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.
Felippe et al. (Thu,) studied this question.