Chemicals used for weed control are termed herbicides. The use of chemicals to control weeds is not a new practice; some compounds, such as sodium chloride and sulphuric acid, were used during the late nineteenth century to kill unwanted vegetation. The intensive use of herbicides for weed control however began around 1945 with the discovery of the selective phytotoxicity of [2-4-D (2,4-dichloro-phenoxy) acetic acid]. This compound kills plants of some species with little or no injury to others. This characteristic stimulated much interest among scientists, and research soon thereafter demonstrated that this selectivity allowed use of 2,4-D to control many dicotyledonous weeds without injury to grass crops such as corn and other grains. Subsequent research demonstrated that treatment with 2,4-D or some other herbicide might cause highly variable plant responses. This variability was soon traced to such soures as different rates at which the herbicide was applied.

Selective herbicide: selective herbicides are those to which different plant species show a wide range of response. One species may be killed, whereas another may not be injured by a particular application of the herbicide. Selectivity allows use of a particular herbicide to remove weeds growing among crop plants; the herbicide is applied to all plants uniformly, but the crop plants are not injured, whereas the weeds are killed. Many herbicides show selectivity, but 2,4-D is among the most widely known for its relative toxicity to broadleaf weeds and harmlessness to narrow leaf plants. That characteristic enables 2,4-D to serve many uses, among the more notable of which is killing ragweed growing in cornfields. Other herbicides show selectivity for other groups of plants. The spectrum of weed species susceptible to a given herbicide is an important aspect of selectivity. A narrow-spectrum herbicide is toxic to only one or a very few species of plants, and all other plants are tolerant. A broad-spectrum herbicide has relatively little value in a program designed to control many weed species. These herbicides are justified only when a particularly noxious weed is to be controlled; example would include purple nutsedge (Cyperus rotundus L.) or Johnson grass (Sorghum halepense L) on cultivated lands or water hyacinth (Eichornia crassipes) in aquatic sites.

A broad-spectrum herbicide is used for simultaneous control of many weed species, as commonly exist in a typical plant community. Many broad-spectrum herbicides are used in combinations of two or more to extend even further the spectrum of weed control. Almost all herbicides in common use are broad-spectrum.

Selectivity is perhaps a misnaming, because the herbicide does not "select" the plants it should kill among those being treated. Instead, phytotoxicity is based on the different responses of individual plants to that herbicide. Some plants tolerate a given herbicidal treatment very well and others do not. Such tolerance is based largely on morphological, physiological, and genetic plant characteristics. Most selective herbicides are translocated within the plant from the point of entry to the point of accumulation and ultimate toxicity. Such materials do not commonly cause immediate phytotoxicity. They are absorbed and move some distance inside the plant, away from the point of entry, before becoming toxic. These materials often accumulate in the root, stem, or leaf-growing point, or in some other specific location inside the plant. Such points of accumulation most commonly are areas of high meristematic activity. When the accumulation reaches a certain level, toxicity occurs and the treated plant ultimately dies.

Contact herbicides are toxic to the plant protoplasm, and kill only those parts of the plant they contact. These compounds are not translocated, because the protoplasm is killed before translocation begins. Contact herbicides are usually nonselective, but a few show some selectivity, depending on the plant part that is contacted. Paraquat (1,1’-dimethyl-4, 4’ –bipyridinium ion), for example, is highly toxic to plant parts that contain chlorophyll, but has few or no ill effects on plant stems or on parts of the plant that do not contain chlorophyll.

Herbicides are applied as sprays to the foliage of growing plants or to the soil surface before or shortly after planting. Herbicides that are absorbed into the plant system through the leaves are applied when the plants are in an active stage of growth. Weeds are most easily killed by foliar sprays during the early postemergence growth period, but applications may be made at any time after plant emergence, depending on specific objectives. For example, herbicidal sprays may be applied during early stages of seedling growth to eliminate weed competition with crop plants; or an herbicide may be applied in corn fields as this crop is approaching maturity to eliminate various weeds that would interfere with mechanical harvesting procedures.

Herbicides that affect the plant primarily through the root or that kill dormant or germinating seed are applied to the soil. Many of these herbicides have little or no effect on plants if applied only to the foliage. Some herbicides that exhibit primarily contact toxicity when applied to the foliage develop more selectivity when applied to the soil. For example, dinoseb (2-sec-butyl-4, 6-dinitrophenol) exhibits contact toxicity to plant foliage, but it exhibits selectivity when applied before emergence; it thus allows satisfactory emergence of some important crop plants, such as peanuts, but kills the germinating seedlings of many annual weeds. These materials can also be applied to the soil at different times relative to plant emergence.

Herbicides applied to the soil surface before the crop is planted are known as preplant treatments. Herbicides applied in this manner are most commonly toxic to weed seed that are in a dormant or germinating condition in the soil. The materials are usually toxic to crop seed or seedlings also, but applications before the crop is planted avoid injury to crop seedlings. Most herbicides applied before planting are incorporated, by disking or similar means, into the soil to a depth of about six inches. Such incorporation increases the probability that the herbicide will come into contact with the weed seed to be killed.

Ag.
Technologies
(Weed Control)