Herbicides are used to control the growth of unwanted plants (weeds). Modern herbicides generally act by restricting growth. They inhibit the action of one or more of the many receptors that catalyze reactions which are essential to the growth of the plant. There is one group however, the auxins, that kill by overstimulating growth. With selective herbicides, either the target in the weed is affected more than that of the crop, the herbicide is degraded more quickly within the crop, or the uptake or translocation of the active ingredient differs from that of weeds. Non-selective herbicides kill crops as well as weeds.The herbicide is absorbed by the leaves and then translocates to the roots. By inhibiting the action of an enzyme it prevents the production of aromatic amino acids needed by the plants for protein synthesis. As this enzyme is absent in mammals, glyphosate has little toxicity. It is very effective and is rapidly degraded in the soil and by light. It is marketed under many names including Roundup and Tumbleweed. Although glyphosate is a non-selective herbicide, selective seeds of corn (maize) and soya have been developed (via genetic engineering) whose plants can withstand the herbicide. Thus, fields can be sprayed with the herbicide and these transgenic crops will be the only ones that will not be affected. These seeds are used in many parts of the world but have not been allowed in most EU countries, except in very tightly controlled trials because of public concern about the effect of transgenic crops on the environment.
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