Reliability of the sulfur hexafluoride tracer technique for methane emission measurement from individual animals: an overview

Measurements of enteric methane (CH4) emissions from individual animals have traditionally been made with indirect calorimetry techniques, which are both accurate and reliable. However, the expense and need for animal training and the extent to which calorimetric results can be extrapolated to free-ranging animals have been questioned and stimulated the development of the sulfur hexafluoride (SF6) tracer technique. The tracer technique is now widely used in New Zealand and many other countries for CH4 emission measurements on grazing and pen-fed cattle, sheep, deer and alpacas. Few studies with cattle and sheep have examined the validity of the SF6 tracer technique. Most of these studies have concluded that estimations of CH4 emission by this technique do not differ from those of calorimetric techniques, though some exceptions have been reported. There is general agreement that the tracer technique is associated with large between-animal variability in the CH4 emission estimates from animals on the same diet, but it remains unknown whether this is due to the environment, housing conditions or the technique itself. High within-animal variability has also been reported from tracer CH4 measurements. There is growing evidence that CH4 emission estimates by the tracer technique are positively influenced by the permeation rate (PR) of the SF6 gas from permeation tubes and it has been suggested that fate of the tracer in the rumen rather than unrepresentative breath sample collection is the likely reason for the latter. It is concluded that although some issues related to the tracer technique need to be clarified, using a narrow range in PR and balancing of PR between treatments should be practised in order to overcome the relationship between PR and CH4 emission estimates.