Factors Affecting the Efficacy of Herbicides

The success of herbicides in controlling weeds depends on several environmental, biological, and operational factors. Understanding these ensures better weed management and crop safety.

a. Weed Factors

• Species and growth stage: Young, actively growing weeds are more susceptible; mature or dormant weeds are harder to control.

• Weed morphology: Leaf thickness, waxy cuticle, hairiness, and size affect herbicide absorption.
• Weed density: High weed density may require higher doses or repeated application.

b. Herbicide Factors

• Selectivity: Whether the herbicide targets specific weeds or is non-selective.
• Formulation: Liquid, granular, or wettable powder affects uptake and spread.
• Mode of action: Contact vs systemic herbicides differ in movement within the plant.
• Stability: Herbicides sensitive to light, heat, or moisture may degrade before action.

c. Environmental Factors

• Soil properties: Organic matter, texture, pH, and moisture affect absorption and persistence.
• Temperature and humidity: High temperatures and moderate humidity often enhance foliar absorption; very dry or hot conditions may reduce efficacy.
• Rainfall and irrigation: Rain immediately after spraying can wash off herbicides; adequate moisture is required for some soil-applied herbicides.
• Sunlight: Certain herbicides (e.g., contact herbicides) may degrade under strong sunlight; light can also trigger germination for pre-emergence herbicides.

d. Crop Factors

• Crop stage and variety: Rapidly growing or dense crops may shade weeds and reduce herbicide contact.
• Crop tolerance: Herbicide selectivity depends on crop susceptibility.

e. Application Factors

• Method of application: Foliar, soil, granular, or aerial applications affect uniformity.
• Equipment calibration: Improper calibration leads to under- or over-dosing.
• Spray timing: Correct timing relative to weed emergence is critical.
• Spray coverage: Poor coverage reduces effectiveness; overlapping or missed areas lead to survival of weeds.

f. Resistance Development

• Repeated use of the same herbicide or mode of action may lead to herbicide-resistant weed populations, reducing long-term efficacy.