Methane Production by Ruminants

Methane production by ruminants: its contribution to

global warming

Angela Moss, Jean-Pierre Jouany, John Newbold

To cite this version:

Angela Moss, Jean-Pierre Jouany, John Newbold. Methane production by ruminants: its con-tribution to global warming. Annales de zootechnie, 2000, 49 (3), pp.231-253. <10.1051/ani-mres:2000119>. <hal-00889894>

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Review article

Methane production by ruminants:its contribution to global warming

Angela R. MOSSa*, Jean-Pierre JOUANYb, John NEWBOLDc

a ADAS Nutritional Sciences Research Unit, Alcester Road, Stratford Upon Avon,Warwickshire CV37 9RQ, UK

b INRA, Centre de Recherches de Clermont-Ferrand-Theix,63122 Saint-Genès-Champanelle, France

c Rowett Research Institute, Bucksburn, Aberdeen, AB21 9SB, UK

(Received 15 November 1999; accepted 5 April 2000)

Abstract —The aim of this paper is to review the role of methane in the global warming scenario andto examine the contribution to atmospheric methane made by enteric fermentation, mainly by rumi-nants. Agricultural emissions of methane in the EU-15 have recently been estimated at 10.2 milliontonnes per year and represent the greatest source. Of these, approximately two-thirds come fromenteric fermentation and one-third from livestock manure. Fermentation of feeds in the rumen is thelargest source of methane from enteric fermentation and this paper considers in detail the reasons for,and the consequences of, the fact that the molar percentage of the different volatile fatty acids pro-duced during fermentation influences the production of methane in the rumen. Acetate and butyratepromote methane production while propionate formation can be considered as a competitive pathwayfor hydrogen use in the rumen. The many alternative approaches to reducing methane are considered,both in terms of reduction per animal and reduction per unit of animal product. It was concludedthat the most promising areas for future research for reducing methanogenesis are the developmentof new products/delivery systems for anti-methanogenic compounds or alternative electron acceptorsin the rumen and reduction in protozoal numbers in the rumen. It is also stressed that the reasonruminants are so important to mankind is that much of the world’s biomass is rich in fibre. Theycan convert this into high quality protein sources (i.e. meat and milk) for human consumption and thiswill need to be balanced against the concomitant production of methane.

methane / ruminants / global warning / reduction strategies

Résumé — Production de méthane par les ruminants : sa contribution au réchauffement de laplanète.Cet article examine le rôle du méthane dans le processus de réchauffement de la planète etévalue la contribution au méthane atmosphérique des gaz d’origine digestive issus principalement desruminants. Les émissions annuelles de méthane d’origine agricole dans l’Europe des quinze ont étéestimées récemment à 10,2 millions de tonnes et représentent la principale source des entrées

Ann. Zootech. 49 (2000) 231–253 231© INRA, EDP Sciences

* Correspondence and reprintse-mail: [email protected]

A.R. Moss et al.232

1. THE GREENHOUSE EFFECTAND METHANE CONTRIBUTION

1.1. Evolution of the Earth’s atmos-phere during the last century

There has been much interest in the com-position of the Earth’s atmosphere over thelast few decades as a result of the observedincrease in atmospheric temperatures. Theobserved increase in concentration of manygases in the troposphere has been related tothe increase in global temperatures. The past

and current concentrations of the maingreenhouse gases, rates of increase andatmospheric lifetimes are summarised inTable I.

1.2. Description of the greenhouse effect

The greenhouse effect is thought to bedue to the absorption of solar infrared (IR)radiation by gases and the earth’s surface,which, as a result, are heated and then re-emit IR radiation at low frequency with a

atmosphériques de méthane. Parmi celles-ci, approximativement les deux tiers proviennent des fer-mentations entériques et un tiers des lisiers. Le méthane ruminal représente environ 90 % de l’ensembledes fermentations digestives. Le présent article analyse en détail l’impact des orientations fermentairessur la production de méthane dans le rumen. L’acétate et le butyrate favorisent la production deméthane tandis que la formation de propionate constitue une voie alternative d’utilisation de l’hydro-gène dans le rumen. Les différentes possibilités offertes actuellement pour diminuer les émissions deméthane sont analysées, à la fois en terme de réduction par animal et par unité de produit animal. Lesvoies d’approche les plus prometteuses pour réduire la production ruminale de CH4 consisteraient àrechercher de nouveaux produits doués d’activité antiméthanogénique ou à favoriser la formationd’accepteurs d’électrons autres que CO2 ou le formate, ou à agir dans le sens d’une réduction de lapopulation de protozoaires. Enfin, cette réflexion globale sur la contribution des ruminants à l’effetde serre doit tenir compte du fait que ces animaux jouent un rôle essentiel dans l’équilibre de notreécosystème en transformant l’importante biomasse végétale mondiale en protéines animales (viandeet lait principalement) qui constituent la base de l’alimentation humaine. Cet aspect doit contrebalancerles aspects négatifs liés à la production de méthane et à ses conséquences.

méthane / ruminants / réchauffement de la planète / stratégies de réduction

Table I. The tropospheric concentrations, residence times and atmospheric trend of various green-house gases. Source: IPCC [54, 55].

CO2 CH4 CFC-111 CFC-122 N2O

Atmospheric concentration (ppmv) (ppmv) (pptv) (pptv) (ppbv) Pre-industrial 280 0.8 0 0 288 Present (1990) 355 1.72 280 484 310 Current rate of change 0.5 0.9 4 4 0.25 (% per year)

Atmospheric lifetime (years) 50–200 10 65 130 150

Relative radiative effectiveness Per molecule 1 21 12 400 15 800 206 Per unit mass 1 58 13 970 5 750 206

1 chlorofluorocarbon 11; 2 chlorofluorocarbon 12. ppmv: parts per million volume; ppbv: parts per billion vol-ume; pptv: parts per trillion volume.

Methane production by ruminants

greenhouse effect is that average global tem-peratures will rise, along with many conse-quences on human life. The degree to whichthese changes are projected to occur isdependent upon a reliable greenhouse gaspolicy model and a range of scenarios forthe levels of greenhouse gas emissions. Bythe year 2 030 the world is likely to be1–2 °C warmer than today, although giventhe full range of uncertainties, the rangecould be from 0.5 °C to 2.5 °C. The con-comitant rise in global mean sea level is 17to 26 cm, with a full range of 5 to 44 cm,due mainly to thermal expansion of theoceans and increased melting of ices in theArctic and Antarctic areas.

1.4. Consequences on humansand animals

The projected climatic changes in thenext century due to the greenhouse effectare likely to have an effect on water sup-plies and the increase in temperature willinduce a new distribution of deserts and wetareas in the world and will alter the rangeor numbers of pests that affect plants or dis-eases that threaten animals or human health.Also of interest are the effects on unman-aged ecosystems, mainly forests.

1.5. Contribution of methaneto the greenhouse effect

While carbon dioxide receives the mostattention as a factor in global warming, thereare other gases to consider, includingmethane, nitrous oxide (N2O) and chlo-rofluorocarbons (CFCs).

The presence of methane in the atmo-sphere has been known since the 1940’swhen Migeotte [90] observed strong absorp-tion bands in the infra-red region of the solarspectrum which were attributed to the pres-ence of atmospheric methane. Numerousmeasurements since have demonstrated theexistence of an average temporal increase

high absorptive power. In fact greenhousegases in the atmosphere are essential formaintaining life on earth, as without themthe planet would be permanently frozenbecause all of the incoming heat from thesun would be radiated back into space bythe earth’s surface (see Moss [98] for areview). The threshold concentration ofthese gases at which their greenhouse effectwould be minimised is not known, but it isaccepted that their concentrations in theatmosphere should not be allowed to con-tinue to rise. As a result of this acceptanceinternational organisations like the IPCC(Intergovernmental Panel on ClimateChange) have asked the governments ofdeveloped nations to evaluate the amountof gases produced in their country and todevelop research to limit emissions further.

Warming of the earth’s surface isachieved by solar energy being radiated,mainly in the visible part of the spectrum(wavelength 0.4 to 0.7 µm) and passingthrough the atmosphere of the earth with-out being absorbed. Some of the solarenergy is reflected back into space by cloudsand about 7% is radiated in the ultra-violetregion of the spectrum (below 0.4 µm)which is absorbed by the ozone layer in theatmosphere. The solar energy reaching theearth’s surface warms the earth and is radi-ated back from the surface in the infra-redregion of the spectrum (4–100 µm). Approx-imately 70% of this radiation is in the wave-length band between 7 and 13 µm, whichcan pass back through the atmosphere intospace. The remaining radiation is absorbed,essentially by water vapour and carbon diox-ide, thus there is warming of the lower layerof the atmosphere (troposphere), which inturn radiates heat, keeping the earth warmerthan it would otherwise be [46].

1.3. Consequences of the greenhouseeffect on our environment

The consequences of the increases in con-centration of the gases that generate the

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of atmospheric methane during the period1980–1990 of about 18 ppbv (parts per bil-lion per volume) per year [119]. The cur-rent rate of increase in atmospheric methaneconcentration has subsequently slowed toabout 10 ppbv per year [129], but the reasonfor this is uncertain [152]. The current globalaverage atmospheric concentration ofmethane is 1720 ppbv, more than double itspre-industrial value of 700 ppbv [8]. Theconcentration of methane in the Northernhemisphere is about 100 ppbv more than inthe Southern hemisphere, indicating eithergreater source or lower sink strength in theNorthern hemisphere [152].

The rising concentration of methane iscorrelated with increasing populations andcurrently about 70% of methane productionarises from anthropogenic sources and theremainder from natural sources. Agricul-ture is considered to be responsible for abouttwo-thirds of the anthropogenic sources [36].Biological generation in anaerobic envi-

ronments (natural and man-made wetlands,enteric fermentation and anaerobic wasteprocessing) is the major source of methane,although losses associated with coal andnatural gas industries are also significant.The primary sink for methane is reactionwith hydroxyl radicals in the troposphere[23, 24, 42], but small soil [97, 130, 153]and stratospheric [23, 24] sinks have alsobeen identified. The major sources and sinksof methane are shown in Figure 1.

Agriculture contributes about 21–25, 60and 65–80% of the total anthropogenic emis-sions of carbon dioxide, methane and N2Orespectively [36, 56, 152]. Agriculture isalso thought to be responsible for over 95%of the ammonia, 50% of the carbon monox-ide and 35% of the nitrogen oxides releasedinto the atmosphere as a result of humanactivities [56].

The release of an estimated 205 to245 million tonnes of methane per year fromagricultural sources is shown in Table II.

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Figure 1.Sources and sinks for methane on the earth and atmosphere.


1.6. Recommendations in Europeanpost-Kyoto policy

Facing the serious visible signs of globalwarming, the United Nations (UN) createdthe Framework Convention on ClimateChange (UNFCCC) with the missions ofpreparing the Conferences of the Parties(COP) for international decisions on gaseousemissions and collecting information on cli-matic changes through the Global Impactof Environmental Change (GIEC). Dele-gates from nearly all the countries in theworld work in COP according to the UNrules, with the exception that only countrieswhich ratified the Rio Convention areallowed to vote. The other countries are onlyallowed to propose amendments to the textssubmitted to COP for approval. The Sub-sidiary Body for Implementation (SBI) wascreated to elaborate recommendations forthe COP and to control the enforcement ofdecisions. It collaborates with the SubsidiaryBody for Scientific and TechnologicalAdvances (SBSTA) which is in charge ofco-ordinating scientific studies with theinformation given by international organi-sations and the needs of the COP.

The COP1, COP2, COP3, COP4 metrespectively in Berlin (1995), Geneva(1996), Kyoto (1997) and Buenos-Aires(1998) to decide on strategies of reduction ofradiatively active trace gases. After a14 day-meeting, 174 countries took the fol-lowing decisions registered in an agreementcalled the “Kyoto Protocol” produced dur-ing COP3, which now have to be applied:

– a decrease of greenhouse gas emissionsby an average of 5.2% below 1990 levelduring the period 2008–2012 in industri-alised countries;

– the level of allowed emissions duringthis period varies according to the countries:+8% for Australia; –8% for EU; +10% forIreland; –6% for Japan; +5% for Norwayand – 7% for USA;

– the agreement is applied only to6 greenhouse gases: carbon dioxide,methane, nitrous oxide, two fluorocarbonsand sulphur hexafluoride.

The soil sink strength for methaneappears to have been reduced by changesin land use, chronic deposition of nitrogenfrom the atmosphere and alterations in nitro-gen dynamics of agricultural soils [62, 97,126, 130]. Ojima et al. [110] estimated thatthe consumption of atmospheric methaneby soils of temperate forest and grasslandeco-systems has been reduced by 30%.Without the temperate soil sink for methane,the atmospheric concentration of methanewould be increasing at about 1.5 times thecurrent rate.

Since atmospheric methane is currentlyincreasing at a rate of about 30 to 40 mil-lion tonnes per year, stabilising globalmethane concentrations at current levelswould require reductions in methane emis-sions or increased sinks for methane ofapproximately the same amount. This reduc-tion represents approximately 10% of cur-rent anthropogenic emissions. The majoragricultural sources of methane are floodedrice paddies, enteric fermentation and animalwastes. Decreasing methane emissions fromthese sources by 10 to 15% would stabiliseatmospheric methane at its present level andis a realistic objective [35].

In 1990, agricultural emissions of methanein the EU-15 were estimated at 10.2 mil-lion tonnes per year and were the greatestsource (45%) of methane emissions in theEU. Of these, approximately two-thirdscame from enteric fermentation and one-third from livestock manure.

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Table II. Methane emission rates from agricul-tural sources. Source: Watson et al. [152].

Agricultural sources Methane emission rates(million tonnes per year)

Enteric fermentation 80 Paddy rice production 60–100 Biomass burning 40 Animal wastes 25 Total 205–245

A.R. Moss et al.

Since the objective is an effective reduc-tion at a global level, the protocol intro-duced a very complex system allowingindustrial countries to exchange or postponein time, part of their reduction. According totheir economics, countries are allowed tosell or buy some emission rights providedthat the joint obligations on total reductionsare respected.

Such decisions raise several questions:

1) guidelines for national greenhouse gasinventories must be proposed;

2) unanimous decisions must be taken atthe European level to reach a total decreaseof 8% during the 1st decade of the third mil-lennium;

3) appropriate controls must be put intoplace to supervise the effective reduction ineach country;

4) Penalties must be applied to infringers.

Because about 60% of the methane arisesfrom agricultural activities in Europe, muchof the effort in the near future will concernthis economic sector. Anaerobic digestionin the forestomach of ruminants is a majorsource of methane emissions. The contri-bution of other livestock such as horses, rab-bits, pigs or poultry is much less significant.As indicated below, mitigation scenariosbased on a scientific knowledge of methano-genesis must be proposed to the EuropeanCommission and European Parliament toprovide guidance on how to fulfil the man-date outlined in the Kyoto protocol.

2. METHANE PRODUCTIONAND HYDROGEN SINKSIN THE RUMEN

2.1. Fermentative reactionsin the rumen and caecum involvingH2 production and H2 sinks

2.1.1. Fermentation in the rumen

Ingested feed macromolecules aredegraded in the digestive tract into smallmolecules that are then transferred into the

blood flow through the digestive mucosa.Such hydrolysis is performed by enzymesof both endogenous and microbial origin.Although the anatomy and physiology ofthe digestive tract varies widely in the ani-mal kingdom, enzymatic digestion is gen-erally located at the beginning of the diges-tive tract while microbial digestion takesplace at the end. Ruminants and some otheranimals considered as pseudo-ruminantslike camelidae, other animals like the birdHoatzin have in addition large anaerobicfermentative chambers located at the begin-ning of the tract. Such anatomical charac-teristics with a small intestine flanked bytwo microbial compartments at both endsare much more efficient for the digestion ofcarbohydrates and for the degradation ofplant cell walls. Furthermore, microbial pro-tein synthesised in the forestomachs is thenavailable for digestion in the small intestinewhere they supply more than 50% of theamino acids entering the blood stream.

Fermentation of glucose equivalentsreleased from plant polymers or starch, isan oxidative process under anaerobic con-ditions occurring in the Embden-Meyerhof-Parnas pathway and giving reduced co-fac-tors like NADH (see Fig. 2). These reducedcofactors have to be re-oxidised to NAD tocomplete the fermentation of sugars. NAD+

is regenerated by electron transfer to accep-tors other than oxygen (CO2, sulphate,nitrate, fumarate). Electron transport-linkedphosphorylation inside microbial bodies is away of generating ATP from the flow ofgenerated electrons through membranes, ifthe required co-factors are present [37]. Pro-duction of H2 is a thermodynamicallyunfavourable process that is controlled bythe potential of the electron carrier [158].Even traces of H2 inhibit the hydrogenaseactivity, but more H2 is tolerated if bacte-ria have ferridoxin-linked pyruvate oxi-doreductases [92].

Although H2 is one of the major endproducts of fermentation by protozoa, fungiand pure monocultures of some bacteria, itdoes not accumulate in the rumen because

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fermentation pathways can be summarisedas follows:

2H producing reactions:

Glucose → 2 pyruvate + 4H(Embden-Meyerhof-Parnas pathway)

Pyruvate + H2O → acetate (C2) + CO2 + 2H

2H using reactions:

Pyruvate + 4H → propionate (C3) + H2O

2 C2 + 4H → butyrate (C4) + 2H2O

CO2 + 8H → methane (CH4) + 2H2O

When H2 is not correctly used by metha-nogens, NADH can be re-oxidised by dehy-drogenases of the fermenting bacteria toform ethanol or lactate. This situation whichoccurs in animals fed large amounts ofrapidly fermentable carbohydrates, is con-sidered as abnormal and illustrates a realdysfunction of the ruminal ecosystem.

Assuming that the amount of 2H pro-duced (2Hp) is equal to 2H used (2Hu) ona molar basis, Demeyer and Van Nevel(1975) proposed the following equationobtained from the previous reactions:

2 C2 + C3 + 4 C4 = 4 CH4+ 2 C3 + 2 C4 (1)

If production of H2, lactate (L), valerate (V),and consumption of O2 (O) are considered,

it is immediately used by other bacteriawhich are present in the mixed microbialecosystem. The collaboration between fer-menting species and H2-utilising bacteria(e.g. methanogens) is called “interspecieshydrogen transfer” [51]. Some physicalassociations between fermentative speciesand H2-users may facilitate interspeciestransfer in the rumen. Attachment ofmethanogens to the external pellicle of pro-tozoa has been reported by Krumholz et al.[66] and Stumm et al. [133].

In the rumen, formation of methane isthe major way of hydrogen eliminationthrough the following reaction:

CO2 + 4 H2 → CH4 + 2 H2O

The hydrogen transfer towards methanogensis beneficial to the degradation of cell wallcarbohydrates as shown in vitro by Wolinand Miller [159] with bacteria, by Bauchopand Mountfort [4] with fungi, and by Ushidaand Jouany [139] with protozoa. Theseresults were confirmed in vivo in gnotox-enic lambs with or without methanogens[40].

Metabolic hydrogen in the form ofreduced protons (H) can be also used duringthe synthesis of volatile fatty acids or incor-porated into microbial organic matter. Thestoichiometry of the main anaerobic

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Figure 2.Metabolism of NADH H+ and the electron sink products* in anaerobiosis.

A.R. Moss et al.

equation (1) can be converted into:

2 C2 + C3 + 4 C4+ L +3V = 4 CH4+ 2 C3+ 2 C4 + 4V + 2H2 + L + 2O (2)

The recovery rate of metabolic hydrogenwhich is calculated as 2Hu/2Hp, variesbetween 78 and 96% in the rumen forroughage diets [26]. Considering a meanhydrogen recovery of 90%, then equation(1) allows the calculation of methane pro-duction:

CH4 = (1.8 C2 – 1.1 C3 + 1.6 C4) / 4= 0.45 C2 – 0.275 C3 + 0.40 C4 (3)

Clearly, equation (3) indicates that the molarpercentage of volatile fatty acids (VFAs)influences the production of methane in therumen. Acetate and butyrate promotemethane production while propionate for-mation can be considered as a competitivepathway for hydrogen use in the rumen.Such theoretical calculations have been con-firmed in vitro where the end products canbe easily quantified. Methane productionwas measured when the molar proportionsof individual VFAs, was altered by addingmonensin to the diet of animal donors(Fig. 3).

Methane was not correlated to C2 pro-duction (r2 = 0.029) but, there was a goodnegative correlation between methane andC3 (r2 = 0.774). The correlation betweenmethane and C2/C3 ratio (r2 = 0.772) wasslightly lower. The ratio (C2 + C4)/C3,which accounts for acetate and butyrate bothof which are involved in H2 production, andpropionate which is involved in H2 utilisa-tion, improved the relationship slightly(r2 = 0.778). This result is consistent with theidea that propionate production andmethanogenesis are competing, and arealternative pathways for regenerating oxi-dised co-factors in the rumen. However, thisresult alone gave no information on the reg-ulating mechanisms involved. Van Kesseland Russell [144] observed in vitro, usingrumen fluid sampled from animals fed onroughage-based diets, that ruminalmethanogens lose the ability to use H2 atlow pH, giving rise to free H2 in the gasphase when the pH was less than 5.5. Thuson roughage diets a low pH leads to adecrease in methanogenesis independentfrom propionate formation. On the contrary,starch-fermenting bacteria can competeagainst methanogens for hydrogen use by

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Figure 3. Relationship between methane and (C2 + C4)/C3 ratio.


Breath tests were used to screen a largehuman population. About 0.33 of the indi-viduals have 108 to 1010 methanogens per gdigesta. The latter concentration givesenough methane production (30 ml to 3 litresof methane per day) for a breath detection.

As in the rumen, methanogens of colonicfermentation use H2 to reduce CO2 tomethane [93]. When non-methanogenic fer-mentation occurs in the hindgut, H2 is usedto reduce CO2 into acetate [33] accordingto the following reaction:

2 CO2 + 4 H2 → CH3 – COOH + 2 H2O

Such use of H2 is of interest for the nutritionof animals since acetate is absorbed into theblood and used as a major source of carbonand energy by ruminants, while methane islost from the animal. Accordingly, severalattempts have been made by scientists toreduce methane production from digestivefermentation and to increase acetogenesisby the microbial community of the rumen.Although the concentrations of acetogenicbacteria in the bovine rumen are similar tothose of methanogens [71], Prins andLankhorst [115] did not observe any for-mation of acetate from the reduction of14CO2 by rumen contents. Contrary tomethanogens, acetogenic bacteria are able touse sources other than hydrogen for theirenergy supply, which explains why theirconcentration can be high in the rumen whileacetogenesis is negligible.

Demeyer and De Graeve [27] showedthat the addition of H2 to the gas phase offermenters inoculated with rumen digestahad little impact on VFA production (from– 4% to + 7%), but stimulated methano-genesis significantly (+ 94%). When addedto caecal digesta sampled from the samecattle, H2 significantly stimulated VFA pro-duction (+ 10% C2, + 14% C3, + 14% C4)as well as methane production (+ 67%).However, the increase in methane was lowerthan that noted with rumen digesta. Thehydrogen recovery rate was always muchlower in the caecum or colon than the rumen

producing large amounts of propionate [123].However, H2 accumulated and propionatedecreased dramatically while acetateincreased when the pH reached non-physio-logical values below 5.3. This means that themicrobial ecosystem involved in propionateformation differs with the dietary conditions.The cellulolytic bacteria Fibrobacter suc-cinogenesis the major propionate producersthrough the succinate pathway in roughagediets, while lactate is the main intermediatein the conversion of starch to propionate.Unlike cellulolytic bacteria and metha-nogens, lactic bacteria are known to be tol-erant to low pH making them able to useH2 and be competitive with methanogenseven in unfavourable pH conditions.

2.1.2. Fermentation in the hindgut

In ruminants, large amounts of organicmatter can by-pass the rumen and bedigested in the hindgut if there is no diges-tion in the small intestine or if the digestionis incomplete. So ground roughage diets anddiets rich in maize starch can supply largequantities of digestible organic matter to thehindgut. It has been estimated that 10 to30% of digestible organic matter can bedigested there. Because the large intestine isthe only compartment of fermentation in thedigestive tract of simple-stomached species,it plays an essential digestive role, espe-cially in herbivore monogastrics.

The anaerobic bacteria in the hindgut arenot very different from those found in therumen [61]. Other species of protozoainhabit the large intestine of equines [9], butprotozoa are missing from the hindgut ofruminants. Anaerobic fungi are absent inthe human large intestine, although theyhave been found and identified in the largeintestine of Equidae [10].

There is no clear information on theoccurrence of significant methanogenic fer-mentation in the hindgut of species otherthan humans and pigs. Only some individ-uals in the rat population and termite popu-lation are able to produce methane [12, 68].

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(0.50 to 0.60 vs. 0.85 to 0.90). Such resultsindicate that methanogenesis is the majorpathway for H2 use in the rumen comparedto the use for propionate or butyrate syn-thesis. When methanogens are present,methanogenesis still remains the major H2sink in the caecum, but its contribution isminor when compared to the rumen. Con-versely, the contribution of VFAs is larger inthe caecum. In experiments carried out inFrance (Jouany, unpublished data) usingincubations with human bowel contentssampled from methane producers or non-methane producers, it was observed thatmethane emissions from methane produc-ers were low (5% of total C6 fermented esti-mated as C2/2 + C3/3 + C4) and, as a con-sequence, the other end products offermentation were not statistically differentbetween the two groups. The hydrogenrecovery rate calculated from equations (1)and (2) was low (0.33 in non-methane pro-ducers vs. 0.38 in methane producers) whichindicates that hydrogen sink reactions otherthan those considered in the rumen exist inthe hindgut, and that these reactions can rep-resent more than 60% of the total hydrogensink reactions. The recovery values notedabove for humans are close to those calcu-lated from work with rabbit digesta [27].

Individual determinations of methaneproduction indicate that large variationsoccur between animals under the same con-ditions within a herd [59]. Subtle balancesbetween microbes involved in hydrogentransfer must exist in the digestive ecosys-tems to explain such variations. Feedingbehaviour and animal physiology (rumenmotility, flow of digesta, mastication, sali-vation) are probably determinants of themicrobial populations involved in produc-tion and use of hydrogen, which explainssuch an animal effect. This aspect has beenconfirmed by the higher accuracy in pre-diction of methane production when mech-anistic models are used rather than simpleregression equations as shown by Benchaaret al. [5]. The former models [30] integratesome parameters derived from animal

characteristics while the latter are generallyassociated only with dietary characteristics.

2.2. Micro-organisms involvedin digestive H2 metabolism

Methane is produced by strict anaerobesbelonging to the sub-group of the Archaedomain [155]. Archaea have no peptido-glycan polymer in their cell walls. Alsointracellular lipids are different in compo-sition from other bacteria. Triacylglycerolis replaced by ether linkages between glyc-erol and polyisoprenoid chains. RibosomalRNA nucleotide sequences of Archaea andother bacteria show an early divergence ofthe two types of cells during evolution.There is a large phylogenetic diversity ofmethanogens in natural media. Also, the dif-ferent genera and species of methanogenshave various shapes and physiological char-acteristics: cocci, rods, spirilla, thermophylicand mesophylic species, motile and non-motile cells.

Rumen methanogens grow only in envi-ronments with a redox potential below–300 mV [131]. More than sixty specieswere isolated from various anaerobic habi-tats like sanitary landfills, peat bogs, water-logged soils, salt lakes, thermal environ-ments, and intestinal tracts of animals. Onlyfive of these species belonging toMethanobrevibacterand Methanosarcinagenera, were isolated from rumen digesta.Only two of these species have been foundat a population level greater than 106 ml–1.

Although H2, formate, acetate, methanol,mono-, di- and tri-methylamine are allpotential substrates for methanogens, onlyH2/CO2 and formate to a lesser degree, areused as methane precursors in the rumen[91]. The reactions involved in methane pro-duction in the rumen which have beendescribed by Rouviere and Wolfe [121] aretheir sole energy-generating mechanism.They show that specific co-factors areneeded for the methane to be produced andinhibition of some of them could be a way to

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Johnson and Johnson [59] showed from 118experiments that digestibility of dietaryenergy explained only 5% of the variation inproportion of gross energy lost as methane.Digestible energy does not take into accountthe nature of fermented OM (FOM) and apossible shift of digestion from the rumen tointestine.

2.3.2. Residence time in the rumenand level of intake

A reduction in methane production isexpected when the residence time of feedin the rumen is reduced since ruminal diges-tion decreases and methanogenic bacteriaare less able to compete in such conditions.Furthermore, a rapid passage rate favourspropionate production and the relevant Huse. According to Kennedy and Milligan[63] and Okine et al. [111], a 30% decline inmethane production is observed when theruminal passage rate of liquid and solidphase increased by 54 to 68%. Mean reten-tion time was shown to explain 28% of thevariation in methane emissions [111].

An increase in feeding level induceslower methane losses as a percentage ofdaily energy intake [6, 7, 100]. Johnson andJohnson [59] noted that methane lossesexpressed as the proportion of gross energyintake declined by 1.6 percentage units foreach multiple of intake. The major effect offeeding level is explained by its conse-quences on passage of feed particles out ofthe rumen [113].

2.3.3. Source of C and patternof fermentation

Because proportions of the individualVFAs is influenced by the composition ofOM of the diet, mainly by the nature andrate of fermentation of carbohydrates, thesedietary characteristics will have large effectson methane production. Diets rich in starchwhich favour propionate production willdecrease the methane/FOM ratio in therumen. As discussed before, the effect of

reduce the activity of methanogens. In theintestine, methanogens are able to use otherprecursors. As an example, M. smithiimakesmethane only by reducing methanol withH2, methanol being produced from hydrol-ysis of pectins and other methylated plantpolysaccharides.

Acetogens are the major bacteriainvolved in H2 utilisation in the hindgutwhile their population rarely exceed the con-centration of 105 ml–1 found in the rumenof adult ruminants [96]. They appear in therumen soon after the birth of lambs in herdconditions [95] and their populationdecreases during the growth of methanogens,confirming the strong competition betweenthe two H2-users. When inoculated into gno-tobiotic lambs isolated without methanogens,they reached the concentration 108–109 ml–1

which were maintained for the entire exper-iment. Free hydrogen accumulated and rep-resented 10% of the total gas production inthe rumen of gnotobiotic lambs, which indi-cates that acetogens have a low efficiency inhydrogen use. Inoculation of methanogensin these gnotobiotic animals induced a dropin the concentration of acetogens and aquasi-complete use of H2 since free hydro-gen disappeared. This means that acetogensand methanogens compete for H2 use, andthat methanogens always derive advantagefrom this competition as confirmed byDemeyer et al. [28], Le Van et al. [73] andLopez et al. [75].

2.3. Effect of feeding characteristicson methane production

2.3.1. Digestible OM or energy

Methane emissions are closely related tothe amount of rumen fermented OM or theamount of digestible OM since more than50% of digestion occurs in the rumen. Whenthe digestibility of energy increases by 10%,energy losses as methane increase by0.47 points in a roughage diet and by0.74 points in a mixed diet [6]. However,

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such diets on ruminal pH can also explainthe observed effect on methane emission.Conversely, a roughage-based diet willincrease the ratio. As an example, the levelof methane losses was 6–7 or 2–3% ofenergy intake when forages were fed atmaintenance or when high grain concen-trates were fed ad libitum respectively [59].Some other feed characteristics can affectmethane production. It increases whenmature dried forages are fed [134] or whenthey are coarsely chopped rather than finelyground or pelleted [50, 99], and decreaseswhen forages are preserved in ensiled form[99]. Because they stimulate the rumendegradation of plant cell walls, alkali-treat-ments of poor-quality forages have beenshown to increase the amount of methaneemissions [100].

3. MITIGATION SCENARIOS FORMETHANE EMISSIONS FROMRUMINANTS

3.1. Methane inhibition

3.1.1. Direct inhibition

Direct inhibition of methanogenesis byhalogenated methane analogues and relatedcompounds has been widely demonstrated invitro [146] and some have been tested invivo. Chloroform reduced methanogenesisin vitro and in vivo [3, 19], but is obviouslynot suitable for use in practice. Chloralhydrate, which is converted to chloroform inthe rumen [114, 117], inhibited methaneproduction in vivo [86] but lead to liverdamage and death in sheep after prolongedfeeding [70]. Amichloral (a hemiacetal ofchloral and starch) appeared to be safer andincreased liveweight gain in sheep [137],but unfortunately its antimethanogenic activ-ity declined with prolonged feeding [21,57]. Similarly the effects of trichloroac-etamide and trichloroethyl adipate on rumi-nal methanogenesis were apparently tran-sient [19, 20, 138]. The anti-methanogenicactivity of bromochloromethane was also

reported to be transient [125], however Mayand colleagues [88, 89] suggested that acombination of bromochloromethane andα-cyclodextrin was more stable and capableof suppressing methane emissions in sheepand cattle over a prolonged period.

2-bromoethanesulfonic acid (BES), abromine analogue of coenzyme F involvedin methyl group transfer during methano-genesis, is a potent methane inhibitor [80,156]. BES is a specific inhibitor of metha-nogens and does not appear to inhibit thegrowth of other bacteria [124, 128]. How-ever, unfortunately when tested in vivo theinhibition in methanogenesis was transientsuggesting that adaptation of the metha-nogenic population occurred [146].

Recently, 9,10-anthraquinone has beenshown to inhibit methanogenesis by mixedrumen micro-organisms in vitro [43, 67]and to depress methane production in lambsover a 19 day period [67]. Garcia-Lopez et al.[43] speculated that 9,10-anthraquinoneinhibited the reduction of methyl co-enzymeM to methane by uncoupling electron trans-fer in methanogenic bacteria.

3.1.2. Ionophores

Inhibition of methane production is nor-mally accompanied by an increase in pro-pionate production, and a negative rela-tionship between methanogenesis andpropionate production has been clearlyestablished in work on interspecies hydrogentransfer [157]. Ionophoric antibiotics such asmonensin have been shown to depressmethane production by mixed rumenmicrobes in vitro [145]. This decrease inmethanogenesis is not due to a direct effectof the ionophores on methanogenic bacteriabut rather results from a shift in bacterialpopulation from gram positive to gram neg-ative organisms with a concurrent shift inthe fermentation from acetate to propionate[18, 104]. Van Nevel and Demeyer [146]found that in vivo monensin depressedmethane production by 25% when averagedover 6 studies, however unfortunately some

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3.1.4. Stimulation of acetogens

An alternative strategy to reduce rumi-nal methanogenesis would be to re-channelsubstrates for methane production into alter-native products. As noted above acetogenicbacteria, in the hindgut of mammals and ter-mites, produce acetic acid by the reductionof carbon dioxide with hydrogen and reduc-tive acetogensis acts as an important hydro-gen sink in hindgut fermentation [27, 68].Reductive acetogenesis occurs in the intes-tine of non-ruminants, sometimes along withmethanogenesis and sometimes replacingmethanogenesis [11, 34]. Bacteria carryingout reductive acetogenesis have been iso-lated from the rumen [44, 45, 95], but theyare few in number, and attempts to increaseacetogenesis have not been successful,largely because under rumen conditions thereductive acetogens have been unable tocompete with the methanogenic archaea [28,53, 107, 108]. Lopez et al. [74] found thatacetogens depress methane production whenadded to rumen fluid in vitro and suggestedthat even if a stable population of acetogenscould not be established in the rumen itmight be possible to achieve the samemetabolic activity using the acetogens as adaily fed feed additive.

3.1.5. Methane oxidisers

Global methane accumulation is the dif-ference between methane production andmethane oxidation. Methane oxidising bac-teria have been isolated from a wide range ofenvironments [47], including the rumen[132]. Studies with 13CH4 tracers suggestthat oxidation of methane to CO2 is of littlequantitative importance in the rumen [142]but may be more important in the gut of pigs[49]. Valdes et al. [143] isolated a methaneoxidising bacterium from the gut of youngpigs which decreased methane accumula-tion when added to rumen fluid in vitro,however the validity of this approach in vivohas yet to be tested.

long term in vivo trials have shown that theinhibition of methanogenesis by monensindid not persist [58, 122]. This appears to bein conflict with the observation that alteredpatterns of volatile fatty acid productionpersist in monensin treated animals duringlong term trials [118, 120]. The effect ofsalinomycin on methane production how-ever seemed to be more persistent [149].

3.1.3. Propionate enhancers

Awareness over antibiotic residues inanimal products and the threat of bacterialantibiotic resistance in the wider environ-ment has lead to an increasing interest inalternatives to antibiotics as growth pro-moters. Martin has suggested that dicar-boxylic organic acids such as malate mayalter rumen fermentation in a manner simi-lar to ionophores [79]. Lopez et al. [75]observed that when fumarate, a precursorof propionate, was added to rumen simulat-ing fermentors, propionate productionincreased with a stoichiometric decrease inmethane production. Ouda et al. [112] foundthat acrylate, an alternative precursor of pro-pionate, also depressed methane productionin rumen simulating fermentors, but to alesser extent than an equimolar addition offumarate. Asanuma et al. [1] also foundfumarate depressed methane production invitro and suggested that fumarate could bean economical feed additive in Japan.Malate, which is converted to propionatevia fumarate, also stimulated propionateproduction and inhibited methanogenesis invitro [83]. However Carro et al. [14] foundthat malate actually increased methane pro-duction in a rumen simulating fermentor,although this was largely explained by stim-ulation in fibre digestion and methane pro-duced per unit of dry matter fermented actu-ally fell. Malate failed to stimulate rumenpropionate concentrations in the rumen ofcattle and did not affect estimated methaneproduction [85, 94] although malate didstimulate average daily gain in steers [85].

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3.1.6. Defaunation

Methanogenic bacteria have beenobserved on the exterior surface of rumenciliate protozoa [148] and as endosymbiontswithin the ciliates [39]. Newbold et al. [105]estimated that methanogens associated withciliate protozoa were responsible forbetween 9 and 25% of the methanogenesisin rumen fluid and the removal of protozoafrom the rumen (defaunation) has been asso-ciated with decreases in methane produc-tion [141]. However these effects are appar-ently diet modulated with greater responseson high concentrate as opposed to high for-age diets [140]. A variety of techniques toremove protozoa from the rumen have beentested experimentally, but none is used rou-tinely, because of toxicity problems, eitherto the rest of the rumen microbial populationor to the host animal [154]. Recently, therehas been an increased interest in plant sec-ondary metabolites for use as possible defau-nating agents. In particular, saponin-con-taining plants show promise as a possiblemeans of suppressing or eliminating proto-zoa in the rumen without inhibiting bacterialactivity. Saponins are glycosides whichapparently interact with the cholesterol pre-sent in eukaryotic membranes but not inprokaryotic cells [17]. A decrease in proto-zoal numbers was reported in the rumen ofsheep infused with pure saponin [76] or fedsaponin-containing plants [29, 102, 106,109, 135, 136]. However, even if a practicalon-farm method to remove protozoa fromthe rumen can be found, the effects of defau-nation on methane emissions can not be con-sidered in isolation. Rumen ciliate protozoaplay an active role in ruminal fibre break-down [22] and defaunation has been shownto adversely impact fibre digestion in therumen [60]. However, protozoa also havea negative impact on animal productivity inthat the engulfment and digestion of bacte-ria by protozoa [150] significantly lowersthe flow of microbial protein leaving therumen [60]. Thus the use of defaunationto decrease methane production from

ruminants would have to be balanced againstthe effects on fibre and protein metabolismin the rumen.

The inclusion of fat in ruminant dietsdepresses protozoal numbers [25, 52] andthe use of lipids as a defaunating agent hasbeen suggested [104]. Fat inclusion in thediet causes a marked decrease in methaneproduction by rumen fluid, with the effectbeing at least partly governed by the fatsource used [32, 78]. However, the effects offat on methane production are not limitedto those mediated via the rumen protozoaand lipids have been shown to inhibitmethanogenesis even in the absence ofrumen protozoa [13, 31], possibly due tothe toxicity of long chain fatty acids tomethanogenic bacteria [48, 116]. However,as with defaunation the effect of fat supple-mentation can not be viewed in isolation.Fat inclusion in the diet (particularly at lev-els above 5 g.kg–1 DM) can significantlyinhibit fibre breakdown in the rumen [65,77], and again the severity of the effectvaries with the fat used [78].

3.1.7. Probiotics

The most widely used microbial feedadditives (live cells and growth medium)are based on Saccharomyces cerevisiae(SC)and Aspergillus oryzae(AO). Their effecton rumen fermentation and animal produc-tivity are wide ranging and this has beenreviewed by several authors [82, 103]. Thereis very limited information on their effecton methane production and all of this isin vitro. AO has been seen to reduce methaneby 50% [41] which was directly related toa reduction in the protozoal population(45%). On the other hand, addition of SC toan in vitro system reduced the methane pro-duction by 10% initially, though this wasnot sustained [101]. In other experimentswith AO and SC, an increase in methaneproduction has been reported [81, 84], whileMathieu et al. [87] reported that SC addi-tion did not affect methane in vivo. Thissuggests that more research is required

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addition of soluble carbohydrates gave ashift in fermentation pattern in the rumenwhich give rise to a more hostile environ-ment for the methanogenic bacteria in whichpassage rates are increased, ruminal pH islowered and certain populations of proto-zoa, ruminal ciliates and methanogenic bac-teria may be eliminated or inhibited. Thework of Lana et al. [69] supports this theoryconfirming that low rumen pH regulatesmethane production.

3.2.2. Forage type and supplementation

Supplementing forages whether of lowor high quality, with energy and protein sup-plements, is well documented to increasemicrobial growth efficiency and digestibil-ity (see Moss [99] for a review). Milk andmeat production will increase as a result.The direct effect on methanogenesis is stillvariable and unclear, but indirectly, methaneproduction per unit product will decline.The area was recently reviewed [99].Increasing the level of non-structural car-bohydrate in the diet (by 25%) would reducemethane production by as much as 20%, butthis may result in other detrimental effectse.g. acidosis, laminitis, fertility problems.Also with the implementation of quotas formilk production in the EU, many produc-ers are optimising milk production fromhome-grown forages in order to reduce feedcosts. Supplementing poor quality foragesand chemically upgrading them are goodoptions for increasing productivity and inturn reducing methane emissions per unitproduct. Reductions of total emissionswould only result if livestock numbers arereduced correspondingly.

Feeding of ruminants to optimise rumenand animal efficiency is a developing areaand the efficient deployment of this infor-mation to all livestock producers would ben-efit the environment in terms of bothmethane and nitrogen emissions. This wouldlead to best practise information and wouldrequire good technology transfer. Manyfarmers within the EU have to pay for

before it can be concluded that yeast cul-tures or AO extracts decrease methane pro-duction in vivo.

3.1.8. Immunisation

Baker [2] has proposed that it may bepossible to immunise ruminants against theirown methanogens with associated decreasesin methane output. Shu et al. [127] haveshown that such an approach can success-fully reduce the numbers of Streptococciand Lactobacilli in the rumen.

3.2. Increase in animal productivity

The concept of increasing animal pro-ductivity to reduce methane emissions fromruminants is based on the maintenance ofoverall production output and as a result,increased production of useful productwould mean methane production per unitproduct would decline. A reduction in totalemissions of methane would only result iftotal output levels (e.g. total milk or beefproduced) remained constant and livestocknumbers were reduced. Possible options forincreasing ruminant productivity are dis-cussed in the following sections.

3.2.1. Diet type

The type of feed offered to a ruminantcan have a major effect on methane pro-duction. The forage to concentrate ratio ofthe ration has an impact on the rumen fer-mentation and hence the acetate:propionateratio (declines with F:C ratio). It wouldtherefore be expected that methane produc-tion would be less when high concentratediets are fed [38]. Johnson and Johnson [59]reported a methane energy loss of 6 to 7% ofgross energy intake when forages were fedat the maintenance plane of nutrition andthis reduced to 2–3% when high grain con-centrates (> 90%) were offered at near adlibitum intake levels. Moss et al. [100] founda similar effect when grass silage was sup-plemented with barley. Van Soest [147] indi-cated that a high grain diet and/or the

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unbiased nutritional advice. If this advicewas freely available, there would likely bean increase in productivity and an improve-ment in the impact of emissions from live-stock into the environment. For some of themore productive member states (e.g. Den-mark and the Netherlands for milk produc-tion) this approach may not be so benefi-cial.

3.2.3. High genetic merit dairy cows

Improving the genetic merit of dairycows has escalated in the last decade withthe import of Holstein genetic material fromUS and Canada for use on the EU nativedairy breeds. As a result, average nationalyields have increased. One of the majorimprovements is the ability of the cow topartition nutrients into milk preferentiallyto maintenance and/or growth. This hasundoubtedly resulted in increased efficiency.The UK dairy herd has increased its averageyield by 8.8% from 1995 to 1997 and thetop 10% of herds are averaging 8351 litresper cow. There are additional benefits whichinclude the following:

(i) a cow’s lifetime production can beachieved in less lactations, therefore thereare less maintenance costs e.g. lifetime pro-duction of 30 000 litres achieved as 5 lac-tations of 6 000 litres or 3 lactations of10 000 litres;

(ii) a 100 cow herd producing averageyield of 6 000 litres = 600 000 l.y–1 or60 cow herd producing 10 000 litres, there-fore less cows to maintain;

(iii) less replacement heifers to maintain.

Kirchgessner et al. [64] suggested thatincreasing milk production of dairy cowsfrom 5 000 to 10 000 litres milk annuallywould only increase methane production by5% (i.e. from 110 to 135 kg methane peryear). Leng [72] indicated that Holstein cat-tle fed a high quality ration would produceonly about 15% as much methane per litre ofmilk as native Indian cattle on traditionalfeed.

This could reduce methane emissions by20 to 30% through reduced numbers. Thegenetic merit of livestock within the EU israpidly improving and this will undoubt-edly bring with it increased efficiency. Themanagement of these high genetic meritcows will also become more complex andthe overall implementation of this may bestalled by animal welfare implications. Highgenetic merit cows can have increased prob-lems with fertility, lameness, mastitis andmetabolic disorders. All these issues willhave to be addressed if genetic progress is tobe successfully continued.

3.2.4. Ionophores

The use of ionophores gives rise toimproved animal productivity (on averagean 8% improvement in feed conversion effi-ciency [15]) and a possible direct reductionin methane production. This option is in usethroughout the EU for beef animals only asits use is not permitted in dairy cows becausethe product requires a withdrawal period.Its effect is therefore impacting on less than50% of the methane emissions. As with allmeasures that reduce methane productionby increasing animal productivity, the ben-efit is only seen if animal numbers arereduced correspondingly.

The use of chemicals/antibiotics toincrease animal productivity are increas-ingly becoming unpopular to the consumersof animal products. It is therefore envisagedthat the use of ionophores to reduce methaneproduction is not a sustainable option.

3.2.5. Bovine somatotropin

Bovine somatotropin (BST) is a geneti-cally engineered metabolic modifierapproved for use in some countries toenhance milk production from dairy cows.BST does not affect digestibility, mainte-nance requirements or the partial efficiencyof milk synthesis, nor does it act directly onthe mammary gland. BST affects mammary

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percentage reduction necessary to stabilisethe other major greenhouse gases. Addi-tionally, because methane has a shorteratmospheric lifetime and greater radiativeabsorption capacity than carbon dioxide,methane reduction strategies offer an effec-tive means of slowing global warming inthe near term.

Methane is an end-product of fermenta-tion of carbohydrates in the rumen. The gen-eration of this can be decreased by promot-ing a shift in fermentation toward propionateproduction, but cannot be eliminated com-pletely without adverse effects on ruminantproduction. Increasing animal productivityseems to be the most effective means ofreducing methane release in the short term.It must be borne in mind that this methodis only successful if overall productionremains constant. The means to achieve thisincrease in productivity have been discussed,but nearly all involve the increased use offeed containing higher quality/lower fibresources of carbohydrate. However, the rea-son that ruminants are so important tomankind is that much of the world’s biomassis rich in fibre and can be converted intohigh quality protein sources (i.e. meat andmilk) for human consumption only by rumi-nants.

The most promising areas for futureresearch for reducing methanogenesis arethe development of new products/deliverysystems for antimethanogenic compoundsor alternative electron acceptors in the rumenand reduction in protozoal numbers in therumen.

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