A "weed" is usually considered to be any plant that interferes with the productivity of a desired crop plant or some other human purpose, even though in other contexts weed species may have positive ecological and economic values. Weeds exert this effect by competing with the crop for light, water, and nutrients. Studies in Illinois demonstrated an average reduction of yield of corn or maize (Zea mays) of 81% in unweeded plots, while a 51% reduction was reported in Minnesota. Weeds also reduce the yield of small grains, such as wheat (Triticum aestivum) and barley (Hordeum vulgare), by 25–50%.
Because there are several herbicides that are toxic to dicotyledonous weeds but not grasses, herbicides are used most intensively used in grain crops of the Gramineae. For example, in North America almost all of the area of maize cultivation is treated with herbicides. In part this is due to the widespread use of no-tillage cultivation, a system that reduces erosion and saves fuel.
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