Weed Science

Weeds are classified into three morphological groups: (1) Grassy weeds (monocots, narrow leaves, parallel venation) — e.g., Echinochloa colonum, Phalaris minor, Cynodon dactylon; (2) Broadleaf weeds (dicots, broad leaves, reticulate venation) — e.g., Parthenium, Amaranthus, Chenopodium; (3) Sedges (monocots, triangular stem, solid) — e.g., Cyperus rotundus (purple nutsedge), Cyperus esculentus. This classification determines herbicide selection.

Pre-emergence herbicides are applied after sowing but before crop and weed emergence — they form a chemical film in the soil that kills germinating weeds (e.g., pendimethalin, atrazine, butachlor). Post-emergence herbicides are applied after weeds have emerged and are growing actively — they may be contact (burn tissue on contact: paraquat) or systemic (translocated throughout the plant: glyphosate, 2,4-D). Timing is critical for efficacy.

Weed Index (WI) = [(Weed-free yield − Weedy yield) / Weed-free yield] × 100. A WI of 30 means that weed competition reduced crop yield by 30% compared to weed-free conditions. Related indices: Weed Control Efficiency (WCE) = [(weedy check weed biomass − treatment weed biomass) / weedy check weed biomass] × 100.

In wheat: 2,4-D (broadleaf control), isoproturon (against Phalaris minor), clodinafop-propargyl (against Phalaris minor and other grasses), metsulfuron methyl (broadleaf). In paddy: butachlor or pendimethalin (pre-emergence grassy weeds), bispyribac sodium (post-emergence grassy and sedge weeds), 2,4-D sodium salt (broadleaf and sedge control in transplanted paddy).

Allelopathy is the chemical inhibition of one plant by another through the release of allelochemicals (phenolic acids, terpenes, alkaloids) into the soil or atmosphere. Examples: (1) Sorghum (jowar) releases sorgoleone — a potent allelopathic compound used in natural weed management. (2) Parthenium hysterophorus releases parthenin and caffeic acid, inhibiting germination of crops planted in its vicinity. Positive allelopathy (where one crop suppresses weeds) is used in cover crops.

Weed Science is directly tested in IBPS AFO (Professional Knowledge), NABARD Grade A, ICAR JRF (Agronomy and Plant Protection), Pre-PG entrance exams (IARI, BHU, ANGRAU), and state PSC Agriculture Officer exams. Herbicide names, classification, weed index formula, and allelopathy examples are among the most repeated topics.