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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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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DM, dry matter • ERUCT, Emissions from Ruminants using a Calibrated Tracer • GC, gas chromatography
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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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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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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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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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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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).
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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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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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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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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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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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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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