Evaluation of Nutritional Value and Rumen Degradation Rate of Six Unconventional Feeds Using In Vitro and In Situ Methods

Objective: This study systematically evaluated the nutritional compositions and bioactive compounds of six unconventional feed resources (Pepper residue (PR), Grape marc (MC), Pepper straw (PS), Lycium barbarum branches and leaves (LBBL), Licorice straw (LS), and Cyperus esculentus leaves (CES)). It also assessed the rumen degradability and rumen fermentation characteristics at different substitution levels through in vitro and in situ methods, to explore their potential application in sheep diets. Methods: Samples were analyzed considering nutrient composition, amino acids, polyunsaturated fatty acids (PUFAs), and bioactive compounds. In situ degradation was measured using rumen-fistulated sheep, and in vitro batch fermentation culture was conducted at varying substitution levels (0–100%) to measure gas production, pH, VFAs, NH3-N, and microbial crude protein (MCP). Results: The six unconventional feed resources showed significant differences in nutrient composition, bioactive compounds, and fermentation performance. Crude protein (CP) ranged from 4.45% to 15.76%, with LS highest in total amino acids. LBBL contained 4.24 g/kg Lycium barbarum polysaccharides, LS had 9.24 g/kg liquiritin, GM was richest in proanthocyanidins, and PS had more capsaicin than PR. PR exhibited the highest DM degradation (74.77%, p < 0.001), followed by LS; CEL was lowest. PR and LS also had the highest CP degradation. In vitro fermentation revealed significant differences in fermentation characteristics among the six feeds. At 100% replacement, PR and LS exhibited high cumulative gas production, elevated MCP concentrations, and total VFAs of 54.41 and 64.02 mmol/L (p < 0.001), respectively. At 25% replacement, GM and CEL achieved high concentrations of VFAs and maintained MCP levels of 27.84 and 31.57 mg/dL (p < 0.001). PS reached its maximum total VFAs and MCP at 50% replacement, while LBBL reached 64.90 mmol/L total VFAs and 32.63 mg/dL MCP at 75% replacement. Conclusions: Nutrient composition and degradation kinetics varied significantly among substrates. PR had the highest DM degradability, while CEL had the lowest. PR and LS maintained stable fermentation at 100% substitution. GM and CEL were most effective at 25%; PS at 50%; and LBBL at 75% substitution levels.