A chemical used to kill or control the growth of plants. Some herbicides (such as synthetic auxins and triazine) selectively kill broad-leaved plants while leaving grass-leaved plants (i.e., cereal crops) unharmed. Other herbicides, such as paraquat, kill all plants. Herbicide use has dramatically increased crop yield worldwide but has caused serious environmental problems, polluting soil and water and causing health hazards for humans and animals. See also pesticide.