Assessing the ecotoxicological effects of environmentally relevant concentrations of binary mixtures of 2,4 D-amine, glyphosate, and paraquat herbicides on monocotyledonous plants: An experimental approach with Sorghum saccharatum

In environmental situations, non-target plant communities are exposed to pesticide mixtures rather than single pesticides, and the effect of pesticides can either be additive, synergistic, or antagonistic. Data on the ecotoxicological end-points of herbicide mixtures on non-target terrestrial plants are scarce, limiting ecotoxicological risk assessment. This study assessed the phytotoxic effects of binary mixtures of environmentally relevant concentrations of glyphosate, 2,4-D–amine, and paraquat on the monocot, Sorghum saccharatum, using a composite experimental design to explore a wide range of mixture concentrations. After three days of incubation in darkness at 25 °C, glyphosate exhibited the highest root growth inhibition with an IC50 of 0.09 mg/L (0.035 mg/kg soil) and glyphosate was 41 and 3390 times more toxic than 2,4-D amine (IC50 = 3.7 mg/L/1.4 mg/kg soil) and paraquat (IC50 = 305.1 mg/L/118 mg/kg soil) respectively. All binary mixtures increased root growth elongation inhibition at environmentally relevant concentrations to levels much higher than could be explained by the additive effects of the individual mixture components. Effective mixture concentrations from 0.01 IC50 caused ≥70 % root growth inhibition, particularly glyphosate-containing mixtures (glyphosate-paraquat and glyphosate-2,4- D amine). At lower mixture concentrations (0.01 IC50), the paraquat - 2,4-D amine mixture effect was low but from 0.1 IC50 this mixture also exhibited higher than additive mixture effects on root growth. The Concentration Addition (CA) and Independent Action (IA) models grossly underestimated the mixture toxicity, giving a predicted IC50 of 1.16 Toxic Units (TU) compared to the observed IC50s of 0.0052 and 0.0054 TU respectively, underscoring the limitations of these models for predicting mixture effects of pesticides with dissimilar modes of action. The resulting model deviation ratios of 223.01 and 214.43 for CA and IA respectively, indicated synergism. The synergistic effects of herbicide mixtures have important implications for weed management.