Crop–Weed Competition

Concept

• Competition = the struggle between crop and weed plants for limited growth factors (nutrients, moisture, light, space, CO₂).
• Weeds are often faster growing, highly adaptive, and more efficient at resource utilization than crops.
• This results in yield reduction, poor crop quality, and higher production costs.

Factors of Crop–Weed Competition

i) Competition for Nutrients

• Weeds have vigorous root systems and faster nutrient uptake capacity.
• Many weeds act as luxury consumers (absorb more nutrients than needed).
• Examples:
  • Amaranthus → accumulates >3% nitrogen (called nitrophyll).
  • Chenopodium and Portulaca → accumulate >4% potassium.
  • Setaria lutescens → accumulates zinc (585 ppm), 3× more than cereals.
• Effect on crops: Nutrient deficiency, stunted growth, reduced yield.

ii) Competition for Moisture

• Weeds use more water due to higher transpiration rates and larger leaf area.
• In dryland farming, weeds are major competitors for scarce soil moisture.
• Examples:
  • Chenopodium album uses ~550 mm of water, while wheat uses ~479 mm.
  • Weeds deplete soil moisture up to 90 cm depth, while wheat roots exploit mainly top 15 cm.
• Effect: Crops suffer water stress, reduced biomass, poor grain filling.

iii) Competition for Light (Solar Radiation)

• Tall, fast-growing weeds overshadow short crops → cause shading.
• Leads to reduced photosynthesis, weak stems, delayed maturity.
• Early season shading is critical; crop growth loss cannot be recovered even if weeds are removed later.
• Examples:
  • Weeds smother crops like cotton, potato, sugarcane in early growth.
  • In rice, Echinochloa crus-galli often overtops seedlings.

iv) Competition for CO₂ (Space)

• Weeds capture space, preventing proper crop stand establishment.
• C₄ weeds (e.g., Amaranthus, Echinochloa, Setaria) are more efficient in CO₂ fixation than C₃ crops (wheat, rice, pulses).
• In dense stands, CO₂ availability becomes limiting.
• Effect: Faster weed growth → shading + nutrient/moisture drain → crop suppression.

3. Critical Period of Crop–Weed Competition (CPWC)

• Definition: The specific time during crop growth when weed presence causes maximum yield loss; weed control must be done within this window to prevent irreversible damage.
• If weeds are controlled before or after this period, yield loss is minimal.

Examples of CPWC (Weed-Free Period Required):

• Rice (direct-seeded / transplanted): 0–6 Weeks After Sowing (WAS) / Transplanting (WAT).
• Maize: 2–6 WAS.
• Wheat: 3–6 WAS.
• Soybean: 3–6 WAS.
• Groundnut: 3–6 WAS.
• Sugarcane: 30–120 Days After Planting (DAP).
• Cotton: 2–8 WAS.
• Sorghum: 2–6 WAS.
• Pulses (e.g., chickpea, pigeon pea): 3–5 WAS.

👉 Beyond CPWC, crop canopy closes, and weeds have less effect.

4. Outcomes of Crop–Weed Competition

• Yield Reduction: 30–50% in most crops; up to 80% in direct-seeded rice & groundnut if weeds uncontrolled.
• Nutrient Mining: Weeds act as nutrient sinks, reducing soil fertility.
• Increased Cost of Cultivation: More tillage, herbicide use.
• Poor Quality Produce: Contamination with weed seeds (e.g., Argemone in mustard, Cuscuta in lucerne).

5. Weed–Crop Competition Strategies (Weed Superiority)

Weeds generally win due to:

• Faster germination (emerge before crops).
• Rapid early growth rate.
• Prolific seed production.
• Diverse rooting patterns (shallow + deep roots).
• High allelopathic potential (e.g., parthenin from Parthenium).
• Efficient C₄ photosynthesis in many weeds.

6. Management Implications

• Weed control should be timely, especially during CPWC.
• Methods:
  • Pre-emergence & post-emergence herbicides timed to CPWC.
  • Cultural practices (high seed rate, narrow spacing, competitive varieties).
  • Mechanical/hand weeding during critical stages.
  • Cover crops & mulching to reduce early weed emergence.

Factors Affecting Weed–Crop Interference

1. Period of Weed Growth

• Concept: The time of weed emergence relative to the crop determines its competitive impact.
• Weeds that germinate simultaneously with or slightly earlier than crops are the most damaging.
• Early-emerging weeds establish better root systems and canopy, reducing crop vigor.
• Example:
  • In sugarcane, early flushes of weeds like Cyperus rotundus cause severe yield loss, while late-emerging weeds are less harmful.

2. Weed/Crop Density

• Concept: The intensity of interference increases with weed density. The relationship between crop yield and weed density is usually sigmoidal (S-shaped):
  • Low weed density → negligible yield loss.
  • Moderate density → rapid decline in yield.
  • Very high density → yield loss plateaus.
• Higher crop density can suppress weed growth by limiting light, space, and nutrients.
• Examples:
  • In rice, Joint vetch at >10 plants/m² reduces yield by ~20 q/ha.
  • Barnyard grass at the same density reduces yield by ~11 q/ha.

3. Plant Species Effects (Weed Species)

• Concept: Competitive ability varies with weed type, growth habit, and life cycle.
• Perennials with deep roots and stored reserves are more competitive than annuals.
• Examples:
  • Canada thistle (deep-rooted perennial) competes strongly with wheat and oats.
  • Broad-leaved weeds like cocklebur and wild mustard compete more aggressively than grasses due to rapid canopy development and shading.

4. Plant Species Effects (Crop Species and Varieties)

• Concept: Crops differ in their competitive ability depending on height, growth rate, canopy spread, and rooting pattern.
• Tall, fast-growing, spreading crops suppress weeds better than short, slow-growing crops.
• Examples:
  • Barley and maize (tall crops) are more competitive than wheat and pulses.
  • Groundnut varieties: Spreading type (TMV-3) suppresses weeds better and yields higher than bunch type (TMV-2).
  • Sorghum and pearl millet are more competitive against weeds than rice or wheat.

5. Soil and Climatic Influences

• Soil Fertility: High fertility may favor weeds (luxury consumption). Example: Amaranthus proliferates in nitrogen-rich soils.
• Soil Moisture:
  • Xerophytic weeds (Chenopodium album) thrive under dryland conditions.
  • Hydrophytic weeds (Eichhornia, Scirpus) dominate in waterlogged soils.
• Soil pH:
  • Acidic soils → Cyperus iria, Fimbristylis miliacea.
  • Alkaline soils → Chenopodium album, Convolvulus arvensis.
• Climatic Extremes:
  • Drought → favors hardy weeds (e.g., Tribulus terrestris).
  • Floods → aquatic weeds (Eichhornia, Hydrilla) dominate.
  • Temperature extremes → shift in weed flora (cool-season vs warm-season weeds).

6. Cropping Practices

• Time of Planting:
  • If crops are sown with the first flush of weeds, interference is maximum.
  • Stale seedbed technique → allowing weeds to germinate and then destroying them before sowing crop → reduces interference.
• Method of Planting:
  • Line sowing → facilitates weeding and better crop stand.
  • Broadcasting → favors weeds by reducing crop competitiveness.
  • Zero-tillage → alters weed flora (favors Phalaris minor in wheat).
• Crop Rotation: Rotations break weed cycles; monocropping encourages dominance of specific weeds (e.g., Phalaris minor in rice–wheat).
• Irrigation Practices: Over-irrigation favors aquatic weeds; limited irrigation favors drought-tolerant weeds.