Prior to this, the release rate of SF6 from each permeation tube was determined via weekly gravimetric weighing over an 8-wk period to produce an 8-point regression curve (R2 >0.999). The mean release rate of SF6 from the permeation tubes at the start of the experiment was 5.40 (±0.82 SD) mg/d. To correct for the known decline in the rate of release of SF6 from the permeation tubes within the rumen during the course of the experiment, 10 surveillance tubes, with similar release rates as the tubes used in the experiment, were maintained at 39°C in an incubator and monitored weekly until 6 wk after the completion of the final measurement period. The release rate of SF6 from the surveillance tubes was found to decline by 0.15% per day, and this value was subsequently used to adjust the release rates for the experimental tubes during periods 2, 3, and 4, as described by Lassey et al. (2001).