This Link does not exist in your language, View in: English (en),
Or use Google Translate:  
বাংলা (bn) | Change Language (Change Language)

Weeds are important biotic constraint to food production. Weeds compete with crops for the same resources, basically water, nutrients, light and carbon dioxide. Furthermore, they are alternate hosts for crop pests and pathogens. Moreover, some of them lack autotrophy and fully develop only by parasitizing crops or wild hosts. Globalization of trade and weak legislations on weed control in most of the developing countries has favored spread of Invasive Alien Plants (IAP).

Weeds, in general, precede crops on farming lands and are major yield reducing factors. In ecological terms, most annual weeds are r-strategists, establishing populations with high relative growth rate (r). Thus, they produce numerous viable seeds and their populations quickly build up, in an exponential pattern. Weed competition with crops reduces agricultural output (quantity and quality), and increases external costs by spreading them across farm boundaries. It is also a major constraint to increased farmers’ productivity, particularly in developing countries where weed control claims 20 to 50 % of farmers’ time and keep smallholders in a vicious circle of poverty.

From FAO Integrated Weed Management