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Ecotoxicology - Pesticide Classification - Herbicides
It is really only in the last 50 years that use of chemical weedkillers or herbicides has become widespread. Prior to this, the control of weeds in crops was carried out largely by manual weeding, crop rotation, ploughing and various ways of stopping weed seeds being dispersed in crop seed. Today, the heavy use of herbicides is confined to those countries that practice highly intensive, mechanised farming. In 1971 it was estimated that more energy was expended on weeding crops than on any other single human task (Brain 19 ). Herbicides are also used extensively in non-crop and amenity situations such as industrial sites, roadsides, ditch banks, recreational areas etc. Table 3: Classification of Herbicides gives a summary of the the major chemicals currently being used as herbicides and their classification according to Chemical Group, uptake/mode of action, persistence and timing/site of application. Herbicides can be classified in a number of different ways. The main classification used here is according to chemical class but they can also be classified according to their selectivity, the way that they affect the plant, the timing of application and the area covered by an application. Herbicides are classed as selective if they kill some plant species but not others, for instance they may kill the weeds but not the crop and as non-selective if they kill all vegetation. Herbicides may be intrinsically selective in that they are active against some species of weed but not others but they may also be used selectively, that is in such a way that they only come into contact with the weeds and not the crop. There are two main ways in which herbicides affect the plants they are applied to: contact herbicides kill parts of the plant that they come into contact with. These are generally used against annual weeds, if they are to be effective need complete coverage of the target weed with the chemical. Systemic or translocated herbicides are absorbed either by the roots or foliage of the plant and then move within the plants system to areas remote from the site of application. Translocated herbicides tend to be slower acting than contact ones and while they can be used against annual weeds they are more commonly aimed at perennial weeds. With translocated herbicides a uniform, although not necessarily complete, coverage of the target weeds is necessary. Finally, herbicides can be classified according to the timing of application in relation to the crop they are being used in. Pre-plant, or pre-sowing herbicides must be applied to an area before the crop is planted. Pre-emergence herbicides are applied before the crop has emerged, this may allow an added level of selectivity as a herbicide can be applied to growing weeds while the crop itself is still protected by the oil. Finally, post-emergence herbicides are applied after the crop has emerged from the soil. Again, a level of selectivity may be introduced by applying a germination inhibitor to prevent further germination of weed seeds - after the crop itself has germinated. Phenoxy Herbicides Their physical properties vary greatly according to formulation. For instance, as alkali salts they are highly water soluble (can be formulated as aqueous solutions) whereas when as simple esters they have low water solubility and are lipophilic (generally formulated as emulsifiable concentrates). The main hazard they present is mainly posed by unwanted spray drift but they have also sometimes been contaminated with the highly toxic compound TCDD (or dioxin). Other related compounds, also with plant growth regulating properties include phenoxypropionic acids (e.g. CMPP) and phenoxybutyric acids (e.g. 2,4DB). Table 3: Classification of Herbicides
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