Factors Affecting Competitive Ability of Crops against Weeds

a. Density of weeds:

• Higher weed density reduces crop yield, but the effect is sigmoidal, not linear.
• Some weeds have stronger competitive impact than others.

b. Crop density:

• Increasing crop population suppresses weed growth until self-competition occurs.
• Row spacing and plant arrangement affect weed competition; square planting reduces intra-crop competition.
• Wide rows with dense intra-row plants can encourage dense weed growth.

c. Type of weed species:

• Weed species differ in competitiveness; some affect crops more than others.
• Examples: Echinochloa crusgalli in rice, Setaria viridis in corn, Xanthium in soybean.
• Some weeds, like Flavaria australasica, are more competitive than grasses.

d. Crop species and variety:

• Different crops and varieties vary in weed competitiveness.
• Example order: barley > rye > wheat > oat.
• Taller and spreading varieties suppress weeds better; dwarf/semi-dwarf varieties are more vulnerable.
• Longer-duration cultivars (e.g., rice) are more competitive than short-duration ones.

e. Soil factors:

• Soil type, fertility, moisture, and pH influence crop-weed competition.
• High soil fertility may favor weeds more than crops, reducing crop yield.
• Proper irrigation can help crops suppress weeds if applied after crop establishment.
• Soil reactions may favor specific weed species, affecting competitive dynamics.

f. Climate:

• Adverse weather (drought, excessive rain, extreme temperatures) favors weeds.
• Crop susceptibility to stress increases competition from weeds, especially in marginal lands.

g. Time of germination:

• Crop-weed competition is strongest when crop germination coincides with first flush of weeds.
• Planting methods that delay weed emergence (e.g., drying top soil) help crops establish first.

h. Cropping practices:

• Planting methods, crop geometry, density, and choice of species/varieties strongly affect competition.

i. Crop maturity:

• Older, well-established crops compete better against weeds.
• Early-stage weeding is critical for enhancing crop yield.