Weed & Its control:

      There are approximately 250,000 species of plants worldwide; of those about 3% or 8000 species behave as weeds. Weeds are troublesome in many ways. Primarily, they reduce crop yield by competing for water, light, soil nutrients, and space. Other problems associated with weeds in agriculture include:

                                                                                   

• Reduced crop quality by contaminating the commodity.
• Interference with harvest.
• Serve as hosts for crop diseases.
• provide shelter for insects to overwinter.
• Limit the choice of crop rotation sequences and cultural practices.
• Allelopathy :Production of chemical substances which are toxic to crop plants, animals, or humans.

      No matter what definition is used, weeds are plants whose undesirable qualities outweigh their good points, according to man. Our human activities create weed problems since no plant is a "weed" in nature. Though we may try to manipulate nature for our own good, nature is persistent. Through the manipulation process, certain weeds are controlled, while other more serious weeds may thrive because favorable growing conditions for them also have been met. Weeds are naturally strong competitors and those weeds that can best compete always tend to dominate. Both humans and nature are involved in plant breeding programs. The main difference between the two programs is that man breeds plants for yield, while nature breeds plants for survival.

For more information: http://www.seedbuzz.com/knowledge-center/article/weed-its-control