Michaelis–Menten kinetics predict the rate of SF6 release from permeation tubes used to estimate methane emissions from ruminants

The sulphur hexafluoride (SF6) technique used to estimate methane emissions from ruminants involves placement of a permeation tube into the reticulo-rumen of animals. The permeation tube releases a trace amount of SF6 across a polytetrafluoroethylene membrane. The animals emit in exhaled breath enteric methane along with a trace amount of the SF6 gas enabling the rate of methane emission to be calculated from the ratio of methane to SF6. The SF6 release rate from individual permeation tubes is generally determined over a 60–90 day period immediately prior to their dosing into the reticulo-rumen of experimental animals. However, the release rate of SF6 from permeation tubes slowly declines over time. This decline can result in substantial error in calculated methane emissions unless gas measurements are made within 30 days of tube dosing or a correction is applied to the pre-determined rate of SF6 release. The research presented here shows that the declining rate of SF6 release can be described by Michaelis–Menten kinetics for up to 800 days, that Michaelis–Menten parameters can be predicted from the initial mass and loss rate of SF6, and that Michaelis–Menten parameters thus determined can be used to accurately predict the future release rate of SF6 from permeation tubes for at least 1 year. The two important implications of this research are that application of Michaelis–Menten kinetics to predict SF6 release rates can: (1) increase the accuracy of estimation of methane emissions from ruminants, and (2) extend the period during which estimates of methane emissions can be accurately estimated by the SF6 technique to 1 year after insertion of permeation tubes into the reticulo-rumen.