Herbicides Chemical Weed Control Herbicides are chemicals used to suppress or kill plants or to interrupt their normal growth processes (Cobb and Reade, 2010) Of all groups of pesticides (including insecticides, fungicides, and rodenticides), herbicides are the leading group in terms of tons produced, dollar value from sales, and total acreage treated Extensive and widespread use of herbicides in agriculture continues because of their high level of effectiveness and low cost relative to other methods of weed control Use of herbicides has resulted in improved control of weeds that grow within crop rows where cultivation is not possible Herbicides have replaced frequent tillage operations in some systems, which conserve energy, reduce crop damage, and minimize damage to soil structure With herbicides, crop production is less dependent on weather and human labor, such that greater flexibility in choice of crops and management methods is possible In the US today, the abundance of relatively inexpensive food and fiber is due in large part to the benefits afforded by herbicides for weed control in the last half century However, the use of herbicides also carries risks, including injury to crops and nontarget plants, herbicide residues in soil or water, toxicity to nontarget organisms, and concerns for human health and safety For this reason, the benefits and risks of each method must be weighed carefully when developing a weed control program, particularly in wildland ecosystems In the US, all pesticide development and use is subject to strict regulation by the federal government 89 planting), preemergence (prior to crop or weed emergence), and postemergence (after crop or weed emergence) A herbicide classification scheme based on method of application is summarized in Table Other classification schemes are based on plant responses to herbicides Selective herbicides are more toxic to some plant species than others (e.g., monocotyledonous vs dicotyledonous plants), whereas nonselective herbicides are toxic to all plant species Selectivity is one of the most important and useful characteristics of herbicides, for it allows applications to be made to weeds without the risk of injury to crops or desirable vegetation Factors related to herbicide chemistry (structure, formulation), the plant (age, size, surface characteristics, morphology, physiology), and the environment (humidity, temperature, soil moisture) determine the selectivity of a particular herbicide Other categories of plant response used to classify herbicides are the pathway of herbicide movement in plants and the mechanisms by which herbicides kill plants (see Table 2) Systemic herbicides are those that move in plants; movement occurs in the phloem (symplast), xylem (apoplast), or both Herbicides that not move in plants, but rather exert their effect at the site of application, are called contact herbicides Herbicides classified by mechanism of action include growth regulators; inhibitors of photosynthesis, pigments, lipid synthesis, cell wall synthesis, amino acid synthesis, and cell division; and cell membrane destroyers Fate of Herbicides in the Environment Herbicides There are approximately 150 herbicide active ingredients, which are formulated into hundreds of commercial products (Senseman, 2007) Most are organic compounds, containing carbon, hydrogen, oxygen, and various other chemical elements Each herbicide has a chemical name that describes its structure and a common name, which is often a simplified version of the chemical name Formulated herbicides also have a trade name assigned by the manufacturer for marketing purposes Manufacturers formulate herbicides to enhance their handling and weed control properties Formulated compounds include the herbicide active ingredient plus inert ingredients such as solvents, diluents, and various additives When the same herbicide active ingredient is formulated in more than one way, each is assigned a different trade name Herbicides can be classified in several different ways, which provide users with a convenient means of selecting herbicides for various purposes Herbicides are often classified according to similarities in chemical structure, which often, but not always, result in similar effects on plants A more useful classification scheme is based on where they may be used In agriculture, herbicides are registered for use in agronomic and horticultural crops, turfgrass, and landscape and ornamental plantings In noncrop areas, herbicides are used in pastures and rangelands, aquatic habitats, rights-of-way, utility sites, recreation areas, forests, and wildlands Herbicides are also classified according to the method and timing of application Application methods include soil and foliage treatments, depending on where in the plant the chemical is most readily absorbed Timings of herbicide application include preplant (prior to crop Herbicide fate in the environment is an issue of public concern and an important consideration when herbicides are registered for legal use For herbicides to be effective, they must persist long enough to kill the weeds for which they were intended Persistence beyond that time, however, may result in injury to nontarget plants and other organisms, residues in crops, and environmental contamination Herbicides that enter plants generally move to a site of action and cause a toxic reaction Over time, most herbicides in plants are transformed into relatively less toxic forms by biochemical processes If a herbicide is not degraded, it may remain in the plant or end up in the soil as a contaminant Eventually, however, all herbicides that enter a plant, the soil, water, or atmosphere will be degraded by the same chemical and physical reactions that act on biologically derived compounds (Cobb and Reade, 2010) Once a herbicide reaches the soil, several processes, including adsorption to soil particles, movement to another location, and decomposition, will determine its persistence Soil and herbicide characteristics regulate adsorption of herbicide molecules onto particles of clay and organic matter Herbicides that are tightly adsorbed are not available for plant uptake, movement to other sites, or decomposition Herbicides that are loosely adsorbed or located in the soil solution may move by leaching (vertical and lateral movement by water) The potential for movement of herbicides into groundwater through leaching is a concern for a few very mobile herbicides, which consequently are subjected to strict monitoring and regulation Herbicides may also enter the environment during application by drift (movement of herbicide particles in air) or from the soil surface by