Co-inoculation with Lactiplantibacillus plantarum and Bacillus amyloliquefaciens enhances the fermentation quality and in vitro ruminal digestibility of whole-plant millet silage

Whole-plant millet is a climate-resilient alternative forage requiring ensiling optimization. This study evaluated single and combined bacterial inoculants on the fermentation quality and in vitro ruminal degradability of millet silage. Forage harvested at the grain-filling stage was ensiled for 60 days with sterile water (Negative Control), Lactiplantibacillus plantarum (L), Lactobacillus mucosae (M), or their combinations with Bacillus amyloliquefaciens (LB, MB) at 1 × 10 6 colony-forming units/g fresh matter, followed by a 72-h in vitro rumen assay. Compared with the control, bacterial inoculation significantly reduced silage pH (4.876 vs. 4.683–4.788, P < 0.001) and increased lactic acid concentration (2.543 vs. 2.938–3.450% of dry matter, P < 0.001) after 60 d ensiling. Inoculated silages also showed lower ammonia nitrogen concentrations (4.731 vs. 3.868–4.055 mg/dL, P < 0.001) and reduced neutral detergent fiber contents (59.675 vs. 58.213–58.574% of dry matter, P < 0.001). Consequently, inoculated silages yielded higher in vitro dry matter and fiber digestibility, alongside elevated volatile fatty acid concentrations. Notably, co-inoculating L. plantarum and B. amyloliquefaciens exhibited greatest fermentation efficiency, showing the highest asymptotic gas production and average gas production rate (AGPR) among all treatments ( P < 0.05). While a transient reduction in the proportion of ruminal methane occurred during early incubation (24–48 h, P < 0.05), it was not sustained by 72 h. Overall, multispecies bacterial inoculation improves the fermentative efficiency and nutritional degradability of whole-plant millet silage, optimizing sustainable roughage resources.