History - Advanced Level

The auxinic herbicides were among the first selective organic herbicides developed and were discovered independently by British and American scientists during the 1940’s. 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 in use. The auxinic herbicide family contains four major chemical groups (Figure 1).

phenoxyalkanoic acid structures
pyridinecarboxylic acid structures
benzoic acid structure

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

This family of herbicides is termed the ’auxinics’ because they

  1. Resemble the structure of the natural plant hormone auxin (indole-3-acetic acid; IAA) (Figure 2) and
  2. Cause physiological effects in sensitive plants similar to those caused by IAA.


Fig. 2: Structural similarities between the natural auxin, IAA, and an auxinic herbicide, 2, 4-D  (Image credit: Tracy Sterling, Deana Namuth, Jeremy Steele, and Smitha Kasinadhuni)

The term auxin is from the Greek word, auxein, meaning ’to increase’ and was first used by Frits Went in 1926 to describe the compound that caused the curvature of oat coleoptiles toward light. This compound was later found to be the endogenous plant hormone, indole-3-acetic acid (IAA).