Methods for Measuring and Estimating Methane Emission from Ruminants

Animals 2012, 2, 160-183; doi:10.3390/ani2020160 OPEN ACCESS animals ISSN 2076-2615 www.mdpi.com/journal/animals Review Methods for Measuring and Estimating Methane Emission from Ruminants Ida M. L. D. Storm 1, Anne Louise F. Hellwing 2, Nicolaj I. Nielsen 3 and Jørgen Madsen 1,* 1 2 3 Department of Large Animal Sciences, University of Copenhagen, Grønnegårdsvej 2, DK-1870 Frederiksberg C, Denmark; E-Mail: Department of Animal Science, Aarhus University, AU-Foulum, P.O. Box 50, DK-8830 Tjele, Denmark; E-Mail: AgroTech, Agro Food Park 15, DK-8200 Aarhus N, Denmark; E-Mail: * Author to whom correspondence should be addressed; E-Mail: ; Tel.: +45-3533-3092; Fax: +45-3533-3022. Received: 9 February 2012; in revised form: 8 March 2012 / Accepted: 2 April 2012 / Published: 13 April 2012 Simple Summary: Knowledge about methods used in quantification of greenhouse gasses is currently needed due to international commitments to reduce the emissions. In the agricultural sector one important task is to reduce enteric methane emissions from ruminants. Different methods for quantifying these emissions are presently being used and others are under development, all with different conditions for application. For scientist and other persons working with the topic it is very important to understand the advantages and disadvantage of the different methods in use. This paper gives a brief introduction to existing methods but also a description of newer methods and model-based techniques. Abstract: This paper is a brief introduction to the different methods used to quantify the enteric methane emission from ruminants. A thorough knowledge of the advantages and disadvantages of these methods is very important in order to plan experiments, understand and interpret experimental results, and compare them with other studies. The aim of the paper is to describe the principles, advantages and disadvantages of different methods used to quantify the enteric methane emission from ruminants. The best-known methods: Chambers/respiration chambers, SF6 technique and in vitro gas production technique and the newer CO2 methods are described. Model estimations, which are used to calculate national budget and single cow enteric emission from intake and diet composition, are also discussed. Other methods under development such as the micrometeorological Animals 2012, 2 161 technique, combined feeder and CH4 analyzer and proxy methods are briefly mentioned. Methods of choice for estimating enteric methane emission depend on aim, equipment, knowledge, time and money available, but interpretation of results obtained with a given method can be improved if knowledge about the disadvantages and advantages are used in the planning of experiments. Keywords: methane; ruminants; estimation methods; limitations 1. Introduction Livestock and mainly ruminants account for up to one third of the emitted methane worldwide [1], and methane has a greenhouse potential 25 times that of CO2 [2]. Therefore methane accounts for a great part of the emitted CO2-equivalents from agriculture. Over the last 100 years several different methods have been developed with the purpose of measuring and estimating methane emissions from ruminants. These methods have various scopes for application, advantages and disadvantages—but none of them are perfect: Some are expensive, some cheaper; some suited for grazing animals, some for housed livestock; some can handle many animals, some only few. This all affects the measuring results and our interpretation of them. It is therefore important to know the possibilities and limitations of each method. This applies to the understanding of current research results and to the planning of future projects. The present literature within techniques for estimating greenhouse gas emissions from livestock is primarily concerned with individual methods and their validation. Apart from a review with emphasis on grazing livestock was published in 2007 [3], literature comparing a range of different estimation approaches is scarce. This review briefly presents the most common methods for estimating and measuring methane emissions from ruminants, including newly developed techniques. The focus is on methods at the individual animal scale. Each method is presented and advantages and disadvantages emphasized. Finally, the descriptions are summarized to facilitate comparison. 2. Measuring Methane by Means of Chambers Different chamber systems or respiration chambers have been used for the last 100 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.g., the methane concentration. 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 Animals 2012, 2 162 chamber. 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 [5,8–10], more simple types with no insulation of chambers and fresh air inlet from the room [11–13], systems where just the head of the animal is placed in the chamber [14–16] and systems developed to measure grazing animals [17–19]. Figure 1. Diagram of open circuit respiration chamber. 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.g., dry matter intake (DMI). 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.g., animals on pasture (...truncated)