Assessing Uncertainty in the CO₂-Based Method for Evaluating Dietary Effects on Enteric Methane Emissions in...

The CO₂-based method is a high-throughput technique for quantifying methane (CH₄) emissions by combining predicted CO₂ emissions with the CH₄:CO₂ concentration ratio in exhaled air. When applying this method to evaluate dietary CH₄ mitigation, uncertainty in predicted CO₂ emissions remains a concern. Therefore, we aimed to quantify the impact of this uncertainty on the evaluation of dietary CH₄-reducing effects. A database of published dairy cow studies reporting CH₄ emissions was screened, yielding 36 control-treatment comparisons from 19 studies. For both the control and treatment records, CH₄ emissions were calculated using CO₂ emissions predicted by the two existing equations. Dietary CH₄-reducing effects were expressed as percentage reductions relative to the control. The agreement between the CH₄ reductions derived from the observed and predicted CH₄ emissions was evaluated. The mean bias was not significantly different from zero for either equation, indicating no substantial systematic bias. The standard deviation of the differences between the observed and predicted CH₄ reductions was around 3 percentage points. These results provide a quantitative basis for understanding the prediction uncertainty and suggest that the CO₂-based method can serve as a preliminary tool for evaluating the relative CH₄ mitigation when direct gas measurements are unavailable.