Interactions between Pre-Emergence Herbicides and Nematicides on the Soybean Growth and Nematode Population

In recent years, the combined pressure from nematodes and weeds has increased considerably. Consequently, the spray of mixed nematicides and pre-emergence herbicides has become a common practice in soybean fields. However, the interactions between these products are poorly understood. We hypothesize that additive, synergistic, or antagonistic effects may occur due to their simultaneous application. Therefore, this study aimed to investigate the interactions between nematicides and herbicides applied at soybean pre-emergence on the control of <i>Pratylenchus brachyurus</i> populations and the initial development of soybean plants. Three independent experiments were conducted, each corresponding to one nematicide treatment, and each experiment was repeated twice under similar field conditions. All experiments were carried out in a greenhouse under a 2 × 5 + 2 factorial design. The first factor consisted of the presence or absence of a nematicide. The second factor included five pre-emergence herbicides. Two additional control treatments were also included. Biometric variables of soybean plants and nematode population densities were evaluated. The results indicated that the combination of [flumioxazin + imazethapyr] increased nematode populations, whereas [flumioxazin + <i>S</i>-metolachlor] and [fomesafen + <i>S</i>-metolachlor] reduced nematode populations and enhanced soybean root biomass accumulation. The [flumioxazin + imazethapyr] mixture negatively affected the nematicidal activity of <i>Bacillus amyloliquefaciens</i> BV03, both when applied as a seed treatment and as a foliar spray. In contrast, the antagonistic effect of [flumioxazin + pyroxasulfone] on <i>B. amyloliquefaciens</i> BV03 was observed only when the nematicide was applied as a seed treatment. Some herbicides reduced <i>P. brachyurus</i> populations independently of nematicide presence, while others showed additive effects when combined with the nematicide. The interactions observed in this study underscore the importance of carefully selecting products within an integrated management context, considering not only their individual efficacy but also their combined effects on soil and plant health.