Herbicide Background

The use of herbicides in North American agriculture is wide-spread because the high cost of human labor makes manual weeding prohibitive. This lesson will present information on herbicides with modes of action based on the photobiology of the plant. This will not be a comprehensive coverage of herbicides, neglecting many widely used herbicides with modes of action based on mimicry of phytohormones or inhibition of metabolic pathways for amino acids, lipids and other molecules. Also, you should understand that by focusing on herbicides using photobiology as a mechanism of action, you will not be looking at molecules that act in a highly specific manner. Rather, this class of herbicides takes advantage of the enormous energy in light and the dreadful consequences it has on plant cells when not properly controlled, providing the effect of a molecular hand grenade. The power of sunlight is truly awesome.

Herbicides have been classified by weed scientists according to their modes of action. For the purpose of this course, we will rely upon the classification of the Weed Science Society of America (WSSA), but you should be aware that this is not universally used. For example, the Herbicide Resistance Action Committee (in collaboration with the WSSA) has developed an alternate classification scheme for herbicides based on their modes of action.

Another source of confusion is the naming of herbicides. The chemical names are often long and cumbersome and the trade names contain no information. For example, atrazine is the chemical compound 6-chloro-N-ethyl-N’-(1-methylethyl)-1,3,5-triazine-2,4-diamine, but is also sold world-wide as Aatrex, Aktikon, Aktinit, Argezin, Atazinax, Atranex, Atrasine, Atratol A, Atred, Candex, Cekuzina, Crisatrina, Cyazin, Farmco atrazine, Fenamin, Fenatrol, Geigy 30027, Gesaprim, Griffex, Hungazin, Inakor, Oleogesaprim, Primatol, Primaze, Radazin, Strazine, Triazine, Vectal, Weedex A, Wonuk, Zeazin, and Zeazine. The following resource may be useful in providing you with more information about a herbicide when all you know is an unfamiliar name: Guide for Weed, Disease and Insect Management in Nebraska.

This lesson will deal with four general herbicide modes of action related to photobiology and outlined in Table I. Be aware that this is a simplification. If you were to take a graduate course in herbicide mode of action there would be much more complexity apparent.

Table 1
 Mode of Action Class of Molecule Example WSSA Classification HRAC Classification
Inhibition of propoporphyrinogen oxidase Diphenylethers Phenyl heterocycles Acifluorfen sulfentrazone 14 E
Inhibiton of carotenoidbiosynthesis Triazole Pyridazinone Isoxazole Isoxazolidinone amitrole norflurazon isoxaflutole clomazone 11, 12, 13 and 28 F
Inhibition of Photosystem II Triazines Nitriles Ureas atrazine bromoxynil diuron 5, 6, and 7 C
Photosystem I electron diversion Bipyridyliums diquat 22 D