Herbicide and additive impacts on Bradyrhizobium japonicum growth in solution

Abstract Plant biostimulants include beneficial fungi and bacteria, and are often applied to foliage to improve crop growth, yield, and/or crop quality. Crop improvements due to biostimulant addition may be modest; therefore, solo applications may not be economical or climate smart. However, biostimulants combined with other postemergence treatments, such as herbicides, may provide an alternative application method, if mixtures do not harm the living organism(s). The growth of Bradyrhizobium japonicum , as a biostimulant surrogate, was assessed in solutions of glyphosate [ N ‐(phosphonomethyl)glycine] and dicamba (3,6‐dichloro‐2‐methoxybenzoic acid), with and without common spray additives (ammonium sulfate [AMS] and nonionic surfactant) in laboratory studies over 72 h. Solution turbidity, using optical density as a surrogate of bacterial growth, was measured at 600 nm at 24, 48, and 72 h after inoculation, and colony forming units (CFUs) per milliliter were estimated. Growth was not detected in either the glyphosate or AMS solutions, most likely due to the low pH and high electrical conductivity of the solutions, respectively. When herbicides were mixed with a nonionic surfactant, CFUs per milliliter were about 25% greater than the positive control. These data suggest that mixing bacteria with postemergence herbicide + surfactants/additives combinations can hinder or maintain growth when preparing for agrochemical applications. Biostimulant type and the agrichemical combination(s) should be evaluated prior to tank mixing to determine if detrimental interactions occur. After application, an evaluation of the effectiveness of the biostimulant to the crop and efficacy of the agrichemical to the target organism should be conducted.