Northern Prairie Wildlife Research Center
Monsanto, The Agricultural Group, 800 North Lindbergh Boulevard, St. Louis, MO 63167
Herbicides are one of the integrated pest management tools available for vegetation control and management in wetland restoration projects. However, many wetland managers are reluctant to use herbicides because of concerns about the impact on the entire ecosystem. This paper provides information on the use, environmental impact, and fate of glyphosate herbicide in aquatic and wetland ecosystems.
Glyphosate is formulated as the isopropylamine salt and sold for aquatic weed control under the tradename ®Rodeo Aquatic Herbicide. The labeled sites include all bodies of fresh and brackish water, including estuaries. Glyphosate is a broad-spectrum foliar applied-herbicide that works by being absorbed through green tissue. When desirable plants are present, selective application methods such as directed sprays, wipers, cut stem treatments, and direct injections may be used.
The fate of glyphosate that is not absorbed by plants is dominated by the fact that it is strongly adsorbed on most types of soil or sediment particles. The amount of glyphosate adsorbed correlates with the amount of vacant phosphate binding sites. In clay soils, the high levels of metallic cations also increase the amount of glyphosate adsorbed. In contrast, the pH and percent organic matter have no significant effect on the amount of glyphosate that is adsorbed on mineral soils. In aquatic environments, adsorption on soil and sediments is a major route of initial dissipation in water. Adsorption of glyphosate by soil and sediment particles results in the lack of residual herbicidal activity as well as an extremely low leaching potential.
Glyphosate is rapidly and extensively degraded in soil, under both aerobic and anaerobic conditions, by indigenous soil microflora. In laboratory studies, the half-lives for glyphosate in various soils under aerobic conditions are typically less than 25 days, but ranged from 1.85 to 130 days. In natural waters, half-lives ranged from 6.5 to approximately 21 days. In field studies, measured half-lives average less than 60 days in soils and 1.5 to 14 days in water. The rate of glyphosate dissipation is thought to be related to the microbial composition and the extent of binding to the soil. Studies show that glyphosate does not accumulate in the soil following multiple applications.
The principal soil metabolite of glyphosate degradation is aminomethylphosphonic acid (AMPA). AMPA is also degraded by soil microflora. The final end-products of glyphosate degradation are carbon dioxide, water, nitrogen, and phosphate. When glyphosate is burned in air, the decomposition products are carbon ash (28%), water (25%), and acetonitrile (4%), with the remaining 43% coming off as carbon dioxide and phosphorus pentoxide.
Extensive research has shown that when glyphosate is used according to labeled directions, there is no significant direct environmental impact on mammals, birds, invertebrates, or fish, and no bioaccumulation in the food chain. The primary mode of action of glyphosate is the blocking of aromatic amino acid synthesis, a synthesis process that only occurs in plants.