Determining the Toxicity of Herbicides Using a Novel Method

A simple method to measure toxic chemicals has been used. This assay uses the bacterium Rhizobium meliloti as the indicator organism. This assay was used to measure the toxicity of 30 herbicides used on the New Mexico State University farms. One of the herbicides, Poast (sethoxydium), was found to be toxic at less than 10 ppm (mg l-1). Nine were found to be toxic at less that 100 ppm, thirteen to be toxic at 100 to 1,000 ppm and seven were found to be toxic at concentrations greater than 1,000 ppm. The toxicity using this test was compared with values from the manufacturers using animal tests and Daphnia tests. Three herbicides were tested for their stability when mixed with three soil types. Soil was taken from a cotton field, from an alfalfa field and from an uncultivated desert soil. Sethomydim was found to have a half life (that is the toxicity decreased by 50%) of 8.8 to 21.1 hours in the three soils. Glyphosate was found to have a half life of 2.4 to 7.8 days. Bromxylnil was found to be stable for at least 15 days, the toxicity decreased less than 50% in this interval. The assay can be used to determine the toxicity of herbicides and to follow the fate of herbicides in the soil.