Solid-state fermentation of milk thistle meal with complex probiotics and cellulase as a sustainable cattle feed from Chinese herb residues

The huge milk thistle meal (MTM) production requires proper processing. The study utilized response surface methodology to optimize fermentation conditions and assessed complex probiotics and cellulase impacts on nutrients, fermentation, digestibility, and in vitro ruminal features of fermented MTM (FMTM) for advancing clean bioresource technologies and feed utilization. Optimal solid-state fermentation conditions for maximum neutral detergent fiber reduction (14.25 %) were: 1.0 g/kg complex probiotics, 7.1 g/kg enzyme, 38°C, 456.9 g/kg moisture, and 96 h. Under fermentation, the contents of crude protein (254.0 vs. 268.8 g/kg DM; P = 0.002), total amino acid (201.7 vs. 249.2 g/kg DM; P < 0.001), ammonia-N (9.92 vs. 19.2 g/kg TN; P < 0.001), and in situ degradation rates of DM (371.0 vs. 410.9 g/kg; P = 0.006) and crude protein (576.7 vs. 614.2 g/kg; P < 0.001) of FMTM were elevated. In vitro gas production experiments have shown that compared with TMR including MTM, TMR involved FMTM had lower methane concentration (200.40 vs. 182.54 g/kg; P = 0.095), higher ammonia-N concentration (40.06 vs. 44.18 mg/dL; P < 0.001), total volatile fatty acid production (79.72 vs. 83.63 mmol/L; P = 0.0031), and molar ratios of propionate (24.41 vs. 24.96 %; P = 0.0002) in the rumen. Moreover, TMR containing FMTM increased microbial diversity, and enrichment of norank_f__F082, Succiniclasticum, Christensenellaceae_R_7_group, Ruminococcus, NK4A214_group, and unclassified_f__Lachnospiraceae. In conclusion, solid-state fermentation using complex probiotics and cellulose enables sustainable MTM utilization, making it a high-quality protein source for ruminants and providing a reference for other Chinese herb residue applications.