History and Uses - Introductory Level

The auxinic herbicides were among the first selective organic herbicides developed and were first discovered independently by British and American scientists during the 1940s. The selective control of broadleaf weeds in cereal grain crops by auxinic herbicides has made this group one of the most widespread and important herbicide families. Table 1 contains the four major classes of the auxinic herbicides, their trade names and the crops on which they are used. Figure 1 shows their structures. They are applied postemergence. Depending on the class, they may have soil as well as foliar activity (Table 2).

Common features among the auxinic herbicides are:

  1. They are applied POST emergence for the control of broadleaf weeds in grass crops or systems,
  2. Their soil residual activity varies with individual herbicides,
  3. Some are volatile,
  4. They translocate from leaves or roots to growing points, and
  5. Selectivity in tolerant grasses is due to restricted translocation, enhanced metabolism, and the lack of a vascular cambium. Selectivity in broadleaf species is due to metabolism of the herbicide to a non-toxic form.

Table 1. Auxinic herbicides belonging to different chemical families and the crops in which they are used.

Herbicide Class

 Active Ingredient Trade Name Major Use
Phenoxyalkanoic acids 2,4-D Various Corn, wheat, barley, turf, etc.
2,4-DB Butyrac Soybeans, peanuts, forage legumes
Dichlorprop (2,4-DP) Various Brush control
MCPA Various Wheat, barley, corn, turf, sorghum, flax
MCPB Various Peas
Mecoprop MCPP Various Cereals, turf
Benzoic acids Dicamba Banvel, Clarity, Banvel SGF Corn, wheat, barley, fallow, sorghum, range
Pyridinecarboxylic acids Clopyralid Stinger, Reclaim, Transline, Lontrel Wheat, barley, canola, range, corn, oats, sugar beets
Fluroxypyr Starane, Attain Wheat, barley
Picloram Tordon Wheat, barley, range
Triclopyr Garlon, Grandstand, Remedy Rangeland, turf, rice
Quinolinecarboxylic Acids Quinclorac Facet, Paramount Rice  

Table 2. Foliar and soil activity of auxinic herbicides.

 

Herbicide Class

Foliar Activity

Soil Activity
Phenoxyalkanoic acids 2,4-D 2,4-DB MCPA Little to none
Benzoic acids Dicamba Dicamba (very little)
Pyridinecarboxylic Acids Clopyralid Picloram Triclopyr Clopyralid (< one year) Picloram (> one year) Triclopyr (very little)
Quinolinecarboxylic acids Quinclorac About one year
the structure of 2,4-D, 2,4-DP, 2,4-DB, 2,4,5-T, MCPB, MCPA, and mecoprop. All of them are phenoxyalkanoic acids
The chemical structure of four pyridinecarboxylic acids (fluroxypyr, clopyralid, triclopyr, and picloram)
the chemical structure of dicamba, a benzoic acid.
The chemical structure of quinmerac and quinclorac, both quinolinecarboxylic acids.

Fig. 1: Structures for the Four Major Chemical Groups of Auxinic Herbicides

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