Potential impact of low levels of chlorsulfuron and other herbicides on growth and yield of nontarget plants

The influence of low application rates of chlorsulfuron on the growth and reproduction of four taxonomically diverse plant species (canola, smartweed, soybean, and sunflower) were examined. Exposures examined ranged from 1 × 10−3 to 8 × 10−3 of the recommended field rates for cereal crops and were approximately 1,000 times less than the highest exposure recommended by the U.S. Environmental Protection Agency. Each species received a single application at one of three different stages of reproductive development. Effects were determined by measuring the height and yield of mature plants. The comparative effects of four different herbicides (atrazine, chlorsulfuron, glyphosate, and 2,4-D) were determined in the same manner by exposing each test species to a single low dose at one of three critical stages of reproductive development. Chlorsulfuron reduced the yield of all plants tested, with the amount of reduction depending on the time and rate of application. Most noteworthy was its influence on canola and soybean, in which at critical stages in development, applications of 9.2 × 10−5 and 1.8 × 10−4 kg/ha, respectively, reduced seed yields (dry wt) to 8 and 1% of those of controls without causing a significant change in vegetative growth. These low application rates are within the range of reported herbicide drift levels and suggest that chlorsulfuron may cause severe reduction in the yields of some nontarget crops if they are subjected to exposure at critical stages of development. Application of other herbicides at comparable rates and stages of plant development had no influence on either canola or soybean.