**1. Introduction**

Global warming has been attributed to the increment of atmospheric concentration of greenhouse gases (GHGs). Since 1750, concentrations of CO<sup>2</sup> , CH<sup>4</sup> and N2 O had increased by 40, 150 and 20%, respectively, until 2014, and the rate of increment of GHG per year from 2000 to 2010 was approximately 2.2% [1]. Of various anthropogenic activities contributing to global

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

warming, agriculture is an important source. This sector is responsible for 18% of the total anthropogenic GHG emissions annually [2]. Livestock represents the most important cause of GHG from agriculture contributing approximately to 80% of these emissions [3] and more than 25% of global GHG emissions [4].

Herrero et al. [5] estimated the total emissions from livestock were in the range of 5.6 – 7.5 GtCO<sup>2</sup> –eq/year (5.6 to 7.5 × 1012 kg CO<sup>2</sup> –eq) between 1995 and 2005. They observed that the main sources were enteric CH<sup>4</sup> (~32.2%), N<sup>2</sup> O emissions associated with feed production (~27.45%) and land use for animal feed and pasture (~24.42%). Havlík et al. [6] opined that ruminants represent more than 80% livestock emissions; particularly, beef and dairy sector contribute to about 60% [7]. Emissions from enteric fermentation contribute to 8% of total CH<sup>4</sup> emissions and are estimated to increase to 30% between 2000 and 2020 [8]. Enteric fermentation is the normal process of feed digestion in ruminants and is mediated by the microbial activity in the rumen and in the large intestines. Significant amount of methane is produced by methanogens residing within the rumen (87%) [9], which is released principally through eructation, approximately 10–15% is emitted by normal respiration and via flatus [10].

The continued growth of human population and consequent demand for food are potential drivers of GHG emissions. International climate negotiators have been focused to reduce GHG emissions by the improvement of engineering processes, energy efficiency and investments on alternative energy generation technologies. However, the abatement of ruminant GHG emissions has not received adequate attention by the United Nations Framework Convention on Climate Change [11]. Even so, several research groups have been working to develop strategies to optimize ruminal functions in order to achieve the desired levels of production by enhancing feed conversion efficiency and simultaneously reducing methane emissions by manipulating the rumen microorganisms. It is essential to have a detailed knowledge of ruminal microbiome, their interactions among themselves and with the host to achieve these objectives, and to identify the new approaches for mitigation of GHGs emissions [12].
