What is a chemical that is used to destroy weeds called? 

The undesirable plants that grow along with the main crops are known as weeds.

The process of removing unwanted plants from a garden, lawn, or farm is referred to as weeding. 

Weeding is usually done to keep unwanted plants from competing for resources like sunlight, water, and nutrients with desirable plants. It can be done manually with hoes and cultivators or chemically with weedicides (chemicals that help in destroying the weeds). 

Weedicides only destroy weeds and do not affect the main crop. Some examples of weedicides are dalapon, metachlor and siziazine.