Co-inoculation in soybean with Azospirillum brasilense and Bradyrhizobium spp. is a consolidated practice to ensure nitrogen supply and maximize productivity and sustainability. This study aimed to evaluate the agronomic efficiency of the co-inoculation in soybean, via seed or in-furrow, of doses of the mixed inoculant containing Bradyrhizobium spp. and A. brasilense in the same formulation. The experiments were conducted under field conditions in four different edaphoclimatic regions to compare seven treatments: Control without nitrogen (N) and inoculant (T1), 200 kg ha-1 of mineral N fertilizer (T2), 100 mL of standard liquid inoculant of Bradyrhizobium spp. (T3) in 50 kg-1 seeds, 100 mL (T4) or 200 mL (T5) of the mixed liquid inoculant in 50 kg-1 seeds, 200 mL (T6) or 300 mL (T7) ha-1 of the mixed inoculant applied in-furrow. Nodulation characteristics (number and mass of nodules) and carbon and N metabolism (dry mass, N content, and N accumulation in the aerial part) were evaluated at the phenological stage R2 (full flowering—there is an open flower at one of the two uppermost nodes). At maturity (R8—full maturity—95% of the pods have reached their full mature color), yield, the mass of 1,000 grains, N content in the grains, and grain N export were measured. The data were submitted to analysis of variance using the F test and comparison of means was performed using the Duncan test (5%). Under field conditions, the mixed inoculant at doses of 200 mL 50 kg-1 seed or 300 mL ha-1 applied in-furrow were equivalent or superior to the standard inoculant for the total number of nodules, N content in the grains, grain N export, productivity, and mass of 1,000 grains. The use of the mixed inoculant containing Bradyrhizobium spp. and A. brasilense in the same formulation meets the demand of grain N export and resulted in increases of 28%, 18% and 7.4% in soybean productivity compared with control without nitrogen, mineral N fertilizer and standard liquid inoculant of Bradyrhizobium spp. respectively.