Theory:

Weeds are unwanted plants.
Weeds grow  along with crops and compete with them for nutrients, water, space as well as for light.
Thus, weeds affect the growth and development of the crop.
The removal of weeds is called weeding.
Weeding is an absolute necessary to increase the productivity of the crops. The best time for the removal of weeds is before they produce flowers and seeds.
 
Types:
There are different types of weeds, some affects the growth of the plant, some weeds interfere during harvesting and some weeds are poisonous.
 
Controlling mechanism:
Farmers use many controlling methods to remove weeds.
There are two major ways to control weeds, such as
  • physical methods and 
  • use of chemicals.
Physical methods :
Tilling / ploughing before sowing of crops can help in uprooting and killing of weeds, which may then dry up and get mixed with the soil. 
Removal of weeds by uprooting or cutting them close to the ground, from time to time. This is done with the help of a khurpi - a weed remover. Seed drills are also used to uproot weeds.
 
weeding needpix.jpg
   
Chemical methods:
Weeds are also controlled by using certain chemicals, called weedicides. These are diluted and sprayed in the fields to kill the weeds using a sprayer. They kill only the weeds and leave the main crop.
 
sprayer from pikrepo.jpg
 
Ill effects of weedicides:
 
The weedicides applied in the crop land stays as a residue in the plant's leaves, fruits and seeds and cause many unfavourable health hazards if consumed. It also affects the farmers while spraying. So farmers need to take extreme care while spraying the weedicides. Many weedicides are banned due to their ill effects on human health.
 
Reference:
Image credit :
 
Physical weeding : https://www.needpix.com/photo/992293/work-in-the-garden-garden-digging-plucking-weeds-weeding-planting-tools-earth-plants (free image)
Chemical weeding :  https://www.pikrepo.com/ftblu/man-wearing-white-dress-shirt-holding-yellow-sprayer  (free image)