mixed growth of plant species, it kills some species without injuring the others.
E.g. Atrazine

Non-selective herbicide:

It destroys the majority of treated vegetation
g. Paraquat

Based on mobility

Contact herbicide:

A contact herbicide kills those plant parts with which it comes in direct contact
g. Paraquat

Translocated herbicide:

The herbicide tends to move from treated part to untreated areas through the xylem/phloem depending on the nature of its molecule.
g. Glyphosate

Based on the Time of application

Pre-plant application (PPI)

Application of herbicides before the crop is planted or sown.
Soil application as well as foliar application is done here.
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 any crop.

Pre-emergence application

Application of herbicides before a crop or weed has emerged.
In the case of annual crops, the application is done after the sowing of the crop but before the emergence of weeds, and this is referred to as pre-emergence to the crop while in the case of perennial crops, it can be said as pre-emergence to weeds.
For example, soil application by spraying atrazine on 3rd DAT to sugarcane can be termed as pre-emergence to cane crop while soil application by spraying the same immediately after a rain to control a new flush of weeds in an inter-cultivated orchard can be specified as pre-emergence-to-weed.
g. Atrazine, Pendimethalin, Butachlor, Thiobencarb, Pretilachlor

Post-emergence application

Herbicide application after the emergence of a crop or weed is referred to as post-emergence application.
When the weeds grow before the crop plants have emerged through the soil and are killed with a herbicide then it is called early post-emergence.
For example, spraying 2,4-D Na salt to control parasitic weed Striga in sugarcane is called post-emergence while spraying paraquat to control emerged weeds after 10-15 days after planting potato can be called early post-emergence.
g. Glyphosate, Paraquat, 2,4-D Na Salt.

Based on molecular structure

Inorganic compounds
Organic compounds

Methods of applying herbicides

Spraying
Broadcasting

Factors influencing the methods of applying herbicides are

Weed-crop situation
Type of herbicides
Mode of action and selectivity
Environmental factors
Cost and convenience of application

Depending on the target site, the herbicides are classified into;

soil-applied herbicides
foliage applied or foliar herbicides

Soil application of herbicides

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. Fluchloralin; Left undisturbed under irrigated condition

Incorporated under rainfed condition

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

Nitralin herbicides to control Convolvulus arvensis

Band application

Application to a restricted band along the crop rows leaving an untreated band in the inter-rows. Later inter-rows are cultivated to remove the weeds. Saving in cost is possible here.

For example, when a 30 cm wide band of herbicide is applied over crop rows that were spaced 90 cm apart, then two-thirds cost is saved.

Fumigation

The application of volatile chemicals into confined spaces or into 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.

E.g. Methyl bromide, Metham

Herbigation

It is the application of herbicides with irrigation water both by surface and sprinkler systems. In most countries, farmers apply fluchloralin for chillies and tomatoes, while in Western countries the application of EPTC with sprinkler irrigation water is very common in Lucerne.

Foliar applied herbicides

Blanket spray

It is the uniform application of herbicides to standing crops without considering the location of the crop. Only highly selective herbicides are used here. E.g. Spraying 2,4-Ethyl Ester to rice three weeks after transplanting

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 the use of a protective shield or hood. For example, spraying glyphosate in between rows of tapioca using a hood to control Cyperus rotundus.

Protected spray

It is a method of applying non-selective herbicides on weeds by covering the crops that 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, and banana.

Spot treatment

It is usually done in small areas having serious weed infestation to kill it and to prevent its spread. A rope wick applicator and Herbicide glove are useful here.

Formulations of herbicides

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

Objectives in herbicide formulations are;

Ease of handling
High controlled activity on the target plants Need for preparing herbicide formulation
To have a product with physical properties suitable for use in a variety of types of application equipment and conditions.
To prepare a product that is effective and economically feasible to use
To prepare a product that is suitable for storage under local conditions?

Types of formulation

Emulsifiable concentrates (EC):

A concentrated herbicide formulation containing an organic solvent and adjuvants to facilitate emulsification with water e.g., Butachlor

Wettable powders (WP):

A herbicide is absorbed by an inert carrier together with an added surface-acting agent. The material is finely ground so that it may form a suspension when agitated with a required volume of water e.g., Atrazine

Granules (G):

The inert material (carrier) is given a granular shape and the herbicide (active ingredient) is mixed with sand, clay, vermiculite, and finely ground plant parts (ground corn cobs) as carrier material. e.g. Alachlor granules.