Weeds extend the harmful effects slowly, steadily and inconspicuously and the effect is almost unchangeable.

• If no restriction is imposed they compete with crop plants and the yield reduction of individual crops varies and that of cropping system 5 to 50%.
• Presence of weeds increases the cost of agriculture and hinders the progress of work.
• It increases the irrigation requirement.
• They reduce the value of produce or otherwise adds the cost of cleaning.
• Some weeds when eaten (Cleome viscosa) by milch animals will produce an undesirable odour in the milk. At times death/disorder/disformity may occur. eg: Datura stramarium
• The fruits and seeds of Xanthium strumarium and Achyranthes aspera entangle with wool which fetch lower prices.
• They harbour insect pests, pathogen and parasites
• They reduce the value of the land
• Presence of weeds will impair the purity of varieties by chance of cross pollination
• Weeds cause health hazards to man and animals
• Weeds cause allelopathic effect