Types of herbicides
There are two main types of herbicides: hormone herbicides and contact herbicides.
Hormone herbicides are systematic chemicals that kill weeds by modifying some aspect of growth or development in a way that is harmful to the plant, e.g. causing unrestricted growth until the plant runs out of food reserves and dies.
Particular hormones only affect certain plant types, so they can be used selectively, e.g. to kill broad leaf weeds in cereal crops.