Since an important purpose of plowing is to reduce the abundance of weeds, the notillage system would be impracticable if not accompanied by herbicide use. The most important herbicides used in maize cultivation are atrazine, propachlor, alachlor, 2,4-D, and butylate. Most of the area planted to other agricultural grasses such as wheat, rice (
Oryza sativa), and barley is also treated with herbicide, mostly with the phenoxy herbicides 2,4-D or MCPA.
The intended ecological effect of any pesticide application is to control a pest species, usually by reducing its abundance to below some economically acceptable threshold. In a few situations, this objective can be attained without important nontarget damage. For example, a judicious spot-application of a herbicide can allow a selective kill of large lawn weeds in a way that minimizes exposure to nontarget plants and animals.
Of course, most situations where herbicides are used are more complex and less well-controlled than this. Whenever a herbicide is broadcast-sprayed over a field or forest, a wide variety of on-site, nontarget organisms is affected, and sprayed herbicide also drifts from the target area. These cause ecotoxicological effects directly, through toxicity to nontarget organisms and ecosystems, and indirectly, by changing habitat or the abundance of food species of wildlife.
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