Published online 9 August 2006
Published in J Environ Qual 35:1686-1691 (2006)
DOI: 10.2134/jeq2006.0054
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Assessment of the Sulfur Hexafluoride (SF6) Tracer Technique for Measuring Enteric Methane Emissions from Cattle
S. M. McGinna,*,
K. A. Beauchemina,
A. D. Iwaasab and
T. A. McAllistera
a Agriculture and Agri-Food Canada, P.O. Box 3000, Lethbridge, AB, Canada T1J 4B1
b Agriculture and Agri-Food Canada, Swift Current, SK, Canada S9H 3X2
* Corresponding author (mcginns{at}agr.gc.ca)
Received for publication February 7, 2006.
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ABSTRACT
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A commonly used method of measuring enteric methane (CH4) emissions from ruminants is the SF6 tracer technique that measures respired and eructated CH4. However, within the animal, a small proportion of CH4 is produced post-ruminally and some of this may escape through the rectum. The comparison of emissions using a chamber technique that measures all enteric CH4 losses, and the SF6 tracer technique, could give some insight into the magnitude of post-ruminal emission. The objective of our study was to assess the precision and accuracy of the SF6 tracer technique against a chamber technique for cattle fed a range of diets. Using a repeated-measures design, eight beef heifers were offered a high grain or high forage diet for ad libitum or restricted (65% of ad libitum) feed intake to vary the site of digestion within the gastrointestinal tract (n = 24). The SF6 tracer technique underestimated CH4 emissions on average by 4% relative to the chamber technique. This difference was not significant (P > 0.05) and suggests low post-ruminal CH4 emissions. There was a trend for greater accuracy and precision of the SF6 tracer technique when used with cattle fed a high forage diet at a restricted level of intake. The high forage diet corresponds to the conditions of cattle grazing pasture, suggesting the SF6 tracer technique is most reliable for the grazing system.
Abbreviations: DM, dry matter • ERUCT, Emissions from Ruminants using a Calibrated Tracer • GC, gas chromatography
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INTRODUCTION
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THE SF6 tracer technique (ERUCT technique; Emissions from Ruminants using a Calibrated Tracer) in which sulfur hexafluoride (SF6) is released in the rumen has been widely adopted to estimate enteric methane (CH4) emissions from ruminants (Johnson et al., 1994; Lassey et al., 1997; McCaughey et al., 1997; Boadi and Wittenberg, 2002; Boadi et al., 2002a, 2002b; Pinares-Patiño et al., 2003; Ulyatt et al., 2005). This technique quantifies CH4 released through respiration and by eructation from the ruminant. Although the ERUCT technique has been widely used, assumptions about its application require further validation (Ulyatt et al., 1999), including the use of different feeding conditions.
The ERUCT technique requires that the majority of CH4 is released from the ruminant via eructation and respiration, and does not capture any CH4 released from the rectum. Head enclosure techniques, such as ventilated hoods and face masks, make a similar assumption (Johnson and Johnson, 1995). However, little work has been done to quantify the proportion of CH4 released from the rectum under different feeding conditions. In the often cited study of Murray et al. (1976), about 90% of the CH4 was reported to originate in the rumen and 10% in the large intestine in sheep. However, about 90% of the CH4 produced in the large intestine was absorbed into the blood and released by respiration, with the result that only 1% of the enteric CH4 loss was through the rectum. Murray et al. (1976) has been cited by numerous studies (Lassey et al., 1997; Boadi et al., 2002a) as justification for measuring only the CH4 emissions from the anterior of the ruminant. However, the amount of CH4 released from the rectum may depend on the site of digestion of feed within the gastrointestinal tract. Although most feed is substantially digested in the rumen, many dietary factors increase the proportion of feed digested post-ruminally, which could potentially affect the route of CH4 loss. For example, decreasing the particle size of feeds, increasing the level of intake, use of treatments such as heating to reduce ruminal degradability, and use of resistant starches, can increase post-ruminal digestion (Galyean and Owens, 1991). Diets that increase the extent of post-ruminal digestion may create uncertainty regarding the appropriateness of the ERUCT technique. Any significant release of CH4 through the rectum would result in an underestimation of enteric CH4 production with the ERUCT technique, as well as from head enclosure techniques.
The objective of our study was to assess the precision and accuracy of the ERUCT technique for estimating enteric CH4 emissions from cattle. This was done by examining the influence of diet and level of feed intake, which can shift the site of fermentation within the gastrointestinal tract and possibly affect the accuracy of the ERUCT technique. Simultaneous measurements of enteric CH4 from whole animal chambers, calibrated using a known standard, were used as a basis for evaluating the ERUCT technique.
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MATERIALS AND METHODS
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Impact of Diet and Intake Level
An experiment was conducted to assess the accuracy and precision of the ERUCT technique for estimating enteric CH4 emission from beef cattle fed a range of diets that could result in varying post-ruminal production of CH4. The cattle fed these diets were placed into large chambers and the chamber CH4 emissions were compared to those measured with the ERUCT technique.
ERUCT Technique
The ERUCT technique involves placing a permeation tube containing ultra pure SF6 into the animal's rumen several days before starting an experiment. A Teflon membrane controls the release of SF6 from the permeation tube. In the present study, each permeation tube was filled with about 1.1 g of SF6 and weighed weekly for at least 2 mo to establish the SF6 release rate. The SF6 release rates of the eight permeation tubes (one per animal) varied between 4.30 and 4.93 mg d–1. The permeation tubes were expected to deliver a constant flow of SF6 for at least 3 mo beyond the completion of the study. Two permeation tubes, that were filled at the same time as the eight in the animals, were kept at 39°C and weighed at the end of the study to adjust the SF6 flow rate of the in-animal permeation tubes (as these tubes were not recovered at the end of the study).
In the ERUCT technique, air from around the nasal cavity, consisting of respired and eructated gases, is drawn through tubing into an evacuated canister located usually around the neck of the animal. The flow into the canister over 24 h was restricted using an in-line capillary tube such that the vacuum in the canister was reduced by about 50%. Our canisters were made from a poly vinyl chloride tube similar to the yoke-type described by Lassey et al. (1997). The canister was evacuated daily and SF6 and CH4 concentration was analyzed using gas chromatography (GC). Standard curves for the GC were generated throughout the study using five gas standards (18.27–299.50 nmol mol–1 for SF6 and 1.57–100.00 µmol mol–1 for CH4). The correlation coefficient exceeded 99.9% for all standard curves.
After each 24 h of sampling, the canisters were over-pressurized with pure nitrogen. Three 20-mL air subsamples from each canister were taken with a syringe (via a septum port on the canister) and injected into three corresponding 6.8-mL exetainers, as outlined by Rochette and Bertrand (2003), for analysis on a GC. At least 1 h was needed between over-pressuring and subsampling to ensure the canister air sample was well mixed (data not shown).
The SF6 and CH4 mixing ratio (µmol mol–1) in the canisters (CSF6 and CCH4, respectively), and the pre-determined SF6 release rate (QSF6; g d–1) were used to determine the CH4 emission (QCH4; g d–1) using Eq. [1]. The background SF6 and CH4 values measured in the middle of the chamber using another canister (CSF6b and CCH4b, respectively) were subtracted from CSF6 and CCH4, respectively. The ratio of molecular weights (MW) was used to account for the difference in density between the gases:
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The logic of using the ERUCT technique in a chamber requires some consideration. In the chamber, as would be true for a barn, feedlot, or even in a pasture where livestock are often in close proximity, there may be multiple sources of CH4 being sampled by the animal's canister. This background gas is subtracted in Eq. [1] to determine the proportion of the captured gas in the canister that is attributed solely to the respired and eructated gas. In the chamber, where the air is well mixed, background concentrations are easily measured using a second canister. In our chambers, there was a large dilution of chamber air with fresh air and the difference between the background and the animal's canister concentration was large. However, it should be noted that in chambers where the volume exchange is small, background concentration can approach the animal's canister concentration, and cause large variability in the SF6 to CH4 ratio. In our study, the ERUCT technique measured only the respired and eructated emission of the animal. Thus, CH4 released through the rectum of the target animal and emissions from the paired animal co-located in the chamber, did not contribute to the ERUCT measurements.
The alternative approach to using the ERUCT technique in a chamber is to use it in an open environment, both before and after placing the animals into a chamber. However, we were concerned that differences attributed to the animal's environment could confound the comparison of the two measurements, and thus we did not adopt that approach.
Chamber Technique
Four identical chambers measuring 4.4 m wide by 3.7 m deep by 3.9 m tall (volume = 63.5 m3; Model C1330; Conviron, Winnipeg, MB, Canada) were used (Fig. 1
). Each chamber housed two animals in stalls bedded with comfort mats. The chambers were vented using fresh-air intakes and chamber exhaust ducts (i.d. = 30.5 cm) with dedicated fans on each duct. Before calibrating the chambers, the flow rates were adjusted in the intake and exhaust ducts to generate a positive pressure inside the chamber of 2 Pa or less (PX653-0.1D5V; Omega Engineering, Stamford, CT). Fresh-air flow (approximately 0.28 m3 s–1) was fed into the recycling fan unit that kept the air moving within the chamber. The recycled air flowed through three raised floor vents running the length of the chamber; one vent located between the animal stalls and one on each side of the stalls adjacent the walls of the chamber. The loop of the recycled chamber air was completed after it passed through filters and a temperature controller. Air within the chamber was maintained at 15°C, but humidity was not controlled. The well-mixed air inside the chamber ensured that air leaving the chamber through the exhaust duct (approximately 0.22 m3 s–1) was representative of the entire air volume of the chamber. The inlet and exhaust flow rates were difficult to estimate due to curvature in the ducts, and therefore the measuring device was fixed at one position throughout the study and corrections were applied (see below) to compensate for inaccuracy in the measurement. The air volume of each chamber was exchanged every 5 min.
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Fig. 1. Schematic diagram of the chamber used to measure methane emissions. The insert shows the vertical cross-section of the rubber flap sealing the chamber air from the manure removal track.
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Below and to the rear of each stall (two per chamber) was a hole in the floor leading to the manure removal track (Fig. 1). The floor of the chamber was cleaned each morning to remove the accumulated manure. A flexible rubber mat hanging vertically effectively sealed the chamber from the manure removal track. Because the chamber was under slight positive pressure, in-flow of gases into the chamber was prevented.
During the operation of the chambers, CH4 concentrations in the intake and exhaust air streams were monitored. Air was pumped sequentially from each duct of each chamber at 1 L min–1 (TD3LS7; Brailsford and Company, Rye, NY) and passed through an infrared gas analyzer via a set of solenoids that were controlled by a datalogger (CR23X; Campbell Scientific, Logan, UT). The air from the intake and exhaust of a single chamber were connected to a CH4 analyzer (Model Ultramat 5E; Siemens, Karlsruhe, Germany). The air stream entering was dried using magnesium perchlorate to a dew point of below –50°C. After switching the air stream, data for the first 3 min after switching were ignored to compensate for the volume in the sample line. After 3 min, the average concentration of CH4 was recorded each minute on the datalogger. It took 30 min to complete a cycle of all four chambers, where each chamber was sampled for 7 to 8 min in total. Each morning the zero (using N2 gas) and span (using primary standards) of the analyzer was checked and small adjustments were made to account for any drift. The difference between the incoming and outgoing flow of CH4 was used to calculate the amount generated in each chamber by the two animals using the same method as reported by Beauchemin and McGinn (2005).
The CH4 emissions calculated using chambers were calibrated by releasing known amounts of CH4 into each chamber and calculating the recovered amount using the mass balance of incoming and outgoing CH4. For each chamber (with no cattle inside), a three point regression was developed where 0, 0.2, and 0.4 L min–1 of CH4 was released sequentially into each chamber using a mass-flow controller (Omega Engineering, Stamford, CT). Following each release of gas, the CH4 concentration in the exhaust air was allowed to reach steady-state before the emission was calculated. The slope of these best fit linear relationships (actual against the calculated emission) indicated the between chamber variability was 6 to 12% (Table 1), where all chambers underestimated the actual release rate. Thus, the chamber emission data were corrected using these calibration factors.
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Table 1. Corrections for each chamber based on a three-point calibration against a known release of CH4 within each chamber.
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Testing the Recovery of the ERUCT Technique
The mass-flow controller used for calibrating the chamber technique was also used to release 0.40 L CH4 min–1 into one chamber to test the recovery of gas by the ERUCT technique. Two permeation tubes with a combined release rate of 9.41 mg SF6 d–1 were placed in a water bath (39°C) within the chamber during the time that CH4 was released. A total of 18 canisters were set up to sample the well mixed chamber air for SF6 and CH4 over a 3-d period (six canisters per day).
Dietary Treatments and Animal Management
Eight spayed beef heifers (initial body weight of 328 ± 28 kg, mean ± standard deviation) that were previously conditioned to the chambers were sub-divided to four pairs. Each pair was assigned to a chamber and each pair was considered an experimental unit. The experiment was designed as a double 2 x 2 Latin Square in a split plot design with level of feed intake as the sub-plot. All animals received two diets over two 8-wk periods at two levels of feed intake. The diets were either (dry matter [DM] basis): (i) high forage (70% forage) or (ii) high grain (70% grain). The high forage diet contained mostly whole crop barley silage and barley grain while the high grain diet was based on coarsely ground corn grain and barley silage. Ingredient composition of the diets is given in Table 2. Within each period (Table 3), the cattle were fed unrestricted (ad libitum feed intake), followed by restricted (65% of ad libitum intake) feed intake. The two levels of feed intake were used to span a range in CH4 emissions within diet, as CH4 emissions increase as a function of feed intake (Johnson and Johnson, 1995).
During the 8-wk experimental periods (Table 3), Weeks 1 to 3 were used to adapt the animals to the diets. The transition from the high forage diet to the high grain diet was done during this phase using a series of diets with increasing proportions of grain. Transition from the high grain diet to the high forage diet was done more rapidly. During the first 6 wk of each period, feed was allocated for ad libitum (unrestricted) intake whereas allocation of feed was restricted during Weeks 7 and 8. During the restricted feeding phase, the amount of feed allocated was calculated to be 65% of unrestricted intake, based on intakes measured during Week 6.
The diets were prepared daily using a feed mixer (Data Ranger; American Calan, Northwood, NH) and offered once daily. Quantities of feed offered and refused were recorded daily for each animal. Samples of diets were analyzed weekly for DM content to calculate daily DM intake for each heifer. Samples of each diet and the component ingredients were taken weekly and pooled by period for chemical analysis.
The cattle were cared for according to the guidelines of the Canadian Council on Animal Care (1993). They were housed in individual stalls in a metabolism barn, except during Weeks 6 and 8, when they were transported to the chamber facility for gas emission measurements. Within each chamber, each pair of animals received the same diet, which was offered once daily at 1030 h. To allow the animals to acclimatize to the chambers, CH4 concentration was monitored for a 3-d period beginning 12 h after the animals were put into the chambers. The animals were transported back to the metabolism barn the following day in preparation for a new period.
Statistical Analysis
To determine the treatment effects on CH4 emissions, the data for each pair of animals were analyzed using the Proc Mixed procedure of SAS (SAS Institute, 1999) using repeated measures. The model included the fixed effect of diet, and the random effects of period and chamber, with day of sampling (1 to 3) and level of intake (ad libitum vs. restricted) within each period treated as a repeated measures. Treatment effects were declared significant at P < 0.05.
To compare the ERUCT and chamber techniques for measuring CH4, the differences between paired samples were calculated and the mean difference for each diet was compared to zero (P < 0.05). A concordance test was also performed using the output from the mixed model with repeated measures (Lin, 1989; Lin, 1992). The correlation coefficient accounting for repeated measures was used to calculate the concordance correlation coefficient (Hamlett et al., 2004). Also used in this calculation were the overall mean and variance for each method of measuring CH4. The correlation coefficient was used to determine precision over the range of CH4 measurements by determining the deviation of the data from the best-fit line. The concordance correlation coefficient was used to determine accuracy by determining how much the best-fit line deviated from the line y = x. The data from the ERUCT method were plotted on the y axis and the data obtained from the chamber technique were plotted on the x axis. Initially, the entire dataset was analyzed for an overall estimate of precision and accuracy of the ERUCT technique. The analysis was then done within diet and within level of feeding to evaluate the effect of these dietary conditions.
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RESULTS AND DISCUSSION
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The ERUCT technique failed on 2 d within one chamber for an animal fed unrestricted amounts of the high grain diet. Thus, the data for the ERUCT technique for the pair of animals on these 2 d were excised, and a total of 46 comparisons of the two techniques were used. Mean emission averaged over all diets was 135 g d–1 using the ERUCT technique compared to 142 g d–1 for the chamber technique. Therefore, the estimates from the ERUCT technique were about 4% lower than those from the chamber technique (Table 4). Differences between the two techniques over the range of dietary conditions in this study were not different from zero (P > 0.40 in all cases). The difference between the ERUCT and chamber techniques was similar to the range of 5 to 7% suggested by Ulyatt et al. (1999) and Johnson et al. (1994).
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Table 4. Comparison of the methane emissions from cattle fed high grain and high forage diets, in unrestricted and restricted amounts, measured using the chamber and ERUCT techniques.
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The repeated measures correlation coefficient (0.81) indicated some deviation of the dataset from the best-fit linear line, but overall precision was high. The concordance coefficient (0.79) indicated agreement between the two techniques over the entire range of CH4 emissions, that is, close to a 1:1 fit (Fig. 2
). The location shift, which indicates how the y intercept of the plotted data differs from the y intercept of the line y = x, was negative, but small (the line y = x would have a location shift of zero). This location shift indicated that the ERUCT technique slightly underestimated CH4 relative to the chambers. Since the chamber technique measured all enteric methane production, and the ERUCT technique only measured respired and eructated losses, it follows that the difference (the underestimation) by the ERUCT technique could have been due in part to the difference between site of emission from the animal. The scale shift, which indicates the discrepancy in slope between the plotted data and the line y = x, was 0.99 (1:1 line would equal 1.0). Thus, over the range of diets and feeding conditions used in this study there was good agreement between the ERUCT and chamber techniques.
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Fig. 2. Comparison of methane emissions using the chamber and sulfur hexafluoride (SF6) tracer techniques. The correlation coefficient was 0.81 indicating moderate deviation of the data set from the best-fit linear line. The concordance coefficient was 0.79 indicating agreement between the two techniques (i.e., close to a 1:1 fit).
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To test the recovery of gas by the ERUCT technique, 18 canisters were used over 3 d to recover a known release of CH4 inside the chamber. The ERUCT technique measured 0.42 L CH4 min–1 (±0.02), which was higher (P < 0.05) than the actual release rate of 0.40 L CH4 min–1. This 5% difference could be due in part to analytical factors, such as capture of gas within the canister, sampling, and gas analysis. The release rate of CH4 due to the mass-flow controller was checked in the laboratory over several days by measuring the mass loss of the cylinder. Therefore, the release rate did not contribute to the difference between techniques.
The impact of diet (i.e., high forage vs. high grain; restricted vs. unrestricted intake) on the relationship between ERUCT and chamber techniques was examined to determine if the relationship was consistent within feeding condition (Table 4). We hypothesized that greater differences between the techniques would exist when cattle are fed diets that are extensively fermented post-ruminally compared to diets that are extensively digested in the rumen. With more post-ruminal digestion, there may be greater opportunity for CH4 release through the rectum, which is not accounted for in the ERUCT technique. In this study the forage diet based on barley and barley silage represented a diet that would be extensively fermented in the rumen. Barley grain is rapidly and extensively digested in the rumen of cattle (Yang et al., 1997) and the relatively long retention time of forages in the rumen ensures extensive ruminal digestion (Galyean and Owens, 1991). In contrast, the high grain diet based on corn grain represented a diet with greater potential for post-ruminal fermentation. Corn grain is less extensively fermented in the rumen than barley (Yang et al., 1997), and this effect is enhanced by the coarse processing of corn (Rémond et al., 2004). Furthermore, feed is retained in the rumen for a shorter period of time as the proportion of long forage in the diet is reduced and as feed intake increases (Galyean and Owens, 1991). Although only 1% of the enteric CH4 loss is thought to exit via the rectum (Murray et al., 1976), it is not clear whether this amount increases when cattle are fed diets with greater post-ruminal digestion.
The agreement between the ERUCT and chamber techniques was higher for the high forage diet compared to the high grain, and higher for restricted versus unrestricted feed intake (Table 4). However, for all feeding conditions the mean difference between techniques was not different (P > 0.40 in all cases) from zero. The high forage diet was expected to result in a greater proportion of CH4 produced in the rumen than the high grain diet and it follows that the difference between techniques would be greater for the high grain diet. The ERUCT technique (respired emission) is expected to underestimate emissions relative to the chamber technique (respired and hind gut emissions), particularly when animals are fed diets that result in a greater proportion of post-ruminal digestion. The unrestricted level of feed intake was also expected to produce a greater proportion of CH4 post-ruminally relative to restricted feeding, due to reduced retention time of feed in the rumen. The treatment interactions also follow this line of logic, where the high forage, restricted feeding combination resulted in the best agreement between techniques, that is, where the repeated measures correlation and concordance coefficients were 0.87 and 0.84, respectively. The worst agreement between techniques was found for the high grain, unrestricted combination, where the repeated measures correlation coefficient and concordance coefficients were 0.26 and 0.21, respectively. The two remaining interactions were intermediate; high forage, unrestricted feeding (repeated measures correlation coefficient = 0.82; concordance coefficient = 0.62) showed greater agreement than the high grain, restricted combination (repeated measures correlation coefficient = 0.69; concordance coefficient = 0.44). Thus, it is evident that the type of diet fed should be considered when employing the ERUCT technique.
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CONCLUSIONS
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The ERUCT technique underestimated CH4 emissions over a range of diets by an average of 4% relative to the chamber technique. Some of the underestimation of the ERUCT technique can be attributed to the post-ruminal loss of CH4 that is captured in the chamber technique, but not the ERUCT technique. Another factor that contributes to this difference is associated with gas recovery within the canisters.
While there was general agreement between the ERUCT technique and measurements made with the chamber technique, agreement between the two techniques is improved when used with a high forage diet fed at a restricted level of intake. This suggests that the ERUCT technique is most appropriate for use with grazing cattle. In situations where high grain diets are fed, such as beef feedlots and intensive dairy operations, the ERUCT technique can be used, but with greater uncertainty.
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ACKNOWLEDGMENTS
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This work was supported by the National Carbon and Greenhouse Gas Emission Accounting System (NCGAVS), a project developed by the Government of Canada. The technical assistance of Trevor Coates (gas measurements), Karen Andrews (animal care), Darrell Vedres (gas analysis), Leanne Thompson (gas measurements), and Dave Pittman (animal care) is acknowledged as an important contribution to this study.
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[Abstract]
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ABSTRACT
INTRODUCTION
