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Enteric fermentation is a digestive process by which carbohydrates are broken down by microorganisms into simple molecules for absorption into the bloodstream of an animal. Because of human agricultural reliance in many parts of the world on animals which digest by enteric fermentation, it is one of the factors in increased
methane emissions
Increasing methane emissions are a major contributor to the rising concentration of greenhouse gases in Earth's atmosphere, and are responsible for up to one-third of near-term global heating. During 2019, about 60% (360 million tons) of methane r ...
.
Ruminants
Ruminant animals are those that have a
rumen
The rumen, also known as a paunch, is the largest stomach compartment in ruminants and the larger part of the reticulorumen, which is the first chamber in the alimentary canal of ruminant animals. The rumen's microbial favoring environment al ...
. A rumen is a multichambered stomach found almost exclusively among some
artiodactyl
The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...
mammals, such as
cattle
Cattle (''Bos taurus'') are large, domesticated, cloven-hooved, herbivores. They are a prominent modern member of the subfamily Bovinae and the most widespread species of the genus ''Bos''. Adult females are referred to as cows and adult mal ...
,
sheep
Sheep or domestic sheep (''Ovis aries'') are domesticated, ruminant mammals typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus ''Ovis'', in everyday usage it almost always refers to domesticated s ...
, and
deer
Deer or true deer are hoofed ruminant mammals forming the family Cervidae. The two main groups of deer are the Cervinae, including the muntjac, the elk (wapiti), the red deer, and the fallow deer; and the Capreolinae, including the reindeer ...
, enabling them to eat
cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
-enhanced tough plants and grains that
monogastric
A monogastric organism has a simple single-chambered stomach (one stomach). Examples of monogastric herbivores are horses and rabbits. Examples of monogastric omnivores include humans, pigs, hamsters and rats. Furthermore, there are monogastric ...
(i.e., "single-chambered stomached") animals, such as
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
s,
dog
The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...
s, and
cat
The cat (''Felis catus'') is a domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it from the wild members of ...
s, cannot digest. Although
camel
A camel (from: la, camelus and grc-gre, κάμηλος (''kamēlos'') from Hebrew or Phoenician: גָמָל ''gāmāl''.) is an even-toed ungulate in the genus ''Camelus'' that bears distinctive fatty deposits known as "humps" on its back. C ...
s are thought to be ruminants they are not true ruminants.
Enteric fermentation occurs when methane (CH
4) is produced in the rumen as microbial fermentation takes place. Over 200 species of microorganisms are present in the rumen, although only about 10% of these play an important role in digestion. Most of the CH
4 byproduct is
belched by the animal. However, a small percentage of CH
4 is also produced in the
large intestine
The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in tetrapods. Water is absorbed here and the remaining waste material is stored in the rectum as feces before being r ...
and passed out as
flatulence
Flatulence, in humans, is the expulsion of gas from the intestines via the anus, commonly referred to as farting. "Flatus" is the medical word for gas generated in the stomach or bowels. A proportion of intestinal gas may be swallowed environm ...
.
Methane emissions are an important contribution to global
greenhouse gas emissions
Greenhouse gas emissions from human activities strengthen the greenhouse effect, contributing to climate change. Most is carbon dioxide from burning fossil fuels: coal, oil, and natural gas. The largest emitters include coal in China and lar ...
. The IPCC reports that methane is more than twenty times as effective as CO
2 at trapping heat in the atmosphere - though note that it is produced in substantially smaller amounts. Methane represents also a significant energy loss to the animal ranging from 2 to 12% of gross energy intake. So, decreasing the production of enteric CH
4 from ruminants without altering animal production is desirable both as a strategy to reduce global greenhouse gas emissions and as a means of improving feed conversion efficiency. In Australia ruminant animals account for over half of their green house gas contribution from methane. Australia has implemented a voluntary immunization program for cattle in order to help reduce
flatulence
Flatulence, in humans, is the expulsion of gas from the intestines via the anus, commonly referred to as farting. "Flatus" is the medical word for gas generated in the stomach or bowels. A proportion of intestinal gas may be swallowed environm ...
-produced CH
4.
However, in Australia there are ruminant species of the
kangaroos
Kangaroos are four marsupials from the family Macropodidae (macropods, meaning "large foot"). In common use the term is used to describe the largest species from this family, the red kangaroo, as well as the antilopine kangaroo, eastern gre ...
that are able to produce 80% less methane than cows. This is because the gut microbiota of Macropodids, rumen and others parts of their digestive system, is dominated by bacteria of the
family
Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
Succinivibrionaceae
The Succinivibrionaceae are Gram-negative bacteria and belong to the Gammaproteobacteria.
They are rod-shaped and obligate anaerobe
Obligate anaerobes are microorganisms killed by normal atmospheric concentrations of oxygen (20.95% O2). Oxy ...
. These bacteria are able to produce succinate as a final product of the
lignocelluloses degradation, producing small amounts of methane as end product. Its special metabolic route allows it to utilize other proton acceptors, avoiding the formation of methane.
Experimental management
Enteric fermentation was the second largest anthropogenic source of methane emissions in the United States from 2000 through 2009. In 2007, methane emissions from enteric fermentation were 2.3% of net greenhouse gases produced in the United States at 139
teragram
The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially. ...
s of carbon dioxide equivalents (Tg CO
2) out of a total net emission of 6087.5 Tg CO
2. For this reason, scientists believe that, with the aid of
microbial
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
engineering, the use of microbioma to modify natural or anthropogenic processes, we could change the microbiota composition of the rumen of strong methane producers, emulating the Macropodidae microbiota.
Recent studies claim that this technique is possible to perform. In one of these studies scientists analyze the changes of human microbiota by different alimentary changes. In other study, researchers introduce a human microbiota in
gnotobiotic mice in order to compare the different changes for developing new ways to manipulate the properties of the microbiota so as to prevent or treat various diseases.
Another approach to manage methane emissions from enteric fermentation involves using diet additives and supplements in cattle feed. For example, ''
Asparagopsis taxiformis
''Asparagopsis taxiformis'', (red sea plume or limu kohu) formerly ''A. sanfordiana'', is a species of red algae, with cosmopolitan distribution in tropical to warm temperate waters. Researchers have demonstrated that feeding ruminants a diet ...
'' (also known as red seaweed) is a species of algae that when fed to cattle has shown to substantially reduce their methane emissions. A second example that has been shown to reduce methane emissions from cattle significantly involves using the compound
3-nitroxypropanol (3-NOP) which inhibits the final step of methane synthesis by microorganisms in the rumen. Some of these methods have already been approved for farmer usage, while others continue to be evaluated for safety, efficacy, and other concerns.
See also
*
Environmental impact of meat production#Greenhouse gas emissions
*
Atmospheric methane
Atmospheric methane is the methane present in Earth's atmosphere. Atmospheric methane concentrations are of interest because it is one of the most potent greenhouse gases in Earth's atmosphere. Atmospheric methane is rising.
The 20-year globa ...
References
Un-cited references
#M. J. Gibbs and R. A. Leng, "Methane Emissions From Livestock", Methane And Nitrous Oxide, Proceedings Of The International IPCC Workshop, Amersfoort, The Netherlands, pp. 73–79, February 1993.
#State Workbook: Methodology For Estimating Greenhouse Gas Emissions, EPA 230-B-92-002, U. S. Environmental Protection Agency, Office of Policy, Planning and Evaluation, Washington, DC, 1995.
#International Anthropogenic Methane Emissions: Estimates for 1990, EPA-230-R-93-010. U. S. Environmental Protection Agency, Global Change Division, Office of Air and Radiation, Washington, DC, 1994.
# {{cite journal , last1=Crutzen , first1=Paul J. , last2=Aselmann , first2=Ingo , last3=Seiler , first3=Wolfgang , title=Methane production by domestic animals, wild ruminants, other herbivorous fauna, and humans , journal=Tellus B: Chemical and Physical Meteorology , date=June 1986 , volume=38 , issue=3–4 , pages=271–284 , doi=10.3402/TELLUSB.V38I3-4.15135 , bibcode=1986TellB..38..271C
#Anthropogenic Methane Emissions In The United States: Estimates For 1990, Report to Congress, U. S. Environmental Protection Agency, Office of Air and Radiation, Washington, DC, 1993.
#Greenhouse Gas Inventory Workbook, Intergovernmental Panel On Climate Change/Organization For Economic Cooperation And Development, Paris, France, pp. 4.1-4.5, 1995.
Animal physiology
Climate change and agriculture