ABSTRACT The CO 2 ‐based method is a high‐throughput technique for quantifying methane (CH 4 ) emissions by combining predicted CO 2 emissions with the CH 4 :CO 2 concentration ratio in exhaled air. When applying this method to evaluate dietary CH 4 mitigation, uncertainty in predicted CO 2 emissions remains a concern. Therefore, we aimed to quantify the impact of this uncertainty on the evaluation of dietary CH 4 ‐reducing effects. A database of published dairy cow studies reporting CH 4 emissions was screened, yielding 36 control–treatment comparisons from 19 studies. For both the control and treatment records, CH 4 emissions were calculated using CO 2 emissions predicted by the two existing equations. Dietary CH 4 ‐reducing effects were expressed as percentage reductions relative to the control. The agreement between the CH 4 reductions derived from the observed and predicted CH 4 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 CH 4 reductions was around 3 percentage points. These results provide a quantitative basis for understanding the prediction uncertainty and suggest that the CO 2 ‐based method can serve as a preliminary tool for evaluating the relative CH 4 mitigation when direct gas measurements are unavailable.
Oikawa et al. (Thu,) studied this question.
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