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

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

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

,2008). Recentenvironmental studies estimate that approximately 14% of greenhouse gasesresult from agriculture (Cole et al., 2013). In particular, the livestockmanagement of cattle is known to produce large amounts of methane gas. Methaneproduced from ruminants has been identified as the single largest contributorof methane because ruminants emit vast amounts of methane as part of theirnatural digestive processes (Iqbal et al.

, 2008).  Ruminant are responsible for a majority ofmethane emissions, especially in areas dominated by grasslands where “ruminantsare the most important natural converters of fibrous biomass to valuableprotein for human nutrition” (Kulling et al., 2002). Worldwide, there are about1.5 billion cows and on average, a single cow releases between 70 and 120 kg ofmethane gas annually (Ishler, 2017).  Forthis reason, cattle are one of the most significant contributors to greenhousegas accumulation in the atmosphere. Cows, classified asruminants, possess four stomachs. Ruminants store their food in the firstchamber of their stomachs, referred to as the rumen.

The rumen is home tohundred of different microbes that are essential for the digestion of cellulose(Ishler, 2017). Many of the microbes used in the digestion process producemethane as a byproduct. When cattle eat, methane gas builds up insideof their stomachs and must be expelled into the atmosphere, known as entericmethane.

Due to the abundance and large size of the cattle, cowsare known to emit more methane gas than all other ruminants combined. Cattletypically 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 fromcattle proposes two different benefits. The first benefit being that loweremissions of methane would ultimately lower the concentration of methane in theatmosphere. The second benefit being that by emitting less methane, cows wouldincrease efficiency and productivity, which would increase profits for farmers(Iqbal et al.

, 2008). Understanding thedigestive physiology of the cow is important to understanding how cattleproduce enteric methane gas emissions. Cattle are classified as ruminants.Ruminants are characterized by their cloven hooves, stomachs composed of fourdifferent compartments, and chewing cud (Hall and Silver, 2009).              The cow stomach can be divided intofour different compartments: rumen, reticulum, omasum, and abomasum. The rumenis the largest compartment of the cow’s stomach and contains billions ofbacteria, protozoa, molds, and yeasts that live in a symbiotic manner with thecow (Hall and Silver, 2009). It is estimated that roughly 25-50 billionbacteria and 200-500 thousand protozoa can be found in every milliliter ofrumen fluid (Hall and Silver, 2009). The microbes residing in the rumen of thecow produce enteric methane emissions as aby-product of anaerobic fermentation, in a process known as methanogenesis(Bell et al.

, 2014). These microbes arethe reason why cattle can digest cellulose fibers in roughage (Hall and Solver,2009). Rumen microorganisms digest the plant fibers ingested by the cow andproduce volatile fatty acids (Hall and Silver, 2009). The fatty acids producedby the rumen microorganisms are then absorbed through the rumen wall andprovide anywhere from 60-80% of the cow’s energy (Hall and Silver, 2009).

Additionally, the microbes found in the rumen synthesize essential amino acidsfrom 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 thatwould typically kill non-ruminant mammals (Hall and Silver, 2009).      The second compartment of the cow stomachis the reticulum. The reticulum is primarily involved with rumination. Duringrumination, the cud is chewed and regurgitated as a bolus on incompletelychewed 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, cattleregurgitate and re-chew their food several times. It is during the process ofrumination that the cow will erucate, or belch, releasing the carbon dioxideand enteric methane gas that accumulates during digestion.      The third compartment of the stomach isthe omasum. The omasum is the gateway to the final compartment of the stomachand functions as a filter to return large particles back into the reticulorumen(Hall and Silver, 2009).

The final compartment of the cow stomach is theabomasum. The abomasum is considered the true stomach, producing acid andenzymes that digest proteins. The remainder of cow digestion takes place in thesmall and large intestines.      With knowledge ofcattle anatomy and digestive physiology, proper nutrition can be established tosupport the rumen microorganisms, reduce digestive problems, and promote ahealthy population of microbes within the rumen. Rumen microorganisms areadaptable to the extent that cattle can digest a large variety of feedsincluding hay, grass, corn, brewers grain, corn stalks, silage, and urea (Halland Silver, 2009). However, different diets can alter the microbial populationsin the rumen, which can alter the cow’s performance and level of methane gasemissions (Jones, 2014).      Over time, cattle have evolved to rely onlarge amounts of fiber; however, cattle do not do well on diets composed of allgrain or diets high in fat (Hall and Silver, 2009).

Ultimately, theproductivity of the cow and the amount of methane gas it emits are influencedby the composition of the cow’s diet. The type of carbohydrate ingested, theamount of fat ingested, the processing of the forages, and the level of feedintake are factors that are known to contribute to cow performance and thelevel of methane gas that the cow emits (Jones, 2014).      Typically, cattle diets that are high incarbohydrates have a higher composition of grain (Jones, 2014). Cattle moreeasily digest feeds made from corn and distillers grain. On the contrary, feedsmade from grass or hay, cannot be easily digested by cattle (Jones, 2014).  The cow’s ability to digest these differentfood sources differs because different microbes are involved in the digestionof each. “Microbes involved in digesting cellulose-rich rich diets (grass andhay) or carbohydrate-rich diets (corn and distillers grain)” are different andproduce different levels of methane (Jones, 2014).

Research has shownthat diets rich in carbohydrates reduce the amount of methane gas emitted bythe cow (Jones, 2014).  Overall, researchhas found that the proportion of concentrate within a cow’s diet is negativelycorrelated with methane emissions (Iqbal et al., 2008).

  A study conducted by Lovett et al. 2005 showedthat a diet with increased fiber-based concentrate reduced emissions of entericmethane per kilogram ofanimal product (Iqbal et al., 2008). Beauchemin and MGinn (2005) also linked apositive response to high levels of starch-based concentrate in the form ofgrains on methane production (Iqbal et al., 2008). The positive correlation isfound because increased proportions of starch in a cow’s diet alters itsruminal volatile fatty acid concentrations so more propionate and less acetateis formed. A decreased supply of acetate results limits the supply of hydrogenthat is available for methanogenesis. As the proportion of propionateincreases, the pH of the rumen decreases.

A decreased pH of the rumen reducesthe enteric methane output by reducing the microbial populations within therumen. Studies have alsoshown that either grinding or pelleting forages increases the passage rate ofdigestion and reduces methane emissions (Jones, 2014). Cows have an unusualarrangement of teeth that allows them to chew large amounts of fibrousmaterial.  A cow has a total of 32 teeth,consisting of 6 incisors on the lower jaw, 2 canines, 24 molars and a dentalpad on the top (Hall and Silver, 2009).

The cow’s configuration of teeth isprimarily suited for grinding so they use their tongues to gather grass whilegrazing. The cow cannot effectively cleave the grass because it lacks incisorson the top of their mouths. Despite their dental anatomy, research hasshown that decreasing the particle size of cattle feed affects the rate ofdigestion as well as the concentration of enteric methane released.

If the foodparticles are too large, the total intake and energy consumed may decrease dueto an increase in ruminal retention time. On the contrary, if the feedparticles are too small, they will reduce chewing and rumination and negativelyinfluence animal performance by reducing the buffer capacity (Shain et al.,1999).

Chewing in cattle is associated with saliva production, which isessential for buffering acids produced in ruminal fermentation (Shain et al.,1999). If the food particles are too small and decrease chewing activity, theycan cause health conditions such as acidosis and bloat.An optimal medianparticle size that is smaller than unprocessed feed improves and increases thepassage rate and decreases the pH of the rumen (Shain et al., 1999). A studyconducted by Herrmann et al. assessed the influence of varying particle sizesachieved by laboratory chopping on methane production.

The study using choppingas a mechanical treatment to reduce feed particle size and enhancemanageability of the feed material (Herrmann et al., 2012).  The study found that amedian chopping length was beneficial and decreased methane output.

   The smaller the particle was, the moresugars were made available through plant cell rupture (Herrmann et al., 2012).The increased availability of sugar allowed lactic acid bacteria by in therumen to produce 4.5 % more organic acids, which led to a faster drop in pH anda faster rate of passage (Herrmann et al., 2012). Additionally, the decrease inpH caused a depressed rate of methanogenesis and methanogenic activity,reducing methane output. An increased rateof passage is ideal because it allows for more efficient fiber digestion and anincreased supply of nutrients and energy. Increased feed efficiency not onlyresults in increased yields but increased profits for farmers (Shain etal.

,1999). Another factor thathas been identified as being a factor contributing to the amount of methaneemitted is the amount of fat incorporated into the cow’s diet. While althoughfats are high in energy, studies have shown that they can have an inhibitoryeffect on methane production because they can be toxic to the methane producingmicrobes found in the rumen (Jones, 2014). Unsaturated fats remove hydrogen gasfrom methane production in order to saturate the fats (Jones, 2014). Sauer et al. 1998conducted a study in which the effects of the addition of monensin, anunsaturated fat, on milk production, milk composition, feed intake, and theamount of methane produced in Holstein dairy cows. The study consisted of twotrials, the first using 109 Holstein cows.

The first trial revealed that theaddition of monensin to the cattle diets increased milk production, decreasedfeed intake, anddecreased enteric methane production (Sauer et al., 1998). The composition ofthe fatty acids in the milk produced by the cattle showed an increase in theconcentration of conjugated dienes and the ruminal biohydrogenation beingdepressed by the addition of unsaturated fats to the cattle diets (Sauer etal., 1998). Unsaturated fats affect ruminal fermentation, having the ability todecrease fiber digestion and decrease ruminal methane production (Hall andEastridge, 2014). In the secondtrial, the monensin feeding experiment was repeated 161 days later on a herd of88 cattle (Sauer et al., 1998). The second trial used 67 cattle from the firsttrial that had previously been exposed to the monensin-supplemented diet and 21cattle that had never been exposed to monensin (Sauer et al.

, 1998). The 21cattle that were exposed to the monensin for the first time also exhibiteddecreased enteric methane emissions; however, the 67 cattle that had previouslybeen exposed to the monensin appeared to have undergone adapted changes. Thepreviously exposed cattle no longer had the same response of decreased entericmethane emissions (Sauer et al., 1998).             Another study conducted by Kullinget al. (2002) studied the effects of adding 40g lauric acid dry matter (C12) tocattle feed on the methane emissions of dairy cows.  As a control, the study supplemented thecontrol feed with steric acid, a fatty acid that is assumed to not havemethane-suppressing potential (Kulling et al.

, 2002). The study found that theenteric methane released and measured in respiratory chambers was 20% less thanthat released from the control (Kulling et al., 2002). The decrease in entericmethane emissions was thought to have been the result of a reduced feed intakeand a lower rate of fiber digestion (Kulling et al., 2002).

Modernmitigation methods to reduce cattle methane production can be classified in twomajor categories. The first category seeks to increase the production of theindividual cow through the use of improved nutrition strategies. Improvedproduction would decrease the number of cattle needed to fulfill the product demand andultimately produce less methane per unit of meat or milk (Iqbal et al.,2008).  The second category of mitigationmethods seeks to directly modify ruminal fermentation so that less methane isproduced in total (Iqbal et al., 2008)Numerousstudies have indicated that methane emissions by dairy cows vary depending ontheir feed intake and diet compositions. However, it was not until recentlythat a study demonstrated that even when cattle were fed the same diet, thevariation among cows in methane emissions could be substantial (Bell et al.

,2014). One study conducted by Bell et al. assessed the variation among cows inemissions of enteric methane during lactation on commercial dairy farms. Thestudy examined 1964 cows from 21 different farms for a total of seven days andmeasured their enteric methane emissions using methane analyzers at roboticmilking stations (Bell et al., 2004).The studyconfirmed that enteric emissions vary among cows on commercial dairy farms.

These findings suggest that there is potential to select for individual cows inorder to reduce enteric methane emissions (Bell et al., 2014). Dietmanipulation can alter the production of enteric methane immediately; however,other mitigation options such as selective breeding show a greater potential aslong-term solutions to reduce methane emissions (Bell et al., 2014). Whilealthough diet manipulation has been shown to immediately decrease methaneemissions, a recent study discovered a tendency for a compensatory increase inmethane formation in manure. Given that a majority of the methane released intothe atmosphere is emitted in the enteric form, methane emitted from cow manureis rarely addressed despite its contribution to global warming. Kulling et al.

was the first study to formulate a direct comparison of methane emitted fromcows enterically and methane emitted from their manure in a single study. Theresults of the experiment indicated that there was a compensatory increase inmanure methane formation when enteric methane emissions were reduced throughdietary alterations (Kulling et al., 2002).

It still remains unclear whether ornot methane mitigation strategies that are effective in managing the entericemissions of the cattle are prone to the compensatory increase in methanerelease form manure, as observed in this particular study (D. Kulling et al.,2002). The long duration of manure storage accompanied by the consideration ofenteric methane emissions introduced new insights on the negative long-termimplications of diet alterations being used as mitigation strategies (Kulling etal., 2002).

The geneticselection of cows shows potential as a future method of mitigation. Under thismethod, cows with above average performance would be selected to increaseanimal production efficiency (Iqbal et al., 2008). Increased production and yieldsof 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 productionof dairy cows was increased from 4,000 kg/cow/ year to 5,000 kg/cow/year, therewould be an increase in annual methane output; however, there would be adecrease in output of emissions per kg of milk produced (Iqbal et al., 2008).

Additionally, cattle could be selected based off of their feed intake. Cattlewith relatively lower feed intakes compared to peers of equivalent mass aremore feed efficient (Iqbal et al., 2008). Therefore, by selecting for more feedefficient cows, there would be a decrease in methane emissions. Othermitigation strategies are being tested at the microbial level. Probiotics arecurrently being examined as a potential solution to reducing methane emissionsfrom cattle.

A probiotic is a microbial feed supplement that impacts rumenfermentation and improves the animal’s overall productivity (Iqbal et al.,2008). Probiotics are hypothesized to decrease methane emissions in fourdifferent ways: increasing the production of propionate, reducing thepopulation of protozoa, promoting the process of acetogenesis, and improvinganimal productivity (Iqbal et al., 2008). 

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