Tolerance of Transgenic Soybean Cultivars to the Herbicide Picloram

Abstract The cultivation of transgenic soybean has expanded in Brazil, often in areas previously used for pasture, where auxin herbicides, such as picloram, are widely used. This herbicide is highly persistent in the soil, which can compromise subsequent sensitive crops, such as soybean. Thus, the present study evaluated the tolerance of transgenic soybean cultivars to picloram. Two experiments were conducted in a randomized complete block design, with four replicates, in a 2 × 10 factorial scheme, involving two cultivars (Olimpo IPRO and Tormenta CE) and 10 doses of picloram, ranging from 0 to 4.80 g a.i. ha–1. The evaluations included visual symptoms of injury (7 to 56 days after emergence-DAE), leaf area, and dry matter accumulation (leaves, stems, pods, roots, and total) at 30 and 60 DAE. The quantification of picloram residues in the soil was performed by UHPLC in the same periods. Both cultivars showed progressive symptoms of poisoning with increasing dose and exposure time to the compound, resulting in significant reductions in leaf area and dry matter. Detectable picloram residues remained in the soil up to 60 DAE at the highest doses applied (4.80 g a.i. ha–1). Although the Tormenta CE cultivar is tolerant to auxinic herbicides, it did not show tolerance to picloram, despite its belonging to this mode of action. These results highlight the importance of choosing the right cultivar in areas with a history of picloram application, to reduce the risks of injury and minimize adverse effects on soybean growth and reproductive development.