Methane measurements

Daily CH4 emissions of deer were calculated using the SF6 tracer technique. This technique was developed by Johnson et al. (1994) and applied for use in New Zealand (Lassey et al., 1997; Ulyatt et al., 1999). The SF6 technique utilises the dilution of SF6 in gases expired or eructed from the mouth and nose of an animal to calculate the emission of CH4. The method assumes firstly, that the SF6 gas is inert and as such there are no detrimental effects or interactions with substances in the animal or experimental environment. Secondly, the method assumes that the emission of SF6 exactly simulates that of CH4 and therefore the rate of dilution of the two gases are identical. This assumes that gases are eructated from the digestive tract by force and therefore dilution by turbulence is more important than dilution by molecular diffusion and that more than 95% of the gases produced by enteric fermentation are expelled through mouth and nose (Murray et al., 1976). Thirdly, it is assumed that the release of the SF6 gas from the permeation tube is at a constant and known rate.

Breath samples from the deer were continuously collected from around the mouth and nose for 24-hours periods over five consecutive days. To enable breath sampling, each deer wore a modified halter and a pre-evacuated polyvinyl chloride (PVC) yoke that was attached to a harness (Plate 2.1a & b). The halter had tubing attached to it, with one end resting above the nose and the other end connected to the yoke via a QuickConnect® valve. The flow of air into the yoke was controlled by a short length of small-diameter metal capillary tubing, situated on the halter. Yokes were changed each morning at approximately 8.30 am and the pressure was checked. Yokes were considered to have a successful breath sample when the pressure in the yoke was between

49 0.4 and 0.8 of atmospheric pressure. In addition, two background samples to determine ambient air concentrations of both SF6 and CH4 were collected daily. For each yoke a gas sample was extracted and analysed by gas chromatography (Hewlett Packard 5890 Series II) using flame ionisation and electron capture detectors to determine the concentration of CH4 and SF6 gases, respectively (Lassey et al., 1997). Accuracy and consistency of gas analysis was based on a calibration curve of three standard gas mixtures (New Zealand Institute of Water and Atmospheric Research), where concentrations of SF6 and CH4 ranged from 15 – 1000ppt and 2 – 200ppm, respectively. All three standards were run at the start and end of each day of sample analysis and the mid-range gas standard was run after every tenth experimental sample, to account for any drift in the detection of SF6 or CH4.

The SF6 permeation tubes were provided by New Zealand Institute of Water and Atmospheric Research. This is a brass tube around 30 mm in length that has a Swagelok® nut fitted on one end. The escape of SF6 from the permeation tube is restricted by a Teflon® membrane. A stainless steel frit and nylon washer were placed either side of the Teflon® membrane to protect it from the internal

pressure and distortion during the tightening of the Swagelok® nut. The

permeation tubes were charged at liquid nitrogen temperature with 0.8 to 0.9 grams (sheep) SF6 liquid.

The permeation tubes used in this experiment were all from the same batch and were filled with SF6 gas on the 5th February 2004. After charging with SF6 the permeation tubes were maintained at a 39°C dry air environment and individually weighed weekly for 8 weeks prior to placement in the animal. A linear regression was fitted to the weights of each tube to determine the SF6 release rate for each tube. Only those tubes that had a linear regression fit of R2 > 0.998 were used. The mean release rate of SF6 for all 20 deer at the start of the experiment was 1.24 ± 0.356 (SD) mg/day. Seven days prior to the first CH4 measurement, a permeation tube was inserted into the rumen of each deer via the mouth using an applicator gun.

Chapter 2: CH4 emissions from immature deer

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Total CH4 production (QCH4; g/day) for each animal in a 24 hour period is calculated from the mixing ratio (μmol/mol) of SF6 to CH4 gas concentrations in the breath sample and the background concentrations of CH4 and SF6 (CbCH4 and CbSF6). These concentrations are expressed in relation to SF6 gas (μmol/day (QSF6)) release from the permeation tubes per day where MW is the molecular weight of the gases (Equation 4). Methane production and yield were calculated from the mean CH4 production rate across the five-day collection period.

2.3.3.1 SF6 release rate correction

Permeation tubes were recovered from 13 deer after slaughter. Ten tubes were recovered shortly after the final CH4 measurement (October), and three after the second CH4 measurement (May). The three permeation tubes recovered in May included two from deer whose permeation tubes appeared to be emitting SF6 gas, and one from an animal with a permeation tube that did not appear to be emitting SF6 gas. This latter permeation tube appeared to be emitting SF6 gas when monitored in the laboratory after retrieval. Release rates of SF6 gas from the recovered permeation tubes were re-determined. This involved cleaning the tube, with water, and reweighing each tube weekly for at least 4 weeks. After retrieval from an animal, the regression of the SF6 release rates based on emissions rates before and after placement in the rumen was no longer linear and therefore a curvilinear linear regression was applied to determine the ‘new’ release rate of SF6 during the measurement periods (Lassey et al., 2001). Methane emissions calculated using the corrected SF6 release rate or the uncorrected release rates were compared. Seven permeation tubes were unable to be retrieved as these deer were retained for future experiments. For these tubes a mean correction factor based on the curvilinear regression curves was applied (Lassey et al., 2001) for each measurement period.

51 Plate 2.1 (a) & (b): Methane (CH4) collection equipment, halter and yoke, used to

collect breath samples from red deer during CH4 measurements, when deer are

standing (a) and grazing (b).

(a)

Chapter 2: CH4 emissions from immature deer

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