Intercropping of Soybean and Common Millet—A Rational Way of Forage Biomass Quality Enhancement

Sustainable agriculture supports environmental protection, climate change mitigation, and forage security to meet the growing demands of livestock production. Given the critical role of macro- and microelements in animal health, diversified and balanced feed production is essential and can be achieved through the sustainable integration of legumes and cereals. This research evaluated the impact of soybean–common millet intercropping and biofertilizer application on the elemental composition and yield performance of forage biomass. Three intercropping patterns were tested: S1M1—alternating rows, S2M2—alternating two-row strips, and S2M4—alternating two-row soybean with four-row millet strips, alongside monoculture controls. The biofertilizer Coveron (BF) was also assessed. The S2M2 combination provided the highest land equivalent ratios for both fresh and dry biomass (1.10 and 1.12, respectively), despite a reduction in millet yield. Considering the elements, the S2M2 combination notably enhanced the accumulation of Ca and B (by 13.2% and 13.0%, respectively, compared to S1) in the soybean vegetative part and Cr and Mn in the reproductive part (by 53.5% and 17.1%, respectively). In contrast, sole soybean showed the highest P levels in both vegetative (3.45 g kg−1) and reproductive parts (4.56 g kg−1). Regarding Al, its accumulation was reduced in intercropped millet. The S1M1 combination increased Mg and S concentrations in both parts of millet biomass (up to 17.3% and 18.4% in the vegetative part, compared to M1). While BF generally had a limited impact on forage biomass yield and elemental accumulation, it increased Mg, P, and S concentrations in soybean pods, as well as concentrations of B, Mn, and Mo in the panicle, simultaneously decreasing P, Cr, and Zn concentrations in the vegetative part of millet. Accordingly, soybean–common millet intercropping in the S2M2 configuration offers a sustainable solution for efficient land utilization and element-enriched forage production.