Agriculture I Weeds

Definition of Weeds

Weeds are unwanted and undesirable plants that grow along with main crops and compete for water, nutrients, sunlight, space, and carbon dioxide, thereby reducing crop yields.
They are usually found in areas where they are not intentionally sown.

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Classification of Weeds

Weeds can be classified based on the following parameters:

1. Based on Morphology (External Appearance)

   • Grasses: Narrow-leaved, parallel-veined leaves.
     Example: Cynodon dactylon (Doob grass)

   • Sedges: Triangular stem, grass-like appearance.
     Example: Cyperus rotundus

   • Broad-leaved weeds: Net-veined leaves.
     Example: Chenopodium album

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2. Based on Life Cycle

   • Annual Weeds: Complete life cycle in one season/year.
     Example: Echinochloa crusgalli (Barnyard grass)

   • Biennial Weeds: Complete life cycle in two years.

   • Perennial Weeds: Live more than two years.
     Example: Cynodon dactylon

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3. Based on Origin

   • Indigenous Weeds: Native to the region.
     Example: Eclipta alba

   • Introduced/Exotic Weeds: Introduced from other regions or countries.
     Example: Parthenium hysterophorus, Eichhornia crassipes

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4. Based on Association with Crops

   • Crop-bound Weeds: Grow specifically with particular crops.

   • Crop-associated Weeds: Found across multiple crops.

   • Aquatic Weeds: Found in water bodies.
     Example: Hydrilla, Eichhornia crassipes

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5. Based on Nature of Stem

   • Herbaceous Weeds: Soft, green stems.

   • Woody Weeds: Hard, thick stems.

   • Semi-woody Weeds: Intermediate characteristics.
     Example: Corchorus trilocularis

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6. Based on Soil Type

   • Acidophiles: Thrive in acidic soils.
     Example: Rumex acetosella

   • Basophiles: Prefer alkaline soils.
     Example: Trianthema portulacastrum

   • Neutrophiles: Grow in neutral soils.
     Example: Amaranthus viridis

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7. Special Classifications

   • Problematic Weeds: Highly competitive and toxic.
     Example: Parthenium hysterophorus

   • Obligate Weeds: Only found in cultivated fields.

   • Facultative Weeds: May grow in both cultivated and wild areas.

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Expected UPSC Questions and Model Answers

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Q1. What are weeds and why is their classification important in agriculture? (150 words)

Answer:
Weeds are unwanted plants that grow in association with crops and compete for essential resources such as water, nutrients, light, and space, thereby significantly reducing crop productivity. Their presence not only affects the yield but also increases the cost of cultivation due to additional weed management efforts.

Classification of weeds is essential because it helps in the adoption of targeted weed control measures. Different weeds vary in morphology, life cycle, ecological requirements, and impact on crops. For instance, knowing whether a weed is annual or perennial can guide the timing and method of control. Similarly, understanding whether a weed is native or exotic helps in formulating biosecurity and quarantine regulations. Hence, classification aids in better understanding of weed behavior and management strategies, ultimately leading to sustainable agricultural practices.

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Q2. Write a short note on aquatic weeds. Mention two examples. (100 words)

Answer:
Aquatic weeds are those plants that grow in water bodies such as ponds, lakes, canals, and paddy fields. These weeds interfere with water flow, reduce oxygen levels, hinder aquatic life, and obstruct navigation and irrigation systems. They may be floating, submerged, or emergent based on their growth pattern.

Examples:

1. Eichhornia crassipes (Water hyacinth) – floating aquatic weed.

2. Hydrilla verticillata – submerged aquatic weed.

Control methods include mechanical removal, biological control using herbivorous fishes, and chemical herbicides.

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Q3. Explain how weeds are classified based on morphology with suitable examples. (150 words)

Answer:
Morphological classification of weeds is based on their external appearance, primarily leaf shape and stem structure. They are broadly categorized into three types:

1. Grasses: These have narrow, parallel-veined leaves with hollow or solid cylindrical stems.
   Example: Cynodon dactylon (Doob grass).

2. Sedges: Characterized by triangular stems and grass-like leaves, but they differ in having solid stems and no nodes.
   Example: Cyperus rotundus (Nut grass).

3. Broad-leaved weeds: These possess broad leaves with reticulate venation and may be herbaceous or woody.
   Example: Chenopodium album (Bathua).

This classification is significant in selecting appropriate herbicides since some herbicides are selective and target specific groups of weeds.

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Q4. What are exotic weeds? Discuss their impact on the Indian ecosystem. (150 words)

Answer:
Exotic or alien weeds are those species that are introduced from outside their native geographical region. These may be introduced intentionally or accidentally through trade, transportation, or agricultural activities.

In India, some exotic weeds like Parthenium hysterophorus and Eichhornia crassipes have become invasive and pose a major threat to biodiversity, agriculture, and human health. Parthenium causes dermatitis and respiratory problems in humans and reduces crop productivity by releasing allelopathic substances. Eichhornia chokes water bodies, affecting aquatic life and water transport.

These weeds often have no natural predators in the new environment, allowing them to spread aggressively and outcompete native species. Thus, they disrupt the ecological balance and require dedicated control measures including mechanical removal, biological agents, and public awareness.