On d 1 of each of the 4 measurement periods, each cow was restrained within a head-locking gate, at approximately 1400 h (before the p.m. milking), and fitted with a halter (to support the nose piece and CH4 sampling line) and a polyvinyl chloride (2.5-L) collection canister. The equipment used was described by Johnson et al. (2007), with the exception of the capillary tubing, which was modified as described below. Prior to exiting the gate, the sample line was connected to the collection canister (which had previously been evacuated to over 90 kPa) using a quick-connect fitting (Swagelok Co., Solon, OH), thus allowing a gas sample to be drawn up into the evacuated canister at a rate of between 0.6 to 0.7 mL/min (approximately). This flow rate was regulated via a length (approximately 5.0 cm) of capillary tubing (0.102 mm i.d.; Alltech Associates Applied Science Ltd., Lancashire, UK) that had been crimped at several points. Crimping was part of the calibration process necessary to achieve the correct flow rate, with flow rate measured using a digital flow meter (Cole-Parmer Instrument Co., Vernon Hills, IL). The gas sample drawn into the evacuated canister was collected from the area around the cow’s nostrils, and contained eructated gas (a mixture of normal atmospheric gases, SF6, and CH4).