Chemicals Used to Control Weeds and Methods of Application.
Chemicals Used to Control Weeds and Methods of Application.
Herbicide is a chemical used to kill targeted plants or weeds.
Principles of chemical weed control
They exhibit selectivity in controlling weeds associated with cultivated crops, without affecting the crops themselves, forms the basis for chemical weed control.
Such selectivity may be due to differences in morphology, differential absorption, differential translocation, differential deactivation, and other factors.
CLASSIFICATION OF HERBICIDES BASED ON;
-
the Method of Application
1. Soil-applied herbicides:
- Herbicides act through the roots and other underground parts of weeds.
- g. Fluchloralin
2. Foliage-applied herbicides:
- Herbicides are primarily active on the plant foliage
- g., Glyphosate, Paraquat
the Mode of Action
1. Selective herbicide:
- A herbicide is considered selective when, in a mixed growth of plant species, it kills some species without injuring the others.
- g. Atrazine
2. Non-selective herbicide:
- It destroys the majority of treated vegetation
- g. Paraquat
Mobility
1. Contact herbicide:
- A contact herbicide kills those plant parts with which it comes in direct contact
- g. Paraquat
2. Translocated herbicide:
- The herbicide tends to move from treated parts to untreated areas through the xylem/phloem, depending on the nature of its molecule.
- g. Glyphosate
the Time of Application
1. 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.
2. 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 the 3rd DAT to sugarcane can be termed as pre-emergence to the 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
3. 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.
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 into the soil.
- Incorporation is done to prevent the volatilization and photo-decomposition of the herbicides.
- E.g., Fluchloralin;
- Left undisturbed under irrigated conditions
- Incorporated under rainfed conditions
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 on 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 fumigants.
- These are good for killing perennial weeds and 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 widely 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
- Highly 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.
