Weed Management
What the exam asks
Weed Management tests weed biology, crop-weed competition, critical period of competition, cultural, mechanical, biological and chemical control, herbicide selectivity and dose calculations. UP examples include Phalaris minor in wheat, Echinochloa in rice and Cyperus rotundus in many crops.
Core rules
- Weeds compete for light, water, nutrients and space; they also harbour pests and increase harvesting costs.
- The critical period of crop-weed competition is the interval when weeds most reduce yield; it differs by crop and sowing method.
- Cultural control includes timely sowing, competitive varieties, crop rotation and mulching; mechanical control includes hand weeding and intercultivation; biological control uses host-specific agents.
- Herbicides are selective or non-selective, and pre-emergence or post-emergence. Dose is expressed as active ingredient (a.i.) per hectare; product dose = a.i. dose ÷ formulation concentration.
- Integrated weed management combines methods and rotates herbicide modes of action to slow resistance.
Worked example
A recommendation is 0.5 kg a.i. ha−1 of a herbicide supplied as 80% wettable powder. Product needed = 0.5 ÷ 0.80 = 0.625 kg ha−1. For 0.4 ha, product = 0.625 × 0.4 = 0.25 kg. Reasoning: the product is not pure active ingredient, so divide by the fraction. Apply in the correct growth stage and water volume; a wrong stage can injure the crop.
Traps
Do not confuse a.i. with commercial product; do not use the same herbicide every year; pre-emergence does not mean before crop sowing in every label. Hand weeding late may remove weeds but cannot recover yield already lost. Cyperus rotundus is a sedge, not a grass, so grass herbicides may fail.