Evaluation of In Vitro Rumen Fermentation and Methane Emission Using Cashew Nut Shell Liquid as a Mitigation Agent

This study evaluated the effects of different inclusion levels of cashew nut shell liquid (CNSL) on in vitro rumen fermentation and methane production to explore its potential as a methane-mitigating feed additive. An experimental diet (70% commercial concentrate, 30% timothy hay) was incubated with rumen fluid mixed with buffer for 48 h in vitro, with CNSL added at 0%, 0.02%, 0.2%, 1%, or 2% of substrate dry matter. Total gas production was measured throughout incubation, and gas samples were analyzed by gas chromatography for methane content. In vitro dry matter digestibility (IVDMD) was determined after fermentation, and fermentation end-products including volatile fatty acid (VFA) profiles and ammonia nitrogen (NH₃-N) concentration were analyzed. Increasing CNSL levels led to a linear reduction in cumulative gas production (with significantly lower gas volume at 2% CNSL, particularly after 36–48 h; p<0.05). Methane production was dose-dependently suppressed: the proportion of methane in total gas decreased linearly as CNSL inclusion increased, and methane yield per unit substrate was reduced by 20% at the 2% CNSL level compared to the control. CNSL inclusion had no significant effect on IVDMD, NH₃-N, or total VFA production. However, higher CNSL shifted the VFA profile by decreasing the acetate proportion while increasing the propionate proportion, resulting in a lower acetate-propionate ratio (with no clear effect on butyrate). In conclusion, CNSL supplementation reduced ruminal methane emissions without adverse effects on nutrient digestibility or overall fermentation, indicating that CNSL is a promising natural feed additive for methane mitigation in ruminant.