15.6 Herbicides that Inhibit ACCase

Monocots and dicots have fundamentally different ACCase enzymes, and these differences create the basis for herbicide selectivity.  The multidomain form of ACCase, found only in graminaceous monocots, is the only one completely effected by the ACCase-inhibiting herbicides, which allows them to specifically control certain weedy members of Poaceae, mostly in dicot crops (ie controlling grassy weeds in soybeans).  Further subtle differences between the ACCase enzymes from small grain crops (wheat and barley) and grassy weeds allow the use of selective herbicides like fenoxaprop  to control wild oats in these crops.

Here are time lapse videos of susceptible plant response to these herbicides:

For the full list of Ohio State University’s Weed Science herbicide time lapse videos, go to: Herbicide Injury Time Lapse – Mode of Action

Two herbicide families with very different chemical structures, inhibit ACCase, the first enzyme of fatty acid synthesis. These herbicides are collectively known as graminicides, or sometimes ‘grass killers.’

“Fops” – Group 1

The aryloxyphenoxy propionate herbicides are informally termed ‘fops,’ reflecting the suffix on most of their chemical names. Fops were first developed in Japan and later in Germany, and diclofop-methyl was the first one commercially marketed in 1979. These herbicides are lethal to many grasses, with the exception of cultivated oats, wheat, red fescue, and annual bluegrass. Since most fops have very low water solubility, they are formulated and sold as esters. Once absorbed into plant tissues, the esters are rapidly cleaved to produce the free acids, which are the herbicidal form of the molecules. The links to photos of herbicide injury symptoms were graciously provided by Dr. Jack Dekker, Iowa State University.

Aryloxyphenoxy  propionate Family (‘fops’)  – Group 1

“Dims” – Group 1

The cyclohexanedione herbicides are chemically quite different from the fops and yet also inhibit ACCase.  These herbicides are termed ‘dims,’ reflecting the suffix on their chemical names.  Sethoxydim (Poast) was the first dim commercially developed in the early 1980’s, by BASF.   Unlike the fops, sethoxydim is very labile (easily broken down) in the environment and in plants; however, many of the metabolites are phytotoxic and so herbicidal activity is usually retained.

Cyclohexanedione Family (‘dims’) – Group 1

Dens – Group 1

The dens are the newest chemical family in the Group 1 Mode of Action.  The only active ingredient in this particular chemical family is pinoaxaden, hence the name “dens”.  Like other Group 1 herbicides these are systemic.