Methane is primarily emitted through anaerobic decomposition of organic matter. Recent research indicates that plants themselves may emit small amounts of CH₄, although the mechanism is not currently known [39] [40] . This implies that croplands could provide an emission of CH₄ under certain circumstances. Soils can emit CH₄ in two ways. First, decomposition of organic matter by microorganisms in poorly aerated soils (rice paddies, wetlands) can lead to emissions of CH₄ rather than CO₂. Throughout the year, portions of agricultural fields can be saturated with water, creating anaerobic microsites. This may cause minimal amounts of CH₄ to be emitted. However, over an entire year, agricultural soils typically are well-aerated, and CH₄ is oxidized by soil microorganisms [40] .

Interest in measuring enteric CH₄ emissions has moved from a focus on nutritional inefficiency to one of contributing to GHG concentrations in the atmosphere and climate change [29] [41] . Measurement of CH₄ emissions due to enteric fermentation must be also taken under conditions as close as possible to typical as found in farming systems.

Enteric CH₄ is a by-product of ruminant digestion produced by methanogenic microorganisms Archaea in a process called fermentation or methanogenesis. The rate and type of fermentation is influenced by animal factors such as chewing, salivation and digesta kinetics [42] [43] . Cattle produce about 7 and 9 times as much CH₄ as sheep and goats, respectively. Enteric CH₄ is produced mainly in the rumen (87% - 90%) and, to a smaller extent (13% - 10%), in the large intestine [44] [45] .

Animal releases CH₄ into the atmosphere by exhaling the gas mainly through the mouth and nostrils [23] . Of the CH₄ produced by enteric fermentation in the forestomach 95% was excreted by eructation, and from CH₄ produced in the hindgut 89% was found to be excreted through the breath and only 11% through the anus [44] . Work [46] recorded 3% from the anus (from the total CH₄ enteric emissions released through mouth, nostrils, and rectum). The concentration in the breath is variable with a relatively low concentration when the breath comes from the lungs and a higher concentration when the “breath” is gases belched from the forestomachs, although breath from lungs also contain absorbed CH₄ and inhaled together with air [24] . In a barn or larger room the concentration will to a large extent be influenced by the air exchange, but the concentration of CH₄ will be a total mix of the CH₄ from breath, belch and fart [24] .

The rumen ecosystem is an anaerobic environment, in which the degradation of plant material occurs in a very short time frame compared with other anaerobic ecosystems such as wetlands and estuaries, and the fermentation products are different. Some of the microbial species have coevolved with ruminants and hindgut- fermenting mammals and do not exist in any other environment (e.g., rumen protozoa) [47] [48] . Digestion of feed components by the microbiota (bacteria, protozoa, fungi) results in the production of volatile fatty acids. These acids, mainly acetate, propionate, and butyrate are used by the animal as source of energy. During the process gases are also formed and their production eliminated mainly through eructation. CO₂ and H₂ are using to form CH₄, and thus reducing the metabolic H₂ produced during microbial metabolism [4] [49] . Fermentation is an oxidative process, during which reduced cofactors (NADH, NADPH, FADH) are re-oxidized (NAD-1, NADP-1, FAD-1) through dehydrogenation reactions releasing hydrogen in the rumen. As soon as produced, hydrogen is used by methanogenic archaea, a microbial group distinct from Eubacteria, to reduce CO₂ into CH₄ [11] .

Note that enteric methane produced by ruminants is a loss of feed energy from the diet and represents inefficient utilization of the feed [23] . In addition to environmental implications, ruminant methanogenesis represents a loss of 2% to 12% of the gross energy intake [29] [47] [50] [51] . However, it was found that small amounts of CH₄are formed also in birds. Methanobrevibacter-related phylotypes have been found to be the most prevalent methanogens in the foregut and in the hindgut of the chicken of the hoatzin (Ophistocomushoatzin). The hoatzin is the only bird species to have evolved a foregut fermentation system, analogous to the rumen [13] .

In addition to enteric (animal-derived) CH₄, excreta are another source of CH₄, especially when stored an aerobically [52] . Methane generated from manure from ruminant and no ruminant livestock contributes 2% and 0.4% of global CH₄ and GHG emissions, respectively. In regions with low input is enteric fermentation undoubtedly the main emission source. However, in industrialized regions with high production and food processing is manure important a source of emissions [12] . Manure CH₄ emissions are a larger proportion of total farm CH₄ emissions in intensively managed dairy operations with manure storage systems, and much lower in extensive or grazing operations [48] .

Manure emissions are relatively high in areas where manure from the dairy sector is managed in liquid systems that produce greater quantities of CH₄ emissions [12] . During manure storage, CH₄ is generated through a reaction similar to that of enteric fermentation. Cellulose in the manure is degraded by microbes, with products of this process serving as substrates for methanogenesis [40] .

Livestock manure contains portion of organic solids such as proteins, carbohydrates and fats that are available as food and energy for growth of anaerobic bacteria. Obvious benefit from methane production could be the energy value of the gas itself. But the gas production from manure depends mainly upon the efficiency of operating system for it. Gas yield can be a certain amount of gas produced per unit of solids degraded by the anaerobic bacteria [53] . Anaerobic digestion is a natural process in which the microorganisms consume organic matter under an oxygen-free environment. It results in production of microbial biomass and greenhouse gases (CO₂ and CH₄). The composition of volatile solids contained in manure influence the anaerobic decomposition of organic matter and the production of CH₄. The manure volatile solids are mainly composed of fatty acids, proteins and carbohydrates of which fatty acids, proteins and a part of carbohydrates are easily biodegradable [54] .

Study [55] concluded that feed intake is the superior factor of total CH₄ production. The amount of enteric CH₄ is mainly related to the type and amount of feed [15] [56] - [59] . Gross energy (GE) is negatively related to feeding level and dietary fat concentration and positively to diet digestibility, whereas dietary carbohydrate composition has only minor effects. As the daily feed intake increases, CH₄ production also generally increases [15] . Most studies agree that dry matter intake (DMI) is the main driver of daily methane output, although methane output per kilogram of DMI decreases with increasing feeding level [60] , diet digestibility, and with increasing proportions of concentrates or lipids in the diet [8] [61] .

There were found higher variability in the quantity of CH₄ emitted per unit of feed intake in grazing ruminants [51] [62] - [65] . The work of [66] suggests that non-grazing low forage feeding system result in the lowest enteric CH₄ emissions・kg⁻¹ energy corrected milk, with about 13% less enteric CH₄ compared to a high forage feeding system at the same farm. Body weight and milk yield accounted for significant proportions of variation in CH₄ emissions. Both parameters were positively related to methane concentrations [67] .

The composition of feed or the quality of forage influences CH₄ production in ruminants. Digestion in the rumen is dependent on the activity of microorganisms, which need energy, nitrogen and minerals [8] [15] . Therefore, the quality of forage affects the activity of rumen microbes and CH₄ production in the rumen. Forage species, forage processing, proportion of forage in the diet, and the source of the grain also influence CH₄ production in ruminants. Methane production tends to decrease as the protein content of feed increases, and increases as the fiber content of feed increases [15] [29] . CH₄ production was positively related to diet digestibility and negatively related to dietary fat concentration, whereas dietary carbohydrate composition had only minor effects [68] . Production of CH₄ has a negative impact on animal productivity, resulting in lost energy ranging from 2% to 12% of the animal’s GEI [55] [69] .

Authors [73] calculated enteric CH₄ emission rates using a procedure that reflects the development of the calves’ rumen as well as German national animal performance data and representative diet properties. Calves (birth weight of 41 kg, final LW of 125 kg and a mean weight gain of 0.67 kg・day⁻¹) had the emission rate of 9.4 kg CH₄ per place and year.

Methane emissions by dairy cows vary with body weight, feed intake, diet composition, and milk yield. When cows are fed the same diet at the same intake, however, variation between cows in CH₄ emissions can be substantial [74] . Study [45] estimated by using the sulfur hexafluoride tracer technique adapted to collect breath samples over 5-day periods expressed methane emission in grazing dairy cows as absolute value (368 g・day⁻¹ or 516 L・day⁻¹). Work [75] evaluated enteric CH₄ emissions from 1964 Holstein cows across 21 farms for at least 7 days using CH₄ analyzers at robotic milking stations. Cows were fed the same feeding systems during sampling. Methane concentrations (in milligrams per L of air sampled by the analyzer) were quantified in gas released by eructation during milking. The average CH₄ concentration across farms was 2.9 mg・L⁻¹ which would equate to 418 g・day⁻¹ of eructed CH₄. [67] using the relationship between CH₄ emission rate during milking and daily CH₄ emissions measured in respiration chambers observed for cows on the same dietary regimen the overall mean CH₄ emissions was 369 g・d⁻¹ and the range was 278 to 456 g・day⁻¹. Reference [40] found annual CH₄ emissions calculated on livestock unit (LU is 500 kg) from 58 kg・LU⁻¹ in dry cows to 106 kg・LU⁻¹ for lactating cows. Lactating cows emit approximately twice the amount of CH₄ as compared to either dry cows or heifers; this is largely due to their increased feed intake, although ration and animal size also have an effect. These emission factors may include emissions from feces deposited on the barn floor, which would be much less than emissions from enteric fermentation [40] . Authors [20] recorded annual CH₄ emissions from enteric CH₄ fermentation 107 kg for dairy cow with a milk yield of 7870 kg・head⁻¹. The corresponding value for dairy ewe was 8.4 kg・head⁻¹. Study [76] evaluated dairy cows fed a diet with forage: concentrate ratio of 500:500 or 900:100 g・kg⁻¹ of DM of total DMI. Mean CH₄ yields did not differ between diets, being 16.9 and 20.2 g・kg⁻¹ DMI for the 500:500 and 900:100 diets, respectively. Methane productions were 267 and 339 g・day⁻¹・cow⁻¹, respectively. Article [46] found at the DMI of 17.5 kg・d⁻¹ and milk yield of 22.9 kg・d⁻¹ CH₄ measured by sulfur hexafluoride technique of 469 g・d⁻¹ (292 - 647), and CH₄ measured by respiration chamber 422 g・d⁻¹ (275 - 577). They calculated ratios during measuring by respiration chamber technique CH₄: DMI of 24.3 g・kg⁻¹ (14.1 - 29.2) and CH₄: milk yield of 19.9 g・kg⁻¹ (6.9 - 54.2).

The study of [77] recorded from lactating and dry cows and heifers on pasture under tropical conditions, using the tracer gas technique that Holstein produced more CH₄ (299.3 g・d⁻¹) than the Crossbred (264.2 g・d⁻¹). Lactating cows produced more CH₄ (353.8 g・d⁻¹) than dry cows (268.8 g・d⁻¹) and heifers (222.6 g・d⁻¹). Dairy cows emit approximately 430 g・d⁻¹ at peak lactation down to 250 g・d⁻¹ as milk yield declines [78] [79] . Holstein cows produced less CH₄ per unit of dry matter intake (19.1 g・kg⁻¹) than the Crossbred (22.0 g・kg⁻¹). Methane emission by heifers grazing fertilized pasture was higher (222.6 g・day⁻¹) than that of heifers on unfertilized pasture (179.2 g・day⁻¹) [77] . Authors [80] measured CH₄ emissions using the sulfur hexafluoride tracer technique in grazing Jersey × Friesian dairy cows in mid lactation. The average DMI was estimated to be 17.1 kg・d⁻¹. Daily CH₄ emissions ranged from 151 to 497 g・day⁻¹ with an average of 311 g・day⁻¹ or 18.2 g・kg⁻¹ DMI.

In the [81] study, data from purebred Holstein, Simmental and Jersey cows were analyzed to test the assumption that there are genetically low methane-producing animals. Methane emission of cows offered forage ad libitum and some concentrate was measured in open-circuit respiration chambers The Holstein, Simmental and Jersey cows emitted on average 25 g・kg DMI⁻¹, 25 g・kg DMI⁻¹, and 26 g・kg DMI⁻¹, respectively. There was no indication of individual cows with persistently low or high CH₄ yield per kg DMI and per kg of milk. Authors [51] in an experiment involving 302 lactating dairy cows revealed that CH₄ emissions per DMI ranged from 11 to 31 g・kg⁻¹ DMI, with mean CH₄ emissions from the lower and upper quartile being 16 and 23 g・kg⁻¹ DMI, respectively. Reference [82] calculated 15.4 g・kg⁻¹ FPCM enteric CH₄ emitted. Methane emissions expressed as absolute value are according to [45] 368 g・day⁻¹ or 516 Ld⁻¹. The variation in CH₄ yield remained among the animals (26.4 ± 3.6 g・kg⁻¹ DMI). The change in CH₄ yields may have resulted from lower feed intakes of lower quality pasture compared with grazing. Regression analysis showed that absolute CH₄ emission (g・day⁻¹) was best described by DMI and rumen acetate concentration [51] .

Sometimes it is stated that increased production of methane causes bloating cows. Study [83] selected Friesian, Jersey, and Friesian × Jersey cross dairy cattle for high or low susceptibility to pasture bloat. They found that cattle with low or high bloat susceptibility are also differing in CH₄ emissions. So, bloat susceptibility is a genetically inherited trait. At the research of [84] , the mean absolute CH₄ emissions from low bloat cows were significantly higher than from high bloat cows (144.5 vs. 107.4 g・d⁻¹ and 147.9 vs. 119 g・d⁻¹) during both periods (5 or 4 consecutive measuring days). However, on per unit of LW basis, CH₄ emissions from low and high bloat animals were not different from each other either at period 1 (346 vs. 312 mg・kg・LW⁻¹) or period 2 (345 vs. 347 mg・kg・LW⁻¹).

Primary sources of CH₄ on dairy farms are not only animals but also manure storage, with smaller contributions from field-applied manure, feces deposited by grazing animals, and manure on barn floors. Reference [40] calculated in simulating representative 100-cow dairy farm annual emissions of 142 kg CH₄ per Holstein cow and 6.4 kg CH₄ per m³ of slurry manure in storage. Feed intake was the primary predictor of total CH₄ production.

Reference [85] showed in steers of average LW 325 kg based on the animal-scale method, the average CH₄ emission rate over 9 days 161 ± 20 g・day⁻¹. There was a significant difference between two contrasting diets (Lucerne silage diet, cereal, Lucerne, and straw mixed ration) in daily CH₄ production, with mean methane production of 124.3 g・day⁻¹ and 169.8 g・day⁻¹ [86] . On average, mature beef cows emit CH₄ from 240 g・day⁻¹ to 350 g・day⁻¹ [78] [79] . Authors [14] found that the CH₄ emission rates corresponding to values of 190 g・day⁻¹ per beef cattle head and [20] recorded annually 60 kg・head⁻¹. The results of [87] show that daily CH₄ emissions differed about 7% according to technique (185 vs. 199 g・day⁻¹ per animal).

Average daily CH₄ emissions were 323 g・day⁻¹ per animal in feedlot. Emissions from the runoff retention pond associated directly with the feedlot operation were approximately 2.7 g of the daily average feedlot emissions of CH₄ [69] . Animals kept in feedlots, as opposed to pasture, emit less CH₄ per kilogram of weight gain due to decreased forage consumption, increased grain in the diet and decreased activity [69] . Authors [88] compared CH₄ emissions from a typical feedlot in Australia and in Canada. The average CH₄ emission was 166 ± 90 and 214 ± 61 g・day⁻¹ per animal for the feedlot in Queensland (AU) and in Alberta (CA), respectively. The lower CH₄ emission at the Queensland feedlot was attributed to the lighter weight of the cattle, and consequently their lower intake, and supplementation of the diet with lipids. Authors [89] studied beef feedlots in the north (Queensland) and south (Victoria) of the Australia. The data show a range of CH₄ emissions from 146 ganimal^?1・day⁻¹ in Victoria to 166 ganimal⁻¹・day⁻¹ in Queensland. Values obtained in the study of [90] on feedlot cattle included both enteric CH₄ and CH₄ derived from the manure deposited. The CH₄ emissions encompass a range from a low of 79 g・day⁻¹ to a peak of 395.8 g・day⁻¹ [90] .

Suffolk sheep emit 22 - 25 g・day⁻¹ [78] [91] . The bison’s annual CH₄ emissions based on calorimeter measurements at 411 kg of LW, 3.4 t DMI per head and year were 72 kg・head⁻¹ [92] . Reference [93] found in developed countries estimated CH₄ emission rate 150.7, 137, 21.9 and 13.7 (ganimal^?1・day⁻¹) per cattle, buffaloes, sheep, and goat, respectively.

Methane production from manure management was 33.2, 2.0 and 0.3 kg・head^?1・year^?1 for dairy cattle, beef cattle and dairy ewes [20] . Stored liquid animal manure is an important emissions source of CH₄ globally [94] . Fluxes were measured from a circular concrete tank 11.25 m in diameter storing liquid dairy manure. Monthly average CH₄ flux ranged from 11 µgm^?2・s^?1 in June after the tank had been emptied, to 153 µgm^?2・s^?1 in July [94] .

It was observed that the excreta of animals grazing in the morning emitted much more CH₄ than that of steers grazing in the afternoon [95] . The CH₄ emission depends on the physical form of the feces (shape, size, density, humidity), the amount of digestible material, the climate (temperature and humidity) and the time they remained intact [95] -[97] . Expected amount of excreta for steers may be around 1.5 - 2.2 kg of dry matter per day, so the measured values must be 730 kg・year^?1. Consequently, the total CH₄ emission can be estimated to be from 0.012 kg・head^?1・year^?1 to 0.067 kg・head^?1・year^?1.

Source [2] quantified summer CH₄ emissions from wastewater lagoons of a commercial dairy farm. Methane concentrations over three lagoons (total area of 1.8 ha) and background concentrations were measured. Methane concentrations in the air over the lagoons ranged from 3 to 12 ppm, and averaged 5.6 ppm, with a background CH₄ concentration of 1.83 ppm. Methane flux density (emission rate. unit area^?1) ranged from 165 to 1184 g・m^?2・s^?1, with a mean daily flux density of 402 kg・ha^?1・d^?1. Methane emission rate averaged 0.211 kg・head^?1・day^?1. Reference [53] estimated the average potential CH₄ production from the livestock manure, and found the production of 692, 946, 125 and 6.4 cm³ daily from dairy cattle (545 kg), beef cattle (450 kg), swine (68 kg) and poultry (1.8 kg), respectively.

Authors [98] measured CH₄ concentrations over the pens, wastewater storage pond, and composting area on a 700-cow open-lot dairy farm. Average CH₄ concentrations over the pens, storage pond, and composting area ranged from 2.07 to 2.80 parts per million by volume (ppmv), 1.87 to 2.15 ppmv, and 1.71 to 1.76 ppmv, respectively. Combined CH₄ emissions for the pen and storage pond areas were 0.34, 0.55, 0.21, and 0.20 kg・cow^?1・day^?1 for the January, March, June, and September. The annual emissions from the pens and storage pond at this dairy operation were 120 kg・cow^?1.