💧Herbicide Formulations and Application Methods

From Laboratory to Field

The previous lesson covered herbicide types, trade names, and the 10 classification systems. Now we move from what herbicides are to how they are prepared and applied in the field. A herbicide’s formulation determines how it mixes with water, how it reaches the target weed, and how effective it will be.

Pure (technical grade) herbicides may be solid crystals, oily liquids, or volatile compounds. Imagine trying to spray pure pendimethalin — a thick, sticky paste — uniformly across a 5-acre wheat field. It is practically impossible. This is why herbicides must be processed into formulations: carefully engineered products that are safe to handle, easy to mix with water, and effective when applied in the field.

This lesson covers:

1. 9 formulation types — EC, WP, G, WSC, SP, D, WDG, F, and pellets
2. Active Ingredient and Acid Equivalent — calculation methods
3. Absorption pathways — apoplast (root) vs symplast (shoot)
4. Selectivity mechanisms — differential absorption, translocation, deactivation
5. Application methods — 6 soil methods and 5 foliar methods
6. Adjuvants — surfactants, stickers, activators, and more

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Types of Herbicide Formulations

Each formulation type is designed for specific application conditions. The abbreviation on the product label tells you the formulation type.

Key Comparison: EC vs WP

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Active Ingredient (AI) and Acid Equivalent (AE)

The Active Ingredient (AI) is the chemical responsible for herbicidal activity, expressed as % by weight or volume.

• Example: Atrazine 80 WP = 80% active ingredient + 20% inert carriers/additives.

The Acid Equivalent (AE) is the theoretical yield of parent acid from a salt or ester formulation. AE is always less than or equal to AI.

• 2,4-D sodium salt AE = 92.5%

Calculation Examples

Q: Product has 80% AE. How much formulation for 2 kg AE/ha? A: 100/80 x 2 = 2.5 kg/ha

Q: Apply 0.5 kg AI/ha of Atrazine 80%. Quantity needed? (a) 0.40 (b) 0.48 (c) 0.52 (d) 0.62 kg/ha A: 100/80 x 0.5 = 0.625 = 0.62 kg/ha

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Mode of Action: Absorption Pathways

How herbicides enter and move through plants determines their effectiveness.

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Herbicide Selectivity Mechanisms

Selectivity is the differential response of plants to a herbicide. More toxicant must reach the site of action in active form inside target plants than in non-target plants.

Three Mechanisms of Selectivity

Metabolism

Change in molecular structure inside the plant yields non-toxic compounds.

Conjugation

Coupling of intact herbicide with plant cell constituents, locking it in a non-toxic, immobile form.

• Maize and millets deactivate Atrazine/Simazine by conjugation via enzyme Glutathione-S-Transferase
• Grasses and Convolvulus conjugate 2,4-D with glucose (glucoside)

Protoplasmic Resistance

Intrinsic resistance of cell protoplasm to a specific herbicide — the plant’s cells are inherently tolerant.

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Methods of Herbicide Application

Knowing the formulation and selectivity of a herbicide is only half the picture. The method of application determines whether the herbicide reaches the target weed effectively while sparing the crop. Herbicides are applied in two fundamentally different ways depending on the target:

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Soil Application Methods

A. Surface Application

Herbicide is sprayed or broadcast uniformly on the soil surface. It may be left undisturbed or incorporated by tillage to prevent volatilisation and photo-decomposition.

Example: Fluchloralin — left undisturbed under irrigated conditions (moisture keeps it stable) / incorporated under rainfed conditions (to prevent gaseous loss).

B. Subsurface Application

Herbicide applied in a concentrated band at 7-10 cm below the soil surface using special nozzles under a sweep hood. Targets perennial weeds with deep root systems.

Examples:

• Carbamate herbicides to control Cyperus rotundus
• Nitrate herbicides to control Convolvulus arvensis

C. Band Application

Herbicide applied in a restricted band along crop rows, leaving inter-row space untreated. Inter-rows are then cultivated mechanically.

Cost saving calculation: When a 30 cm wide band is applied over rows spaced 90 cm apart, only one-third of the field receives herbicide — saving two-thirds of herbicide cost.

D. Fumigation

Volatile chemicals applied into soil produce toxic gas that destroys nearly all organisms (weeds, seeds, nematodes, pathogens). A drastic treatment for high-value crops.

Examples: Methyl bromide, Metham — used in nursery beds and greenhouse soils.

E. Lay-by Application

Herbicide applied after last cultivation (e.g., after earthing up). Used in crops like sugarcane where no further mechanical weeding is possible.

F. Herbigation

Herbicides applied along with irrigation water through surface or sprinkler irrigation systems. Saves labour and ensures uniform distribution.

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Foliar Application Methods

A. Blanket Spray

Uniform application over the entire standing crop. Since both crop and weeds are contacted, only highly selective herbicides can be used.

Example: Spraying 2,4-D Ethyl Ester in rice three weeks after transplanting — kills broadleaf weeds but spares rice (a grass).

B. Directed Spray

Spray directed onto weeds between crop rows while carefully avoiding the crop canopy using protective hoods or band devices.

Example: Spraying glyphosate (non-selective) between sugarcane rows using a hood to control Cyperus rotundus without damaging the cane.

C. Protected Spray

Non-selective herbicides applied after physically covering crop plants with polyethylene covers. Effective but expensive and laborious — used only for high-value crops.

Example: Spraying glyphosate in jasmine, cassava and banana plantations.

D. Spot Treatment

Targeted application on specific spots or patches with serious weed infestation. Uses specialised equipment like the rope wick applicator and herbicide glove (wipe-on technique).

E. Basal Application

Bark is removed at the stem base (30 cm) and a drenching herbicide spray is applied to kill trees. Used for woody weed and tree removal in plantation management.

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Comparison: Foliar Application Methods

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Adjuvants

No herbicide formulation is complete without adjuvants — the support chemicals that make the active ingredient work better in field conditions. Adjuvants are substances added to herbicides to improve performance without increasing innate herbicidal activity.

Uses of Adjuvants

• Improve selectivity to non-target plants
• Make herbicide safer for users
• Prolong shelf life
• Reduce drift hazards

Types of Adjuvants

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Summary Cheat Sheet