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Finally, the descriptions are summarized to facilitate comparison. Different chamber systems or respiration chambers have been used for the last years with the main purpose of studying the energy metabolism of animals [ 4 , 5 ]. Methane loss is an inherent part of the energy metabolism in ruminants, and various types of chambers are valuable tools in the investigation of mitigation strategies for methane emissions. The principle of the chambers is to collect all exhaled breath from the animal and measure e. Animal calorimetric systems, where air composition is measured, are divided into two main types: The closed-circuit [ 6 ] and the open-circuit, with the latter being the dominating one [ 5 ].
In Figure 1 an outline of an open-circuit system is shown. A pump pumps air from the chamber through a flow meter and different gas sensors. Fresh air for the animal is drawn from outside. In some systems fresh air is drawn through an air conditioning system to control humidity, temperature and mixing of air in the chamber but air can also simply be taken from outside the chamber.
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The methane emission is calculated from flow and gas concentration in inlet and outlet air from the chamber, but more complex calculations have been developed that also take into account the small differences in inflow and outflow and changes in chamber concentration of gases [ 7 ].
The difference between the outgoing and incoming amount of methane corresponds to the methane emission. Many different chambers have been constructed on the basis of this principle including insulated chambers with controlled temperature and humidity [ 5 , 8 , 9 , 10 ], more simple types with no insulation of chambers and fresh air inlet from the room [ 11 , 12 , 13 ], systems where just the head of the animal is placed in the chamber [ 14 , 15 , 16 ] and systems developed to measure grazing animals [ 17 , 18 , 19 ].
Chambers are regarded as the standard method for estimation of methane emission from ruminants, because the environment can be controlled and the reliability and stability of instruments can be measured [ 5 , 20 ]. However, there is a risk of creating an artificial environment, which affects animal behavior e.
As DMI is one of the main drivers of methane emission a decrease in DMI would not only effect total emission but also the derived estimates like loss of gross energy [ 21 ]. Therefore, it has been queried that results obtained in chambers cannot be applied to free ranging animals e. Investigations have shown that chambers give more precise estimates of methane emissions than the SF 6 tracer technique [ 11 ].
Classical chambers for energy metabolism with air conditioning, internal mixing of air and careful tightening to reduce the risk of air loss to the surroundings [ 8 ] are expensive to build. Therefore less expensive systems have been developed with methane measurements as the main purpose [ 11 , 12 , 13 ]. In Denmark four chambers based on open circuit calorimetry have been built.
The chambers are 1. The chambers are constructed of a metal frame covered with transparent polycarbonate walls Figure 2.
They are placed so cows can have visual contact with other animals in an existing barn to ensure animal welfare and dry matter intake. DMI measured before and during chamber stays have shown that feed intake is unaltered Hellwing, unpublished data. It clearly shows that design and placement of chambers can reduce the risk of creating an artificial environment and eliminate the risk of reduced DMI. Furthermore, data on methane emission can be combined with data on rumen metabolism and digestibility [ 24 ], increasing our understanding of methane production and metabolism.
Respiration chambers constructed at Aarhus University of a steel frame covered with polycarbonate. Nearly all aspects of feeding and nutrition can be investigated in a chamber system. The level of feeding, effect of feedstuff, effect of chemical and physical composition, restricted versus ad libitum feeding, different feeding schedules, different additives etc.
Also changes in emission during the day can be described with the system, but resolution will depend on the number of measurements during a given day. The variation in measurements is affected by instrument variation as well as within and between animal variations. Within animal variation or day-to-day variation will affect the number of days needed for measuring. The day-to-day coefficient of variation CV has been reported to be 7. Increasing the number of measuring days will decrease the random error [ 27 ]. For methane or energy metabolism studies of three to five days has been used [ 11 , 25 , 26 ].
A higher between animal variation during ad libitum feeding than during restricted feeding corresponds well with the findings of Thorbek [ 28 ] who studied CO 2 and CH 4 production by growing calves at high and low feeding levels. A high between animal variation will increase the number of animals needed to document that treatments are significantly different.
Considerations about design and placement of the chambers can eliminate the risk of reduced feed intake. There is no doubt that this system gives quantitative measurements of methane emission with low tolerance but establishment costs and limited capacity of the system restricts the number of animals, which can be examined experimentally. This method is relatively new and was first described in — [ 22 , 29 ]. The main purpose of the method was to investigate energy efficacy in free ranging cattle [ 29 ], because it had been queried that results obtained in respiration chambers could not be applied to free ranging animals [ 22 , 23 ].
The basic idea behind the method is that methane emission can be measured if the emission rate of a tracer gas from the rumen is known. For this purpose a non-toxic [ 39 ], physiologically inert [ 40 ], stabile gas is needed. Furthermore, the gas should mix with rumen air in the same way as methane. SF 6 was chosen [ 29 ], because it fulfills the above criteria, is cheap, has an extremely low detection limit and is simple to analyze. SF 6 is filled into small permeation tubes.
The permeation tube is then placed in the rumen of an experimental animal and collection of air can start. The sampling apparatus consists of a collection canister, a halter and capillary tubing. The capillary tubing is placed at the nose of the animal and connected with the evacuated canister Figure 3. The tubing regulates the sampling rate. The sampling time is typically one day [ 22 , 29 , 41 ], but emission estimates from shorter time intervals have been published [ 30 , 36 ].
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The concentration of SF 6 and CH 4 in the canister is determined by gas chromatography. For more detailed description of equipment and guidance see [ 41 ]. The methane emission is calculated from the release rate of SF 6 and concentration of SF 6 and CH 4 in the containers in excess of background level [ 31 ] as described in Equation 1. Illustration of the SF 6 tracer technique.
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Reprinted with permission from [ 22 ]. Copyright American Chemical Society. Results based on direct measurements of gas composition in gas head space in the rumen of cannulated animals have also been published [ 36 , 42 , 43 ]. The system can be used to investigate nearly all aspects of feeding and nutrition e.
Using the method for investigation of dynamics of methane emission is debatable. The method has been carefully tested during the last two decades and a number of difficulties have been described. The following problems will briefly be discussed: Maintaining a constant release rate from permeation tubes, effect of release rate upon emission rate of methane, background level determination, inconsistency between methane measurements determined in chambers and with SF 6 and within and between animal variation.
The release rate is important and will affect emission estimates if not correctly determined.
However, permeation curves have been shown to be slightly curvilinear under laboratory conditions [ 46 , 47 ]. Tests of permeation tubes pre- and post-experiments have also shown differences in permeation rate. Different methods to account for this are described by [ 46 , 47 ].
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The permeation tubes are weighted in a laboratory in dry air and the release rate should be the same in the rumen. It has also been shown that permeation tubes with high release rates give higher methane emissions than tubes with low release rates. It is therefore recommended to use permeation tubes with nearly the same release rate when comparison of different treatments is needed [ 48 , 49 ].
The measured concentration should be corrected for background levels of both SF 6 and CH 4 [ 31 ]. Measuring a representative background concentration under field condition can be difficult, because wind direction and other animals in the field can affect the concentrations [ 50 ]. Johnson et al. However, others have shown slightly higher values with the SF 6 technique than chambers [ 26 , 55 ], and yet other studies have found much higher values with the SF 6 technique than chambers [ 11 , 44 , 46 , 56 , 57 ].
Both within and between animal CV is much higher in experiments with the SF 6 technique than with the chambers. The within CV was 4. Also the between animal CV was twice as high with the SF 6 technique as with the chambers. The correlation between the different methods is also inadequate.