Ecotoxicological risk assessment for Roundup® herbicide

Abstract

Roundup® Herbicide (RU) and its active ingredient glyphosate have been extensively investigated in ecological toxicity studies to support registrations in various countries and also in many scientific investigations independent of regulatory considerations. The purpose of this study was to review the current state of knowledge on the ecological toxicity of Roundup and glyphosate and to consider this information in a comprehensive ecological risk assessment. A conservative hazard quotient method was used to evaluate risk. The hazard quotient (HQ) was calculated by dividing the maximum environmental exposure concentration derived from modeling or environmental monitoring data by the greatest level of Roundup or glyphosate found to have no effect on survival, growth, or reproduction of the most sensitive nontarget organisms. Roundup, which contains glyphosate and a polyethoxylated tallowamine surfactant (POEA), was used for acute HQ determination, because exposure to the total formulation was a reasonable assumption. Because of differential rates of dissipation, the individual components of the formulation (either glyphosate or the surfactant) were considered separately for chronic HQs. The risk assessment evaluated both terrestrial and aquatic uses of Roundup. Conclusions of the analysis are as follows. 1. Roundup® Herbicide contains the isopropylamine salt of glyphosate as the active ingredient and a surfactant (typically polyethoxylated tallowamine) to facilitate plant uptake of the active ingredient. Roundup® is a broad-spectrum herbicide and will control many types of herbacious and woody plants. Several formulations of Roundup® are used worldwide and may have different amounts of the components. 2. Glyphosate in the environment tends to bind tightly to soil and paniculate matter and is essentially unavailable to plants or other soil organisms. As a result, glyphosate has little activity in soil, and the herbicidal effects are limited to foliar contact. In addition, glyphosate is very water soluble and does not partition into animal fats. Consequently, glyphosate does not bioconcentrate in fish or other animals. Less is known about the environmental fates of surfactants, but rates of degradation appear to be similar to glyphosate. 3. The risk assessment described here considered terrestrial and aquatic uses of Roundup®. Acute risk characterization assumed that organisms were potentially exposed to the intact formulation, whereas chronic risk characterization considered potential exposure to the components of Roundup® (glyphosate and the surfactant). This approach allowed the acute assessment to consider acute effects of the surfactant that are greater than glyphosate alone for aquatic animals. 4. For terrestrial uses of Roundup® (agricultural, forestry, rights-of-way, residential, etc.), minimal acute and chronic risk was predicted for potentially exposed nontarget organisms. This conclusion is based on the conservative hazard quotient analysis that resulted in no HQ values greater than 1. The following taxa were evaluated using the hazard quotient method: aquatic microorganisms, aquatic macrophytes, aquatic invertebrates, warm water and cold water fish, amphibians (tadpoles), soil microorganisms, soil invertebrates, birds, and mammals. Honeybees and other beneficial arthropods and nontarget terrestrial plants were not evaluated using the hazard quotient method. 5. Honeybees are not affected by glyphosate formulations, either by ingestion or direct overspray, at maximum use rates. The majority of other beneficial arthropods are unaffected by Roundup®. Although screening tests under extreme exposure conditions indicate toxicity of glyphosate formulations to some beneficial arthropods at the maximum use rates, these effects were reduced or eliminated when more realistic exposure conditions were used. These data demonstrate minimal risk to beneficial arthropods in areas adjacent to treated fields. Within treated fields, vegetation changes resulting from herbicide use can lead to significant changes in beneficial arthropod populations. 6. Nontarget terrestrial plants in areas adjacent to the treated areas may be exposed to Roundup®. From ground applications, small amounts of herbicide may move downwind from treated areas to adjacent nontarget areas via spray drift. However, as glyphosate is not active in soil, potential herbicidal effects will be limited to only those plants very near the treated area that are in a sensitive growth stage at the time of treatment. Aerial applications can result in increased drift relative to ground applications, but recent technological advances have significantly reduced aerial spray drift. 7. Greater exposure of aquatic organisms to Roundup® is likely during use to control emergent aquatic macrophytes compared to terrestrial uses. Acute and chronic HQ values are less than 1.0 (minimal risk) for all taxa for direct addition of Roundup® to 2-m-deep water at the maximum use rate. Acute HQ values can approach or exceed 1.0 in shallow water (0.15 m). Examination of assumptions reveals that degradation, sorption, and interception by target vegetation of greater than 50% will mitigate the potential for effects in shallow waters. Even in shallow waters, the chronic HQ did not exceed 1. Use of Roundup® for aquatic habitat restoration can be safely carried out, but requires consideration of items such as application rate, depth of water, and vegetation coverage. © Springer-Verlag 2000.