# Theoretical Methane Emission Estimation from Volatile Fatty Acids in Bovine Rumen Fluid
## 1 Introduction  
Food and Agriculture Organization (FAO) statistics show that the worldwide supply of animal protein has risen from 34 to 43 kg per capita per year between 1993 and 2013 [1]. Large inequalities in protein consumption between countries mean that the annual consumption of animal proteins in many wealthier nations far exceeds this amount. For example, in 2013, the annual per capita animal protein supply in North America, the European Union, and Australia and New Zealand stood at 113 kg, 81 kg, and 114 kg, respectively [1]. To set our massive demand for meat and dairy products, concentrated animal feeding operations (CAFOs) have inevitably served the long-term viability of the livestock industry over the last three decades [2, 3, 4]. Despite their economic benefits and production efficiencies, CAFOs have triggered unwanted environmental problems due to the large production of manure waste, far exceeding the capacity of land to assimilate the loadings of organic carbon and nutrients [5, 6]. One of the recent urgent issues associated with CAFOs is the loss of gaseous species to the ambient air stream, which has contributed to climate change by means of emitting potent greenhouse gasses such as CH\({}_{4}\) and N\({}_{2}\)O [7, 8]. For instance, animal agriculture contributes 9% of anthropogenic CO\({}_{2}\) emissions, 37% of CH\({}_{4}\) emissions, and 65% of N\({}_{2}\)O emissions, and the combined emissions expressed as a CO\({}_{2}\) equivalent amounts to about 18% of anthropogenic greenhouse gas (GHG) emissions [7]. Despite the well-defined guidelines to estimate GHG emissions from the Intergovernmental Panel on Climate Change (IPCC), the whole rationale for estimating GHG emissions from the livestock sector is only sensible with the support of robust and accurate data sets [9, 10]. To this end, it is highly desirable to establish technical advancements to provide robust and accurate data sets for estimating GHG emissions from the livestock sector. Among the various GHGs from the livestock industry, CH\({}_{4}\) is a major ubiquitous GHG during the normal digestive process in ruminant animals, and its global warming potential is 25 times that of CO\({}_{2}\)[11]. For instance, ruminal methanogens use the methanogenesis pathway to maintain low H\({}_{2}\) partial pressure and to facilitate fiber digestion in the rumen by converting H\({}_{2}\) to CH\({}_{4}\)[12]. There are two strategies to reduce CH\({}_{4}\) emissions from the livestock sector. One strategy is dietary manipulation [11], and another strategy is to improve the efficiency of ruminal function and to mitigate methane release [12]. However, quantification of CH\({}_{4}\) emissions from the livestock industry is challenging because a limited number of individual animals are monitored in any study, and correction factors are required to calculate actual CH\({}_{4}\) emission values [10]. Moreover, direct quantification of individual animal CH\({}_{4}\) emissions in open-circuit respiration chambers or using the sulfur hexafluoride (SF6) tracer technique requires considerable investment in infrastructure and technical support and may impact animal feeding behavior [10; 12]. VAF quantification methods through a catalytic esterification reaction (CER) requires a biochemical methane potential (BMP) test that allows alternative measurements compared with the conventional methane estimation methods such as respiration chamber techniques or direct stomach portable treatment through the livestock. It is urgent that a quantitative methodology for estimating CH\({}_{4}\) emissions from the livestock sector should be developed. Better accuracy in CER VFA measurements will provide compensation to methane emission inventory, especially from livestock. This case study aims to contribute to the development of more accurate methods for the quantification and specification of volatile fatty acids (VFAs) in bovine rumen.