module 14
Herbicide mode of action
Objectives
At the end of this module, you will:
- understand the importance of knowing a herbicide's mode of action
- know that certain herbicides are related chemically and that these have similar effects on plants
- understand how major herbicide groups kill plants
and the implications that this has for herbicide application
Module Outline
Introduction
Herbicides are chemicals that kill plants principally by interfering with the normal biochemical processes of plants. Organic herbicides have been used in agriculture for over 50 years and there are a large number of products sold. It is often important to have some knowledge of how herbicides work as this can affect how a herbicide is applied. For example, a herbicide that is adsorbed by germinating seedlings, such as trifluralin, will have low efficacy if applied to the foliage of large plants.
Equally, a herbicide that must be absorbed by leaf tissue, such as glyphosate, will not be effective if applied to bare soil. Despite the hundreds of individual products and the large number of active ingredients available for use, most herbicides can be grouped by chemical similarity into relatively few chemical groups.
Often several herbicide chemistries will have similar biochemical effects on the plant and when these actions of herbicides are considered an even fewer number of “Modes of Action” occur. The following is intended as a brief survey through the herbicide modes of action with space spent on the more important modes of action. There are numerous publications about herbicide mode of action, and several books devoted to the use of herbicides. These should be consulted for more information.
Herbicide mode of action classification
Classification of herbicides by mode of action is now accepted by the herbicide industry in Australia and all products sold carry a letter on the label indicating the mode of action. The list of modes of action currently used is shown below.
Table 14.1 Mode of action classification of herbicides
| Letter code | Mode of action | Herbicide chemistries |
| A | Inhibitors of acetylcoenzyme A carboxylase (lipid synthesisers) | Aryloxyphenoxypropanoate, Cyclohexanedione |
| B | Inhibitors of acetolactate synthase | Sulfonylurea, Imidazolinone, Sulfonamide, Pyrimidinylthiobenzoate |
| C | Inhibitors of Photosystem II | Triazine, Substituted urea, Uracil, Triazinone, Benzothiadiazole, Nitrile, Phenylpyridazine |
| D | Inhibitors of microtubule formation | Dinitroaniline, Benzoate |
| E | Inhibitors of mitosis | Carbamate, Thiocarbamate, Organophosphate |
| F | Inhibitors of carotenoid biosynthesis | Aminotriazole, Nicotinanilide, Pyridazinone, Pyrazole, Isoxazolidinone, Picolinamide |
| G | Inhibitors of chlorophyll synthesis | Diphenyl ether, Oxadiazole, Triazolinone |
| I | Disrupters of plant cell growth | Phenoxyacetic acid, Benzoate, Pyridine, Carboxylate |
| J | Inhibitors of fat synthesis | Alkanoate |
| K | Diverse modes of action | Amide, Carbamate, Benzofuran, Phthalamate, Aminopropanoate |
| L | Disrupters of Photosystem I | Bipyridyl |
| M | Inhibitors of EPSP synthase | Glycine - Glyphosate |
| N | Inhibitors of glutamine synthetase | Glycine - Glufosinate |
Summary
The vast range of available herbicides can be grouped into relatively few modes of action. Each mode of action has a distinct effect on plants with inhibition of a single enzyme by a herbicide usually sufficient to cause death of the plant. Many herbicides are selective in crops and selectivity is often, although not always, the result of rapid herbicide metabolism in the crop plant. Knowledge of the mode of action of a herbicide is important for the design of management strategies to limit herbicide resistance. This issue will be considered in more detail in Module 16 and Module 17A 'Integrated Weed Management for Cropping'.
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References and further reading
Tahrens, WH, (ed.) 1994, Herbicide handbook, 7th edn, Weed Science Society of America, Champaign, Il.
Caseley, JC & Walker, A 1990, ‘Entry and transport of herbicides in plants’, in RJ Hance & K Holly, (eds), Weed control handbook: Principles, Blackwell Scientific Publications, Oxford, pp. 183-200.
Cobb, A 1992, Herbicides and plant physiology, Chapman & Hall, London.
Cobb, AH & Kirkwood, RC 2000, Herbicides and their mechanisms of action, Sheffield Academic Press, Sheffield.
Devine, MD, Duke, SO & Fedke, C 1993, Physiology of herbicide action, PTR Prentice Hall, Englewood Cliffs, NJ.
Dodge, AD 1990, ‘The mode of action of herbicides’, in RJ Hance & K Holly, (eds) Weed control handbook: Principles, Blackwell Scientific Publications, Oxford, pp. 201-216.
Duke, SO, Jydon, J, Becerril, JM, Sherman, TD, Lehnen, LP, Jr. & Matsumoto, H 1991, ‘Protoporphyrinogen oxidase-inhibiting herbicides’, Weed Science, vol. 39, pp. 465-473.
Gronwald, JW 1991, ‘Lipid biosynthesis inhibitors’, Weed Science vol. 39, pp. 435-449.
Hance, RJ & Holly, K 1990, ‘The properties of herbicides’, in RJ Hance & K Holly (eds), Weed control handbook: Principles, Blackwell Scientific Publications, Oxford, pp. 75-125.
Hatzios, KK (ed.) 1998, Herbicide handbook, supplement to 7th edn, Weed Science Society of America, Lawrence, KS.
Kearney, PC & Kaufmann, DD, (eds) 1975, Herbicides: Chemistry, degradation and mode of action, Vol 1, Marcel Dekker Inc, New York.
Kearney, PC & Kaufmann, DD (eds) 1976, Herbicides: Chemistry, degradation and mode of action, Vol 2, Marcel Dekker Inc, New York.
Kearney, PC & Kaufmann, DD (eds) 1988, Herbicides: Chemistry, degradation and mode of action, Vol 3, Marcel Dekker Inc, New York.
Kirkwood, RC, (ed.) 1991, Target sites for herbicide action, Plenum Press, New York.
Parsons, JM, (ed.) 1995, Australian Weed Control Handbook, 10th edn, Inkata Press, Melbourne.
Schmidt, A, Linden, H & Böger, P 1991, ‘Phytoene desaturase, the essential target of bleaching herbicides’, Weed Science, vol. 39, pp. 474-479.
Stidham, MA 1991, ‘Herbicides that inhibit acetohydroxyacid synthase’, Weed Science, vol. 39, pp. 428-434.
Vaughn, KC & Lehnen, LP Jr. 1991, ‘Mitotic disrupter herbicides’, Weed Science, vol. 39, pp. 450-457.
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