Dissipation of nicosulfuron and rimsulfuron in surface soil

Many factors influencing herbicide dissipation have been studied; however, the effect of the presence of one herbicide on another herbicide's dissipation rate has not been extensively investigated. Field studies were established in 1997 and 1998 on a Sequatchie silt loam in Knoxville, TN. Treatments applied to tilled, bare ground included nicosulfuron (0.046 kg ai ha^-1), rimsulfuron (0.046 kg ai ha^-1), nicosulfuron (0.046 kg ai ha^-1) + rimsulfuron (0.046 kg ai ha^-1), and an untreated control. Samples (0-8 cm) were collected from 0 to 31 days after treatment (DAT) in 1997 and 1998. Laboratory studies were also conducted using the same soil. Soil was fortified at 50 ppb (µg g^-1) nicosulfuron, rimsulfuron, and nicosulfuron + rimsulfuron. Samples for the laboratory study were removed from the incubator 0, 1, 2, 3, and 7 DAT and frozen until extraction. All samples were extracted twice with 90:10 (v:v) 0.1 M aqueous ammonium carbonate/ acetone for 20 minutes. Solid phase extraction C₁₈ and silica columns were used for sample cleanup and samples were analyzed using an HPLC. Neither rimsulfuron nor nicosulfuron dissipation was influenced by the presence of the other herbicide. Field studies in 1997 and 1998 determined that both herbicides alone and in mixture disappeared quickly. Rainfall within 12 hr of application in each year and a soil pH of 5.7 encouraged rapid degradation. In 1997, the half-life (DT₅₀; of nicosulfuron was 5.3 d and the DT₅₀ of rimsulfuron was 3.1 d. When the two herbicides were applied in combination, the DT₅₀ of nicosulfuron was 4.2 d and the rimsulfuron DT₅₀; was 3.5 d. In 1998, all DT₅₀ were < 2.2 d. Rapid degradation was observed in the soil fortification experiments with DT₅₀ for all treatments < 3.5 d.