Synergistic mechanism of Lactiplantibacillus plantarum combined with cellulase on nutrient retention, fermentation quality and bacterial metabolic function of Hulunbuir native grass silage

Objective To investigate the effects of adding Lactiplantibacillus plantarum and cellulase, either individually or in combination, on the fermentation quality, bacterial community, and predicted metabolic functions of native grass silage from the cold pastoral zones of Hulunbuir, and to clarify their synergistic effects. Methods Fresh native grass was ensiled for 60 days under four treatments: no additive control (CK), single inoculation of L. plantarum (LAB), single addition of cellulase (CEL), and a mixture of L. plantarum plus cellulase (MIX). After ensiling, fermentation quality, conventional nutrient composition, bacterial community structure, and 16S-based predicted metabolic enzyme profiles were determined. Results Dry matter (DM) content ranged from 358 to 369 g/kg fresh matter across treatments after 60 d, with no significant differences among groups. Compared with CK, LAB only significantly increased lactic acid (LA) content (from 11.8 to 15.9 g/kg DM), while CEL mainly reduced neutral detergent fiber (NDF) and acid detergent fiber (ADF) contents. The combined treatment (MIX) demonstrated the greatest overall improvement in fermentation quality and nutrient preservation among the treatments tested: it achieved the highest crude protein (CP; 81.4 vs. 73.7 g/kg DM), water-soluble carbohydrate (WSC; 8.95 vs. 4.11 g/kg DM), LA (18.9 vs. 11.8 g/kg DM), and acetic acid (AA; 16.1 vs. 14.1 g/kg DM) contents, together with the lowest pH (4.01 vs. 4.38) and ammonia nitrogen (NH3-N; 1.81 vs. 5.85% total nitrogen). MIX also significantly enriched beneficial Bacillota and Lentilactobacillus, suppressed spoilage Clostridium, and showed the highest predicted abundances of carbohydrate-degrading and lactate-synthesizing enzyme gene families. Conclusion The combined addition of L. plantarum and cellulase achieved the best fermentation quality and nutrient preservation in Hulunbuir native grass silage among the treatments tested, associated with reshaped bacterial community structure and higher predicted abundances of fermentative metabolic enzymes.