Residue of Picloram in New Agricultural Areas: Impacts, Detection, and Mitigation Measures

The expansion of grain production into former pasture areas in Brazil, one of the biggest grain producers' countries worldwide, has intensified concerns regarding the residual effects of auxinic herbicides historically used for broadleaf weed control. Among these, picloram (4-amino-3,5,6-trichloropicolinic acid) stands out due to its high persistence and mobility in soil, posing a significant risk to sensitive rotational crops. This review discusses the implications of picloram use in pasture systems and its impact on the incorporation of these areas into soybean-based grain production systems. Owing to its low adsorption coefficient and moderate water solubility, picloram exhibits considerable leaching potential and a prolonged half-life, which may extend beyond 300 days depending on soil and climatic conditions. Its primary dissipation pathway occurs via aerobic microbial degradation, and reduced oxygen availability at deeper soil layers can further prolong its residual activity. Soybean, dry beans, and tomato are highly sensitive to picloram residues, with phytotoxic symptoms including epinasty, leaf deformation, root thickening, growth inhibition, and yield reduction. Even low soil concentrations have been associated with significant grain yield losses, characterizing a carryover effect that may persist for more than two growing seasons. Additionally, the herbicide may remain biologically active after passage through livestock digestive systems, raising concerns regarding manure contamination. Detection strategies include herbicide application history analysis, bioassays using sensitive indicator species, and chromatographic techniques, each presenting methodological limitation when used individually. Mitigation approaches include rational herbicide use, avoidance of picloram in areas intended for future grain production, phytoremediation strategies, and crop rotation with more tolerant species such as corn and sorghum. Given the projected increase in soybean cultivation in areas formerly under pasture, understanding picloram behavior in soil and its agronomic implications is essential to ensure sustainable land-use transition and prevent yield losses associated with herbicide carryover.