Diquat, a nonselective herbicide, was developed for vegetation control, resulting in new methods in agriculture that conserved resources and protected the environment.
In 1955, R. J. Fielden, a chemist with Imperial Chemical Industries (ICI) of England who was synthesizing compounds for the company’s dyestuffs division, produced a new compound by combining ethylene dibromide and 2,2’-bipyridine. The resulting soluble salt, diquat dibromide, was sent to the company’s Jealott’s Hill Research Station in Berkshire, where R. C. Brian determined that it had a toxic effect on plants. The chemical structure was subsequently determined by R. F. Homer, who also correlated the structure with the chemical’s herbicidal properties. ICI thereupon marketed diquat as well as its chemical relative paraquat. These new herbicides filled an agricultural need and caused profound changes in agricultural practices.
Diquat was proven to be benign to the environment. Although a powerful plant desiccator and contact-type herbicide, it does not have residual abilities, nor does it accumulate in the environment or in organisms. Tests of its herbicidal properties indicated that it was powerful even at very low concentrations; moreover, diquat was sold as an aqueous solution containing 20 to 25 percent of the active ingredient, which was further diluted prior to use. In part, diquat is harmless to the environment because it is rapidly inactivated by sunlight; within one week or less, diquat degrades completely, leaving no residue. Even at high concentrations, no adverse effects have been observed on soil microorganisms, fungi, or invertebrates.
Diquat and its metabolic by-products have very low mammalian toxicity, and they are relatively nontoxic to grazing animals. The herbicide binds tightly to the insoluble components of a plant, so that little is absorbed by grazing animals. Less than 20 percent of the diquat dose is metabolized by animals that ingest it. Diquat residues in milk and tissue are extremely small—the substance does not accumulate even when it is fed to animals at low doses over a long period of time—and cause no ill effects in animals or in human consumers. Nor does occupational exposure to diquat result in a health hazard, so long as workers use the product as recommended. Most important, diquat is not considered teratogenic, carcinogenic, or mutagenic; that is, it produces no birth abnormalities, cancers, or spontaneous changes in genes.
Diquat is an important herbicide not only because of its extremely low environmental impact but also because of its phenomenal herbicidal effects. The toxicity to plants, which affects only the green parts of the plant, was shown to be the result of the chemical’s electron transfer properties. When diquat combines with an additional single electron it forms a free radical, which reacts with oxygen to re-form diquat and a superoxide anion. It is this superoxide anion that causes cell death. Diquat also interferes with the plant’s photosynthetic mechanisms and effectively destroys enzyme systems that could metabolize and possibly inactivate it; degradation of diquat in the plant is therefore negligible. Light and oxygen are required for diquat to operate, but since it also rapidly degrades in sunlight and adsorbs tightly to soil minerals, there is little environmental impact.
Diquat is used as a plant desiccant to assist with the harvest of many crops. Residues in the harvested crops are very low, and there are no phytotoxic effects on crops planted after the herbicide application. The amount of residual diquat remaining within a crop is usually minimal but is dependent on the crop species, the rate of application, the time interval between application and harvest, and the degree to which the pod or husk was shielded from the desiccant spray. The food tolerance levels set by the U.S. government in the Federal Drug and Cosmetic Act
Diquat does not vaporize or move easily in the wind, so there is minimal spray drift. In the soil, diquat is rapidly and strongly adsorbed to clay minerals, which inactivate its herbicidal activity; how much of the diquat is adsorbed depends on the soil type, in particular on the amount of clay minerals present. Diquat is essentially immobile in soils, which prevents leaching and makes contamination of water supplies from runoff or percolation to the water table unlikely. Free diquat can be degraded by many microorganisms, but when diquat is bound to the soil, it exerts no adverse influence on the soil microorganisms or their metabolic processes.
The speed at which diquat takes effect depends on many factors. Diquat is more effective in bright, sunny, warm, and humid weather. Rainfall does not appear to decrease the effect of the herbicide; in fact, the addition of wetters and surfactants increases its activity, whereas the presence of soil dust particles on the foliage protects the plant. Diquat in a solution of water alone is more effective than diquat in combination with fertilizers, and ground sprays producing small droplets are much more effective than those with large drops.
In water, diquat disappears rapidly (within one to two weeks), since it adsorbs to aquatic vegetation or to the bottom mud. Applications of the herbicide in the normal range are not harmful to fish and other aquatic organisms; if applied at large enough doses, however, it will kill large amounts of vegetation quickly, in turn causing oxygen depletion and posing a threat to fish. Under normal circumstances, though, diquat poses no threat to fish, and there has been no evidence of diquat accumulating within fish or within the food chain.
The introduction of diquat led to a variety of new herbicide uses and new options for the farmer. Diquat was found to be highly effective in controlling common weeds, grasses, and brushwood species. Diquat has been used for the destruction of potato haulms, as a preharvest desiccant of seed crops, and as an effective inhibitor of algal growth. Since the herbicide has no residual activity, it can be used to control weeds before sowing a new crop or before the emergence of the new crop. Because woody bark tissue is unaffected by diquat, it has been possible to use the material to control unwanted vegetation without damaging established plants.
Diquat has been used in orchards, vineyards, plantation crops such as banana, sugarcane, and forests, and as a ground spray for crops. Because diquat can control unwanted vegetation on steep and sloping land, it can remove vegetation without the risk of soil erosion that is associated with traditional tillage methods. The use of a herbicide such as diquat also decreases the tillage and labor costs for farming a given area. Applications of diquat increase the development and yield of crops, which no longer have to compete for resources with weeds; the chemical has also been used to kill the foliage of root and seed crops to facilitate harvest.
Diquat has been used to control the flowering of sugarcane stalks, thereby increasing sugar yields, and to desiccate sugarcane before burning, thus effectively decreasing the amount of trash and again increasing the sugar concentration. Low concentrations of diquat applied to olive trees have been found to facilitate the abscission of fruits, and it has been used to control the growth of raspberry shoots and to aid in mechanical harvest. Diquat can also be used to desiccate crops such as grasses, clover, and alfalfa in preparation for long-term storage.
Initially, there was some fear that the lack of residual control of plant materials would be a major disadvantage, but it was found that diquat used in conjunction with slow-acting herbicides with residual activity resulted in total vegetation control. The introduction of diquat allowed growers to minimize both tillage and fertilizer use while maximizing the production of crops, forests, and pastures. Because diquat was nontoxic to the environment, it was a welcome addition to the arsenal of herbicides. In the United States, the EPA required drinking water to be tested for diquat, setting a maximum contaminant level of 20 parts per billion (ppb), and requiring water suppliers to monitor diquat if its levels rose above 0.4 ppb. A simple and effective decontamination treatment is effected with the use of granular activated charcoal filtering.
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Audus, L. J. Herbicides. New York: Academic Press, 1976. Presents a survey of various herbicides with many details on the action and effects of each material. An excellent reference for the student of weed control, crop management, or toxicology. -
Kearney, P. C., and D. D. Kaufman. Herbicides: Chemistry, Degradation, and Mode of Action. Vol. 2. New York: Marcel Dekker, 1976. Reviews research on the chemistry, degradation, and mode of action of specific groups of herbicides. Not recommended for students without a chemistry background. -
Monaco, Thomas J., Stephen C. Weller, and Floyd M. Ashton. Weed Science: Principles and Practices. 4th ed. New York: Wiley, 2002. Comprehensive discussion of the effects of weeds, methods for their control and elimination, and the role, function, and effects of herbicides. Bibliographic references. -
Pasi, Aurelio. The Toxicology of Paraquat, Diquat, and Morfamquat. Bern, Switzerland: Hans Huber, 1978. A detailed and thorough examination of the environmental effects of applications of diquat and related herbicides. Recommended for advanced students. -
Summers, L. A. The Bipyridinium Herbicides. New York: Academic Press, 1980. A thorough review of the history, use, toxicology, and mode of action of diquat and related herbicides. Excellent for the serious student. -
World Health Organization. Environmental Health Criteria 39: Paraquat and Diquat. Geneva, Switzerland: Author, 1984. Evaluation of the potential hazards of diquat to the environment and human health. Excellent reference.
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