Atrazine dissipation and soybean phytotoxicity following burndown applications of atrazine + mesotrione + triclopyr for Conyza spp. management

This study aimed to evaluate the effectiveness of (atrazine + mesotrione) + triclopyr on Conyza spp. at different intervals between application and soybean sowing, as well as the phytotoxic effects of residual atrazine on soybeans. The field experiment, conducted in a randomized block design with laboratory analyses, included Conyza spp. infestations. The herbicide application intervals were 60, 45, 30, 15, and 0 days before soybean sowing (DBS), with a sequential application of ammonium glufosinate 15 days after the initial treatment. Climatic conditions and the soil matric potential were monitored. The control effectiveness at different Conyza heights and germination flows was assessed visually. Atrazine concentrations in the soil were analyzed using liquid chromatography with diode array detection. For plants in advanced phenological stages (15–20 and 20–25 cm), longer intervals were required to achieve control above 80%. Rainfall and increased soil matric potential accelerated Conyza spp. control, regardless of plant height. At 0 DBS, atrazine concentrations near 300 and 100 g a.i. ha⁻¹ were observed at 0 and 10 days post-application, respectively, with 100 mm of accumulated rainfall, resulting in low phytotoxicity in soybeans. The concentration of atrazine in the soil gradually decreased over time, and higher rainfall volumes after herbicide application accelerated its dissipation, reducing its residual concentration and potentially shortening the plant-back interval required between herbicide application and soybean sowing to avoid phytotoxic effects.