Interference of herbicides in the association of the bacterium Azospirillum brasilense in soybeans and toxicity of herbicides to the bacterium

Seed inoculation with Azospirillum brasilense and Bradyrhizobium japonicum can increase soybean productivity through biological nitrogen fixation and phytohormone production, improving root development and nutrient and water uptake. However, post-emergence herbicides may negatively affect these bacteria. This study evaluated the effects of herbicides applied to soybean plants on A. brasilense and B. japonicum. A greenhouse experiment was conducted in a completely randomized design (CRD) using Enlist® soybean co-inoculated with both bacteria. Herbicide treatments included glyphosate, ammonium glufosinate, 2,4-D, and their mixtures, applied sequentially. Compared with the control without herbicide or inoculants, herbicide treatments increased the number of viable nodules. Glyphosate, glyphosate + glufosinate, and the triple mixture increased the number of grains and pods per plant. In parallel, laboratory assays with A. brasilense were conducted in a 3 × 5 factorial design, evaluating three herbicides at five doses (DC, 2DC, 1.5DC, ½DC, and ¼DC). These assays determined the minimum inhibitory concentration (MIC) and assessed indole-3-acetic acid (IAA) production under herbicide exposure alone and in mixtures. In vitro results showed growth inhibition by glyphosate and 2,4-D and reduced IAA production, especially under herbicide mixtures. Despite this sensitivity, co-inoculation did not impair nodulation or soybean productivity, indicating compatibility between herbicide application and microbial inoculation.