Question

what harmful effect does a crop bare due to weeds

Answer 1

Harmful Effects of Weed
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

Answer 2

weeds will take up the nutrients give to the crops to grow .
weeds can be posinous and if it go with harvested crops it make other crop spoil