Chlorsulfuron persistence and response of legumes in an alkaline soil

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Abstract
Chlorsulfuron was applied to a field experiment on an alkaline soil, pH 8.0 (1:1, water:soil), in June 1983. The chlorsulfuron concentration in the 125 g a.i./ha rate was monitored, using the response of alfalfa roots in the growth chamber, for 28 months after application. Estimated MT50, MT90, and MT99 (months to 50, 90, and 99% dissipation) was 0.22, 8.4, and 24.8, respectively. From the rate of dissipation at the 125 g a.i./ha rate, chlorsulfuron concentrations following a 25 g a.i./ha application of 0.23, 0.04, and 0.01 ng/g were estimated for 2, 3, and 4 years after application, respectively. Residues from the 25 g/ha rate were toxic to alfalfa in the field until 4 years after application or until >99% of the chlorsulfuron had dissipated. The concentration of chlorsulfuron that caused a 50% reduction in the root weight of alfalfa (GR50) in 21 days in the growth chamber was similar to the GR50 for chlorsulfuron incorporated into the surface 10 cm of soil in the field using alfalfa and lentil top growth as the response indicators. Chlorsulfuron reduced alfalfa and lentil growth at concentrations of <0.1 ng/g in both the field and growth chamber.