Herbicides are chemicals used to manipulate or control undesirable vegetation. The most frequent application of herbicides occurs in row-crop farming, where they are applied before or during planting to maximize crop productivity by minimizing other vegetation. They also may be applied to crops in the fall, to improve harvesting. Herbicides are used in forest management to prepare logged areas for replanting; the total applied volume and area covered is greater but the frequency of application is much less than for farming (Shepard et al. 2004). In suburban and urban areas, herbicides are applied to lawns, parks, golf courses and other areas. Herbicides are applied to water bodies to control aquatic weeds that impede irrigation withdrawals or interfere with recreational and industrial uses of water (Folmar et al. 1979).

The potential effects of herbicides are strongly influenced by their toxic mode of action and their method of application. The molecular site of action is challenging to predict because structural associations have not been identified (Duke 1990), but modes of action are well-established. Herbicides can act by inhibiting cell division, photosynthesis, or amino acid production or by mimicking natural auxin hormones, which regulate plant growth, and causing deformities in new growth (Ross and Childs 1996). Methods of application include spraying onto foliage, applying to soils, and applying directly to aquatic systems.