From carbon intake to atmospheric return: quantifying carbon recycling in dairy cows

Enteric CH₄ emissions from ruminants are commonly reported as absolute greenhouse gas outputs without explicitly considering the biogenic origin of the emitted carbon. This study evaluated carbon partitioning in commercial dairy systems by quantifying carbon intake, enteric CH₄ emissions, estimated CO₂ production, and carbon exported through milk in Holstein dairy cows. Data were obtained from 1301 cows distributed across 13 commercial dairy farms located in northern Spain. Methane emissions were measured using a nondispersive infrared detector integrated into automatic milking systems. Carbon intake was estimated from dietary organic matter intake, whereas respiratory carbon losses included CH₄ derived carbon and estimated CO₂ derived carbon using both a fixed CO₂:CH₄ ratio and an independent production based prediction equation. Across farms, carbon partitioning consistently showed residual carbon as the largest fraction of ingested carbon (approximately 55–78%), followed by milk carbon (15–24%), whereas carbon exhaled as CH₄ and CO₂ represented the smallest proportion (6–22%). Ratio based CO₂ estimates were consistently lower than production based estimates, with an average ratio of 0.39 ± 0.11 between methods. A positive association was observed between CH₄ intensity (g CH₄ kg⁻¹ energy corrected milk) and the proportion of carbon exhaled relative to intake, indicating that higher CH₄ intensity was associated with greater diversion of dietary carbon toward respiratory losses. These findings indicate that enteric CH₄ emissions should be interpreted within the broader framework of carbon partitioning and biogenic carbon cycling. The carbon contained in enteric CH₄ originates from atmospheric CO₂ previously fixed by plants and represents the redistribution of recently captured carbon rather than the introduction of new carbon into the atmospheric system. Integrating carbon balance approaches may contribute to a more comprehensive physiological interpretation of enteric CH₄ emissions in commercial dairy production systems.