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Weeds are wild, unwanted plants that compete with normal healthy plants for water, light, soil nutrients and space. Weeds are competitive, persistent, pernicious, and interfere negatively with human activity. There are approximately 250,000 species of plants worldwide; of those, about 3% or 8000.species behaves as weeds.
Negative impacts of weeds are:
• They reduce crop quality by contaminating the commodity.
• They interfere with harvest and reduce the efficiency of farm equipment.
• Weeds reduce the carrying capacity of grazing lands and pastures.
• They serve as hosts for crop diseases or provide shelter for insects during winter. They limit the choice of crop rotation sequences and, cultural practices.
• Weeds produce chemical substances which are toxic to crop plants (allelopathy), animals or humans.
• Weedicides prove to be costly and harmful to the environment. Some weeds are parasites, either partially or totally, on crop plants.
• Weeds impair quality and quantity of plant and animal products. Example: thorny weeds.
• Seeds of some weeds are similar to crop seeds and it is very difficult to separate them, once mixed. Example: Argemone mexicana (Prickly poppy) has seeds resembling to mustard seeds. This adulteration of mustard by poppy has lead to a disease called Dropsy.
Methods of Weed Control
• Chemical control involves the use of herbicides. Herbicides control weed plants either by speeding up, stopping or changing the plant’s normal growth patterns; by desiccating (drying out) the leaves or stems; or by defoliating the plant (making it drop its leaves).
• Mechanical control is the use of powered tools and machinery to manage weeds. It is suitable for larger infestations because it reduces the weed bulk with less manual effort. Care should be taken to minimize soil disturbance.
• Biological control involves the use of insects or pathogens (diseases) that affect the health of the weed. Usually, these bio-control agents are from the same country of origin as the weed species.
• Manual control is the use of the hands or handheld tools to deal with weeds. An advantage of manual control is that it minimizes soil disturbance, and decreases the likelihood of erosion and weed seed germination.