Question

how is it possible that wedicides effect only thevweeds not the main crop?

Answer 1

Weedicide is a synthetic chemical to kill weeds.

Weeds are undesirable plants - grown at unwanted place in unwanted time.

Mode of action of weedicide determines what it will destroy - Entire plant population on which its sprayed or only weeds.

Contact weedicide is of 1st type. It kills whatever plants that comes in contact with weedicide. E.g. Paraquat

2nd type of weedicides has selective action. It destroys undesirable plants i.e., weeds. E.g. 2,4- D , Atrazine. These are called Systemic herbicide or translocated weedicide.