The influence of breath concentration in the gas sample on the accuracy of methane to carbon dioxide ratio using the sniffer method in dairy cows.

The purpose of this study was to investigate the carbon dioxide (CO 2 ) concentration in the sampled gas to avoid low concentration of breath while measuring the methane (CH 4 )/CO 2 ratio using the sniffer method. This study also assessed the effect of selective elimination by applying the threshold of CO 2 concentration to the CH 4 /CO 2 ratio. The gas measurement in the automatic milking system was conducted with 26 multiparous Holstein cows using an electric fan to manipulate the CO 2 concentration in the sampled gas. Four different thresholds of the background-corrected CO 2 concentrations (0, 0.025, 0.05, and 0.1%) were applied to every 1-s value of the individual gas measurement. Subsequently, three different upper limits of the proportion of eliminated values (none, 0.5, and 0.33) were applied to the individual records per milking. The results showed that the sampled gas must contain more than 0.1% of the corrected CO 2 concentration to enable accurate calculation of the CH 4 /CO 2 ratio. It is recommended that at least half of the values in the data be larger than the threshold of the corrected CO 2 concentration for unbiased measurement of the CH 4 /CO 2 ratio.