SF6
The SF6 technique samples breath over 24 hours, whereas other techniques use spot samples of breath over periods of minutes throughout the day, so diurnal variation has to be
considered. The majority of CH4 (87-99%) is released by eructation (Blaxter and Joyce, 1963; Murray et al., 1976), which provides a clear signal for sample processing. Please note that the tracheostomy used in Murray et al. (1976) may have resulted in a higher percentage, but in both publications, it is clear that the majority of the CH4 is released via eructation. The SF6 tracer gas technique was developed in an attempt to measure CH4 emissions by animals without confinement in respiration chambers (Johnson et al., 1994). Air is sampled near the animal’s nostrils through a tube attached to a halter and connected to an evacuated canister worn around the animal’s neck or on its back. A capillary tube or orifice plate is used to restrict airflow through the tube so that the canister is between 50 and 70% full in approximately 24 hours. A permeation tube containing SF6 is placed into the rumen of each animal. The pre-determined release rate of SF6 is multiplied by the ratio of CH4 to SF6 concentrations in the canister to calculate CH4 emission rate.
Many research centres have used the SF6 technique with variations in design of sampling and collection equipment, permeation tubes, and gas analysis (Berndt et al., 2014). Reliable results depend on following standard protocols, with greatest variation coming from accuracy of determining SF6 release rate from permeation tubes and control of sampling rate. With capillary tubes, sampling rate decreases as pressure in the canister increases, whereas an orifice plate gives a steadier sampling rate over 24 hours (Deighton et al., 2014). A source of error that has not been evaluated is that animals might interact and share CH4 emissions when the sampling tube of one animal is near the head of another animal. There is good agreement between CH4 emissions measured by the SF6 technique and respiration chambers, although results from the SF6 technique are more variable (Grainger et al., 2007; Muñoz et al., 2012).
