Why Use Herbicides?

Herbicide, an agent, usually chemical, for killing or inhibiting the growth of unwanted plants, such as residential or agricultural weeds and invasive species. A great advantage of chemical herbicides over mechanical weed control is the ease of application, which often saves on the cost of labor. Most herbicides are considered nontoxic to animals and humans, but they can cause substantial mortality of nontarget plants and the insects that depend on them, especially when applied aerially

Chemical weed control has been used for a very long time: sea salt, industrial by-products, and oils were first employed. Selective control of broad-leaved weeds in fields of cereal crops , Sulfates and nitrates of copper and iron were used, and sulfuric acid proved even more effective. Application was by spraying. Soon sodium arsenite became popular both as a spray and as a soil sterilant. Chemical weed control superseded both plant-disease and insect-pest control in economic impact.

Modern weed killers are put in two categories: selective (affecting specific plant species) and nonselective (affecting plants generally). These, in turn, are classified as foliage-applied and soil herbicides. Contact herbicides (e.g., sulfuric acid, diquat, paraquat) kill only the plant organs with which they are in contact. Translocated herbicides (e.g., amitrole, picloram, and 2,4-D) are effective against roots or other organs to which they are transported from aboveground treated surfaces (i.e., soil). With respect to planting time, herbicides are also classified as preplant, preemergence, or postemergence weed killers. Preplant herbicides may be applied to the soil or to weeds before crop planting.