Effects of cotton straw biochar on methane emissions using rumen simulations

Cotton stalk valorization is limited by high lignification, resulting in waste and environmental issues. Biochar, a pyrolysis-derived material, shows promise in reducing ruminant methane emissions. Aims: This study investigates the effects of cotton stalk biochar (CSB) as a feed additive on rumen fermentation, methane emissions, microbial communities, and nutrient degradation in dairy cows.

Cotton stalks were air-dried, cut into 2-3 cm segments, oven-dried at 105°C for 6 h, ground through an 80-mesh sieve, and pyrolyzed under N2 (0.5 L/min) at 13.3°C/min to 400°C with a 3 h hold to produce CSB. CSB was added to diets at 0%, 3%, and 6% levels. Each treatment was conducted in triplicate, with two fermentation bottles for each replicate. Rumen fermentation was simulated using an in vitro static culture method with rumen fluid from Holstein dairy cows. Fermentation parameters, gas production, nutrient disappearance, microbial populations, and metabolites were analyzed.

CSB addition increased rumen pH and significantly reduced total gas, CH4, CO2, and H2 production (P < 0.05). Total volatile fatty acid concentration decreased with increasing CSB levels. However, no significant changes were observed in microbial diversity or total DNA copy numbers of bacteria and methanogens. Metabolomics revealed that CSB altered key metabolites involved in amino acid (such as citrulline) and fatty acid metabolism (such as pyruvic acid and undecanoic acid). As CSB levels increased, cellulase activity decreased, thereby inhibiting dry matter and fiber loss rates and further reducing gas production. Conclusions: In conclusion, the addition of 3% or 6% biochar to the feed of cows was able to minimise ruminal gas production. However, it may act by inhibiting the ruminal fermentation (NDF degradation) pathway.

The addition of 3% CSB can reduce in vitro gas production in the rumen of dairy cows.