Mango Peel Fermented with Ethanol-Industry Yeast Waste and Urea Modulates In Vitro Rumen Fermentation, Nutrient Degradability, and Methane Concentration

This study evaluated mango peel co-fermented with fresh yeast waste from the ethanol industry and urea (YUFMP) as a ruminant supplement using the in vitro gas-production technique. A 2 × 4 factorial arrangement in a completely randomised design was used, with two roughage-to-concentrate (R:C) ratios (40:60 and 60:40; rice straw as roughage) and four YUFMP levels (0, 2%, 4%, and 6% of substrate dry matter, DM). Co-fermentation raised the crude protein of mango peel from 4.39% to 17.00% DM and reduced its total phenolic content from 100.30 to 73.49 mg GAE/g extract. The 40:60 ratio gave higher in vitro degradability of DM, organic matter, neutral detergent fibre (IVNDFD), and true DM (IVTD) than the 60:40 ratio at 12 and 24 h (p < 0.01), and higher total volatile fatty acids (VFA) at 24 h (65.56 vs. 57.16 mmol/L; p < 0.01). YUFMP increased asymptotic gas production from 87.85 to 100.47–102.58 mL/0.5 g DM (p < 0.01) and cumulative gas production at 12–96 h (p < 0.05), with linear and quadratic trends. At 24 h, IVNDFD and IVTD were highest at 6% YUFMP (47.90 and 69.30%; p < 0.05). The protozoal population declined linearly with YUFMP (12.75 to 9.00 × 105 cells/mL at 24 h; p < 0.01), as did CH4 concentration at 12 h (18.99 to 13.08%) and 24 h (23.21 to 14.93%; both p < 0.01), whereas absolute CH4 volume was unaffected (p > 0.05). YUFMP did not alter total VFA or VFA molar proportions at 24 h and reduced propionate linearly only at 12 h (p < 0.01), indicating that the lower CH4 concentration reflected dilution of CH4 within a larger volume of fermentation gas rather than a redirection of metabolic hydrogen towards propionate. Ruminal pH (6.87–7.04) and ammonia nitrogen were unaffected (p > 0.05). Inclusion at 2–4% DM is recommended for further in vivo evaluation.