Interference of herbicides in the association of the bacterium <i>Azospirillum brasilense</i> in soybeans and toxicity of herbicides to the bacterium

Seed inoculation with <i>Azospirillum brasilense</i> and <i>Bradyrhizobium japonicum</i> 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 <i>A. brasilense</i> and <i>B. japonicum</i>. 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 <i>A. brasilense</i> 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. <i>In vitro</i> 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.