Stratified application of seed treatment components reduces neonicotinoid phytotoxicity and maintains physiological quality of soybean seeds during storage

Industrial seed treatment protects soybean seeds against pests and diseases, but prolonged storage causes progressive physiological quality loss driven by phytotoxicity from neonicotinoid insecticides. To our knowledge, no prior research has evaluated whether stratified application, by applying components in ordered layers, reduces phytotoxicity while maintaining seed quality during storage. We evaluated 10 treatment processes combining fungicide, insecticides, polymer, and finishing powder in stratified or simultaneous sequences on soybean seed physiological quality, stored for 135 days at 20°C. A completely randomized 10 × 4 factorial design (10 processes × four storage periods: 0, 45, 90, and 135 days) with four replications was used. Physiological quality was assessed through germination in paper roll plus vermiculite, seedling emergence, accelerated aging, and modified accelerated aging in substrate. A phytotoxicity index was calculated for each test and analyzed via multivariate analysis and k-means clustering. Simultaneous application of all components (Mix) showed the highest phytotoxicity, reducing germination by 4 percentage points (pp) and vigor by up to 6 pp compared to stratified treatments. Treatments with fungicide in the first stage formed a low-phytotoxicity cluster, while Mix occupied an isolated high-phytotoxicity cluster. Seeds maintained viability for up to 90 days and vigor for up to 45 days under controlled storage. Stratified application, particularly with fungicide and polymer applied first, significantly reduces neonicotinoid phytotoxicity and represents a practical strategy to extend seed safety margins in the soybean seed production chain.