A paper titled “Effects of cotton straw biochar on methane emissions using rumen simulations,” published in PLOS One by Shiqi Zhang, Yaqian Zhang, Yankun Zhao, Nan Zheng, and He Chen, explores a potential method for mitigating greenhouse gas emissions from dairy cows. The research focused on the valorization of cotton stalks—a high-lignin agricultural waste product—by converting them into cotton stalk biochar (CSB) and testing its efficacy as a feed additive. Using an in vitro static culture method with rumen fluid from Holstein dairy cows, the study simulated fermentation to investigate how two different inclusion levels, 3% and 6% of the dry matter feed content, would affect rumen parameters, gas production, nutrient degradation, and microbial metabolism.

The key finding of the study confirms that CSB can significantly reduce ruminal gas production, offering a promising strategy for methane mitigation in the dairy industry. Specifically, total gas production decreased by 6.4% in the 3% CSB group and by 12.9% in the 6% group. More importantly for climate mitigation, methane (CH4​) production decreased by 7.9% in the 3% group and by 10.9% in the 6% group, both significantly lower than the control group (0% CSB). Similarly, hydrogen (H₂​) and carbon dioxide (CO₂​) production also decreased in the CSB-supplemented groups.

However, the mechanism and side effects of this reduction are complex and highly dependent on the dosage. The research demonstrated that the reduction in gas production was not due to a direct negative effect on the microbial populations. In fact, CSB addition did not significantly change the total DNA copy numbers for either total bacteria or total methanogens, nor did it affect the microbial diversity. Instead, the study’s results suggest that CSB reduces methane production indirectly by inhibiting the rumen’s fermentation function.

This inhibition was evidenced by a notable dose-dependent decline in nutrient disappearance and enzymatic activity. The digestion of neutral detergent fiber (NDF) and the dry matter disappearance rate were both significantly lower in the 6% CSB group than in the 0% and 3% groups. Crucially, cellulase activity—the enzyme responsible for breaking down fiber—showed a significant dose-dependent decrease, with the 6% group exhibiting a large 32.1% decline compared to the control. The physical adsorption of substrates, enzymes, and microbes onto the high specific surface area of the biochar is believed to be the primary cause of this reduced enzymatic activity and subsequent impaired fiber degradation. Conversely, crude protein disappearance and protease activity (the enzyme that breaks down protein) were unaffected, suggesting that the biochar’s adverse effects are selective towards large molecules like cellulase or their substrates.

The 3% CSB inclusion level emerged as the most optimal and balanced dosage. At this level, gas production was effectively reduced without significantly affecting the disappearance rates of dry matter and crude protein. Furthermore, metabolomics analysis suggested that 3% CSB promoted amino acid and fatty acid metabolism, with key metabolites such as Pyruvic acid, Citrulline, and Undecanoic acid being altered, which could potentially enhance animal health. The negative consequences on digestion were primarily evident at the higher 6% inclusion level, where the significant reduction in fiber degradation raises concerns about potential negative impacts on animal productivity and weight gain over the long term.

In conclusion, this study validates the potential of using cotton straw biochar as a feed additive to mitigate methane emissions from dairy cows, successfully repurposing a problematic agricultural waste. However, it highlights the importance of optimal dosing to avoid compromising essential digestive functions. The 3% inclusion level achieved the goal of gas reduction while maintaining nutrient utilization, suggesting a practical path forward. Future research is needed to modify the biochar’s surface to reduce non-target adsorption and conduct long-term in vivo trials to confirm these results on animal health and productivity before widespread implementation.

Source: Zhang, S., Zhang, Y., Zhao, Y., Zheng, N., & Chen, H. (2025). Effects of cotton straw biochar on methane emissions using rumen simulations. PLOS One, 20(12), e0336804.