Weed control

What weeds are

A weed is a plant growing where it is not wanted. Weeds compete with crops for water, nutrients, light and space, reduce yield and quality, harbour pests and diseases, and raise production cost. Some are poisonous to animals.

Classification

• By life span: annual (e.g. Phalaris minor in wheat, Echinochloa in rice), biennial, perennial (e.g. Cyperus rotundus or nut sedge, Cynodon dactylon or Bermuda grass).
• By leaf type: grasses (narrow leaves), sedges (triangular stems), broadleaf weeds (e.g. Chenopodium album).
• By habitat: cropland, aquatic (water hyacinth, Eichhornia), parasitic (Striga on sorghum, Orobanche on mustard, Cuscuta or dodder).

Parthenium (congress grass) is a serious weed that also causes allergy in humans.

Methods of control

• Preventive: clean seed, well-decomposed manure, clean tools and irrigation channels.
• Cultural: proper crop rotation, optimum seed rate and spacing, timely sowing, competitive varieties, mulching, stale seedbed.
• Mechanical: hand weeding, hoeing, interculture, tillage, mowing, burning.
• Chemical: herbicides. Selective herbicides kill weeds without harming the crop (e.g. 2,4-D kills broadleaf weeds in cereals). Non-selective herbicides kill all green plants (e.g. glyphosate, paraquat). By timing they may be pre-plant, pre-emergence (before weeds emerge) or post-emergence.
• Biological: using living organisms, e.g. the Mexican beetle *Zygogramma bicolorata* against Parthenium and *Neochetina* weevils against water hyacinth.
• Integrated weed management (IWM): combining two or more methods for economical, eco-friendly control.

The critical period of crop-weed competition is the time during which weeds must be controlled to prevent yield loss; it is often the first one-third to one-half of the crop's life.

Key points

• Phalaris minor is a major weed of wheat.
• 2,4-D is selective against broadleaf weeds.
• Glyphosate is non-selective.
• Zygogramma controls Parthenium biologically.
• Orobanche and Striga are parasitic weeds.

Exam tips

• Pair weeds with crops they infest; such matching questions are common.
• Distinguish selective vs non-selective and pre- vs post-emergence carefully.