Fenugreek seed meal as a partial replacement for soybean meal: Dose-dependent effects on in vitro rumen fermentation and greenhouse gas production.

The use of alternative protein sources in ruminant diets has the potential to reduce feed costs and mitigate environmental impacts associated with conventional protein feeds. This study evaluated the effects of partially or completely replacing soybean meal (SBM) with graded levels of fenugreek seed meal (FSM; 0, 4.3, 8.6, 12.9, and 17.3% of DM) on in vitro ruminal fermentation, gas production, methane (CH4) and carbon dioxide (CO2) emissions, and nutrient degradability. FSM contained lower concentrations of essential amino acids (74.3 g/kg DM) than SBM (157 g/kg DM), whereas total polyphenols were higher in FSM (2014.9 µg/g) than SBM (1453.3 µg/g), particularly catechin, ellagic acid, methyl gallate, and rutin. Ruminal gas production kinetics were altered. The asymptotic gas volume (b) increased with 25% FSM inclusion (323.9 mL/g DM) relative to the control (300.7 mL/g DM; P = 0.005), but decreased at higher inclusion levels, with a significant quadratic effect (P = 0.014). The fractional gas production rate (c) and lag time were also affected (P ≤ 0.03), as was metabolizable energy, which peaked at FSM25 (P = 0.007). In vitro dry matter degradability and fiber degradation decreased linearly and quadratically at higher FSM levels (P < 0.05). Total volatile fatty acids, acetate, and propionate concentrations were higher at FSM25 compared to FSM0 but declined at greater FSM inclusion (P ≤ 0.037). Ruminal pH increased with higher FSM levels (P = 0.007). These results indicate that partial replacement of SBM with FSM (approximately 25% of SBM protein) can enhance ruminal fermentation and metabolizable energy, whereas higher inclusion rates reduce fiber degradability and alter fermentation end-products.