How they work
This technique was devised to answer the question ‘How much methane does an animal grazing freely in a field produce over a given period (usually one 24-hour feeding cycle)?’ Sulphur hexafluoride (SF6) is an inert gas that is easy to detect even in minute amounts. In experiments using this technique, a ‘permeation tube’ containing SF6 is inserted into an animal’s rumen via the mouth. This tube will slowly release SF6 over time at a predetermined rate and the released gas will be emitted via the mouth. The animal is fitted with a lightweight ‘yoke’, which carries an air-evacuated canister that slowly draws air at a steady rate from near the animal’s nostrils.
When the animal belches, it releases both SF6 and methane from its nostrils, and some of this is sucked into the canister (along with air surrounding the animal). Canisters are changed daily so that a series of repeat 24 hour measurements can be obtained. The air accumulatedin the canister is analysed later to determine the ratio of SF6 to methane in the sample.
Since the amount of SF6 that is released from the permeation tube over a given period is known, scientists can use the measured ratio of SF6 to methane to calculate how much methane the animal has released over the same period.
Why do it
Measurements are made under realistic conditions and can continue for longer periods of time.
Challenges
Unlike respiration chambers, there is no easy way to measure exactly what the animals are eating while grazing. Feed intake can be inferred from the animal’s size and weight gain/milk production, or from dung collection, but these are subject to large errors. The SF6 tracer technique is also labour intensive: gas collection canisters are generally changed every 24 hours, for about five to eight days.
Overall, the technique is not as accurate as respiration chambers.
Read more
Methane as a greenhouse gas
NZAGRC-PGgRc methane research programme
Guidelines for use of sulphur hexafluoride (SF6) tracer technique (external website)