On the importance of background sampling in applications of the SF6 tracer technique to determine ruminant methane emissions

Implementations of the sulphur hexafluoride (SF6) tracer technique to determine methane (CH4) emission rates from individual ruminant animals involve measuring levels of both CH4 and SF6 in background air. In well-ventilated settings, including grazing, background sampling is straightforward and the algebraic correction for background levels is then usually minor. In a recent paper in this journal (Vol. 170, p. 265-276), Williams et al. drew attention to the much more careful consideration that is needed for background sampling in experiments that use the SF6 tracer technique with housed animals when both CH4 and SF6 levels can build up unevenly within the housing. This note builds on that study to show specifically and rigorously: (a) what is meant by background air, and that background corrections to CH4 emission estimates are unaffected by the recycling of CH4 and SF6 through inhalation of self-exhaled gases; (b) that in studies of the role of various treatments on CH4 emission rates, the siting of background samplers can crucially impact on findings; and, in particular, (c) that reports of a possible dependence of estimated CH4 emission rates upon the rate of SF6 release in the rumen are called into question due to the sensitivity of those findings to the siting of background samplers.