Review: Enteric Methane Emissions Across Physiological Stages and Dietary NDF/NFC Ratios in Holstein Dairy Cattle-Implications for China's Carbon Mitigation.

Dairy cattle are a substantial contributor to global agricultural greenhouse gas emissions, primarily producing enteric methane through the ruminal anaerobic fermentation of dietary fiber. As China formally pledges to achieve carbon neutrality before 2060, accurately quantifying these emissions and developing localized mitigation strategies within the livestock sector has become a critical priority. Enteric methane emissions in dairy cattle are not a static physiological baseline; rather, they represent a highly dynamic phenotype profoundly influenced by an intricate network of physiological and environmental parameters. These include the animal's age, anatomical and ruminal development, parity, lactation stage, and the precise stoichiometric balance of dietary carbohydrates. This review synthesizes extensive experimental data to construct a robust, scientifically logical framework elucidating the profound physiological mechanisms that govern apparent methane emission parameters. Accordingly, this paper reviews our recent research on methane emissions from Holstein dairy cattle across various ages and lactation stages, including heifers, lactating cows, and dry cows. Furthermore, it extensively evaluates the modulation of methanogenesis under diets with varying neutral detergent fiber to non-fibrous carbohydrate (NDF/NFC) ratios, demonstrating that an increased NDF/NFC ratio is positively correlated with higher enteric methane production, yield, and intensity due to the promotion of acetate-type ruminal fermentation. Ultimately, this review aims to provide robust theoretical support for the accurate quantification of enteric methane emissions and the formulation of precision mitigation strategies tailored to specific physiological states.