Opposite effects of mixtures of commercial formulations of glyphosate with auxinic herbicides on the ten spotted live-bearer fish Cnesterodon decemmaculatus (Pisces, Poeciliidae)

We analyzed the acute toxicity of the 48% glyphosate (GLY)-based Credit^®, the 57.71% dicamba (DIC)-based Kamba^®, and the 83.5% 2,4-dichlorophenoxyacetic acid (2,4-D)-based Weedar^® Full, alone and as mixtures on the fish Cnesterodon decemmaculatus. Mortality revealed the LC_{50 96h} values of 91.73 mg L^-1 (range: 86.80-98.00 mg L^-1), 1401.57 mg L^-1 (range: 1243.78-1527.35) and 678.04 mg L^-1 (range: 639.35-718.04 mg L^-1) for GLY, DIC and 2,4-D, respectively. Mean values for the toxic unit (TU) that induced 50% mortality (TU_{50 96h}) of fish exposed to equitoxic mixtures were 1.67 (range: 1.65-1.69) for Credit^® and Kamba^® and 1.28 (range: 1.20-1.36) for Credit^® and Weedar^® Full suggesting that both mixtures are antagonic. Non-equitoxic combinations demonstrated an antagonistic interaction of herbicides Credit^® and Kamba^®, whereas a synergistic effect was observed for Credit^® and Weedar^® Full formulations. GLY and DIC as a mixture demonstrated lower toxicity on non-target species compared to GLY and 2,4-D in combination, at least for C. decemmaculatus, leading to the conclusion that the former combination could be strongly recommended in further agricultural practices.