Functional Prediction of Bacteria–Enzyme Co-Regulation on Rapeseed Straw Silage: Fermentation Quality and Fiber Degradation

This study utilized rapeseed straw as the raw material and employed a completely randomized design with four treatments: a distilled water control (CK), individual supplementation of Lactiplantibacillus plantarum (1.0 × 106 CFU/g fresh weight) (Lp), individual supplementation of xylanase (50,000 U/g fresh weight) (XY), and a combined bacterium–enzyme treatment (XYLp). Each treatment was replicated five times, vacuum-sealed, and fermented at 25 °C for 60 days to systematically evaluate the effects of different treatments on the fermentation quality, nutritional composition, and microbial community structure of rapeseed straw silage. The results demonstrated that, compared with the CK group, all additive treatments significantly decreased pH and increased lactic acid (LA) content (p < 0.05). Among them, the Lp group exhibited the lowest pH value (4.27), which was significantly lower than all other treatments except XYLp (p < 0.05). Both the Lp and XYLp groups showed significantly higher LA content than the other groups (p < 0.05). Crude protein (CP) content was significantly higher in all additive treatments than in the CK group (p < 0.05). The XYLp group exhibited the most substantial fiber degradation, with acid detergent fiber (ADF) and neutral detergent fiber (NDF) contents being significantly lower than CK and reaching the lowest values among all treatments (p < 0.05). Both the XY and XYLp groups showed significantly lower hemicellulose and holocellulose contents compared to the CK and Lp groups (p < 0.05). Microbial community analysis revealed that the synergistic bacterium–enzyme treatment significantly enriched fibrolytic genera, including Kosakonia and Pediococcus, and upregulated the expression of key fibrolytic enzymes such as cellulase (EC: 3.2.1.4), β-glucosidase (EC: 3.2.1.21), and endo-1,4-β-xylanase (EC: 3.2.1.8). Functional prediction further indicated that the bacterial–enzyme synergy enhanced fibrous structure degradation and fermentable substrate release by activating carbohydrate metabolism pathways and bacterial secretion systems. These findings suggest that the combined application of Lactiplantibacillus plantarum and xylanase has the potential to be a promising strategy for enhancing fiber degradation and overall fermentation quality in rapeseed straw silage.