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

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.