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Behind the
challenge to cease global warming lay the increasing concentrations of
greenhouse gases in the Earth’s atmosphere. A greenhouse gas is defined as a
“gaseous compound in the atmosphere that is capable of absorbing infrared
radiation, thereby trapping heat in the atmosphere” (Haferkamp and Macneil,
2004).  Greenhouse gases ultimately lead
to global warming by trapping heat in the atmosphere and inducing what is known
as the greenhouse effect. 

            Some greenhouse gases, such as
carbon dioxide, are produced as a byproduct of respiration and the burning of
fossil fuels. Other greenhouse gases, like methane, are produced in large
through agricultural practices. While not as widely recognized as carbon
dioxide, methane gas is one of the most potent greenhouse gases.       

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As a contributor to
global warming, methane accounts for 16% of all greenhouse gas emissions
globally (Iqbal et al., 2008). Despite carbon dioxide lingering in the
atmosphere for longer periods of time, methane gas is more threatening to the
environment because of how effectively it absorbs heat.  The global warming potential, or ability to
absorb heat, of methane is 21 times greater than carbon dioxide (Iqbal et al.,

environmental studies estimate that approximately 14% of greenhouse gases
result from agriculture (Cole et al., 2013). In particular, the livestock
management of cattle is known to produce large amounts of methane gas. Methane
produced from ruminants has been identified as the single largest contributor
of methane because ruminants emit vast amounts of methane as part of their
natural digestive processes (Iqbal et al., 2008).  Ruminant are responsible for a majority of
methane emissions, especially in areas dominated by grasslands where “ruminants
are the most important natural converters of fibrous biomass to valuable
protein for human nutrition” (Kulling et al., 2002). Worldwide, there are about
1.5 billion cows and on average, a single cow releases between 70 and 120 kg of
methane gas annually (Ishler, 2017).  For
this reason, cattle are one of the most significant contributors to greenhouse
gas accumulation in the atmosphere.

Cows, classified as
ruminants, possess four stomachs. Ruminants store their food in the first
chamber of their stomachs, referred to as the rumen. The rumen is home to
hundred of different microbes that are essential for the digestion of cellulose
(Ishler, 2017). Many of the microbes used in the digestion process produce
methane as a byproduct. When cattle eat, methane gas builds up inside
of their stomachs and must be expelled into the atmosphere, known as enteric
methane. Due to the abundance and large size of the cattle, cows
are known to emit more methane gas than all other ruminants combined.

typically lose 2-15% of their ingested energy in the form of erucated methane
(Iqbal et al., 2008). For this reason, the mitigation of methane loss from
cattle proposes two different benefits. The first benefit being that lower
emissions of methane would ultimately lower the concentration of methane in the
atmosphere. The second benefit being that by emitting less methane, cows would
increase efficiency and productivity, which would increase profits for farmers
(Iqbal et al., 2008).

Understanding the
digestive physiology of the cow is important to understanding how cattle
produce enteric methane gas emissions. Cattle are classified as ruminants.

Ruminants are characterized by their cloven hooves, stomachs composed of four
different compartments, and chewing cud (Hall and Silver, 2009). 

            The cow stomach can be divided into
four different compartments: rumen, reticulum, omasum, and abomasum. The rumen
is the largest compartment of the cow’s stomach and contains billions of
bacteria, protozoa, molds, and yeasts that live in a symbiotic manner with the
cow (Hall and Silver, 2009). It is estimated that roughly 25-50 billion
bacteria and 200-500 thousand protozoa can be found in every milliliter of
rumen fluid (Hall and Silver, 2009). The microbes residing in the rumen of the

 produce enteric methane emissions as a
by-product of anaerobic fermentation, in a process known as methanogenesis
(Bell et al., 2014).

These microbes are
the reason why cattle can digest cellulose fibers in roughage (Hall and Solver,
2009). Rumen microorganisms digest the plant fibers ingested by the cow and
produce volatile fatty acids (Hall and Silver, 2009). The fatty acids produced
by the rumen microorganisms are then absorbed through the rumen wall and
provide anywhere from 60-80% of the cow’s energy (Hall and Silver, 2009).

Additionally, the microbes found in the rumen synthesize essential amino acids
from the protein and the nitrogen ingested by the cow (Hall and Silver, 2009).

For this reason, cattle can ingest urea and other sources of nitrogen that
would typically kill non-ruminant mammals (Hall and Silver, 2009).

     The second compartment of the cow stomach
is the reticulum. The reticulum is primarily involved with rumination. During
rumination, the cud is chewed and regurgitated as a bolus on incompletely
chewed feed. In order for the feed to be digested by the microbes in he rumen,
the feed has to be broken down into small pieces. For this reason, cattle
regurgitate and re-chew their food several times. It is during the process of
rumination that the cow will erucate, or belch, releasing the carbon dioxide
and enteric methane gas that accumulates during digestion.

     The third compartment of the stomach is
the omasum. The omasum is the gateway to the final compartment of the stomach
and functions as a filter to return large particles back into the reticulorumen
(Hall and Silver, 2009). The final compartment of the cow stomach is the
abomasum. The abomasum is considered the true stomach, producing acid and
enzymes that digest proteins. The remainder of cow digestion takes place in the
small and large intestines.


With knowledge of
cattle anatomy and digestive physiology, proper nutrition can be established to
support the rumen microorganisms, reduce digestive problems, and promote a
healthy population of microbes within the rumen. Rumen microorganisms are
adaptable to the extent that cattle can digest a large variety of feeds
including hay, grass, corn, brewers grain, corn stalks, silage, and urea (Hall
and Silver, 2009). However, different diets can alter the microbial populations
in the rumen, which can alter the cow’s performance and level of methane gas
emissions (Jones, 2014).

     Over time, cattle have evolved to rely on
large amounts of fiber; however, cattle do not do well on diets composed of all
grain or diets high in fat (Hall and Silver, 2009). Ultimately, the
productivity of the cow and the amount of methane gas it emits are influenced
by the composition of the cow’s diet. The type of carbohydrate ingested, the
amount of fat ingested, the processing of the forages, and the level of feed
intake are factors that are known to contribute to cow performance and the
level of methane gas that the cow emits (Jones, 2014).

     Typically, cattle diets that are high in
carbohydrates have a higher composition of grain (Jones, 2014). Cattle more
easily digest feeds made from corn and distillers grain. On the contrary, feeds
made from grass or hay, cannot be easily digested by cattle (Jones, 2014).  The cow’s ability to digest these different
food sources differs because different microbes are involved in the digestion
of each. “Microbes involved in digesting cellulose-rich rich diets (grass and
hay) or carbohydrate-rich diets (corn and distillers grain)” are different and
produce different levels of methane (Jones, 2014).

Research has shown
that diets rich in carbohydrates reduce the amount of methane gas emitted by
the cow (Jones, 2014).  Overall, research
has found that the proportion of concentrate within a cow’s diet is negatively
correlated with methane emissions (Iqbal et al., 2008).  A study conducted by Lovett et al. 2005 showed
that a diet with increased fiber-based concentrate reduced emissions of enteric

per kilogram of
animal product (Iqbal et al., 2008). Beauchemin and MGinn (2005) also linked a
positive response to high levels of starch-based concentrate in the form of
grains on methane production (Iqbal et al., 2008). The positive correlation is
found because increased proportions of starch in a cow’s diet alters its
ruminal volatile fatty acid concentrations so more propionate and less acetate
is formed. A decreased supply of acetate results limits the supply of hydrogen
that is available for methanogenesis. As the proportion of propionate
increases, the pH of the rumen decreases. A decreased pH of the rumen reduces
the enteric methane output by reducing the microbial populations within the

Studies have also
shown that either grinding or pelleting forages increases the passage rate of
digestion and reduces methane emissions (Jones, 2014). Cows have an unusual
arrangement of teeth that allows them to chew large amounts of fibrous
material.  A cow has a total of 32 teeth,
consisting of 6 incisors on the lower jaw, 2 canines, 24 molars and a dental
pad on the top (Hall and Silver, 2009). The cow’s configuration of teeth is
primarily suited for grinding so they use their tongues to gather grass while
grazing. The cow cannot effectively cleave the grass because it lacks incisors
on the top of their mouths.

 Despite their dental anatomy, research has
shown that decreasing the particle size of cattle feed affects the rate of
digestion as well as the concentration of enteric methane released. If the food
particles are too large, the total intake and energy consumed may decrease due
to an increase in ruminal retention time. On the contrary, if the feed
particles are too small, they will reduce chewing and rumination and negatively
influence animal performance by reducing the buffer capacity (Shain et al.,
1999). Chewing in cattle is associated with saliva production, which is
essential for buffering acids produced in ruminal fermentation (Shain et al.,
1999). If the food particles are too small and decrease chewing activity, they
can cause health conditions such as acidosis and bloat.

An optimal median
particle size that is smaller than unprocessed feed improves and increases the
passage rate and decreases the pH of the rumen (Shain et al., 1999). A study
conducted by Herrmann et al. assessed the influence of varying particle sizes
achieved by laboratory chopping on methane production. The study using chopping
as a mechanical treatment to reduce feed particle size and enhance
manageability of the feed material (Herrmann et al., 2012).  The study found that a
median chopping length was beneficial and decreased methane output.   The smaller the particle was, the more
sugars were made available through plant cell rupture (Herrmann et al., 2012).

The increased availability of sugar allowed lactic acid bacteria by in the
rumen to produce 4.5 % more organic acids, which led to a faster drop in pH and
a faster rate of passage (Herrmann et al., 2012). Additionally, the decrease in
pH caused a depressed rate of methanogenesis and methanogenic activity,
reducing methane output.

An increased rate
of passage is ideal because it allows for more efficient fiber digestion and an
increased supply of nutrients and energy. Increased feed efficiency not only
results in increased yields but increased profits for farmers (Shain et

Another factor that
has been identified as being a factor contributing to the amount of methane
emitted is the amount of fat incorporated into the cow’s diet. While although
fats are high in energy, studies have shown that they can have an inhibitory
effect on methane production because they can be toxic to the methane producing
microbes found in the rumen (Jones, 2014). Unsaturated fats remove hydrogen gas
from methane production in order to saturate the fats (Jones, 2014).

Sauer et al. 1998
conducted a study in which the effects of the addition of monensin, an
unsaturated fat, on milk production, milk composition, feed intake, and the
amount of methane produced in Holstein dairy cows. The study consisted of two
trials, the first using 109 Holstein cows. The first trial revealed that the
addition of monensin to the cattle diets increased milk production, decreased

intake, and
decreased enteric methane production (Sauer et al., 1998). The composition of
the fatty acids in the milk produced by the cattle showed an increase in the
concentration of conjugated dienes and the ruminal biohydrogenation being
depressed by the addition of unsaturated fats to the cattle diets (Sauer et
al., 1998). Unsaturated fats affect ruminal fermentation, having the ability to
decrease fiber digestion and decrease ruminal methane production (Hall and
Eastridge, 2014).

In the second
trial, the monensin feeding experiment was repeated 161 days later on a herd of
88 cattle (Sauer et al., 1998). The second trial used 67 cattle from the first
trial that had previously been exposed to the monensin-supplemented diet and 21
cattle that had never been exposed to monensin (Sauer et al., 1998). The 21
cattle that were exposed to the monensin for the first time also exhibited
decreased enteric methane emissions; however, the 67 cattle that had previously
been exposed to the monensin appeared to have undergone adapted changes. The
previously exposed cattle no longer had the same response of decreased enteric
methane emissions (Sauer et al., 1998).

            Another study conducted by Kulling
et al. (2002) studied the effects of adding 40g lauric acid dry matter (C12) to
cattle feed on the methane emissions of dairy cows.  As a control, the study supplemented the
control feed with steric acid, a fatty acid that is assumed to not have
methane-suppressing potential (Kulling et al., 2002). The study found that the
enteric methane released and measured in respiratory chambers was 20% less than
that released from the control (Kulling et al., 2002). The decrease in enteric
methane emissions was thought to have been the result of a reduced feed intake
and a lower rate of fiber digestion (Kulling et al., 2002).

mitigation methods to reduce cattle methane production can be classified in two
major categories. The first category seeks to increase the production of the
individual cow through the use of improved nutrition strategies. Improved
production would decrease the number of cattle needed to fulfill

the product demand and
ultimately produce less methane per unit of meat or milk (Iqbal et al.,
2008).  The second category of mitigation
methods seeks to directly modify ruminal fermentation so that less methane is
produced in total (Iqbal et al., 2008)

studies have indicated that methane emissions by dairy cows vary depending on
their feed intake and diet compositions. However, it was not until recently
that a study demonstrated that even when cattle were fed the same diet, the
variation among cows in methane emissions could be substantial (Bell et al.,
2014). One study conducted by Bell et al. assessed the variation among cows in
emissions of enteric methane during lactation on commercial dairy farms. The
study examined 1964 cows from 21 different farms for a total of seven days and
measured their enteric methane emissions using methane analyzers at robotic
milking stations (Bell et al., 2004).

The study
confirmed that enteric emissions vary among cows on commercial dairy farms.

These findings suggest that there is potential to select for individual cows in
order to reduce enteric methane emissions (Bell et al., 2014). Diet
manipulation can alter the production of enteric methane immediately; however,
other mitigation options such as selective breeding show a greater potential as
long-term solutions to reduce methane emissions (Bell et al., 2014).

although diet manipulation has been shown to immediately decrease methane
emissions, a recent study discovered a tendency for a compensatory increase in
methane formation in manure. Given that a majority of the methane released into
the atmosphere is emitted in the enteric form, methane emitted from cow manure
is rarely addressed despite its contribution to global warming. Kulling et al.

was the first study to formulate a direct comparison of methane emitted from
cows enterically and methane emitted from their manure in a single study. The
results of the experiment indicated that there was a compensatory increase in
manure methane formation when enteric methane emissions were reduced through
dietary alterations (Kulling et al., 2002). It still remains unclear whether or
not methane mitigation strategies that are effective in managing the enteric
emissions of the cattle are prone to the compensatory increase in methane
release form manure, as observed in this particular study (D. Kulling et al.,
2002). The long duration of manure storage accompanied by the consideration of
enteric methane emissions introduced new insights on the negative long-term
implications of diet alterations being used as mitigation strategies (Kulling et
al., 2002).

The genetic
selection of cows shows potential as a future method of mitigation. Under this
method, cows with above average performance would be selected to increase
animal production efficiency (Iqbal et al., 2008). Increased production and yields
of the cow would cause the methane output per unit of meat or milk to decrease.

Kirchgessner et al. (1995) conducted a study that supported that if production
of dairy cows was increased from 4,000 kg/cow/ year to 5,000 kg/cow/year, there
would be an increase in annual methane output; however, there would be a
decrease in output of emissions per kg of milk produced (Iqbal et al., 2008).

Additionally, cattle could be selected based off of their feed intake. Cattle
with relatively lower feed intakes compared to peers of equivalent mass are
more feed efficient (Iqbal et al., 2008). Therefore, by selecting for more feed
efficient cows, there would be a decrease in methane emissions.

mitigation strategies are being tested at the microbial level. Probiotics are
currently being examined as a potential solution to reducing methane emissions
from cattle. A probiotic is a microbial feed supplement that impacts rumen
fermentation and improves the animal’s overall productivity (Iqbal et al.,
2008). Probiotics are hypothesized to decrease methane emissions in four
different ways: increasing the production of propionate, reducing the
population of protozoa, promoting the process of acetogenesis, and improving
animal productivity (Iqbal et al., 2008). 

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