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Ruminant enteric methane mitigation: a review
D. J. Cottle
A,C
, J. V. Nolan
A
and S. G. Wiedemann
B
A
School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia.
B
FSA Consulting, 11 Clifford Street, Toowoomba, Qld 4350, Australia.
C
Corresponding author. Email: dcottle@une.edu.au
Abstract. In Australia, agriculture is responsible for ~17% of total greenhouse gas emissions with ruminants being the
largest single source. However, agriculture is likely to be shielded from the full impact of any future price on carbon. In this
review, strategies for reducing ruminant methane output are considered in relation to rumen ecology and biochemistry,
animal breeding and management options at an animal, farm, or national level. Nutritional management strategies have the
greatest short-term impact. Methanogenic microorganisms remove H
2
produced during fermentation of organic matter in the
rumen and hind gut. Cost-effective ways to change the microbial ecology to reduce H
2
production, to re-partition H
2
into
products other than methane, or to promote methanotrophic microbes with the ability to oxidise methane still need to be
found. Methods of inhibiting methanogens include: use of antibiotics; promoting viruses/bacteriophages; use of feed
additives such as fats and oils, or nitrate salts, or dicarboxylic acids; defaunation; and vaccination against methanogens.
Methods of enhancing alternative H
2
using microbial species include: inoculating with acetogenic species; feeding highly
digestible feed components favouring ‘propionate fermentations’; and modifying rumen conditions. Conditions that sustain
acetogen populations in kangaroos and termites, for example, are poorly understood but might be extended to ruminants.
Mitigation strategies are not in common use in extensive grazing systems but dietary management or use of growth
promotants can reduce methane output per unit of product. New, natural compounds that reduce rumen methane output may
yet be found. Smaller but more permanent benefits are possible using genetic approaches. The indirect selection criterion,
residual feed intake, when measured on ad libitum grain diets, has limited relevance for grazing cattle. There are few
published estimates of genetic parameters for feed intake and methane production. Methane-related single nucleotide
polymorphisms have yet to be used commercially. As a breeding objective, the use of methane/kg product rather than
methane/head is recommended. Indirect selection via feed intake may be more cost-effective than via direct measurement of
methane emissions. Life cycle analyses indicate that intensification is likely to reduce total greenhouse gas output but
emissions and sequestration from vegetation and soil need to be addressed. Bio-economic modelling suggests most
mitigation options are currently not cost-effective.
Additional keywords: Australian red meat industries, carbon price, greenhouse gas emissions.
Introduction
Rumen methanogenesis results in the loss of 6–10% of gross
energy intake (GEI), or 8–14% of the digestible energy intake of
ruminants (Johnson et al.1993; Okine et al.2004). As well as
reducing emissions, reducing methane (CH
4
) production (MP)
improves feed energy use and system efficiency.
Enteric CH
4
is the most significant single source of greenhouse
gas (GHG) emissions from the Australian ruminant industries. In
2005, of the 87.9 Mt CO
2
-e total estimated emissions from the
agricultural sector, 50% were from enteric fermentation in cattle,
16% from sheep and 0.3% from other animals (AGO 2007). On
average, mature beef cows emit ~350 g CH
4
/day in the tropics and
~240 g/day in temperate zones; dairy cows emit ~430 g/day at
peak lactation down to ~250 g/day as milk yield declines. Ewes
emit 22–25 g/day (Eckard 2009).
Enteric emissions as a proportion of total national CO
2
-e
emissions are highest in New Zealand, Ireland, Australia,
France and Sweden (Table 1). Leaving enteric emissions out
of carbon accounting or emission trading schemes would
therefore have most impact in these countries.
Australian livestock-related emissions declined by 5%
between 1990 and 2006 (Fig. 1). The decline was mainly due
to a 48% fall in sheep numbers, offset largely by a 14% rise in
beef cattle numbers that reflected changing relative returns to
each industry.
Mitigation options
Several reviews of enteric CH
4
production and mitigation options
have recently been published (Beauchemin et al.2008; de Klein
and Eckard 2008; McAllister and Newbold 2008; Rowlinson
et al.2008; Buddle et al.2010; Eckard et al.2010; Hegarty et al.
2010; Martin et al.2010; Shibata and Terada 2010). Unlike this
review, they all focus more on non-genetic-based mitigation
options.
While absolute MP per animal (per day) is useful for
context, the focus of life cycle assessment (LCA) research is
CSIRO PUBLISHING Review
www.publish.csiro.au/journals/an Animal Production Science, 2011, 51, 491–514
CSIRO 2011 10.1071/AN10163 1836-0939/11/060491
the estimation of CH
4
emission intensity relative to production
(MI), e.g. kg CH
4
per kg liveweight gain (LWG) or kg beef or milk
(Table 2). Increasingly, this significant distinction is being
recognised by GHG researchers, leading to greater emphasis
on CH
4
relative to feed intake and product output.
For breeding animals, the numbers and LW of weaners are
primary determinants of productivity and have a large impact on
whole herd/flock enteric CH
4
efficiency (Hegarty and McEwan
2010).
Ruminant CH
4
mitigation options (Fig. 2) include: (i)
modifications of the rumen microbial population (e.g. by
vaccines, probiotics, defaunation) or the gastrointestinal tract
(GIT) environment (e.g. by feeding grain, fats, oils, antibiotics,
tannins, seaweed, clays, acids or salts); or (ii) the animal itself;
or (iii) the livestock system. Research is needed to improve
understanding of rumen ecology and animal factors that affect
MP from individual animals. Farm management and breeding
options also need consideration at a flock or herd level.
Some authors have stated that, with appropriate policies,
technologies and management practices, it may be possible to
achieve reductions in MI of up to 75% (Mosier et al.1998).
However, most technologies to control MP from ruminants have
not proved cost-effective (Keogh and Cottle 2009) and some may
result in the under-utilisation of low-cost fibrous feed resources
(van Nevel and Demeyer 1996). For example, the addition of oil to
feed will reduce MP but may also depress dry matter intake (DMI)
(Beauchemin and McGinn 2005) and is a relatively expensive
strategy. It will be important, therefore, to develop cost-effective
strategies for mitigating ruminant CH
4
emissions that do not have
negative effects on ruminant production from both high- and low-
quality forages. A related challenge is to develop ways to deliver
such technologies to livestock in extensive grazing conditions
with limited human intervention.
An important component of the research into all mitigation
options is whole-farm system modelling and LCA (ISO 14040
series). This component is critical to first assess the likely
whole-farm impacts, but also to ensure that the strategy does
not increase emissions elsewhere in the production chain. For
example, feed supplements may improve livestock production
and reduce MI [e.g. Hunter and Neithe (2009)], but the
impacts of growing and processing these supplements must
also be taken into account. Similar considerations apply to
feedlot production.
Table 1. Importance of agricultural and livestock industry emissions
(excluding land-use change and forestry emissions and removals) in
some national inventories in 2005
Source: adapted from United Nations Framework Convention on Climate
Change (2009)
Nation Agricultural
emission as
a % of total
national
emissions
Enteric
emissions as
a%of
agricultural
emissions
Enteric
emissions as
a % of total
national
emissions
New Zealand 48.4 64.0 31.0
Ireland 26.4 49.6 13.1
France 17.4 29.3 5.1
Australia 16.8 65.8 11.1
Denmark 13.4 27.1 3.6
Sweden 12.9 32.9 4.2
European Community 9.3 31.6 2.9
Canada 8.6 39.1 3.4
United Kingdom 6.7 36.6 2.5
USA 6.5 27.8 1.8
Italy 6.5 29.0 1.9
Russia 6.0 28.3 1.7
Japan 2.0 25.7 0.5
0
10
20
30
40
50
60
70
80
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
Year
Mt CO2-e
Cattle Sheep All other livestock types
Fig. 1. Trends in CO
2
-e emissions from Australian livestock 1990–2006. Source: adapted from DCC (2008).
492 Animal Production Science D. J. Cottle et al.
Flock/herd and national objectives
The total MP from the herd or flock can be calculated as:
Number of livestock ·average DMI per head ðkgÞ
·average MP per kg DMI ð1Þ
The number of livestock run on a property in any year is
determined by the farmer’s assessment of pastoral conditions,
their available management options and their attitude to climatic
and financial risk. The DMI per head can be deliberately reduced
by nutritional control, e.g. by increasing stocking rates, or
indirectly by selecting smaller animals, but neither option is
commonly practised. The more desirable approach to reducing
DMI is genetic selection for improved feed-use efficiency, i.e.
breed livestock that produce the same amount and quality of
product (meat or wool) with a lower feed intake.
The main approaches to reducing MP per unit DMI (Y
m
)
are discussed in this review. Y
m
can be referred to as the CH
4
conversion factor, an entity that enables small-scale CH
4
emission estimates to be extrapolated to farm, national and
global inventories (Lassey 2007).
At a national level, when aggregating the outputs from all
flocks and herds, it needs to be clear whether the accounting
or policy objective is to reduce the amount of CH
4
produced
from an industry per year (which is most simply achieved by
reducing stock numbers) or the MI level (which usually requires
either genetic or nutritional approaches). Genetic or nutritional
approaches are needed if the main national objective is to ‘feed
the world’via exports, which requires increases in both animal
numbers and meat production per year, while at the same time
minimising total CH
4
emissions. This approach is far more likely
to improve farm profitability and sustainability than reducing
stock numbers, unless more economic alternate land uses to
livestock production are available.
MI versus LWG is typically a curvilinear relationship because
of the effect of maintenance feed requirements. Increasing DMI
over the maintenance threshold does increase CH
4
emissions, but
increases production to a greater extent. Carbon accounting
Table 2. Enteric methane (CH
4
) emissions from beef cows, steers and heifers
n.a., not applicable
Liveweight
(kg)
Nutrition/management CH
4
emissions
(g/day)
Annual
CH
4
emission
(kg/head.year)
Liveweight
gain (kg/day)
CH
4
production
(kg CH
4
/kg
liveweight gain)
Reference
Cows
580–600 Best grazing management –rotational
grazing + supplementation
–67.5 –n.a. DeRamus et al.(2003)
580–600 Continuous grazing –some restricted
access and weight loss
–86.0 –n.a. DeRamus et al.(2003)
506.2 Rotationally grazed –lucerne 246 89.7 –n.a. McCaughey et al.(1999)
516.2 Rotationally grazed –grass 270 98.6 –n.a. McCaughey et al.(1999)
Heifers
295–480 Continuous grazing –some restricted
access and weight loss
–32 –n.a. DeRamus et al. (2003)
295–480 Rotationally grazing improved pastures –83 0.30 0.76 DeRamus et al. (2003)
300–350 Controlled trial –feeding long copped,
low-quality tropical hay (Angleton grass)
94.5 34.5 –0.88 n.a. Kurihara et al. (1999)
300–350 Controlled trial –feeding long copped,
mid-quality tropical hay (Rhodes grass)
215 78.5 0.57 0.38 Kurihara et al. (1999)
300–350 Controlled trial –feeding lucerne hay and grain 134 48.9 1.30 0.10 Kurihara et al. (1999)
328 Grain ration 148.7 54.3 1.50 0.10 McGinn et al. (2006)
328 Feedlot forage ration 134.8 49.2 1.50 0.09 McGinn et al. (2006)
Steers
280–700 Feedlot –Australian typical grain ration 146 53.3 1.70 0.09 Loh et al. (2008)
265–620 Feedlot –Australian typical grain ration
(predominantly sorghum with some oil)
166 60.6 1.70 0.10 Loh et al. (2008)
265–620 Feedlot –Australian typical grain ration
(predominantly sorghum with some oil)
214 78.1 1.70 0.13 McGinn et al. (2008)
325 Strip-grazed on quality ryegrass 161 58.8 1.20 0.13 Laubach et al. (2008)
323 Hill country grazing in New Zealand 189 69.0 0.80 0.24 Molano, cited in
Laubach et al. (2008)
345 Grazing with supplement 174 63.5 0.80 0.22 Boadi, cited in
Laubach et al. (2008)
356 Grazed (continuous, 2.2 head/ha) 173 63.3 1.26 0.14 McCaughey et al. (1997)
356 Grazed (continuous, 1.1 head/ha) 184 67.2 1.29 0.14 McCaughey et al. (1997)
356 Grazed (rotational, 2.2 head/ha) 159 58.1 1.07 0.15 McCaughey et al. (1997)
356 Grazed (rotational, 1.1 head/ha) 202 73.6 1.48 0.14 McCaughey et al. (1997)
Enteric methane mitigation options Animal Production Science 493
systems based on Intergovernmental Panel on Climate Change
(IPCC) methods are not structured to assess MI but rather to assess
total emissions, taking into account animal types and size and
seasonal and geographical effects on animal and feed attributes.
Rumen manipulation and ecology
In the aerobic metabolism of living cells, excess electrons and H
2
can combine with O
2
to form water, but this reaction is not
possible in anaerobic environments. Anaerobic microorganisms
such as ruminal bacteria, protozoa and fungi ferment dietary
organic matter (OM) components (starch and plant cell wall
polysaccharides, and proteins and other materials) and release
end-products that include volatile fatty acids (VFA), CO
2
,H
2
and
CH
4
. Fermentation also occurs in the caecum and colon of
ruminants but the amount of OM fermented is usually much
less than in the rumen. Contrary to news headlines, even hind gut
MP in ruminants is mostly released via the lungs (Murray et al.
1976).
Fermentation reactions use the coenzyme NAD
+
to oxidise
dietary carbohydrates and NADH/H
+
is formed from NAD
+
.H
2
is
generated when the protons associated with the NADH/H
+
are
reduced by the action of hydrogenases of the microbial ferridoxin
oxidoreductase systems. The H
2
diffuses out of microorganisms
and is either used by other microorganisms, or accumulates in
the rumen gas space. In the final stages of fermentation, H
2
is used
as a reducing agent and NAD
+
is regenerated. In particular,
methanogens oxidise the H
2
(energy content 143 MJ/kg) to
reduce CO
2
to CH
4
(energy content, 55 MJ/kg), thereby
gaining energy for their growth (McAllister and Newbold
2008). This H
2
removal is extremely important because, if H
2
accumulates, reoxidation of NADH to NAD
+
is restricted, and
this inhibits carbohydrate degradation, ATP production and
microbial growth. Forage digestion and the resultant
production of VFA are then restricted (Joblin 1999; Janssen
2010).
Options for reducing MP
Options for reducing MP include: (i) inhibiting H
2
-producing
reactions; (ii) promoting alternative reactions which accept H
+
during reoxidation of reducing equivalents; and (iii) promoting
alternative H
2
-using reactions (Hegarty 1999b).
Redox potential (a measure of the affinity of a substance for
electrons) helps explain the options that have been found to
reduce MP. Some H
2
-utilising reactions that can potentially
remove H
2
(and electrons) and supply NAD
+
are listed in
Table 3. The reactions are ranked according to DG (the change
in free energy of the reactions) based on assumptions of
Ungerfeld and Kohn (2006) about the likely activity of
reactants and products in the rumen, excluding ATP.
A further possibility for reducing MP could be to promote
anaerobic CH
4
oxidation in the rumen. This occurs in anaerobic
sediments 500 m below the ocean surface (Hinrichs et al.1999)
and also occurs to some extent in the rumen. Kajikawa et al.
(2003) calculated CH
4
oxidation from the differences in
13
C
Fig. 2. Potential options for reducing enteric methane production.
494 Animal Production Science D. J. Cottle et al.
enrichment of CO
2
and microbial cells in sheep. Although they
found CH
4
was oxidised in the absence of O
2
, the amounts
were small (<1% of the CH
4
produced). Nevertheless, 9% of
the added
13
C accumulated in the microbes over a 24-h
period, indicating that
13
C from
13
CH
4
was metabolised and
incorporated into microbial polymers. CH
4
oxidation was
inhibited by the addition of tungstate, a known inhibitor of
sulfate and nitrate reduction (Immig 1996), so it is likely that
the CH
4
oxidation was occurring in conjunction with sulfate
or nitrate reduction. The quantitative importance of anaerobic
CH
4
oxidation in the rumen, although apparently small, may be
manipulable and deserves further study in vivo.
Mitigation strategies aimed at reducing populations of
methanogens usually involve inhibition of methanogens and
include alternatives for removal of H
2
so that fermentation is
not impeded.
Inhibiting methanogens
Defaunation
Finlay et al. (1994) suggested that endosymbiotic bacteria
may generate 37% of rumen MP. In addition, an association
between MP and the numbers of protozoa in the rumen has been
observed (Ushida and Jouany 1996). Stumm et al. (1982) have
estimated that, at any one time, 10–20% of rumen methanogens
are attached on the outside of protozoa, particularly on
entodiniomorphs (Vogels et al. 1980). Moreover, removal of
protozoa and feed particles from rumen contents by centrifugation
also removed 76% of the methanogens present (Newbold et al.
1995). Depending on diet, elimination of the protozoa may
reduce MP by up to 50% (Hegarty 1999a).
Hegarty (1999b) refers to a meta-analysis made by Eugene
et al. (2004) of 90 publications dealing with the effects of
eliminating rumen protozoa and notes that for defaunated
animals, in general, there was a reduction in feed intake,
a higher propionate : acetate ratio in total rumen VFA and
a higher microbial protein outflow from the rumen –all
consistent with a lowering of net H
2
generation and
consequent reduction in MP. In addition, turnover of rumen
fluid is faster in defaunated animals, which is also likely to
reduce methanogenesis (Hegarty 2004b). Morgavi et al.
(2008) found that defaunated sheep maintained a lower
(~20%) MP (determined with SF
6
) for more than 2 years,
whereas Hegarty et al. (2008) detected no differences in MP
(in respiration chambers) of lambs reared either free of protozoa,
or chemically defaunated, and those reared concurrently with
normal populations of rumen protozoa. The effectiveness of any
defaunation strategy in reducing MP in commercial situations is
still uncertain.
Antibiotics,monensin. This is a naturally occurring
polyether ionophore antibiotic that is isolated from
Streptomyces cinnamonensis and widely used as a rumen
modifier, especially for cattle given concentrate diets (Grainger
et al. 2008). Monensin reduces MP mainly by reducing
voluntary DMI (Goodrich et al.1984). Monensin dissipates
ion gradients across the membranes of gram-positive bacteria,
which promotes selection for succinate-forming Bacteroides
and S. ruminantium, the latter being a propionate producer that
decarboxylates succinate to form propionate (Chen and Wolin
1979). Monensin also results in selective reduction of acetate
formation and associated H
2
production by inhibiting the release
of H
2
from formate (van Nevel and Demeyer 1977; Slyter 1979).
CH
4
suppression by the inclusion of monensin in the diet is not
premanent (Mbanzamihigo et al.1995,1996; Waghorn et al.
2008). When delivered by controlled release devices, monensin
has been used successfully to minimise bloat in Australian dairy
cattle, but this mode of delivery was not effective in reducing MP
in New Zealand dairy cattle (Waghorn et al.2008).
Bacteriocins. These are naturally occurring peptide toxins
that inhibit the growth of closely related strains of bacteria. Nisin
has effects on ruminal fermentation that are similar to monensin
(Russell and Mantovani 2002) and has been shown in vitro to
reduce MP by 36% (Callaway et al. 1997). As well as exogenous
bacteriocins, however, there are bacteriocins released within the
rumen itself. Kalmokoff et al.(1996) surveyed 50 strains of
Butyrivibrio and found about half exhibited a wide range of
inhibitory activities. Because many lactic acid bacteria produce
bacteriocins, it is possible that part of the reduced MP observed
at low pH is due to bacteriocins rather than a direct effect of
pH. Teather and Forster (1998) have suggested that ruminally
produced bacteriocins could represent a new type of rumen
modifier. Archaea, like bacteria, produce substances referred
to as archaeocins that also inhibit microbial growth (O’Connor
and Shand 2002) but whether archaeocins produced by one
archaeal organism can inhibit the growth of other archea is
unclear.
Feed additives
Synthetic chemicals. Many synthetic feed additives (mainly
antimicrobial compounds) are known to have direct or indirect
effects on MP (van Nevel and Demeyer 1996). These include
halogenated CH
4
analogues, e.g. 2-bromoethanesulfonic acid
(BES) (Immig et al. 1996); dicarboxylic acids, e.g. fumarate
(Asanuma et al.1999); fatty acids, e.g. myristic acid (Odongo
et al.2007); galacto-oligosaccharides and nisin (Santoso
et al.2003); ionophores, e.g. monensin and lasalocid
(Guan et al.2006); nitrite reducers (Sar et al.2005) and
hydroxymethylglutaryl-SCoA reductase inhibitors (Miller and
Wolin 2001). None of these compounds is used routinely in
commercial livestock industries to reduce CH
4
emissions.
Halogens. Halogenated compounds such as chloroform
and BES have direct inhibitory effects on methanogenic
bacteria and reduce MP both in vitro and in vivo (Bauchop
1967; Clapperton 1974). BES inhibits the action of methyl
Table 3. Thermodynamic ranking of some H
2
utilising reactions that
occur in the rumen; from least to most favourable
Source: adapted from Ungerfeld and Kohn (2006)
Reduction reaction Electron acceptor
concentration (mmol/L)
DG
A
(kJ)
Pyruvate + H
2
!lactate 10
6
–5
2CO
2
+4H!acetate + H
+
+2H
2
O 0.016 –9
CO
2
+4H
2
!CH
4
+2H
2
O 0.016 –67
NO
3
–
+H
2
!NO
2
–
+H
2
O15–130
NO
2
–
+3H
2
+2H
+
!NH
4+
+2H
2
O8–371
A
DG is the change in free energy of the reactions.
Enteric methane mitigation options Animal Production Science 495
coenzyme M reductase in the last step of methanogenesis. The
sensitivity of various ruminal methanogens in pure culture to
CH
4
inhibitors including BES is quite variable with
Methanobrevibacter ruminantium being the most sensitive,
Methanomicrobium intermediate and Methanosarcina mazei
the least sensitive (Lee et al.2009). Martin and Macy (1985)
found that 30 mM BES reduced MP by 76% in mixed cultures
of rumen fluid. Though often effective in the short term, these
compounds may lose their inhibitory effects with repeated
administration (van Nevel and Demeyer 1996). Immig et al.
(1996), for example, observed in sheep that MP was
reestablished after 4 days of BES infusion into the rumen. The
inhibitory effect of BES was extended by complexing it to a
cyclodextrin matrix (McCrabb et al.1997).
Dietary nitrate. Renewed recognition that nitrate
supplements in ruminant diets compete successfully for H
2
and
electrons (and decrease MP) is a promising development (Leng
2008). In addition to inhibiting MP, the end-product of nitrate
reduction in the rumen is ammonia –a major source of the N for
microbial protein synthesis in the rumen.
Nitrate has a greater affinity for H
2
than does CO
2
and most
other potential precursors (Table 3; Ungerfeld and Kohn 2006)
and so, when nitrate is present in the rumen, nitrite formation
is favoured over MP. Nitrite reduction to ammonia is also more
favourable than CO
2
reduction but is often less favourable
than nitrate reduction (Iwamoto et al.1999). Nitrates are
potent inhibitors of methanogenesis in other anaerobic systems
including biodigestors and sediments (Hungate 1966; Allison
et al.1981; Akunna et al.1994). There were marked reductions in
MP in vitro after the addition of nitrate or nitrite salts to rumen
contents from animals that were acclimated to dietary nitrate
(Allison and Reddy 1984; Inthapanya et al.2011). As well as
reducing MP, the nitrate-reducing microorganisms should obtain
more energy, and so achieve higher rates of microbial growth.
This been demonstrated in vitro (Guo et al.2009).
There are as yet few in vivo studies of the effect of nitrate on
MP from ruminants, but emissions from Merino crossbred sheep
given chopped hay in respiration chambers were reduced by 23%
in response to the addition of 4% KNO
3
(0.6% NO
3
-N) to the
diet (Nolan et al.2010) with no detectable changes in DM
digestibility or microbial cell outflow from the rumen. The
sheep used were acclimated to dietary nitrate and there was no
indication of nitrite toxicity as judged by blood methaemoglobin
concentrations, which were always below 2%. In Texel lambs,
van Zijderveld et al.(2010b) found similar reductions in MP in
response to dietary nitrate and, in addition, showed that numbers
of rumen methanogens were reduced. They obtained a further
(additive) reduction in MP when sulfate was added to the nitrate-
supplemented diet. Again, feed intake and LWG were unaffected.
Inclusion of nitrate in the diet also reduced MP in dairy cows
van Zijderveld et al.(2010a).
In the rumen, nitrate and urea are converted to ammonia, which
provides N for microbial protein synthesis. In production
studies with goats given a low-protein straw diet, Hao Trinh
Phuc et al. (2009) obtained similar improvements in growth
rate and N retention irrespective of whether the animals were
supplemented with nitrate or urea. In a similar study with cattle,
Le Thi Ngoc Huyen et al. (2010) compared sodium nitrate and
urea as iso-nitrogenous sources of supplementary N in cattle
offered NaOH-treated rice straw, molasses and cottonseed meal.
They found that MP was reduced by nitrate supplementation
while feed intake, digestibility and growth rate did not differ
between treatments. This is particularly noteworthy for the
Australian pastoral industry where nitrate could potentially
replace urea as the major supplementary source of rumen
fermentable N for sheep and cattle grazing on mature, low-
protein pastures. McAllister et al.(1996) have cautioned that
nitrate supplementation might be impractical because of the
risk of (nitrite) toxicity but, after an extensive review of the
literature, Leng (2008) concluded that ‘nitrate can be used as a
source of fermentable N in the rumen and, provided the animal is
acclimated to nitrate, there will be no ill-effects and possibly
improved efficiency of microbial growth’. Leng (2008) suggested
that nitrite accumulation and absorption, the reason for toxicity,
may be avoided if (1) the rumen microbial population has
been acclimated to nitrate, and (2) sulfur : nitrate ratios in the
diet are appropriate to maintain the activity of sulfur-reducing
bacteria that also play a role in reducing nitrite to ammonia.
Systems already employed in Australia to manage the risk of
‘urea toxicity’when supplementing animals, such as molasses-
based blocks or water medication, could also be used to control
nitrate intake. Whether this mitigation strategy is used in practice
will depend on the costs of purchasing nitrate and distributing
it to grazing animals as well as the risk of nitrite toxicity.
Currently, urea-N is less expensive than nitrate-N, so farmers
or graziers would not be likely to adopt nitrate supplementation
without a financial benefit arising from increased rumen
microbial protein production, or from payments for reduced
CH
4
emissions, or both.
Other chemicals. In addition to nitrate and sulfate, there
are other electron ‘sinks’that remove H
2
in the rumen; e.g.
dicarboxylic acids such as malate, fumarate and succinate that
are intermediates in the so-called ‘randomising pathway’, which
can use H
2
to provide the energy for propionate synthesis. As a
result, some of the energy of H
2
is captured and made available
for animal production. In vitro studies with fumarate or malate
have usually resulted in reduced MP (Asanuma et al.1999) but
this has not always been the case (Callaway and Martin 1997).
In vivo data are scarce but, when Bayaru et al. (2001) included
2% fumaric acid in diets for cattle given silage, MP decreased by
23% from 180 to 139 L/day. Unfortunately, dicarboxylic acids are
expensive to synthesise and are unlikely to be affordable in the
foreseeable future.
MP was markedly inhibited in in vitro cultures treated
with nitropropanol, nitroethane, nitroethanol, sodium laurate,
Lauricidin or a finely ground product of the marine algae,
Chaetoceros, or combinations of these compounds (Anderson
et al.2003). However, these compounds also inhibit fermentation
to varying degrees (Bozic et al.2009) and so may reduce DMI and
animal production. Administration of 2-nitro-1-propanol and
nitroethane has been shown to reduce MP in mature ewes by
as much as 94%, but the mechanisms are unclear (Anderson et al.
2006).
Two hydroxymethylglutaryl-SCoA reductase inhibitors,
mevastatin and lovastatin (drugs used in human medicine)
have been found to inhibit the growth in vitro of strains of
Methanobrevibacter isolated from the rumen, and to reduce
their production of CH
4
(Miller and Wolin 2001).
496 Animal Production Science D. J. Cottle et al.
Natural compounds
Proanthocyanidins. These are condensed tannins that occur
naturally in a variety of plants and bind to and precipitate proteins.
They also have antimethanogenic properties (Waghorn and
McNabb 2003; Tavendale et al.2009). Carulla et al.(2005)
found that inclusion of tannins from Acacia mearnsii in the diet
reduced CH
4
release by 13% without affecting body N and energy
retention. Similarly, Tiemann et al.(2008) found that tannins of
Calliandra calothyrsus and Flemingia macrophylla at 25 g/kg
dietary DM reduced MP/GEI by 13% with no change in energy
retention. For example, MP was lower in Angora goats when
eating plants such as Sericea lespedeza that have high tannin
levels than when ingesting crabgrass/tall fescue (6.9 versus 16.2 g
CH
4
/kg DMI) (Puchala et al.2005). Tavendale et al.(2005)
found negative relationships between total phenol, total tannins
or tannin activity and CH
4
concentrations. However, inclusion of
legumes that are rich in tannins, even though reducing MP, can
also reduce DMI and so lower production.
Myristic acid. This is a common saturated fatty acid (14 : 0)
named after the nutmeg, Myristica fragrans. Nutmeg butter
contains 75% trimyristin, the triacylglycerol of myristic acid.
Myristic acid is also found in coconut oil, palm oil and
spermacetin. Odongo et al.(2007) found MP in cows was
36% lower, when the cows had been acclimated for 10 days to
a diet containing 5% (DM basis) myristic acid, than in a baseline
period.
Oils and fats. Inclusion of lipids in the diet reduced MP by
5.6% for each 1% of added lipid (Beauchemin et al.2008) or 3.5%
(Moate et al.2010). This mitigation strategy has been adopted in
Canada but is of questionable value as the small reduction in MP
means the cost : benefit ratio is low. Useful lipids can be found in a
variety of feeds including coconut oil and whole crushed oilseeds
(rapeseed, sunflower seed and linseed). Both long-chain fatty
acids (LCFA) and medium chain-fatty acids (MCFA) reduce
MP (Blaxter and Czerkawski 1966). Research has been mainly
focussed on unsaturated LCFA because they take up H
2
as they
become more saturated. However, LCFA also reduce fibre
digestion (Broudiscou et al.1990) and are less effective in
reducing MP than MCFA (C10–C14), with C12 : 0 and C14 : 0
being most effective (Dohme et al. 2000). High dietary calcium
or high dietary fibre content can reduce the level of CH
4
suppression in response to MCFA (Machmuller et al.2003).
Adding coconut oil, sunflower seed and linseed in vitro reduced
MP and completely eliminated protozoa from rumen fluid
after 4–9 days (Machmuller et al.1998). However, the
reduction in MP was thought to be due to a direct inhibition of
methanogenesis by archaea rather than to the effects of MCFA
on protozoa (Soliva et al.2003).
Other naturally occurring compounds. There will be other
compounds with antimethanogenic properties that have not
been discovered. Others with potential have not yet been
extensively researched, e.g. diallyl disulfide and allicin that
are active ingredients in garlic oil (Allium sativa).These
reduce MP in populations of rumen microbes both separately
and in combination (Busquet et al.2005). García-González et al.
(2008) found that rhubarb and frangula contain active
secondary compounds that inhibit ruminal methanogenic
microorganisms.
Vaccination
A considerable amount of work has been undertaken,
particularly in Western Australia, with the aim of producing
vaccines that trigger the animal’s immune system to generate
antibodies against enteric methanogens. So far vaccine
formulations (using crude methanogen cultures to provide
antigenic materials) have been virtually ineffective when tested
in practical situations (Wright et al.2004). Williams et al.(2009)
evaluated, in 32 sheep, a broader spectrum vaccine targeting
five methanogen phylotypes including more than 52% of
different methanogenic species/strains found in a survey of
sheep in Queensland. The vaccinated sheep showed specific
IgG titers in plasma, saliva and rumen fluid, but CH
4
output
and the total number of rumen methanogens were unaltered. In
New Zealand, workers are using genomic screening to identify
microbial proteins specifically involved in methanogenesis that
could be used to develop antisera and broad-spectrum vaccines
(Buddle et al.2010; see Rumen genomics).
Enhancing non-methanogens
Any strategy aimed at reducing methanogenic populations in
the rumen will be likely to change the balance of species and
improve conditions for growth of non-methanogenic microbial
species (Martin et al. 2010). However, the focus can be placed
specifically on enhancing the conditions for the establishment
and sustainability of non-methanogenic species.
Diet manipulation
The effects of level of feed intake and diet composition on
MP and opportunities for its nutritional manipulation have
been extensively reviewed (e.g. Blaxter and Clapperton 1965;
Pelchen and Peters 1998; Kurihara et al.1999; Hegarty et al.
2010).
Molano and Clark (2008) found MP was highly correlated
with DMI (r
2
= 0.83) but, as Johnson and Johnson (1995)
recognised earlier, when DMI increases, the fraction of GEI
lost as CH
4
declines. Intake of forages tends to increase as
digestibility increases (Freer and Jones 1984; Hegarty et al.
2010) and in sheep, MP was highest when digestibility was
~72% and ME concentration was ~10.5 MJ/kg DM (Pelchen
and Peters 1998). When fibrous forage was replaced with higher
quality immature forage with higher digestibility, MP was
reduced by 15% and for processed forages the reduction was
21% (Benchaar et al.2001). Similarly, MP was 28% lower for
grass forage and 20% lower for good quality silage than for
lower quality hay.
Dietary characteristics affect rumen conditions and so
alter the balance of methanogenic and other species present
(Martin et al.2010) and also the relative importance of
biochemical pathways operating in the microorganisms. Diets
high in highly fermentable grains, for example, will be rapidly
fermented. This will lower pH because of the rapid rate of
production of VFA and possibly lactic acid; lactic acid itself,
or lower pH may then kill protozoa, removing one of the major
habitats of methanogens (see Protozoa section). Lower pH
has other effects on microbial metabolism and inhibits some
species more than others. Thus, different types of fermentable
Enteric methane mitigation options Animal Production Science 497
compounds in the digested component of feed can alter MP
and Moe and Tyrell (1979) were able to explain 67% of the
variance in MP (MJ/day) from dairy cows by accounting for non-
fibre carbohydrate, hemicellulose and cellulose concentrations
in the diet. However, MP is not always so well predicted from
dietary characteristics (e.g. Waghorn et al.2006), in part,
probably because some diets contain chemicals (e.g. nitrate) or
naturally occurring compounds (e.g. tannins) that act as rumen
modifiers.
Fine grinding of fibrous feeds increases the rate of digestion
of carbohydrate materials, but may also increase the rate of
outflow of digesta from the rumen. Factors that increase
rumen turnover or outflow rate (e.g. increases due to inclusion
of roughage in predominately concentrate diets or reductions due
to inclusion of monensin) can also modify rumen conditions. As
turnover is increased, microorganisms that are unable to grow
more quickly (e.g. protozoa and methanogens) will tend to be
eliminated (Hegarty 2004b). Sheep with longer ruminal retention
time (RRT) usually have larger rumen volumes and higher fibre
digestibilities and MP (Pinares-Patiño et al.2003b). Cows with
a low RRT of particles in the rumen produce less CH
4
than
cows with a high RRT (Pinares-Patiño et al.2007). Differences
in RRT change the number, composition, maintenance energy
requirements and diversity of rumen microorganisms and their
VFA pattern. RRT may also affect MP and LCFA synthesis in the
rumen, which may consequently affect the energetic efficiency
of the animal. RRT could be modified in breeding programs
(see Genetics section).
As intake increases above maintenance, animal production
increases proportionally more than MP, so MP per unit of
production tends to decrease (Hunter and Neithe 2009). As
can thus be predicted, pasture-fed cattle produce less CH
4
as
a proportion of GEI when supplemented with grain (DeRamus
et al.2003) and cattle ingesting a highly digestible grass
produced relatively less CH
4
than those ingesting more fibrous
forage (DeRamus et al.2003). Furthermore, cattle on grain-
based diets commonly emit less CH
4
energy as a fraction of
GEI (Johnson and Johnson 1995) than those on high-quality
forages. In this context, it must be recognised, however, that
pasture-fed ruminants will remain important insofar as they are
unique among domesticated livestock in being able to release
the energy of plant cellulose, which represents ~50% of the solar
energy trapped globally by photosynthesis. The challenge is to
provide pasture-fed ruminants with higher digestibility forages
so that their level of production is increased and MP and Y
m
is
reduced.
Farm management practices can affect the quality of pasture
and the amount on offer, both of which have major effects on
MPR per unit of production from livestock. In addition, practices
such as early mating of lambs at ~7 months of age, and provision
of higher quality diets in order to improve fecundity can reduce
CH
4
emissions per unit of saleable product (Hegarty et al.2010)
but can also be detrimental (see Management section). The
GrassGro modelling studies of Alcock and Hegarty (2006)
suggest that pasture improvement may generate an almost
4-fold increase in gross margins from sheep because, even
though annual farm CH
4
emissions are higher when pasture is
improved, the proportionally greater increase in farm profit
outweighs the cost penalty of higher MP. Some caution is
required, however, when making prediction using models such
as GrassGro as these may not accurately predict MP (see
Management section).
Inoculants, acetogens
The negative change in Gibbs free energy (DG) for
methanogenesis (–67 kJ/mol) is more favourable than for
acetogenesis (–8.8 kJ/mol) (Table 3). Indeed, rumen
methanogens have thresholds for H
2
assimilation that are
10–100 times lower than homoacetogens (Fievez et al.2001)
so they normally out-compete the acetogens by keeping the
H
2
concentration below the critical value needed to enable
acetogens to persist. In sheep, acetogens have been reported to
range from less than 10
2
to over 10
8
per mL (Le Van et al.1998;
Joblin 1999). Nevertheless, some other animals, such as termites,
cockroaches, rodents and humans, have ways of suppressing
methanogens (Breznak and Kane 1990; Mackie and Bryant
1994). Phylogenic trees showing microbial evolution indicate
that host animal factors under genetic control can inhibit the
growth of methanogens or foster growth of acetogens (Hackstein
1997), but these mechanisms do not appear to exist normally
in the rumen. Nevertheless, at least 10 species of rumen bacteria
have biochemical pathways that enable them to use H
2
and CO
2
to synthesise acetate by reductive acetogenesis (RA) (Joblin
1999). A variety of other microbes capable of RA have
also been found recently (Henderson et al.2010). They
probably do not normally obtain their ATP for growth from
CO
2
and H
2
, but instead use other substrates (Mackie and
Bryant 1994).
If methanogens could be inhibited sufficiently in ruminants
(e.g. using vaccination or dietary inoculants), H
2
concentration
might rise to the critical level needed for sustainable growth
rates of acetogens (Le Van et al.1998). Circulating antibodies
against methanogens have been identified in sheep (Holloway
and Baker 2002), so it is possible that vaccinated animals could
mount an effective immune response targeting their own rumen
methanogens. Alternatively, acetogens that secrete compounds
toxic to methanogens might be included in inoculants to boost
their numbers in the rumen.
For reasons that are not yet clear, MP is different between
sheep and kangaroos (Kempton et al.1976) and can also differ
between individual sheep (Joblin 1999). Eastern grey kangaroos
and tammar wallabies (von Engelhardt et al.1978) produce
less CH
4
per unit digestible DMI (DDMI) than ruminants even
though they ferment fibrous feeds and generate VFA in a
manner similar to ruminants (Dellow et al.1983). Ouwerkerk
et al.(2005) found that forestomach contents of kangaroos
had appreciable numbers of acetogens but few methanogens.
The situation is similar in the hind gut of pigs, humans and
rats (Joblin 1999), ostriches (Fievez et al.2001) and termites
(Breznak and Switzer 1986), all of which have acetogen
populations that apparently compete effectively with
methanogens. One novel option for reducing ruminant
numbers would be to replace cattle and sheep with kangaroos
for meat production in extensive areas (Wilson and Edwards
2008). Alternatively, a challenge for rumen microbiologists is to
find ways of creating the conditions in ruminants that match
those present in macropods.
498 Animal Production Science D. J. Cottle et al.
Animal breeding
Overview
An alternative to nutritional management is to selectively breed
livestock that can use feed more efficiently or produce less CH
4
per unit DMI (Hegarty 2002; Buddle et al.2010; Hegarty and
McEwan 2010; Hegarty et al.2010; Martin et al.2010; Wall et al.
2010). Direct selection for animals exhibiting lower MP is
currently impractical because it is difficult and costly to
measure MP. It is possible to select for reduced MP indirectly
via correlated traits such as feed intake or rumen digesta retention
time (Hegarty 2002). In future, genetic markers or genes that
affect MP may be discovered.
The animal factors assumed responsible for differences in
MP are rate of digesta passage, microbial activity, fermentation
conditions, anatomical and physiological differences in the GIT
and grazing behaviour (Goopy and Hegarty 2004; Iqbal et al.
2008; Hegarty et al.2010). Between-sheep variation in MP has
been studied using respiration chambers (Blaxter and Clapperton
1965; Hegarty et al.2010), in vitro methods (Demeyer and van
Nevel 1975) and under grazing conditions (Lassey et al.1997;
Ulyatt et al.1999). Ulyatt and co-workers reported that ~85% of
the variation in daily MP from sheep grazing temperate pastures
was due to variation between animals. Pinares-Patiño et al.
(2003a) found that differences in Y
m
of sheep grazing pasture
were maintained in the medium term. The heritability estimate in
sheep of 0.13 for MP/1 h, adjusted for LW, is the only heritability
estimate published for livestock (Hegarty et al.2010).
There are differences in MP between rumen microbial
population genomes (Galbraith et al.2004). The animal’s
genome will affect its rumen microbe population (Guan et al.
2008), digestive function (Hegarty 2004b), feed intake (Arthur
et al.1997; Goopy and Hegarty 2004), and feed-use efficiency
(Arthur et al.1996,2001). These all affect the MP per kg animal
product (MI) and are subject to complex metabolic and hormonal
control. Thus Y
m
will be affected by many genes and a single
nucleotide polymorphism (SNP) chip approach using thousands
of genes or gene markers (Kijas et al.2009) is more likely to be
successful as a molecular approach to reducing MP than a
quantitative trait loci (QTL) approach (Nkrumah et al.2005a).
Vlaming et al.(2008) found the repeatability of Y
m
in dairy
cows was higher on lower quality diets (0.73 on straw) than high-
quality diets (0.47 on silage). Eckard et al.(2010) suggested
animal breeding could achieve a reduction of ~10–20% in Y
m
but
argued that it is better to breed for improved feed conversion
ratio (FCR) as this should be compatible with existing breeding
objectives and is likely to both reduce MP and Y
m
. Breeding for
reduced MP or Y
m
compromises other traits if there are
unfavourable correlations with production traits (Eckard et al.
2010; Hegarty and McEwan 2010; Wall et al.2010). These
interactions have been modelled using selection indexes for
Merinos (Cottle et al.2009), New Zealand sheep (Amer et al.
2009), UK hill sheep (J. Conington, pers. comm.), Angus beef
cattle (D. J. Cottle, pers. comm.) and dairy cattle (Wall et al.
2010).
A two-step selection process is probably best for inclusion of
MP as a trait, with step 1 being the measurement of primary traits
(MP/day, DMI/day, production) and step 2 being the combination
of these primary traits into secondary, derived traits (e.g. Y
m
)if
needed. That is, a breeding objective should include MP and DMI
so that the estimated breeding value (EBV) for MP and DMI can
be calculated separatedly and then divided (EBV
MPR
/EBV
DMI
),
if required, rather than calculating EBV
MPR/DMI
as a single ratio
trait.
Less costly measures for determining primary traits are
required. There is genetic variation in pasture DMI that may
reflect differences in ewe maintenance requirements (Fogarty
et al.2006). A system to measure pasture DMI that includes
animal identification and marker technology (Charmley and
Dove 2007) has been invented (PCT/AU2010/001054).
Selecting livestock which consume less feed for a given
level of production [high net feed efficiency (NFE) or low net/
residual feed intake; (NFI/RFI)] continues to be a global focus
of research (e.g. Crews et al.2010; Hegarty et al.2010). The
RFI trait is moderately heritable (Snelling et al.2010) but
the high cost of its measurement, its questionable relevance
to pasture-fed animals and its correlations with body
composition and reproduction, will continue to limit the use of
RFI selection. The EBV
RFI
of animals determined on ad lib grain
(Herd et al.2006) or hay rations (Meyer et al.2008) may be
poorly correlated with their feed efficiency on lower levels of
intake when at pasture (Lanna 2009) or with their progenies’
EBV
RFI
(Rutherford 2010).
Earlier work by Okine et al.(2001) showed that Canadian
steers with high NFE had 21% lower MP than low NFE steers.
RFI is a trait that reflects energy requirements for maintenance
and growth (Archer et al.1997,1998,1999; Archer and Bergh
2000; Herd et al. 2004). Herd et al.(2002) calculated that, after
two generations of divergent selection, Angus cattle selected for
low RFI produced 15% less enteric CH
4
/day than those selected
for high RFI. The two groups of cattle did not differ in LWG and,
although the difference in Y
m
was small (1.3%), the difference in
MP/kg LWG (MI) was larger (16%).
Hegarty et al.(2010) noted that lifetime performance studies
of animals selected for NFE need to be done. GrassGro modelling
by Alcock and Hegarty (2010) showed that less breeding ewes
would be run when the ewes are more feed efficient which is
counter-intuitive. GrassGro modelling of MP is discussed further
in the Management section of this review.
The uptake of breeding for RFI in the Australian beef industry
has been low and may not be the most economic approach to
breeding (Lanna 2009). van der Westhuizen et al.(2004) found
selecting indirectly for economic value through the use of FCR
was better than use of RFI.
Quantitative genetics
Direct or indirect measurement of CH
4
production.MPis
mainly dependent on diet quality and feed intake (Blaxter and
Clapperton 1965) but differences of 40–60% in Y
m
between
individual sheep on the same diet have been reported (Vercoe
2009). Some of this variation is due to CH
4
measurement error.
Goopy and Hegarty (2004) found that rankings of individual
animals for CH
4
emission characteristics may not persist over
time and that selection of animals for low MP may need to be diet-
specific.
Indirect calorimetry techniques used to measure CH
4
are
reasonably accurate and reliable but expensive, and
Enteric methane mitigation options Animal Production Science 499
extrapolation of results to grazing animals is questionable. The
sulfur hexafluoride (SF
6
) tracer technique (Pinares-Patiño et al.
2003a; Pinares-Patiño and Clark 2008) was developed for field
use and is associated with large within- and between-animal
variability in estimates from animals on the same diet. It also
lacks the accuracy to identify individual animal emission profiles
(Pinares-Patiño and Clark 2009).
Proxies for CH
4
measurement are VFA concentrations and
proportions, 3-min total breath collection, CO
2
:CH
4
ratios in
breath during and after feeding; and short-term calorimetry
estimates (Vercoe 2009). The best proxy predictor of daily MP
has been a 2-h ‘window’of calorimetry, which explained
50–82% of MP variance, depending on the time of day the
window occurred. Propionate production is higher when CH
4
output is lower, as H
2
is used in its formation whereas it is
produced during acetate and butyrate production (Hegarty
2004a). VFA concentrations fluctuate markedly during the day
(Kentaro et al.2003) in response to meals eaten and factors such
as rumen pH, which affect VFA absorption. Hegarty et al.(2010)
reported the use of sealed chambers that can measure MP over
1 h. However, this trait has low heritabililty and its correlation
with daily MP was only 0.5. Hegarty et al.(2010) note selection
on MP without DMI is less likely to achieve a reduction of
CH
4
without production losses. However, this need not be the
case if production traits are included in a selection index.
Cottle et al.(2009) showed that, if MP and production traits
are positively correlated, Australian Merino sheep breeders
would not have MP as a selection criterion even for very high
carbon prices. However, modelling results are very sensitive to
CH
4
: production trait correlations.
Indirect selection via correlated traits. Indirect selection
can be superior to direct selection when the genetic correlation
with the indirect trait is high and the heritability of the indirect
trait is higher than the heritability of the direct trait. The
heritability of MP appears low (Hegarty et al.2010).
There is increased selection accuracy from also including
information on correlated traits, such as DMI and FCR, mainly
dependent on the difference between genetic and environmental
correlations between the traits. The accuracy for a trait with a low
heritability can be significantly improved if the information from
a correlated trait with high heritability is included. Unfortunately,
the genetic and environmental correlations between MP and most
indirect traits, such as DMI and FCR are unknown in both sheep
and cattle (Safari et al.2007).
Wall et al.(2010) noted that genetic improvement to reduce Y
m
could be achieved via: (i) improving productivity and efficiency;
(ii) reducing wastage in the farming system; and (iii) directly
selecting on MP, if or when this is measurable. They showed that
favourable effects on the overall emissions from UK dairy
systems using selection on traits that improve the efficiency of
the system (e.g. RFI, longevity). In contrast to Martin et al.
(2010), who stated nutritional mitigation strategies are the
most developed and ready to be applied, Hegarty et al.(2010)
and Hegarty and McEwan (2010) reported the most effective
strategies for sheep were to increase culling age (longevity),
increase conception and weaning rates and join ewes at a
younger age. The negative implication of increased longevity
on generation interval and therefore annual genetic gains in all
traits was not discussed.
Y
m
should decrease with selection for increased productivity
(Iqbal et al.2008). For example, Kirchgessner et al.(1995)
suggested that increasing milk production of 600-kg dairy
cows from 4000 to 5000 kg/cow.year would increase annual
CH
4
emissions from 103 to 108 kg but would decrease CH
4
emissions/kg of milk from 0.026 to 0.022.
Feed intake,feed conversion and residual feed intake.As
noted previously, MP and DMI are often highly correlated,
e.g. Shibata and Terada (2010) reported that the quadratic
equation relating MP and DMI, that is used for ruminant
livestock in the National GHG Inventory for Japan, had
r
2
= 0.93. Current Australian carbon accounting methods
(DCC 2008; DCCEE 2010) for sheep use an equation from
Williams and Wright (2005) (Eqn 2), which differs slightly
from the equation of Howden et al.(1994).
MPR ðkgÞ¼0:0187 ·DMI ðkgÞ0:0003;r2¼0:87 ð2Þ
O’Hara et al.(2003) noted that the relationship between MP
and DMI can be highly variable between animals. Lassey et al.
(1997) found a correlation of only 0.37 in sheep grazing fresh
pasture. The correlation between Y
m
and DMI for the same dataset
had a stronger relationship (r= 0.6), indicating that as DMI
increases the percentage of digestible energy lost as CH
4
decreases. Animals that produce more products per unit DMI
probably produce less CH
4
per kg product. Lee et al.(1995) found
repeatability of daily estimates of digestible OM intakes of ewes
within a season varied from 0.32 to 0.47 for the four seasons,
while the repeatability across seasons was lower (0.09–0.27).
Thus pasture DMI measurements may need to be made in more
than one grazing period. Kahn (1994) found the heritability and
repeatability of supplement intake at pasture were reasonably
high (0.17 and 0.48, respectively).
If DMI is in the breeding objective, an economic value is
required to reflect differences in the cost of pasture in different
production systems (Ponzoni 1988). Feed costs represent around
half the total cost of production for most classes of livestock, so
DMI should be considered in most breeding programs (Kennedy
et al.1993; Snelling et al.2010).
For individual studs or commercial herd or flock breeding
programs to achieve reductions in CH
4
outputs, one major
decision to be made is whether the selection breeding
objective is MP/head per year, or MP/kg or value of product
produced/head per year (a ratio trait), or MP/kg DMI/head
per year (also a ratio trait). If CH
4
or its metabolic markers
cannot be measured cost-effectively, then traits that are highly
correlated with MP can be used.
An argument for having ‘MP/head per year’as the breeding
objective trait rather than ‘MP/kg DMI per year’is that if DMI
is also included in the breeding objective along with MP, then it is
most efficient to include both traits as selection criteria rather
than selecting for the ratio trait. This approach could still allow
the subsequent calculation of the EBV for MP per kg DMI or per
kg product, if this information was preferred or better understood
by breeders. A similar argument for using MP/year and LW as
breeding objectives and selection criteria can be made for the
trait MP/kg or value of product produced/head per year.
Thompson and Barlow (1986) showed that greater
improvements in enterprise efficiency would result from an
500 Animal Production Science D. J. Cottle et al.
improvement in FCR of the growing animal and reduction in DMI
of the mature dam than selection for just growth rate. Koots et al.
(1994) reported highly negative weighted genetic correlations
between FCR and growth rate and size. These correlations
indicate that selection to improve efficiency (reduce FCR)
would be accompanied by an increase in growth rate but an
increase in mature cow size. A second disadvantage of selection
for FCR relates to suboptimal selection using ratio measurements
(Gunsett 1986) with the two traits concerned (DMI and growth)
having different variances which reduces the efficiency of index
selection.
Considerable genetic variation in feed intake, independent of
size and growth rate, exists in beef cattle. This trait (NFI or RFI), is
usually measured over 50–70 days (Kennedy et al.1993; Archer
et al.1997,1998,1999). Body composition is also included in
RFI calculations in Canadian beef breeding programs (J. Basarab,
pers. comm.). The main advantage of using RFI versus FCR as an
efficiency trait is that it is not a ratio trait and should not reduce
growth or correlated responses in maturity type (Archer et al.
1997,1998; Herd and Bishop 2000). Selection on RFI and
production should lead to identical responses to those from
selection on DMI and production, as RFI adds no new genetic
information (Kennedy et al.1993). Archer et al.(2004) found
profit was maximised where only 10–20% of bulls were selected
by RFI, i.e. two-stage selection was more cost-effective. It can be
argued that DMI is an easier trait for breeders to understand
than RFI. The benefit of high genetic production in cattle may be
outweighed by additional feed costs needed to attain optimal
body energy reserves at calving (Lee et al.2010).
Arthur et al.(2001) found FCR was genetically (r
g
= 0.66) and
phenotypically (r
p
= 0.53) highly correlated with RFI, as were
the correlations between DMI and FCR (r
g
= 0.31, r
p
= 0.23)
and DMI and RFI (r
g
= 0.69, r
p
= 0.72).
The major physiological processes contributing to variation
in RFI are those associated with intake of feed; microbial
populations and their digestion of feed; metabolism; body
composition; physical activity; and thermoregulation (Herd
et al. 2004; Herd and Arthur 2009; Martin et al.2010). Guan
et al.(2008) found feed-efficient steers had specific rumen
bacterial groups producing more butyrate and valerate present
and the breed (host genetics) also affected groups present.
Herd and Arthur (2009) noted that early studies had shown
a multitude of genes to be associated with differences in RFI,
which is expected, given the diversity of physiological processes
involved. Differences in feeding behaviour (Deswysen et al.
1993; Seman et al.1997,1999; Richardson and Herd 2004;
Robinson and Oddy 2004; Wilson et al.2005; Dobos 2007;
Dobos and Herd 2008), such as time spent feeding (RFI r
g
= 0.35,
r
p
= 0.16) and number of feeding sessions per day (RFI r
g
= 0.43,
r
p
= 0.18) also affect RFI. In ram lambs considerable phenotypic
variation was observed in four feeding behaviour traits that were
all moderately heritable (Cammack et al.2005).
Hegarty et al.(2007) found the increase of 13.3 g CH
4
/day per
EBV
RFI
(kg/day) in Angus steers was between that predicted from
DMI alone (18 g CH
4
/day.kg of DMI) and that predicted by a
model incorporating steer mid-test LW and intake level relative
to maintenance (5 g CH
4
/day.kg DMI). Selection for low RFI
reduced MP but RFI explained only a small proportion of variance
in MP. Hegarty et al.(2007, 2009) suggested a high genotype ·
nutrition interaction exists, which could make the use of RFI to
reduce MP difficult and less successful on high-energy (feedlot)
diets.
Concentration of insulin-like growth factor 1 in plasma has
been used to select indirectly for feed intake (Wood et al.2004),
but correlations were low with post-weaning RFI, and results
from subsequent research have suggested that the direction of the
correlation is reversed for RFI in mature animals (Wood et al.
2004; Herd et al.2006). Therefore, at best, the approach would
only potentially be useful as a screening tool for selecting animals
for further RFI or DMI measurements.
Alford et al.(2006) estimated the reduction in MP from the
Australian beef herd resulting from expected use of low RFI bulls
as 568 100 t of CH
4
over 25 years, with annual emissions in Year
25 being 3.1% lower than in Year 1. They concluded that selection
for reduced RFI would lead to worthwhile CH
4
abatement.
Metabolic heat production. An animal’s metabolisable
energy intake (MEI) is partitioned between metabolic heat
production (HP) and retained energy. HP is mainly determined
by maintenance requirements and the heat increment of
production (Brosh 2007,2009). When cattle production is less
intensive and activity (grazing) is greater, HP can represent most
of the energy flux. Brosh (2009) suggested measurement of
individual HP might be used for selection for greater
efficiency, without the need for individual DMI measurements.
Daily HP has been measured by multiplying the daily recorded
heart rate (HR) by short interval measurements of O
2
consumption per heart beat (O
2
pulse). Cows’HR and
respiration rate can be recorded continuously and remotely by
the use of a bolus (Veterix Co., Aqiva, Israel) inserted into
the reticulum. Brosh suggested selecting cattle for lower MP
by using the HR-O
2
pulse method to select for low HP in relation
to the energy in the milk and meat produced.
Rumen retention time. Hegarty (2004b) noted that RRT is
associated with between-animal variation in digestive function
(see Diet selection section). Within-animal variability of RRT
is high and ranking of animals in MP differs with changes in
diet and/or physiological stages (Pinares-Patiño et al.2007)or
between successive measurements for the same diet and same
feed intake (Munger and Kreuzer 2008). The latter authors
estimated the repeatability of RRT as 47–73%, depending on
diet. There are no published reports of selection of animals for
lower RRT, which is measured using infused digesta markers
(Uden et al.2006).
Disease resistance. Ill-health and consequent immune
responses are likely to increase maintenance requirements,
incur loss of nutrients and generally reduce the overall
efficiency of the production system and therefore increase Y
m
(DEFRA 2009).
Molecular genetics
QTL and SNP. Many of the direct and indirect traits related
to MP have low heritability and are difficult to measure. DNA
polymorphisms or variants that explain some of the variation in
traits are called QTL and can be used to improve the accuracy of
selection of traits such as MP. Variation of a DNA marker in the
proximity of a QTL can predict variation at the QTL and
differences in breeding value or phenotype. Until 2004 and the
discovery of SNP, the main DNA markers used for detection of
Enteric methane mitigation options Animal Production Science 501
QTL were microsatellites. In a typical genome scan (GS) ~200
microsatellites and putative QTL near them were associated
with phenotypic differences via linkage analysis. Associations
have to be tested in each new family when markers and QTL are
not physically close and in linkage disequilibrium. There are
difficulties in estimating the effects of different QTL in different
breeds and management systems and incorporating information
on multiple QTL into routine genetic evaluations.
Few QTL have a noticeably large effect on phenotype or
EBV. The additional amount of genetic improvement from using
gene markers varies widely from 2 to 60% (van der Werf 2009).
QTL and SNP for RFI and DMI in beef cattle have been found
(Barendse et al.2007; Sherman et al.2008a,2008b,2008c).
A primary GS for RFI QTL has been demonstrated (Nkrumah
et al.2005b,2007) and five cattle (BTA) chromosomes with
RFI QTL (BTA2, 5, 10, 20 and 29) have been fine mapped to
even smaller confidence intervals (Moore et al.2006). Sherman
et al.(2008a,2008b,2008c) found the combined effects of the
significant SNP explained only 6.9% of the phenotypic variation
of RFI. Nkrumah et al.(2005b) reported polymorphisms in the
coding regions of the leptin gene in cattle that had associations
with DMI and feeding behaviour.
When SNP are used in a GS they are spread through the
genome at a higher density than microsatellites and some SNP
will be functional mutations. Currently available sheep and cattle
SNP chips contain around 56 000 SNP with an average distance
between two markers of only 0.05 cM (Dalrymple et al. 2007;
Matukumalli et al.2009). The current cost of chips is still too
high for commercial use. The low recombination frequency of
0.05% means that mutations will remain with original marker
alleles for many generations allowing prediction of QTL
differences across families and potentially even across breeds.
By simultaneously fitting all markers in GS association studies,
statistical models can accommodate the joint effects of all QTL
affecting a trait (Meuwissen et al.2001; van der Werf 2009).
The EBV of a young dairy bull can be predicted with 60%
accuracy with a 60-K chip, which for many traits approaches
the accuracy of a progeny test. Young bulls could be screened
from a wider pool based on genomic information before progeny
testing.
After calibration of the relative effects of each marker on
specific traits, animals can be selected for several generations on
the basis of their genomic information, as opposed to recording of
phenotypes. This approach clearly has attractions for expensive/
difficult-to-measure traits such as MP. However, uncertainties
about reduction in genetic variance (van der Werf 2009) and the
high initial cost of calibration means SNP chips are most likely to
be used in the dairy cattle industry where the cost of recording for
genetic selection is high (Schaeffer 2006).
Documentation of breed genetic structures will be needed
when mapping the genetic basis of complex traits such as MP.
Although there are probably SNP related to MP, the need to
phenotype at least 2000 animals for MP suggests the use of
SNP chips for MP or DMI is a long way from commercial reality.
Patents have been filed for SNP related to RFI in cattle
(Barendse and Reverter-Gomez 2006; Hayes et al.2006;
Moore et al.2008) but there is little commercial use of them.
Nevertheless, work to validate molecular markers in most
livestock species continues.
Antimicrobial strategies
It may be possible to express a species- or genus-specific
antimicrobial protein that was delivered to the rumen. Such
proteins are found in nature and can be used exogenously to
eliminate a target organism (Schuch et al.2002; Yoong et al.
2004; Entenza et al.2005; Cheng and Fischetti 2007). These
specific antimicrobial proteins are generally products of other
microbes that directly compete with the target organism or are
products of phages that infect the target organism. A mammary
specific lysostaphin transgene provided complete resistance to
Staphylococcus aureus mastitis in both transgenic mice and
dairy cattle (Kerr et al.2001) showing that genetic engineering
could be used with an antimicrobial strategy to target individual
microbe species. Such a strategy to eliminate methanogens from
the rumen (see Inhibiting methanogens section) is a possibility.
Rumen genomics
Attwood and McSweeney (2008) reviewed methanogen
genomics for options for alternative H
2
utilisation in the rumen
and suggested enhancing rumen microorganisms that carry out
RA (see Inoculants, acetogens).
A metagenomic library of the entire rumen microbial
population of a cow has been created and sequenced (Bath
2008) but has not yet appeared in the literature. Less than
0.1% of the genomic sequences matched published sequences,
revealing a unique and complex microbial community. Of the
sequences, 1% showed homology to the Archaeal domain.
Within this group, a large range of different methanogens were
identified, probably including previously unknown species.
These results are being used as the basis for development of a
rumen specific microarray aimed at improving FCR and reducing
MP in dairy cattle. Leahy et al.(2010) recently sequenced the
M. ruminantium genome and identified 71 candidate proteins
for a methanogen vaccine (see Vaccine section).
Plant breeding
Plant breeding can be expected to have a place in reducing
livestock CH
4
emissions but new species are still to be
commercialised. Selecting plants for characteristics that
change rumen conditions could include, for example, high
sugar in perennial rye grasses (Humphreys 1989) or tannins
(Puchala et al.2005).
Increasing the water-soluble carbohydrate (WSC) content in
perennial ryegrass by 33 g/kg reduced MP in vitro by 9%, so the
inclusion of clovers and grasses with high WSC in animal
diets may directly reduce MP/DMI (Lovett et al.2004). When
animals were fed high-sugar grasses, there was a shift in rumen
fermentation towards higher proportions of propionate and
butyrate (Peyraud et al.1997; Lee et al.2001), which may
reduce MP.
A spin-off company of the Molecular Plant Breeding CRC,
Gramina, has used sense suppression technology to prevent
the expression of the enzyme O-methyl transferase to increase
the digestibility of ryegrass without compromising its structural
properties. Sense suppression has also been used to modify lignin
biosynthesis (PCT/AU08/001034). Field trials of new plant
varieties are being conducted before commercial release.
502 Animal Production Science D. J. Cottle et al.
Management
Reproductive rate, turnoff and enterprise mix
Efficiency of production systems depends upon the feed intake of
both the breeding herd or flock and the slaughter generation,
growth rates of various classes of stock and other traits.
Reproductive rates thus greatly influence the age structure and
therefore overall feed efficiency of the system (Archer et al.
1999). In typical beef cattle production systems, the breeding herd
accounts for 65–85% of the total feed requirements (Ferrell and
Jenkins 1984; Montaldo-Bermudez et al.1990) and 65–75% of
this is used for maintenance. As reviewed by Pitchford (2004), the
large maintenance requirement is in contrast to other production
systems such as pigs or poultry, where the breeding animal has
a small intake relative to the total intake of all progeny. Any
improvement in the efficiency with which breeding cows
maintain bodyweight will result in an increase in total meat
production for a given amount of feed. Genetic improvement
for system efficiency (higher reproductive rate, longevity, etc.) is
probably more likely to lower MP than selection for reduced CH
4
per se.
Reducing the number of unproductive animals on-farm has
potential to both improve profitability and reduce MP (Eckard
et al.2010; Hegarty and McEwan 2010). Through earlier
finishing of beef cattle in feedlots, slaughter weights are
achieved at a younger age, with reduced lifetime emissions per
animal and thus proportionately less animals producing CH
4
(Smith et al.2007). Several options therefore exist to minimise
unproductive animal numbers on-farm and possibly shift to
more novel production systems, all of which have potential to
both reduce total CH
4
emissions and improve on-farm
profitability.
Hyslop (cited in DEFRA 2009) demonstrated that efficiency
of the beef production system was paramount in reducing the MI,
and that intensive concentrate-based systems produced the lowest
emissions. However, the externalities of the system such as the
embedded emissions associated with producing concentrate diets
were not included. Hyslop also showed there was a significant
breed difference such that heavier continental breeds of cattle
produced less MP/unit output than the smaller British-type
breeds. Two recent LCA projects have investigated grain and
grass finishing in Australia (Peters et al.2010) and the USA
(Pelletier et al.2010). Both studies indicated that grain finishing
produced beef with lower levels of GHG (in CO
2
-e) per kg of beef
produced where soil carbon levels were considered static and
sequestration in vegetation was not considered. In both cases, the
higher growth rate and lower MP while cattle are in the feedlot
more than compensated for the added GHG associated with
grain production and energy usage in the whole supply chain.
Interestingly, when soil carbon sequestration was included for
pastures, Pelletier et al.(2010) reported lower total GHG for
pasture-fed beef compared with lot-fed beef. This highlights the
possibility of other GHG sinks to counter the higher enteric MI
anticipated from pasture grazing.
Bentley et al.(2008) studied recent innovations to increase
the efficiency with which beef is produced in northern Australia.
Improving herd genetics, property infrastructure, the seasonal
feed-base and its utilisation, as well as promoting feedlot
finishing can all be expected to reduce the number of
unproductive animals and reduce age-at-slaughter and reduce
MI. Charmley et al.(2008) using a herd economic model
showed that an important determinant of MI is reduced days to
market which may be achieved through a range of energy
supplementation and marketing strategies.
Hunter and Neithe (2009) have made theoretical calculations
of feed energy use and MP for breeding, growing and finishing
steers to 650 kg in northern Australia. They calculated that MP
from a breeding cow/steer unit was reduced by ~30% when
weaning rate was increased from 65 to 85% or when growth
rate at pasture was increased from 0.3 to 0.6 kg/day. If the steers
were removed from pasture at 400 kg LW and finished in a
feedlot, MP was reduced by 55%. An increase in weaning rate
of 20% and a doubling of growth rate increased the number of
cattle sold annually (steers and females) by 40% and the
amount of retail beef produced by 47%. Total whole-herd MP
was unaltered, but MI was reduced by ~30%.
While the influence of productive efficiency on MI in cattle is
relatively straight forward, this simple logic cannot be extended to
sheep. The primary reason for the added complexity is that two
products are harvested: wool and meat. This is particularly
relevant in Australia, where the majority of the breeding flock
are of a wool-producing breed (Merinos).
Young sheep have lower Y
m
than mature animals (Knight
et al.2008). Amer et al.(2009) predicted genetic increases in
carcass weight in maternal breeds in New Zealand resulted
in higher MI because of correlated responses in ewe mature
weight associated with selection for growth rate. A genetic
increase in litter size of 10% resulted in a 6% reduction in
MI. For terminal breeds, genetic improvement of carcass
weight reduced MP.
An Australian pasture production and grazing program,
GrassGro3, has been used to identify the impacts of different
mitigation strategies on sheep enterprise MP and profitability
(Vercoe 2009). The main findings reported to date are: (i) the
greatest reduction (20%) in MI and overall emissions resulted
from intensive use of production supplements to finish lambs at
the earliest possible age; (ii) improving reproduction rate reduces
MI by between 2.5 and 5% for each 10% percentage point
improvement in weaning rate. Overall emissions will increase
if there is not a commensurate reduction in ewe numbers; (iii)
joining replacement ewes as lambs generally leads to an increase
in MI and little change in absolute emissions; (iv) lambing away
from optimal time (winter/spring) leads to reduced overall
emissions but increased emissions intensity; and (v) absolute
emissions can only be reduced in situations where total kg of
feed consumed declines.
Farm systems models, such as GrassGro3 and SGS Pasture,
are limited in their ability to model the likely impacts of changes
in flock nutritional management or breeding programs on MP
yet are being used for this, e.g. Alcock and Hegarty (2006).
In GrassGro3, prediction of MP is based on the equations of
Blaxter and Clapperton (1965), who reviewed results from
477 sheep experiments and 138 cattle experiments. Blaxter
developed the following predictive equation:
CH4yield ð%of GEIÞ¼1:30 þ0:112D þLð2:37 0:05DÞ
ð3Þ
Enteric methane mitigation options Animal Production Science 503
where D = DM digestibility of the feed at maintenance and
L = level of feed intake as a multiple of maintenance.
The GrassGro3 model uses the equation:
CH4ðMJÞ¼0:0184 ðIfþIsÞ½ð13:0þ7:52 M=DsolidsÞ
þðLþ1Þð23:73:36 M=DsolidsÞ ð4Þ
where I
f
= feed intake (kg DM per day), I
s
= supplement intake
(kg DM per day), M/D
solids
= ME per kg DM of the solid part of
the diet.
These equations, which associate MP solely with levels of
feed intake and the energy concentration of the diet and do not
incorporate the effects of mitigation factors, e.g. dietary fat
content or feed additives (see Rumen ecology section).
Similarly, the SGS Pasture Model estimates CH
4
energy loss
as a fixed proportion of energy intake. This relationship can be
prescribed manually for various pastures and supplements. The
default values for CH
4
energy production are 6% of GE for forage
(19.89 g CH
4
/kg DMI) and 4% of GE for concentrates (13.26 g
CH
4
/kg DMI), assuming that the GE of forage is 18.45 MJ/kg
and the energy content of CH
4
is 55.65 MJ/kg.
Using DairyMod a mechanistic whole-farm systems model,
which has SGS Pasture as its pasture module, Johnson et al.
(2008) reported that under a higher quality pasture system, a 33%
increase in stocking rate resulted in 11% less CH
4
/cow but 26%
more CH
4
/ha. Thus improving pasture quality may improve
productivity, as well as lower MI, but the whole-farm MP may
also be increased.
While not all the strategies reviewed above are directly
additive, where strategies act at different points in the system,
their cumulative impact on total emissions from a production
system can be significant. For example, dairy cattle bred for
improved FCR (10% less CH
4
), fed on dietary oils (10% less
CH
4
), milked on an extended lactation management system (10%
less CH
4
), with a nitrification inhibitor sprayed on the paddocks
twice per year (61% less N
2
O) could feasibly lead to a cumulative
emission reduction of 40% in whole-farm GHG emissions, but
also significantly improved production from the farm (Eckard
et al.2010).
Wilson and Edwards (2008) proposed running kangaroos
(see Inoculants, acetogens) based on the lower per head MPR
of kangaroos of 0.003 t CO
2
-e/year (Kempton et al.1976; Klieve
and Ouwerkerk 2007) compared with 1.67 for cattle and 0.14
for sheep. The presence of microbes capable of reductive
acetogenesis in kangaroos (Ouwerkerk et al. 2005,2009) may
provide an opportunity to understand and re-direct conversion
of H
2
to products other than CH
4
in ruminants.
The infrastructure, management, stock husbandry and market
acceptance implications of this suggested radical change were not
discussed.
Grazing management
In Australia, most ruminants are raised in continuous grazing
systems where livestock have unrestricted access to pasture.
However, animals soon eliminate the most desirable plant
species because they graze them continually while less edible
plants are undergrazed and may become more dominant in the
pasture. Weeds can become a problem and overgrazing can
accelerate erosion.
Systems of rotational grazing, also referred to as ‘controlled
grazing’or ‘management-intensive grazing’(MiG), can lead to
more efficient use of pasture. In controlled grazing, larger areas of
pasture land are divided into smaller paddocks. Pasture growth
and digestibility can be increased, as can stocking rate and meat
production (kg/ha). DeRamus et al.(2003) reported that moving
cattle in rotation through a group of smaller paddocks produced
forage of higher quality than when cattle were set-stocked in a
conventional system on a similar area of pasture. A comparison
of MP/head per month in the continuous grazing versus MiG
management systems from DeRamus et al.(2003) is given
in Fig. 3but this report does not give values from which
MP/ha or /kg growth can be readily calculated. MiG is
practised by farms in the Eatwild group in the USA and
Canada (Eatwild 2009).
Growing and finishing cattle
Finishing cattle in feedlots. In general, the argument for
finishing beef cattle in intensive feedlot systems is based on
the idea that their faster growth rate and shorter time to market
results in a lower CH
4
per unit of meat production than for cattle
raised in extensive grazing systems (Clemens and Ahlgrimm
2001).
However, opinions on the environmental effects of growing
cattle at pasture or in feedlots vary widely –especially in the non-
scientific literature. Some environmentalists (e.g. Allan Savory)
have argued that finishing beef cattle on maize is more detrimental
to the environment than raising and finishing cattle solely on
pasture. Others (Johnson et al.2000) argue the opposite, pointing
out the environmental consequences of land clearing, cropping
and the associated use of fertilisers and fossil fuel releases
larger amounts of CO
2
than would occur if more ruminants
were present.
The IPCC Tier 2 enteric emission model estimates that feedlot
cattle lose 3.5% of GEI as CH
4
(IPCC 1997). This estimate was
derived from cattle ingesting high-maize diets. CH
4
emissions for
diets based on other cereals (such as barley) have not been as
extensively evaluated, but Johnson et al.(2000) have speculated
that high-grain barley diets would result in greater CH
4
emissions
than those from high-grain maize diets, based on CH
4
losses of
6.5–12% of GE reported previously for barley diets (Whitelaw
et al.1984).
Beauchemin and McGinn (2005) measured CH
4
in growing
beef cattle in Canada fed maize- or barley-based diets typical of
those fed to cattle in North American feedlots. Angus heifer
calves (initial bodyweight 328 kg) were backgrounded on diets
consisted of 70% barley silage or corn silage and 30% concentrate
containing steam-rolled barley or dry-rolled corn. Finishing diets
consisted of 9% barley silage and 91% concentrate containing
barley or corn. All diets contained monensin (33 mg/kg DM).
Cattle were placed into respiration chambers during each phase to
measure MP for 3 days. During the backgrounding phase, DMI
was greater for cattle fed maize than for those fed barley (10.2
versus 7.6 kg/day, but during the finishing phase, DMI was
similar for both diets (8.3 kg/day). CH
4
emissions per kg DMI
and as a percentage of GEI were not affected by the type of grain
504 Animal Production Science D. J. Cottle et al.
during the backgrounding phase (24.6 g/kg DMI; 7.42% of GE),
but were less for maize than for barley during the finishing phase
(9.2 versus 13.1 g/kg DMI; 2.81 versus 4.03% of GE). The results
suggested that barley-based finishing diets generate higher CH
4
outputs than maize-based diets. There does not appear to be any
published work comparing different grains for finishing cattle in
Australia.
In Alberta in Canada, a system of credits for carbon offsets
is legislated and there are three possible protocols for
which credits have been claimed from 2002 onwards, if the
requirements of the system and protocols are met. The system
accounts for productivity improvements throughout the beef
production supply chain such as reduced days to slaughter,
higher reproductive rates and inclusion of feed additives, such
as edible oils, known to reduce emissions. The carbon credit
received for feeding oil is worth much less than the cost of the oil
fed so this protocol may not continue (J. Basarab, pers. comm.).
Selection for RFI (see Genetics section) in beef cattle is a
draft protocol that was submitted for the 5th cycle protocol
development in September 2009. This protocol has completed
all rounds of review and is currently awaiting Alberta
Government review and approval. The protocols are designed
to be additive. Cattle managers who wish to feed edible oils and
slaughter cattle at an earlier age can apply both protocols to their
operation.
Backgrounding on pasture and finishing in feedlots
An alternative to providing high-grain diets to feedlot cattle is
to use the same grain as a supplement to the cattle at pasture.
Cohen et al.(2004) concluded that finishing cattle by
supplementing them with barley at pasture instead of
backgrounding them at pasture and finishing them on a barley-
rich diet in a feedlot would reduce the total emissions of CH
4
and
increase the efficiency of conversion of feed energy to
LWG. These workers compared DMI and CH
4
emissions of
cattle predicted using GrassGro with actual results obtained
in a field experiment. Predicted MP (278 g/day) was
significantly greater than that measured in the field
(196 g/day). CH
4
emissions predicted using GrassGro
compared on the basis of MI were less for supplemented
grazing cattle than for feedlot steers (133 versus 199 g CH
4
/kg
LWG, respectively). In addition, when barley was provided to
cattle at pasture, the total emissions of CH
4
(39 kg) were less
than for cattle unsupplemented at pasture and then finished
on barley in a feedlot (54 kg). The MEI required for LWG was
also less (68 versus 87 MJ MEI/kg LWG).
Mitigation options cost benefit
Models available on the University of Melbourne website
(Eckard 2009) enable the effects of different LWG and
reproduction rates on MP to be modelled. The number of
livestock in each livestock class in the four seasons can be
changed. The reproduction and mortality rates and age structure
of the flock or herd for steady-state numbers in a self-replacing
flock or herd are not automatically calculated thus herd or flock
structures are not easily modelled. The Australian Farm Institute
(AFI) commissioned the development of GHG spreadsheets,
which include the analysis of gross margins and are similar to
Eckard’s sheets, except that they have less problems with N
calculations associated with stubble burning or breakdown. Also
the AFI agro-forestry calculator, unlike Department of Climate
Change methodology, allows the inclusion of mixed-species
biodiversity plantings, which have a much lower carbon ‘yield’
than single-species plantations (M. Keogh, pers. comm.). The
FarmGAS Calculator (Australian Farm Institute 2009)allows
the addition of mitigation options, where the user inputs the
expected effect on emissions of an option to enable a quick cost-
benefit analysis of any option. To complete this hypothetical
0
1
2
3
4
5
6
7
8
9
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month
Methane output (kg/cow.month)
Fig. 3. Comparison of ‘predicted’methane production of cattle in a management-intensive grazing system
and a conventional set-stocked system. Dark shading, set-stocking; light shading, best management practice
(management-intensive grazing). Source: adapted from DeRamus et al.(2003).
Enteric methane mitigation options Animal Production Science 505
scenario, a total treatment cost or per head cost needs to be
determined and the estimated reduction in emissions entered.
The calculator then provides information on the ‘cost’of
emissions with and without this hypothetical application and the
expenditure on mitigation. As these options are not validated by
the United Nations Framework Convention on Climate Change,
their use wouldnot be permitted in any national inventory analysis;
however, they allow cost-benefit analysis of future scenarios for
various mitigation options.
Examples of the outputs of FarmGAS with and without the
use of a CH
4
mitigation option are shown in Table 4. If carbon
prices were ~$A28/tCO
2
e, the price under consideration by the
Australian government in 2009, mitigation options would need
to have low costs per head, and effectiveness levels over ~30%
to be cost-effective.
Conclusions
Large reductions in MI will require the application of an
integrated suite of options. All animals have an obligatory
maintenance energy requirement that results in no production,
yet has an associated MP. ‘Dilution of maintenance’is a major
key to profitably reducing MP. This can be achieved by: faster
turnoffs of slaughter stock, e.g. high-quality pasture or feedlot
finishing; increased reproductive efficiency; early culling of non-
productive animals, minimising periods of low production and
selecting more efficient animals. Research from the field of LCA
has indicated that intensification is likely to reduce total GHG
despite the emissions associated with producing inputs such
as supplementary feed. However, most LCA research has not
thoroughly investigated the impact of emissions and
sequestration from vegetation and soil, which may have a
large influence on results, particularly for low-input grazing
systems. Further research is needed to evaluate mitigation
strategies in the context of the whole agricultural system.
Reduction of stock numbers is neither economic for an
individual producer nor desirable in terms of providing more
feed energy and protein for the world’s population.
Acknowledgements
We thank Meat and Livestock Australia for supporting our work. The authors
acknowledge Professor Roger Hegarty and the paper’s reviewers for their
helpful comments.
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