15.9 Herbicides that Inhibit Very Long Chain Fatty Acids

You may see the Seedling Shoot Growth Inhibitors also referred to as herbicides that inhibit very long chain fatty acids (VLCFA).  These herbicides inhibit several different elongases, each with different enzymatic functions.

Thiocarbamates  (Group 8)

These soil-applied herbicides inhibit early seedling development, primarily the emergence and elongation of primary shoots.  One of the first documented injury symptoms of EPTC herbicide was a reduction in cuticular wax deposition.  Other severe injury symptoms include the failure of leaf emergence from the coleoptile and a general stunting of seedling growth, indicating that inhibition of VLCFA synthesis has many, widespread effects on normal plant growth.  This observation also supports the idea that there may be other functions for elongation enzymes in plants.

Thiocarbamate herbicides are actually pro-herbicides, in that the form of the herbicide applied to plants (the parent molecule) is not toxic.  Once the parent molecule is absorbed into plant tissues, it must be cleaved and metabolically activated by endogenous sulfoxidase enzymes in order to become phytotoxic.

Thiocarbamates are soil applied and can be applied as pre-plant incorporated (PPI) or pre-emergence herbicides.  Crop selectivity depends upon planting crop seeds below the treated soil.  Such manipulations are sometimes hard to achieve with precision, and so crop injury from these herbicides is a recurring problem.  The potential for crop injury led to early research on compounds known as herbicide safeners, antidotes, or protectants.  The first safeners identified were used to protect corn from EPTC injury, and include the compounds naphthalic anhydride, benoxacor, fenclorim, and fluxofenim.    Safeners are coated onto crop seeds or may be mixed with some commercial herbicide formulations.  They commonly induce one or more plant defense mechanisms.  The result is that crop plants are better able to metabolize and detoxify the herbicide.  Interestingly, these compounds protect monocot but not dicot crops from herbicide injury, for reasons that are not entirely clear.

Thiocarbamate herbicides have been shown to cause changes in the makeup of soil microbial communities.  In particular, continuous usage in the same field selects for bacteria and actinomycetes that rapidly degrade these herbicides, to the extent that they are essentially ineffective for weed control.  This phenomenon of rapid herbicide degradation has been termed ‘enhanced degradation’ and ‘preconditioned soil’.

Thiocarbamate Family  – Group 8

Chloroacetamides (Group 15)

The chloroacetamide family is one of the most widely used groups of herbicides in the world, primarily due to the use of metolachlor.  The first member of this family to be commercialized was alachlor by the Monsanto Company in 1969.  They are extensively used in corn and soybeans to control a broad spectrum of annual grasses and some broadleaf weeds.  They are soil applied and usually do not require soil incorporation for activity.

Chloroacetamide Family – Group 15