**3. Rumen environment**

Ruminants are herbivorous mammals considered as latecomers in evolution. Their forestomach is a very complex environment, which allows them to convert plant tissues into nutritious and useful products. The digestive tract of ruminants is formed by various compartments such as reticulum, rumen, omasum, abomasum, small intestine, cecum, colon and rectum [21]. The ruminant stomach is composed by three pregastric fermentation chambers (rumen, reticulum and omasum) [22] (**Figure 1**). Environmental conditions such as temperature (38–42° C), redox potential (250 to 450 mV), pH (5.5–7) controlled by buffer in saliva and osmolarity (260–340 mOsm) [23] provide the ideal conditions for the digestion of plant material by microorganisms. Fibrous components are hydrolyzed and fermented by the interactions among different microbial communities inhabiting the rumen, producing mainly acetate, propionate and butyrate, CO<sup>2</sup> , H2 and CH<sup>4</sup> . VFAs are the most important source of energy for the animal (75% of the total amount of the digested energy) [24]. Moreover, microbial cell biomass is the major source of protein and amino acids [25]. Microbial population also synthetizes vitamins B and K and employs detoxification mechanisms for phytotoxins and mycotoxins [26].

Microbial ruminant ecosystem is composed by a high microbial population density, predominantly obligate anaerobic microorganisms. Bacteria are the most abundant microorganisms and more than 50% of the cell mass in the rumen are comprised of at least 50 bacterial genera (1010 –1011 ml-1), followed by 25 genera of ciliate protozoa (10<sup>4</sup> –106 ml-1), six genera of fungi (10<sup>3</sup> –106 ml<sup>1</sup> ), methanogenic archaea (107–1010 ml1 ) and bacteriophages (108–109 ml1 ) [27–29], nevertheless only 10% of these microbiome have been identified and described [30]. Livestock Methane Emission: Microbial Ecology and Mitigation Strategies http://dx.doi.org/10.5772/65859 55

Emissions by manure management, mostly CH<sup>4</sup>

[13]. They have reported that CH<sup>4</sup>

29%, respectively, from 2000 to 2020.

emission from manure was 62.24 Gg CH<sup>4</sup>

N2

54 Livestock Science

of CH<sup>4</sup>

N2

ture (38–42°

and mycotoxins [26].

–106 ml<sup>1</sup>

fungi (10<sup>3</sup>

and N2

29.49 and 2.42 Gg CH<sup>4</sup>

O due to manure [14].

**3. Rumen environment**

acetate, propionate and butyrate, CO<sup>2</sup>

and N2

O was 53 and 47%, respectively, while FAO [3] estimated global GHG emis-

, respectively [16]. Similarly, FAO [3] reported that Asia, Central and

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

enteric fermentation are higher than those from manure [13, 16], manures also contribute to

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

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

Ruminants are herbivorous mammals considered as latecomers in evolution. Their forestomach is a very complex environment, which allows them to convert plant tissues into nutritious and useful products. The digestive tract of ruminants is formed by various compartments such as reticulum, rumen, omasum, abomasum, small intestine, cecum, colon and rectum [21]. The ruminant stomach is composed by three pregastric fermentation chambers (rumen, reticulum and omasum) [22] (**Figure 1**). Environmental conditions such as tempera-

osmolarity (260–340 mOsm) [23] provide the ideal conditions for the digestion of plant material by microorganisms. Fibrous components are hydrolyzed and fermented by the interactions among different microbial communities inhabiting the rumen, producing mainly

and CH<sup>4</sup>

energy for the animal (75% of the total amount of the digested energy) [24]. Moreover, microbial cell biomass is the major source of protein and amino acids [25]. Microbial population also synthetizes vitamins B and K and employs detoxification mechanisms for phytotoxins

Microbial ruminant ecosystem is composed by a high microbial population density, predominantly obligate anaerobic microorganisms. Bacteria are the most abundant microorganisms and more than 50% of the cell mass in the rumen are comprised of at least 50 bacterial

[27–29], nevertheless only 10% of these microbiome have been identified and described [30].

, H2

genera (1010 –1011 ml-1), followed by 25 genera of ciliate protozoa (10<sup>4</sup>

), methanogenic archaea (107–1010 ml1

C), redox potential (250 to 450 mV), pH (5.5–7) controlled by buffer in saliva and

O emissions due to volatile nitrogen losses, principally in form of ammonia (NH<sup>3</sup>

as animal type, breed, weight, diet and climate conditions. Although CH<sup>4</sup>

and N2

emissions from manure management were 446 million tonnes of CO<sup>2</sup>

sions from manure management were 368 million tonnes of CO<sup>2</sup>

O, are produced during the manure

O emissions from manure would increase by 20 and

, where beef cattle and dairy cow emitted with

. VFAs are the most important source of

–106

) and bacteriophages (108–109 ml1

ml-1), six genera of

)

emissions from



) and NOx

**Figure 1.** Ruminant digestion process. Note: Gastrointestinal tract of ruminants and main biochemical processes occurr ing in it.

The interactions of these microorganisms are widely different, namely mutualism, commensalism, syntrophy, competition and depredation [31, 32].

Hydrolysis of plant polysaccharide material is the first step in the enteric fermentation process, and 80% of plant cell material degradation is carried out by bacteria and fungi, and the rest 20% is by protozoa [33]. In the second stage, monomers are fermented to VFAs, branched chain VFAs, organic acids (lactate), alcohols, CO<sup>2</sup> and H2 . VFAs are absorbed by the rumen and omasal walls of the host animal for its nutrition [10]. Though several parameters such as rumen fluid, volume, pH and VFAs, concentration can disturb this absorption [34]. Free acids can be oxidized by obligate hydrogen producing bacteria to acetate, albeit this reaction is thermodynamically non-favorable, and hence are carried out only in synthropic association with hydrogen consuming bacteria or archaea, which diminish the partial pressure of H2 . When the conditions are not favorable, VFAs are accumulated, decreasing the pH and inhibiting rumen microbiome [35, 36]. NH<sup>3</sup> is produced due to proteolysis and can be used by microorganisms to build their own proteins. The excess of NH<sup>3</sup> is absorbed by the rumen wall and transported by the animal blood [37]. The digested proteins, lipids and the carbohydrate constituents of microbial cells are exploited in the small intestine for the maintenance of the animal and the production of meat and milk. During enteric fermentation, a large quantity of CO<sup>2</sup> is produced due to diverse biochemical processes. A part of this CO<sup>2</sup> produced is released through eructation or normal respiration, and other part is reduced with H2 to CH<sup>4</sup> by hydrogenotrophic methanogens. Methane produced is primarily released through eructation and approximately 10–15% is emitted by normal respiration and via flatus [10]. CH<sup>4</sup> production can be accomplished by the reduction of acetate and methyl-containing C1 compounds, nonetheless these pathways are not common in the rumen [38]. About 2–12% of gross energy intake (GEI) produced in the rumen by fermentation is converted to methane, which apart from leading to the loss of the feed energy, results in the emission and consequently, global warming [39].
