Crop-weed competition - Agrobotany

Crop–Weed Competition

When plants grow close together, they interact in various ways, often leading to competition for essential resources.

Interference:

This refers to the detrimental effects one species has on another due to their interactions. When plants are spaced far apart, they do not affect each other, but closer proximity leads to competition and amensalism.

Commensalism:

This is a relationship between different species where one organism benefits from the association while the other remains unaffected.

Competition (Allelospoly):

Competition occurs when the supply of a growth factor falls below the combined demand of two plants, such as weed/crop, crop/crop, or weed/weed, for normal growth and development. The growth factors involved include water, nutrients, light, space, and air (oxygen and carbon dioxide).

Types of Competition:

• Above-Ground (Aerial) Competition: Occurs in the leaves, involving light and carbon dioxide.
• Below-Ground (Subterranean) Competition: Occurs mainly in the roots, involving water, nutrients, and oxygen.

The primary consequence of competition is a reduction in the economic yield of the affected crop plants.

Forms of Competition:

• Intraspecific Competition: Competition among individuals of the same plant species for growth factors.
• Interspecific Competition: Competition between two different plant species, such as crop/weed, weed/weed, or crop/crop.

Critical Period of Weed Competition/Interference:

This is the minimum period during which the crop must be free of weeds to prevent yield loss. It consists of two components:

• The time weeds can remain in a crop before interference begins.
• The time weed emergence must be prevented to avoid reducing crop yield.

Factors Affecting Weed-Crop Competition:

• Competitiveness of Weed Species
• Weed Density and Weight
• Onset and Duration of Weed-Crop Association
• Growth Factors Availability
• Type of Crop and Seeding Rate
• Spatial Arrangement of Crops
• Plant Architecture
• Cropping Patterns
• Crop Type (C3 or C4 Plants)
• Crop Variety (Tolerance, Resistance, Aggressiveness)

Environmental Factors:

• Climatic Factors: Rainfall, temperature, wind, light, etc.
• Tillage
• Ground Water Management
• Soil (Edaphic)

Amensalism (Allelopathy)

Allelopathy:

Allelopathy refers to the production of chemicals or exudates by living and decaying plants that interfere with the germination, growth, or development of other plant species or microorganisms sharing the same habitat.

Types of Allelopathy:

1. True Allelopathy: Involves the release of compounds that are toxic in their original form as produced by the plant.
2. Functional Allelopathy: Involves the release of substances that become toxic only after transformation by microorganisms.

Common Allelochemicals:

Some commonly encountered allelochemicals in plants include coumaric acid, terpenoids, syringic acid, butyric acid, flavonoids, and phenolic compounds.

Examples of Allelopathic Plants:

1. Black Walnut (Juglans nigra)
2. Gmelina arborea
3. Sorghum bicolor
4. Casuarina
5. Lantana camara
6. Imperata cylindrica (allelopathic on tomato, cucumber, maize, rice, groundnut, okra, cowpea, and pepper)
7. Cyperus esculentus (allelopathic on rice, maize)
8. Cyperus rotundus (allelopathic on barley)

Parasitism

Parasitism:

A relationship where one organism lives as a parasite on or inside another organism.

Parasitic Weeds:

These are plants that grow on living tissues of other plants and derive part or all of their food, water, and mineral needs from the host plant.

Types of Parasitic Plants:

1. Hemi Parasite (Semi Parasite): A plant that is only partially parasitic, possessing its own chlorophyll and photosynthetic ability. It may be facultative or obligate. Example: Striga hermonthica
2. Holo Parasite: A plant that is completely parasitic, lacking chlorophyll, and unable to synthesize organic carbon. Example: Orobanche spp.
3. Obligate Parasite: A plant that cannot establish and develop without a host.
4. Facultative Parasite: A plant that can grow independently but usually behaves as a parasite to obtain some of its nutrition.

Other Ecological Relationships

Predation:

The capture and consumption of organisms by other organisms to sustain life.

Mutualism:

An advantageous relationship between two organisms of different species that benefits both parties. This relationship is obligatory, and both partners are mutually dependent. For example, the relationship between fungus and algae, where the fungus protects the algae, and the algae provide carbohydrates for the fungus.

Neutralism:

A situation where plants exert no influence on each other.

Protocooperation:

A condition where two plants interact and positively affect each other. Both organisms are stimulated by the association but are unaffected by its absence.

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