Accuracy and precision of the techniques used to measure methane emissions from ruminant animals are critically important for the success of greenhouse gas mitigation efforts. This manuscript describes the principles and operation of an automated system to monitor methane and carbon dioxide mass fluxes from the breath of ruminant animals.
Ruminant animals (domesticated or wild) emit methane (CH4) through enteric fermentation in their digestive tract and from decomposition of manure during storage. These processes are the major sources of greenhouse gas (GHG) emissions from animal production systems. Techniques for measuring enteric CH4 vary from direct measurements (respiration chambers, which are highly accurate, but with limited applicability) to various indirect methods (sniffers, laser technology, which are practical, but with variable accuracy). The sulfur hexafluoride (SF6) tracer gas method is commonly used to measure enteric CH4 production by animal scientists and more recently, application of an Automated Head-Chamber System (AHCS) (GreenFeed, C-Lock, Inc., Rapid City, SD), which is the focus of this experiment, has been growing. AHCS is an automated system to monitor CH4 and carbon dioxide (CO2) mass fluxes from the breath of ruminant animals. In a typical AHCS operation, small quantities of baiting feed are dispensed to individual animals to lure them to AHCS multiple times daily. As the animal visits AHCS, a fan system pulls air past the animal’s muzzle into an intake manifold, and through an air collection pipe where continuous airflow rates are measured. A sub-sample of air is pumped out of the pipe into non-dispersive infra-red sensors for continuous measurement of CH4 and CO2 concentrations. Field comparisons of AHCS to respiration chambers or SF6 have demonstrated that AHCS produces repeatable and accurate CH4 emission results, provided that animal visits to AHCS are sufficient so emission estimates are representative of the diurnal rhythm of rumen gas production. Here, we demonstrate the use of AHCS to measure CO2 and CH4 fluxes from dairy cows given a control diet or a diet supplemented with technical-grade cashew nut shell liquid.
Animal production represents a significant source of greenhouse gas (GHG) emissions worldwide, generating CH4 and nitrous oxide either directly (e.g., from enteric fermentation and manure management) or indirectly (e.g., from feed-production activities and conversion of forest into pasture or cropland). Estimates for livestock contribution to the global GHG emission vary from about 71 to 18%2, depending on the boundaries of the analysis and methods used. In the United States, livestock represented 3.1% of the total GHG emissions in 20093.
Enteric CH4 is the largest contributor to GHG emissions from livestock. Therefore, animal scientists have focused their research on discovering mitigation technologies for reducing enteric CH4 production from ruminants. In many cases, results are of questionable scientific value due to inadequate experimental design or measurement techniques1. Thus, the accuracy and precision of the measurement techniques are critically important components of GHG mitigation research. A large body of literature has been published on this topic in recent years4-7. There are several established approaches for measuring enteric CH4 production in ruminants, including respiration chambers (highly accurate but with limited applicability), tracer gases (sulfur hexafluoride; SF6), and head-chambers. Although respiration chambers are considered the “gold standard” for measuring rumen gas emissions, their major disadvantage is that the number of animals on trial is usually limited due to the limited number of chambers available at a particular research facility. The most practical and widely used techniques for measuring enteric CH4 production are the SF6 tracer gas method and more recently, an Automated Head-Chamber System (AHCS, GreenFeed) that can monitor CH4 and CO2 mass fluxes from the breath and eructation gas of ruminants8. Both the SF6 technique and AHCS enable emissions to be analyzed on a large number of animals in free grazing conditions or in free- and tie-stall barns. The SF6 technique utilizes SF6 as a tracer gas, which is continuously released from a permeation tube inserted in the rumen of the animal, collection of a sample of the exhaled gases, and analysis of the gas for SF6:CH4 ratio. AHCS is an automated, head-chamber type system that is also based on the use of a tracer gas (propane). Compared with the respiration chamber method, where animals are confined under abnormal feeding and behavior conditions, and with the SF6 tracer method, which requires special analytical skills and equipment (for gas collection and SF6 analysis) plus extensive animal handling, AHCS is non-intrusive and is less expensive to acquire and operate. Major shortcomings of AHCS include unrepresentative sampling (in applications, such as grazing systems, where the animals have to voluntarily visit the unit) and the use of bait feed, which could represent up to 5% of the animal’s dry matter intake during a gas measurement event. Recent comparative experiments have concluded that AHCS produces emission rates comparable to those estimated using respiration chambers or the SF6 technique9,10.
The stand-alone AHCS system is constructed around a robust automatic feeder that is easily transportable by hand or can be mounted to a trailer equipped with solar panels (or other power sources) for autonomous field operation and long distance travel. The system includes an animal Radio-Frequency Identification system (RFID), a baiting system, an air handling and measurement system, a gas tracer system, electronics and communication system, and a data handling system (Figure 1). More details can be found in the original patent documentation11.
The example AHCS operation protocol described below is for lactating dairy cows housed in a tie-stall barn. The procedure is applicable to other categories of cattle (non-lactating dairy cows, heifers, or beef cattle) housed in similar facilities. The objective of this experiment is to demonstrate the principles and operation of AHCS for the measurement of CH4 and CO2 emissions from ruminant animals.
Animals involved in the experiment described in representative results were cared for according to the guidelines of the Pennsylvania State University Animal Care and Use Committee. The committee reviewed and approved the experiment and all procedures carried out in the study. Details, such as animal and diet composition information and experimental design, can be found in the full publication of this experiment12.
Note: For a list of equipment and supplies needed to conduct the experiment, see Materials Table.
1. Experimental Design
2. Training of the Animals to Use AHCS
3. Calibration of AHCS
Note: The concentration range of the CO2 sensor is 0 to 5%; the range for the CH4 sensor is 0 to 2%. The detection lower limits are 20 ppm for CH4 and 50 ppm for CO2. There are no concerns about high background levels of CH4 and CO2 because the detection limits are far greater than safe high background levels of these gases in animal facilities.
4. CO2 Recovery Test
5. Gas Measurement
Note: Prior to gas measurement, a recent (within a week) calibration of AHCS is required. See steps 3, Calibration of AHCS and 4, CO2 recovery test. Make sure the animal’s RFID tag is in place.
Figure 1: Components of the Automated Head-Chamber System (AHCS, GreenFeed) for measuring CH4 production in ruminant animals.
Methane production in the rumen is a substrate-dependent microbiological process7. Production of CH4 and CO2 increases after the animal is fed and decreases thereafter. Figure 2 demonstrates the increase in CH4 production from a dairy cow fed ad libitum at around 0600 hr (unpublished data by A.N. Hristov, Pennsylvania State University).
Figure 2: Diurnal CH4 emissions from a dairy cow fed once daily measured using AHCS (error bars represent SE; data courtesy of A. N. Hristov, Pennsylvania State University).
The error bars on this figure represent variability in CH4 emission during a sampling event (which includes multiple eructation cycles). It is apparent that in some cases (around 0400 and 0900 hr), variability was larger due to changing concentration of CH4 in exhaled gases. It is also clear that CH4 emissions increased after feeding (which was around 0600 hr in this example). The average daily CH4 emission (i.e., an average of the 13 measurement events) from this cow was 727 ± 22.9 g/days, or 26 g/kg when expressed per kg of diet dry matter intake (DMI).
To demonstrate the range of CH4 emissions from a group of lactating dairy cows measured using AHCS, we include data from a recent crossover design trial conducted at the Pennsylvania State University that utilized technical-grade cashew nut shell liquid as a CH4 mitigation agent (Table 1). The trial was with 8 lactating Holstein dairy cows and 2 experimental periods of 21-days each. Methane data were collected during the last week of each period. Methane emission data were not collected from one cow in period 1 and data for that cow were also not used in period 2. Details of the experiment can be found in Branco et al.12. The average CO2 emission in this study was over 18,000 g/cow per day, or 634 g/kg DMI. Average CH4 emission for this group of cows was 523 g/day or 20 g/kg DMI, which is similar to the average CH4 emissions reported for a dataset of over 370 treatment means (19.1 ± 0.43 g/kg DMI)7. In the study presented in Table 1, compared with the control, technical-grade cashew nut shell liquid tended to decrease CH4 production in the rumen of the cows by about 5% (P = 0.08)12.
The AHCS system combines elements of a dynamic enclosure technique, chamber system, and tracer technique for mass flux measurements of CH4 and CO2. Over the course of days, it collects multiple samples from each animal to define the average total daily gas mass fluxes. To identify an animal and deliver the correct amount of bait, an RFID reader is incorporated into AHCS. The RFID tag is read as the animal begins to place its head into the feeder. Once an animal is identified, AHCS determines if it is eligible to receive a bait reward at that specific time of the day (grazing or free-stall barn applications). The start and end time of each animal’s visit (determined based on the infrared sensors) is automatically recorded. The bait delivery system is used to attract animals to AHCS periodically throughout the day. Typically, the baiting feed is pelletized and may contain grass, alfalfa, grain concentrates, molasses, and vegetable oil. While an animal visits AHCS, a fan pulls air over its head (at rate of about 26 L/min), sweeping emitted CH4 and CO2 into an air intake manifold. The air flow velocity is measured continuously with a hot-film anemometer in the middle of the air collection pipe. A continuous sub-sample of air is extracted and routed into a secondary sample filter, then into two Non-Dispersive Infra-Red analyzers, one sensor for CO2 and one for CH4. AHCS also includes additional sensors for air temperature, air humidity, bait drop, system voltage, atmospheric pressure, propane flow rate, and head position. Pasture and trailer mounted versions for grazing systems include a cup anemometer (local wind speed) and wind vane (wind direction). All sensor data are stored on a local data logger and a computer, enabling AHCS to function automatically and independently. Sensor data are also stored on an internal standard USB (Universal Serial Bus) memory stick. AHCS data are normally transferred through an internet link, once per hour, to an external server where they are permanently logged. Users with internet connectivity can remotely log into AHCS and control the unit, modify baiting schedules, and review historical and real-time data as well as review and monitor AHCS function.
Overall, experiments conducted at the Pennsylvania State University demonstrated that the AHCS system delivers reliable estimates for CH4 and CO2 emissions from dairy cows housed in tie-stall barns. The advantages of AHCS over respiration chambers is that the animal is not restricted and is in its natural environment (i.e., on pasture), or can freely move (in a free-stall barn). AHCS is also less expensive to build than a traditional respiration chamber. This relatively low cost is important, particularly for CH4 mitigation research in developing countries. Compared with the SF6 tracer method, AHCS is simpler to operate and does not require complicated and expensive analytical equipment. Perhaps the most apparent disadvantage of AHCS, compared with chambers and the SF6 methods (particularly when used in grazing or free-stall barn environments), is that the animal has to voluntarily approach the unit and therefore gas measurement events are dependent on animal visits. Within a day, these animal visits may or may not be representative of the diurnal rhythm of CH4 production. Therefore, in applications where the animal visits AHCS voluntarily, the sampling period should be long enough or repeated a sufficient number of times. The tie-stall application used at the Pennsylvania State University alleviates this problem by controlling the number and temporal distribution of gas measurements during a 24 hr feeding cycle. Sufficient sampling of eructation gas during a feeding cycle (as indicated in the above protocol) is important for representative estimation of CH4 production in the rumen of cattle. The amount of bait feed fed to the animals during measurements using AHCS has to be considered in the overall analysis (i.e., must be added to the total amount of feed consumed by the animal), so emission intensity per unit of feed DMI can be accurately estimated. Under normal feeding conditions, the bait feed represents less than 5% of the total DMI of a dairy cows and its effect on the ruminal fermentation and CH4 production is small. It is noted that AHCS (and other similar systems) does not measure CH4 production in the animal’s hindgut. Hindgut fermentation, however, contributes only around 3% of the total CH4 emissions in a ruminant animal7.
Based on experience, there are several important components of measuring enteric rumen gas production using AHCS: (1) the animal has to be accustomed to the baiting feed (and AHCS) and has to like it in order to approach and use the AHCS feeder, (2) the animal’s head has to be inserted all the way into the feeder in order to collect reliable gas emissions data, (3) the AHCS calibration procedure has to be followed strictly, (4) having sufficient time to collect background CH4 and CO2 data between sampling individual animals is important, particularly in tie- or free-stall barns, and (5) it is important that enough data are collected in a sampling cycle (covering a 24 hr period) so emission data generated by AHCS are representative of the actual diurnal CH4 or CO2 emissions by the animal.
Comparative tests with AHCS vs. established CH4 measurement techniques support the above conclusions. For example, a study with growing dairy heifers concluded that AHCS was capable of estimating CH4 emissions from livestock and emission estimates generated by AHCS were comparable to values obtained by respiration chambers9. These authors pointed out that deployment of the AHCS units and replication must be carefully considered to ensure sufficient numbers of measurements are obtained. Based on experience, 8 sampling events, staggered over a 3-day period to cover a 24 hr feeding cycle (see protocol above) are sufficient to obtain accurate measurements of gaseous emissions and relatively low variability in the data (i.e., acceptable precision). In a study with lactating dairy cows, it was concluded that CH4 emissions measured by the AHCS were similar to literature values derived from respiration chambers and between animal variability (CV of 11 to 12%; repeatability of 0.64 to 0.81) was also within the range reported for respiration chambers10. In a recently-published study with lactating cows, AHCS produced a smaller CV than the SF6 method (14.1 to 22.4% vs. 16.0 to 111% for SF6)13. In a 12-week experiment conducted at the Pennsylvania State University with 48 lactating dairy cows, in which rumen CH4 production was inhibited by 30% (P < 0.001), we concluded that AHCS and the SF6 method produced similar CH4 emission results: 319 to 481 g/cow per day (mean = 374 g/d; SEM = 15.9; CV = 13%) and 345 to 485 g/cow per day (mean = 396 g/d; SEM = 29.8; CV = 23%) for AHCS and SF6, respectively14.
In conclusion, accurate, but practical techniques for measuring CH4 production in the rumen are critically important for the success of GHG mitigation efforts. AHCS is an automated gas measurement system that has been proven to deliver reliable and accurate estimates of enteric CH4 and CO2 emissions from beef and dairy cattle.
The authors have nothing to disclose.
The authors would like to thank the staff of the Department of Animal Science’s Dairy Center for their conscientious care of the experimental cows used to generate data for this study.
AHCS | 1 | C-Lock, Inc. | |
Zero, 100 N2 | 1 | Air Liquide | 4 m3 sized tanks filled with 13,790 kPa |
Span, 0.15% CH4 and 1% CO2 | 1 | Air Liquide | 4 m3 sized tanks filled with 13,790 kPa |
Gas sampling bag | 2 | SKC, Inc. | FlexFoil® PLUS Breath-gas analysis bags |
Gas regulator | 2 | Scott Gasses | |
CO2 cylinder | 6 | JT | 90 g CO2 tanks |
Mass scale | 1 | A&D EJ6100 | > 4 kg, with 0.1 g resolution |
Propane cylinder 485 mL | 1 | Coleman | |
ISO 11784/11785 button ear tag | 40 | Allflex USA | One tag per animal |
Alleyway (for free-stalls, tie-stalls) | 2 | Behlen Country | One alleyway per unit |
30 m AC extension cord | 1 | HDX | |
A container with warm water (37-43°C) | 1 | N/A | |
Stopwatch (sec) | 1 | N/A |