Enteric methane emissions from male buffaloes in the eastern Amazon: effects of herd age structure using IPCC Tier 1 and Tier 2 approaches

Abstract Buffalo production in Pará State, eastern Amazon, Brazil, is largely based on extensive pasture systems, in which herd age structure, animal performance, and retention time can influence accumulated enteric methane emissions. In this study, we estimated enteric CH₄ emissions from male buffaloes from 2006 to 2025 using the Intergovernmental Panel on Climate Change Tier 1 and Tier 2 approaches. Herd data stratified by age class were obtained from the Agriculture and Livestock Defense Agency of Pará State. Tier 1 estimates used a fixed emission factor of 68 kg CH₄ animal⁻¹ year⁻¹, whereas Tier 2 estimates used age-specific emission factors derived from animal weight, weight gain, feed intake, digestibility, and energy requirements. Statistical analyses included age-class contribution, Pearson correlation, principal component analysis, hierarchical clustering, and cluster projection. Tier 2 emission factors increased with age, from 15.65 kg CH₄ animal⁻¹ year⁻¹ in animals aged 4–12 months to 58.11 kg CH₄ animal⁻¹ year⁻¹ in animals older than 36 months. Animals older than 36 months accounted for the highest mean contribution to total Tier 2 emissions, reaching 38.0%. The years 2010, 2011, and 2021 showed the strongest influence of adult animal retention on total emissions. The first two principal components explained 91.3% of the total variance and separated 2010 and 2011 as years with atypical adult retention. These results demonstrate that emissions were driven by herd size and age-class composition. Improving pasture quality, nutritional management, zootechnical control, and slaughter planning may lower accumulated CH₄ emissions in Amazonian buffalo systems under regional production conditions.