Development of proxy indicators of methane output by sheep using rapid-throughput laboratory technologies (PhD)
Methane production by ruminants is a significant contributor to agricultural greenhouse gas emissions (Webb et al., 2013). However, current values used to estimate methane output by sheep are default values and do not take into account animal and dietary factors that may affect methane output (Bernstein et al., 2007). Strategies to reduce ruminant methane output are the focus of a large body of research (Iqbal et al., 2008) and, in order to implement these strategies fully, a greater understanding of factors that influence ruminant methane emissions is necessary.
The "gold standard" method for measuring methane output by sheep is the use of respiratory chambers (Blaxter and Clapperton, 1965). However, this method is expensive, time-consuming and labour intensive, making it unsuitable for use in an on-farm situation. The work presented in this thesis explores the potential of three proxies to estimate methane output by sheep, which could be used or adapted to be used as a practical means of estimating methane emissions from sheep on a large scale.
The proxies investigated here are a Laser Methane Detector (LMD), used to take measurements of methane concentration from air expired by sheep, in vitro gas production analysis of feeds offered to sheep, and Fourier-transform infrared spectroscopy (FTIR) analysis of feeds offered to sheep. Predictions of methane output obtained from each of the proxies are validated using respiratory chamber measurements taken from sheep offered a variety of feeds during different experiments.
With further development and validation, all three proxies presented in this thesis demonstrate potential to be used to successfully estimate or predict methane output by sheep as measured in respiratory chambers. A novel and very successful approach to the method for use of the LMD and calculation of daily methane emissions from LMD data is presented in this thesis. However, the methods used were relatively labour intensive and time-consuming. Further work should, therefore, focus on simplifying these methods as much as possible. To my knowledge, the results presented for in vitro gas production and FTIR spectroscopy are also novel, although these are established methods. Both of these methods are rapid-throughput techniques and, therefore, have real potential to be used on a large scale. Further work using larger data sets may provide a more comprehensive idea of the aspects of feeds that affect their methane potentials.
Laser Methane detector
The results obtained during this project suggest the LMD has potential to be used as a means to estimate methane output by sheep. Although the methods used to collect LMD data were simple, they were relatively time consuming, required close contact with animals on a daily basis, and required expensive equipment (i.e. the LMD). However, the work presented in this thesis demonstrates that LMD measurements can be used to estimate daily methane output by sheep that is in the expected magnitude and correlates with daily methane output as measured in methane chambers. Furthermore, the LMD could potentially be used to rank animals in terms of their methane production, thereby facilitating the introduction of breeding programmes for animals that are low methane producers (Hegarty et al., 2007; Pinares-Patiño et al., 2013).
The in vitro gas production technique could provide a quick and simple means of estimating methane emissions by sheep based on the methane production potential of the feeds offered and intake measurements or estimates of feed intake. The technique requires small amounts of feed material, which is freeze-dried, allowing analyses to be conducted several weeks or months after collection. Multiple samples can be analysed at the same time, making the technique practical for use on a reasonably large scale.
Fourier-transform infrared (FTIR) spectroscopy is a rapid-throughput laboratory technique that requires a very small amount of plant or feed material (Allison et al., 2009), making it ideal as a proxy indicator for methane output by sheep provided that it can be used to successfully predict methane emissions using feed samples or faecal matter. The data presented in Section 4.3 demonstrate that it is possible to distinguish between different feed samples on the basis of their FTIR spectra. Furthermore, there is potential for FTIR spectra of feed samples to successfully predict daily methane emissions from sheep as measured in methane chambers.
While further work is required to optimise the methods used to estimate or predict methane output by sheep, the data collected during this project provide evidence for the potential of three proxy indicators for this purpos
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About this project
This project aims to develop proxies, which can be used to estimate methane output by sheep at an on-farm scale.
The potential proxies to be investigated during this project include the methane potentials of various feeds, obtained using an in vitro gas production technique and a selection of plants is currently being collected on a monthly basis for this purpose. The use of a laser methane detector (LMD), which uses infrared spectroscopy to give measurements for methane concentration in the column of gas, is also being explored. Finally, the detection of archaeol, a membrane lipid of the methanogenic archaea, in faeces is another potential proxy.
Methane emissions from sheep are being measured using the chamber technique, and these are being used to calibrate methane production measured using the proxy techniques under investigation. Results to date indicate that the LMD is sensitive enough to detect eructation peaks of methane from the animal’s mouth as well as normal breath concentrations, and investigations are currently focussed on the length of time required to achieve representative data from individual animals, and whether environmental measurements (i.e. from the sheep’s surroundings) provide useful data.