Optimizing Solid-State Mixed-Strain Fermentation of Forage Mulberry for Improved Nutritional Quality via Anti-Nutritional Factor Degradation

To improve the feeding value of forage mulberry as a non-grain feed, solid-state mixed fermentation with Lactobacillus plantarum and Pichia membranifaciens (1:1) was optimized via single-factor tests and an L9 (34) orthogonal design. The optimal conditions (7 days, 28 °C, 30% inoculum, 1:1.2 solid-to-water ratio) reduced crude fiber (CF) content by 44.60%, and increased crude protein (CP), crude fat (EE), and total amino acids (TAA) by 35.77%, 68.85%, and 40.85%, respectively (p < 0.01). Mechanistically, the organic acids produced by L. plantarum lowered the fermentation pH to 4.0–4.5. This pH change was likely associated with the observed 50.84% increase in yeast cellulase activity. Yeast metabolites appeared to promote the proliferation of lactic acid bacteria (LAB). This implies a potential synergistic interaction between the two strains under acidic conditions. The in vitro ruminal dry matter digestibility (DMD) and CP digestibility reached 72.30% and 78.60%, respectively. These values were significantly higher than those of the unfermented control (p < 0.01). Therefore, this pH-enzyme synergistic fermentation strategy can effectively degrade anti-nutritional components. It also improves the feeding nutritional quality of mulberry forage for animal production.