Acute toxicity of Rodeo� herbicide to Rio Grande silvery minnow as estimated by surrogate species: Plains minnow and fathead minnow

Abstract

Use of Rodeo® herbicide (53.8% glyphosate, active ingredient) to control vegetation in or along irrigation canals in the Rio Grande basin in New Mexico has become a concern because of potential for populations of a federally endangered species, the Rio Grande silvery minnow (Hybognathus amarus), to be exposed. To investigate the potential for adverse effects, toxicity of Rodeo® herbicide was estimated by conducting 96-h renewal-acute toxicity tests with a closely related species, plains minnow (Hybognathus placitus), and a standard laboratory animal, fathead minnow (Pimephales promelas). Rodeo® herbicide had no effect on survival of plains minnow or fathead minnow at concentrations as high as 1,000 mg/L; thus, no-observed-acute-effect concentrations (NOAEC) for both species were 1,000 mg/L. Direct estimates of toxicity of Rodeo® herbicide to Rio Grande silvery minnow were not possible because the species was not available for testing. However, the similarity of responses by plains minnow and fathead minnow suggests that 1,000 mg/L Rodeo® herbicide is an acceptable estimate of the 96-h NOAEC for Rio Grande silvery minnow.