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Overall Picture of Auxinic Herbicide Action

Auxin and Auxinic Herbicide Mechanism(s) of Action - Part 1 - Introduction

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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:

They are applied POST emergence for the control of broadleaf weeds in grass crops or systems,
Their soil residual activity varies with individual herbicides,
Some are volatile,
They translocate from leaves or roots to growing points, and
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