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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
E.g. 2,4-D, MCPA, 2,4,5-T
All derivatives of phenoxyalkane carboxylic acids that act as plant growth regulator herbicides. Phenoxy herbicides were the first safe, selective herbicides discovered and they are still used in huge quantities. They act by simulating the action of natural hormones and produce uncoordinated plant growth. Their action is selective as they are toxic to dicotyledonous but not monocotyledonous plants. Hence they can be used to control 'dicot' weeds (broad leaved weeds) in 'monocot' crops (e.g. cereals, grass).

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
Data from: Whitehead, R (1995) The UK Pesticide Guide. CAB International & BCPC.

Chemical group Compound Uptake/action Persistence Timing/site of application Other uses
  alachlor via roots, translocated residual pre/post-emergence  
AMIDES or substituted amides
  napropamide     pre-emergence  
  propachlor     pre-emergence  
BENZOICS or arylaliphatic acids
  dicamba translocated   soil/foliar  
BENZONITRILES or substituted nitriles
  dichlobenil   residual soil  
  bentazone contact   post-emergence  
  diquat contact non-residual foliar pre-harvest, CROP DESICCANT
  paraquat contact non-residual    
CARBAMATES or carbanilates
  asulam translocated   foliar  
  chlorpropham   residual soiltubers POTATO SPROUT SUPPRESSANT
  phenmedipham contact   foliar  
CHLOROALKANOIC ACIDS or chlorinated aliphatic acids
  dalapon   persistent soil?  
DINITROANILINES or nitroanilines
  pendimethalin   residual pre-emergence, soil  
  trifluralin     soil-incorporated  
  bromoxynil contact   post-emergence  
  ioxynil contact   post-emergence  
IMIDAZOLINONES or imidazoles
  imazapyr translocated residual foliar, soil  
OXIMES or cyclohexenones
  cycloxydim translocated   post-emergence  
  sethoxydim     post-emergence  
  MCPA translocated   post-emergence  
  MCPB translocated   post-emergence  
  diclofop-methyl translocated   post-emergence  
  fenoxaprop-P-ethyl     post-emergence  
  fluazifop-P-butyl     post-emergence  
  mecoprop translocated      
  mecoprop-P translocated   post-emergence  
PHOSPONIC ACIDS or phosphona amino acids or phosphates
  glufosinate-ammonium contact non-residual foliar  
  glyphosate translocated non-residual foliar  
  picloram translocated persistent foliar, soil  
  fluroxypur     post-emergence  
  triclopyr     foliar  
  difenzoquat     post-emergence  
  metsulfuron-methyl contact residual post-emergence  
  triasulfuron     post-emergence  
  tri-allate     soil-acting, pre-emergence  
  atrazine   residual pre/post emergence  
  cyanazine contact residual pre-emergence  
  metribuzin contact residual pre-/post-emergence  
  simazine root uptake   soil  
  terbutryn   residual pre-emergence  
  metamitron contact residual post-emergence  
UREAS or phenylureas or substituted ureas
  chlorotoluron contact residual pre-emergence  
  diruon   residual pre-/post-emergence  
  isoproturon     pre-/post-emergence  
  linuron contact residual pre-/post-emergence  
  methabenzthiazuron contact residual pre-/post-emergence  
  metoxuron contact residual post-emergence  
  tebuthiuron root uptake persistent- up to 2 years soil acting  
URACILS or substituted uracils
  bromacil soil-acting persistent    
  lencacil soil-acting residual pre-emergence