Herbicides are chemicals that kill plants. Herbicides are widely used in modern agriculture to control weeds, reduce competition, and increase productivity of crop plants. They are also used by homeowners to control lawn weeds and by turf grass managers, foresters, and other professionals. Herbicides are used not only on land, but also in lakes, rivers, and other aquatic environments to control aquatic weeds.

The modern use of herbicides began in the 1940s, with the development of 2,4-D (2, 4-dichlorophenelyacetic acid). By the end of that decade, herbicide use had grown from a few thousand acres to several million. There are now approximately four hundred different herbicides registered for use in the United States. While the rates of application vary by crop, the vast majority of commercial agricultural crop acreage receives at least one application of herbicide every year.

Herbicides may be applied directly to the soil or to the...