**2. Livestock GHG emissions**

Livestock emissions depend considerably on some of the environmental characteristics such as the mean annual temperature, geographic location and the economic level of the country. It has been observed that in developing and emerging countries, the dietary habits increase meat consumption contributing to these emissions [4, 13], nevertheless developed countries have a greater proportion of intensive animal production, which results in higher emissions of CH<sup>4</sup> , which is estimated to be 150.7 g/cow/day by cattle [4]. Additionally, the size and productivity of animals affect their feed intake and enteric CH<sup>4</sup> emissions [14], which can vary by animal type, growth stage and composition of diet [15, 16]. Castelán-Ortega et al. [17] reported that the average CH<sup>4</sup> emissions by individual dairy cattle are higher in the tropics than in temperate regions, 319.1 and 283 g/day, respectively. This could be attributed to the elevated proportion of cellulose in tropical forages, which is reported to produce three times more CH<sup>4</sup> than hemicellulose.

The estimation of livestock emissions differs considerably between studies as different models are employed for their estimation. Some authors use their own models, but most of the authors follow the guidelines of IPCC [18]. However, the differences on estimations still continue. Tier I utilizes default global or regional emission factors. Tier II utilizes estimated regional or local emission factors and is used in some enteric fermentation studies, nevertheless Tier III is the most reliable model for enteric CH<sup>4</sup> emission and has several advantages compared to Tier II, because it represents mechanisms of enteric fermentation in more detail and can be expected to describe more of the variations caused due to nutritional and animal factors [8, 19].

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

Herrero et al. [5] estimated the total emissions from livestock were in the range of 5.6 – 7.5

(~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

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

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].

Livestock emissions depend considerably on some of the environmental characteristics such as the mean annual temperature, geographic location and the economic level of the country. It has been observed that in developing and emerging countries, the dietary habits increase meat consumption contributing to these emissions [4, 13], nevertheless developed countries have a greater proportion of intensive animal production, which results in higher emissions

vary by animal type, growth stage and composition of diet [15, 16]. Castelán-Ortega et al. [17]

than in temperate regions, 319.1 and 283 g/day, respectively. This could be attributed to the elevated proportion of cellulose in tropical forages, which is reported to produce three times

productivity of animals affect their feed intake and enteric CH<sup>4</sup>

, which is estimated to be 150.7 g/cow/day by cattle [4]. Additionally, the size and

emissions by individual dairy cattle are higher in the tropics

emissions and are estimated to increase to 30% between 2000 and 2020 [8]. Enteric

(~32.2%), N<sup>2</sup>

–eq) between 1995 and 2005. They observed that the

O emissions associated with feed production

emissions [14], which can

than 25% of global GHG emissions [4].

main sources were enteric CH<sup>4</sup>

**2. Livestock GHG emissions**

reported that the average CH<sup>4</sup>

than hemicellulose.

–eq/year (5.6 to 7.5 × 1012 kg CO<sup>2</sup>

GtCO<sup>2</sup>

52 Livestock Science

total CH<sup>4</sup>

flatus [10].

of CH<sup>4</sup>

more CH<sup>4</sup>

Enteric fermentation in ruminants and manure management emissions contributes directly to around 9% of total anthropogenic emissions. In 1990, enteric methane global emissions were 84 Tg/year CO<sup>2</sup> -eq (84 × 109 kg CO<sup>2</sup> -eq), which increased to 92 Tg/year CO<sup>2</sup> -eq in 2005. It is reported that the main sources of global enteric CH<sup>4</sup> emissions are Asia (33%), followed by Latin America (23.9%), Africa (14.5%), Western Europe (8.3%) and North America (7.1%) [14]. Beef trades also have a significant impact on GHG emissions. Emissions from beef trade represented 2% of total emissions traded internationally in 2010 and increased by 19% during the period between 1990 and 2010. The dominant global fluxes in 2010 were the exportation of emissions embodied in meat from Brazil and Argentina to Russia (2.8 and 1.4 Mt CO<sup>2</sup> -eq (2.8 and 1.4 × 109 kg CO<sup>2</sup> -eq), respectively), emissions embodied in US imports of meat from Canada were the same that emissions embodied in US exports to Mexico of 1.2 Mt CO<sup>2</sup> -eq. Australian meat exported to South Korea also embodied substantial emissions of 1.0 Mt CO<sup>2</sup> -eq. In European countries, meat exported from France to Italy and France to Greece embodied 1.4 and 1.2 Mt CO<sup>2</sup> -eq emissions, respectively. Also Italian meat imported from Poland, Germany and Netherlands embodied 0.7, 0.6 and 0.7 Mt CO<sup>2</sup> -eq emissions, while Chinese emissions embodied in beef exported were small in comparison with the other countries. Although emissions due to import of meat are considered insignificant, it is important to consider all livestock sectors that contribute to emissions [13].

With respect to the Mexico, total CH<sup>4</sup> emissions in 2006 were 8954.10 Gg, and agriculture sector was the highest contributor with significant input due to enteric fermentation and manure management [16]. Earlier, Rendón-Huerta et al. [18] has also reported that enteric CH<sup>4</sup> emissions are the major source of GHG emissions in Mexican livestock production systems. They calculated the GHG emissions from dairy cattle in Mexico for a period of time of 30 years using a Tier II of IPCC and reported that emissions of CH<sup>4</sup> , N2 O and CO<sup>2</sup> -eq during 1970 to 2010 increased from 144 to 270, 0.349 to 0.713 and 3704 to 6962 Mt/year, respectively. They observed that methane emissions per cow increased by 11%, while per liter of milk decreased by 30%. In the past 40 years, total N<sup>2</sup> O emission increased by 104%, but N<sup>2</sup> O/cow emissions increased only by 22% in the same period and decreased by 25% per liter of milk. The reduction in GHG emissions per liter of milk means an increase in the efficiency of production systems resulting in an augmentation of milk production per cow and consequentially diminishing the emissions [18]. Hernández-De Lira et al. [16] based on animal census data from 2012, reported that the methane emissions by enteric fermentation in Mexico were 1926.08 Gg CH<sup>4</sup> , of which beef cattle produced 1651.8 Gg CH<sup>4</sup> ; while dairy cows generated only 172.70 Gg CH<sup>4</sup> .

Emissions by manure management, mostly CH<sup>4</sup> and N2 O, are produced during the manure decomposition carried out by anaerobic microbial activities. These emissions depend on specific manure composition and quantity produced which, in turn is dependent on other factors as animal type, breed, weight, diet and climate conditions. Although CH<sup>4</sup> emissions from enteric fermentation are higher than those from manure [13, 16], manures also contribute to N2 O emissions due to volatile nitrogen losses, principally in form of ammonia (NH<sup>3</sup> ) and NOx [13]. They have reported that CH<sup>4</sup> and N2 O emissions from manure would increase by 20 and 29%, respectively, from 2000 to 2020.

Asia, particularly China, Western Europe and North America are the regions with the highest GHG emissions from manure management [14]. According to EPA [20], global GHG emissions from manure management were 446 million tonnes of CO<sup>2</sup> -eq, of which the share of CH<sup>4</sup> and N2 O was 53 and 47%, respectively, while FAO [3] estimated global GHG emissions from manure management were 368 million tonnes of CO<sup>2</sup> -eq. In case of Mexico, CH<sup>4</sup> emission from manure was 62.24 Gg CH<sup>4</sup> , where beef cattle and dairy cow emitted with 29.49 and 2.42 Gg CH<sup>4</sup> , respectively [16]. Similarly, FAO [3] reported that Asia, Central and South America, Sub-Saharan Africa, Western Europe, North America, Eastern Europe and the Commonwealth of Independent States were the regions with the highest emissions of N2 O due to manure [14].
