Bacterial Community Dynamic Regulates Fermentation Quality and Mycotoxin Accumulation in Mulberry Silage Treated with Exogenous Lactic Acid Bacteria Inoculant and Cellulase Enzyme

The global shortage of high-quality protein feed resources continues to widen, and the development of high-value-added woody plants is a key strategy for alleviating feed shortage. The mulberry (Morus alba L.) is a recognized high-protein woody forage resource. However, the inconsistent quality of its natural silage and the unclear risk of mycotoxins represent the core bottlenecks limiting its widespread adoption as feed. Four treatments were set up in this study: (1) control; (2) lactic acid bacteria inoculant (LAB, Lactiplantibacillus plantarum); (3) cellulase enzyme (AC, Acremonium cellulolyticum); (4) a mixture of LAB + AC. After 60 days of ensiling, a systematic analysis was conducted to examine the effects of exogenous microbial inoculant and enzyme preparation on the fermentation quality, bacterial community, and mycotoxin in mulberry silages. Fresh mulberry exhibited a high crude protein content of 23% on a dry matter (DM) basis, making it a high-quality feed resource. Compared to the control, the addition of LAB and AC either alone or in combination, significantly improved (p < 0.001) the fermentation quality and safety of silages: lactic acid content increased from 0.85% DM to 1.41–2.03% DM; pH, ammonia nitrogen, and deoxynivalenol decreased from 4.85, 0.88% DM, and 3.92 μg/kg to 3.53–3.95, 0.40–0.55% DM, and 1.21–3.04 μg/kg, respectively. The combined LAB and AC treatment resulted in the most favorable fermentation performance of mulberry silage. Bacterial community analysis revealed that fresh mulberry exhibited high bacterial alpha diversity, with Gram-negative bacteria as the dominant bacterial community, and Sphingomonas roseiflava as a representative dominant species. After ensiling, bacterial alpha diversity decreased in all the silages. Furthermore, the Lactiplantibacillus plantarum eventually prevailed as the dominant bacteria and exhibiting the highest relative abundance in the LAB + AC-treated silage (57.23%). Bugbase functional prediction indicated that the proportion of potential pathogenic bacteria was significantly higher (p < 0.05) in fresh mulberry than silage. Thus, the synergistic action of LAB + AC treatment effectively optimized the ensiling fermentation process.