Weed Threshold

1. Concept and Definition

• The weed threshold is the minimum weed population or biomass per unit area that causes significant crop yield loss and thus justifies weed control.
• Below this level, the economic damage is negligible, and weed control is not cost-effective.

🔹 In other words: It is the density of weeds at which economic returns from weed control = cost of weed management.

2. Related Terms

• Economic Injury Level (EIL): The lowest population of weeds that will cause economic damage. It is a biological limit, not influenced by management.
• Economic Threshold Level (ETL): The weed population at which control measures should be initiated to prevent weed density from reaching EIL. It is a practical limit used by farmers.
• General Threshold: The minimum weed density or biomass that starts causing measurable crop yield reduction.
• Practical Threshold: The weed level that can be managed with available tools (herbicides, manual weeding, mechanical).

3. Factors Affecting Weed Threshold; Threshold values are not universal; they vary due to multiple factors:

(a) Weed-Related Factors:

• Species: Aggressive weeds (Parthenium hysterophorus, Cyperus rotundus) have lower thresholds.
• Life cycle: Perennials cause long-term issues, reducing threshold values.
• Growth habit: Tall, fast-growing weeds (cocklebur, pigweed) compete earlier.

(b) Crop-Related Factors:

• Crop species: Tall, quick-growing crops (maize, barley) tolerate weeds better → higher threshold.
• Varieties: Spreading groundnut (TMV-3) competes better than bunch variety (TMV-2).
• Stage of crop: Critical period of crop-weed competition strongly influences threshold.

(c) Environment:

• Soil fertility: Fertile soils may support higher weed density before yield loss begins.
• Soil moisture: Under drought, even a few weeds reduce yield drastically (lower threshold).
• pH and temperature: Favorable conditions to weeds reduce crop tolerance.

(d) Management & Economics:

• Input levels: High-input crops (hybrid maize, irrigated rice) require stricter weed control.
• Market value of crop: High-value crops (vegetables, spices) tolerate very few weeds.
• Cost of control: If herbicide/labor is cheap, threshold is kept lower.

4. Examples of Weed Thresholds (Indicative)

• Rice (Echinochloa crus-galli): 10–15 plants/m².
• Wheat (Phalaris minor): 8–10 plants/m².
• Soybean (Amaranthus spp.): 5–10 plants/m².
• Maize: Weed density tolerated up to 2–3 plants/m² during early growth.
  (Thresholds vary with variety, soil, season, and management.)

5. Importance in Weed Management

• Avoids unnecessary weed control (saves labor, herbicides, cost).
• Ensures timely intervention at ETL before weeds cause economic loss.
• Basis for Integrated Weed Management (IWM).
• Helps prevent herbicide resistance by avoiding repeated blanket sprays.
• Supports precision agriculture (site-specific weed control).