Life cycle assessment of the livestock industry - Green house gas emission
Troels Kristensen, Lisbeth Mogensen, John E. Hermansen, Marie Trydeman Knudsen & Anna Flysjö
Aarhus University, Faculty of Science and Technology, Department of Agroecology, P.O box 50, DK8830 Tjele, Denmark
Contact troels.kristensen@agrsci.dk
Life cycle assessments (LCA) provide a systematic, but still in mature method to quantify the
environmental impact of a product from cradle to grave. Livestock products carry a large carbon emission
compared to other foods, with the largest emission coming as methane from enteric fermentation in
ruminants and as laughing gas from handling of manure. As direct emission from use of fossil energy is
only a minor part, the most promising mitigations options are within production efficiency, fed
conversion and new technology to close the N-cycle as well as use of manure in biogas production.
1. Introduction
The agricultural sector world vide is one of the major contributor to green house gas (GHG) emission,
which is associated to the climate change, with an estimated proportion of around 20% of the total
annually emission. The livestock sector is assumed to be responsible for the largest part at nearly 80% of
global agricultural GHG emissions. This is particularly due to methane (CH4 ) emissions from enteric
fermentation in ruminants and manure handling, and due to the intensive nitrogen (N) cycle on livestock
farms leading to direct and indirect laughing gas (N2O) emissions.
With the global demand for animal-sourced foods set to double by 2050, the implications for GHG
emissions are profound. The already large contribution from agriculture to global GHG emissions will
therefore increase in importance unless more effective and climate-friendly systems are adopted.
Furthermore, the agricultural contribution to CO2 emissions from deforestation can only be reduced if the
productivity of existing agricultural land is improved. At the same time has the political system set up
targets to reduce GHG emission by 20% over the same time span, which put pressure on the agricultural
sector and not least the livestock sector to find ways of reducing the emission both in total and per kg of
products.
Life cycle assessment (LCA) is an appropriate and accepted methodology to quantify the environmental
impact of a product through the product chain. At the same time, this tool can efficiently be used to
identify hot spots, and thereby, improvement options in relation to environmental impacts associated with
a certain product. Recently, there has been a boost in the mutual interest from business and policy in
LCA approaches exacerbated by the concern about global warming. It is generally agreed that LCA is an
appropriate tool to address this issue instead of more partial tools like ‘food miles’. This is important
because within the agriculture and food sector, emissions other than CO2 related to energy for transport
are often more important considering their contribution to the total impact on global warming. Thus,
emissions of N2O or CH4 are often the most important contributors to global warming in agricultural
production. In LCA, the total emission of all GHG (CO2, N2O, CH4) are aggregated in CO2 equivalents
(CO2 e.), which are often referred to as carbon footprint (CF), taking into account their different
contribution to the global warming potential, typically with 100-year time horizon. Besides global
warming also other impact categories like eutrophication and acidification can be estimated by LCA, but
in this paper the focus are on global warming.
LCA provides a method where the emission is allocated to the product and thereby to the place of
consumption, in contradiction to the current Kyoto protocol were emission are allocated to the place of
production.
Huge differences in the carbon footprint reported of different foods ready at the retailer exist, with a
general increase from vegetables to animal based foods. De Vries & de Boer (2010) compared different
livestock products in more detail in a review of recently published results. Beef with a range of 15-32 kg
CO2 e. per kg product had the highest carbon footprint followed by pork (4-11 kg CO2 e.) and chicken (4
to 6 kg CO2 e.). The huge difference in carbon footprint published is partly a reflection of different
production systems, but is reflective of the complexity of livestock systems as well.
The purpose of this paper is to
-
Introduce the life cycle assessment methodology
-
Quantify the green house gas (GHG) emission from different livestock sectors
-
Identify important hot spot in the production chain
-
Illustrate the potential mitigations options related to the farm
2. The LCA methodology
The farming stage is crucial in determining the carbon footprint of most foods since often as much as 70 90 % of the emissions in the total chain occur before the products leaves the farm gate. Hermansen &
Kristensen (2011) has illustrated the main aspects to take into account when estimating the carbon
footprint of a product (Figure 1). Before the farm gate, emissions related to the production and transport
of inputs to the farm need to be established. At the farm, major emissions take place related to the
internal turn-over at the farm. Methane is produced during the enteric fermentation and during storage of
manure and needs to be estimated based on total feed use and composition as well as storage conditions
for the manure. N2O emission is related to application of manure and artificial fertilizer to the field as
well as nitrate and ammonia losses during and after the growing season. CO2 is produced following
combustion of fuel. Further change in soil carbon content might also contribute to emission of CO2, but
the net effect is very dependent of the actual production and the approach used for calculation of the
carbon footprint.
The LCA methodology consists of four phases, where each implies several methodological choices which
may affect the result although the ISO guidelines are followed. The first phase, goal and scope definition
will be addressed more intensive here. This phase include definition of the functional units, where the
impact traditionally is expressed per unit product, either mass, energy or protein, with the former being
the most used. As illustrated by De Vries & de Boer (2010) the choice only change slightly the ranking
of different livestock products, but when comparing more diverse foods it might have large impact on the
ranking. However the environmental impact might also be expressed per unit area when the aspects are
more related to local concern (biodiversity or eutrophication), whereas for more global concern, like
global warming impact should be expressed per unit of products.
Emissions to air (CH4, N2O, NH3, CO2)
From feeding, manure storage and application, crop production
OUTPUT
INPUT
LIVESTOCK FARM
Materials
e.g. fertilizer
LIVESTOCK
feed
Residues or co-product
Feed
Energy
e.g. fuel
Manure
Chemicals
e.g. pesticides
Other
FIELD
Main products
Enteric
fermentation
Changes in
soil carbon
Emissions to soil and water (NO3-)
EMISSIONS TO AIR
Transport
Production
of inputs
Transport
Agricultural
production
Transport
Processing
Transport
Packaging
Supermarket
EMISSIONS TO SOIL AND WATER
Figure 1. Illustration of life cycle assessment of livestock products in the chain toward the supermarket
with special attention on the agricultural part of the chain. Hermansen & Kristensen (2011)
In the literature a whole farm approach is commonly used when addressing the impact of livestock
products (Crosson et al., 2011), meaning that the farm physical boundary also forms the input output
barriers for the LCA. Most of the studies in the literature have the objective to quantify the effect of the
current production, which is defined as attributional LCA, in contrast to a consequential LCA, where the
objective is to quantify the effect of a marginal change in the level of the production. The later choice
typically resulting in the largest impact per kg product.
A specific important point dealing with LCA based on a whole farm perspective is how to allocate the
emission between multiple products, like milk and meat from the dairy farm. The standard
recommendation is to allocate based on the physical causal relationship, but often this cannot be
established, and the economic value of the products in question can be used. As illustrated later this may
have significant impact on the estimated impact of the products
3. Different livestock sectors (chain, farm, mitigation).
In the EU livestock sector, beef has by far the highest emission with 22,6 kg CO2 e. per kg followed by
pork 3,5 CO2 e., eggs 1,7 kg CO2 e., poultry 1,6 CO2 e. and milk has 1,3 kg CO2 e. per kg (Lesschen et al
., 2011), with dairy and beef sector being responsible for about 80% of the total GHG emission, with a
total of about 190 Tg CO2 e. each. This could be compared to 32 Tg CO2 e. from the Canadian beef sector
(Verge et al., 2008).
3.1 Pork (feed transport as special focus)
Main stream pork production in the industrialized world is to a large extent based on bought-in feed to the
farm, and even if some feed items are produced at the farm they might be interchanged with other feed. In
that sense one might say that the pork production is a global business drawing primarily on globally
traded feed resources. The main contributors to the carbon footprint of pork for typical pig farming
systems in Northwest Europe was found to be N2O (43 %), followed by CO2 from energy use (29 % ) and
CH4 (28 %) (Nguyen et al., 2011). On-farm emissions (which include enteric CH4 emissions, CH4 and
N2O emissions from in-house manure and outside storage, and N2O emissions from manure application)
accounting for 41% of total emissions.
In terms of hotspots in pork production, the results in Figure 2 show that feed production are the dominant
contributor responsible for around 50 % of total emissions across countries and system. The importance
of this is illustrated by the fact that the quartile of farms with the greatest feed efficiency in Denmark
produces pork with 10% lower emission per kg of product than average farms (Nguyen et al., 2011). In
the conventional system in the Netherlands, the relatively low contribution from feed production is
because of higher amount of by-products used, and that is also the reason for the relatively high share
from transport (Kool et al., 2009). The total emission for organic pork was 25% higher than for
conventional, the main reason being a higher feed conversion factor with 20-30% more feed per kg pork,
although the emission per kg of feed was lowest in the organic systems.
Figure 2. GHG emission per kg pork divided in processes for four countries and two production systems,
conventional and organic. From Kool et al., 2009.
3.1.1.Transport and different type of feedstuff
When comparing different systems and farms, feed composition differ, and in this respect the estimation
of the carbon footprint of the different feeds becomes important. Although transport is only responsible
for a relative low share there are large variation due to distance of transport, but even more on the
transport mode, since truck transport has a much higher impact per km (150-375 g CO2/tkm) than train
(40 g CO2/tkm) and sea freight (9 g CO2/tkm). For examples the emission from transport of 1 ton feed
from Italy to Denmark by a 40t truck is higher than the emission from transport of 1 ton feed from
Argentina to Denmark by ship to Rotterdam and truck from the Netherland to Denmark (308 vs. 271 kg
CO2).
In Table 1 two different protein feeds are compared. The carbon footprint (CF) for producing and
processing 1 kg soybean meal is a bit higher than for 1 kg rapeseed meal, however, for 1 kg protein CF is
nearly the same for rape seed meal and soy bean meal due to a higher protein content in soybean meal .
Taking into account the contribution to the GHG emissions from transport, carbon footprint from the
imported soybean meal becomes more than two times higher per kg feed than the local grown rapeseed
meal. A big discussion right now is how to take into account the contribution to GHG emissions from
land use change (LUC), specially deforestation. According to BIS (2008) if a product’s supply chain
directly caused non-agricultural land to be converted to agricultural use, the GHG emissions associated
with the LUC must be included in the CF calculation. This means that the emissions from deforestation
must be allocated to soybean produced in these countries, where soybean production are expanded into
forested land, like Brazil. Using this method for including LUC might easily cause technical trade barrier
for soy of specific origin and as discussed by Audsley et al, (2009) a more fair way to include LUC could
be to look at all agricultural production as global and interconnected and an assumption that all demand
for land, irrespectively of where in the world, is the driver of deforestation. Using this approach, LUC
from rapeseed meal production is 157 g CO2/kg and 239 g CO2/kg for soybean meal due to a higher land
occupation.
Tabel 1. Example of Carbon footprint (CF) for protein feed fed in Denmark depending on place of
producing and how land use change (LUC) emissions are included.
Rapeseed meal
Soybean meal
Place of origin
Denmark
Brazil
1)
CF, production and processing, g CO2 e. per kg
316
470
2)
CF, transport to Denmark, g CO2 per kg
0
271
Land occupation, m2 per year 1)
1,1
1,67
Total, kg CO2 e. per kg protein
1,0
1,6
- Without LUC
1,0
16,4
- Incl LUC according to BIS, 2008
1,5
2,1
- Incl. LUC according to Audsley et al, 2009
1)
2)
Nguyen et al., 2011
Ecoinvent, 2009
3.2 Dairy (between farm, allocation)
FAO recently reported a global average carbon footprint of 2,4 kg CO2 e. per kg milk of which 93% was
emission from cradle to farm gate (FAO, 2010). Emissions per unit of milk product at farm gate vary
greatly among different regions and countries as illustrated in Figure 3.
12.00
kg CO2-eq. per kg FPCM
10.00
8.00
6.00
4.00
2.00
0.00
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
Output per cow, kg FPCM per year
Figure 3.Relation between GHG per kg milk and annual milk yield per cow. Gerber et al. (2011)
It appears that the carbon footprint per kg milk was markedly reduced with increased level of milk
production until 3.000 to 4.000 kg milk per cow and year. The main reason for the higher carbon footprint
at a low yield is due to a general low feed efficiency combined with a large proportion of low quality
roughage leading to a higher share of methane and a larger proportion of the feed is used to nonproductive purposes such as replacement stock, maintenance requirement, and draught power. One of the
advantages of the low productive systems is a low use of fossil energy and low dependence of imported
concentrates.
Across systems methane is the largest source to GHG emission from the dairy sector accounting for
typically more than 50% of the total emission at farm gate, with more than 2/3 originating from enteric
fermentation in the cow, followed by nitrous oxide accounting for 25-35% of total GHG emission, with
manure management and excreta during grazing accounting for the largest part (Kristensen et al., 2011;
Flysjö et al., 2011). Mitigations strategies reducing the enteric methane emission will therefore often have
a large impact on the total farm gate emission, but awareness of trade off is important. Potential feeding
strategies for reducing the enteric methane emission, like increasing concentrate over roughage ratio,
improving forage digestibly or replacing grass silage for maize silage will also affect the cropping plan,
area for feed production and type of concentrates which might have impact on N utilization and thereby
nitrous oxide emission as well as soil carbon change.
The inter-farm variability in carbon footprint per kg energy corrected milk (ECM) from 67 Danish dairy
farms is shown in Figure 4 showing large variations (SD =0,15) among conventional as well as organic
farms (Kristensen et al., 2011). This might be caused by uncertainty in the data or differences in the
structure and management of the farms. The actual emission is also affected significant due to uncertainty
in the emission factors applied; especially in pasture based lower productive systems as the largest
uncertainty is related to N2O emission factors (Flysjö et al., 2011).
Figure 4. Distribution between farms of GHG emission per kg ECM within production system (Kristensen
et al., 2011)
Kristensen et al. (2011) identified, through factor analysis on data from the 67 farms, that strategies
focusing on high efficiency in the herd or reduced stocking rate (defined by kg milk per ha) are promising
for reducing of carbon footprint per kg milk at farm gate. The difference was 0,13 kg CO2 e. per kg milk
between the top and bottom quartile of farms ranked by “Herd efficiency”, where the main difference was
a difference in feed conversion of 1,32 vs. 1,06 kg milk per kg DMI. Also Christie et al (2011) identified
feed conversion together with N application per ha as the two main parameters in explaining the variation
between GHG emission at farm gate. Improvement in feed conversion both reduces the enteric methane
emission and the nitrous oxide emission from manure as N excretion also is reduced per kg of product.
Better feed conversion might also be obtained by reducing the amount of feed used to non productive
animals – dry cows and replacement stock. Extended lactation in combination with ex tended calving
interval might have a potential to reduce the GHG emission per kg of milk as well as per kg of beef.
Similarly farms with low stocking rate obtained a carbon footprint of 1,04 kg CO2 e per kg milk
compared with 1,14 for farms with high stocking rate, illustrating that with a low stocking rate, it is
easier to have a good resource utilization in the system and, at least in this investigation, that the emission
associated to a farming system based more on imported feed was higher than the emission from a farming
system based on home produced feed.
A number of studies have been performed with highly productive dairy herds. A particular aspect has
been to estimate the carbon footprints in organic and conventional production systems. The overall
picture found was that no major systematic difference seems present ( Kristensen et al., 2011), the lower
milk yield in organic systems being compensated by a higher overall system efficiency.
As mentioned, the method used to divide total farm GHG emissions between the two products meat and
milk has significant impact on the estimated emission of each product, as illustrated in figure 5 based on
Danish production systems (Kristensen et al., 2011).
Figure 5. Emission per kg milk and meat from Danish dairy farms related to method of allocation
(Kristensen et al., 2011).
Without allocation, the emission per kg of milk was 16 to 32% higher than if some of the total farm
emission was allocated to meat. The allocation method had a larger impact on the range in emission per
kg meat than per kg milk, leading to a variation in emission per kg meat from 3,4 to 6,9 kg CO2 e., while
the variation in emission per kg ECM only ranged from 0,91 to 1,06 kg CO2 e. per kg ECM. Based on
farm data and a regression model taking into account total production of milk and meat, 86% of total farm
emission was allocated to milk. The use of the economic value of milk and meat to allocate the total farm
GHG is within the range estimated from the empirical model. The problem with economic allocation is
that it will depend on place and time. In systems with only two products, a more direct use of either
protein mass or biological methods as the one here developed by IDF (2010) based on causal relations
between production and feed requirement seems more appropriate. From a product point of view, protein
mass represents the demand by consumers for protein (De Vries & De Boer, 2010), while the biological
method based on the causal relation between input/output represents the efficiency in energy utilization.
Although it from a theoretical point of view therefore seems ideal to use the biological method, there are
problems with calculating the feed demand in a system where very different types of farm animals form
the basis for the same product as here meat.
3.2.1 Post farm gate
Post farm gate emissions depend to large extent what product is produced, type of energy used, type of
packaging and transportation (e.g. distances and cooled vs. none cooled trucks). Globally, post farm gate
emissions (farm gate to distribution) are on average 0.155 kg CO2 e. per kg milk at farm gate (FAO,
2010). There are however great variations, where Latin America and Canada has lowest emissions (0.110.12 kg CO2 e. per kg milk) and Australia and India has the highest emissions (0.32 kg CO2 e. per kg
milk). Emissions for Europe divided on different processes and after different products are shown in
Table 2. Thus, processing stands for the largest share of post farm gate emissions in average (55%),
followed by packaging (25%) and transport (20%).
Table 2. GHG emission post farm gate in average of Europe expressed per kg milk ab farm. FAO (2010)
Post farm gate emission divided after process,
kg CO2-e. per kg milk at farm gate
Post farm gate emission divided after products in
the chain, kg CO2-e. per kg milk at farm gate
Transport from farm to dairy
Processing in dairy
Packaging
Transport from dairy to retail
Total
Fresh milk and cream
Fermented milk and cream
Cheese
Skimmed milk powder
Whole milk powder
0.016
0.086
0.038
0.014
0.155
0.153
0.304
0.126
0.157
0.171
3.3 Beef (landuse)
Beef cattle systems are traditionally based on pasture on less productive areas and relatively low feeding
intensity compared to the more intensive feeding of bull calves from the dairy production. This, and the
fact that the feed requirement of the mother cows has to be accounted for, result in a high dry matter
intake per kg beef produced in such systems (Verge et al. 2008). This in turn leads to a high carbon food
print of the beef from the extensive pasture based systems with methane accounting for 60-70% of the
total carbon footprint. In more intensive grain-based feeding system the methane emission is lower due to
a higher digestibility of the feed and since a much lower part of the feed is used for maintenance (Peters et
al. 2010). The high feed conversion rate together with the use of low productive type of crop – pasture,
natural areas – also means that the landuse per kg of product in beef is more than three time than the area
for pork produced (Lesschen, et al, 2011).
Nguyen et al. (2010) investigated the environmental profile of four different beef production systems, one
beef cattle system and three categories of dairy based beef. It was assumed that silage was produced on
highly productive grassland; that steers grazed on moderate productive grassland, and beef cattle cows
grazed on low productive grassland. All cereal and roughage was grown within the system while the
supplementary protein feeds was covered by imported soybean meal (Table 3). The carbon footprint
inclusive the contribution from CO2 emissions related to land use was estimated under a number of
different assumptions, some well accepted and some more hypothetical by Hermansen & Kristensen
(2011).
Table 3. Comparative land use and carbon footprint of beef produced in different systems with focus on
emission from land use, per kg meat slaughter weight at farm gate (Hermansen & Kristensen, 2011).
‘Calf’ age at slaughter, month
Land use, total m2 year-1
Grassland, total
- Highly productive
- Moderately productive
- Low productive
Cropland, total
- Cereals
- Soy meal
Carbon footprint, kg CO2 e per kg meat
- Without land use consideration
- Incl. carbon sequestration on direct land use1)
- Incl. CO2 emission from LUC, soy2)
- Incl. CO2 emission from LUC, cereal2)
- Incl. CO2 emission from LUC, grassland3)
Beef cattle
systems 16
42.9
36.9
6.8
0
30
6.0
5.94
0.05
26,8
27,3
27,4
44,0
61,7
Males form dairy herds
12
16
24
16.5
16.7
22.7
0
2.0
18.2
0
2.0
8.3
0
0
9.8
0
0
0
16.5
14.7
4.5
12.39
11.48
4.50
4.11
3.25
0.04
12,2
16,0
27,5
62,2
62,2
14,7
17,9
27,0
59,1
64,2
22,0
19,9
20,0
32,6
79,8
1) Carbon sequestration (soil carbon change): Grassland highly and moderate prod. +1910 kg CO2 e per ha per year; cereals -3080 kg CO2 e per
ha per year; low productive grassland and other crops 0 kg CO2 e per ha per year
2) Land conversion (20 years): From forest to cropland 2.8 kg CO2 e per m2 y-1
3) Land conversion (20 years): From forest to grassland 2.6 kg CO2 e per m2 y-1
Excluding impacts from estimated soil carbon changes on the direct land use or impacts related to land
use changes resulted in a carbon footprint per kg beef ranging from 12,2 to 26,8 kg CO2 e. depending on
systems. Including soil carbon changes from the direct land use narrowed this range, since the carbon
footprint from steers was reduced, while the opposite was the case for intensively reared bull calves.
Thus, a fair comparison needs to take into account the impact of changes related to changes in soil carbon
stocks as influenced by the system. Hermansen & Kristensen (2011) concluded that in the future,
opportunity costs only have to be taken into account for the land areas that in reality represents an
opportunity to produce other foods. In such a situation beef from beef cattle system and steers from the
dairy system may be as competitive as from intensively reared bull calves from the dairy system. In the
beef cattle system a major part of the emission occurs in the breeding herd, following that mitigations
options like feeding oilseeds, corn distillers grain and improving the forage quality in the breeding stock
might reduce the total emission per kg meat by up to 20% (Beauchemin et al., 2011),
4. Perspective
The LCA offers an approach that can relate the emission during the whole production chain to multiple
products, irrespectively of where in the chain the boundaries are placed. The LCA method assigns the
emission at the end user in contrast to the Kyoto protocol placing the emission at the producer country.
Livestock is a global sector with a large part of the production world wide being transported between
countries. In total 25% has to be added to the emission from the livestock sector in EU if taking into
account the total effect of land use change due to imported feed (Lesschen et al., 2011) illustrating also
the quantitative importance.
Reducing the emission from the livestock sector can be done by a combination of improved efficiency,
productivity, introduction of technology that reduce emission and use the potential of agriculture to
produce biogas based on manure and energy crops like maize or willow. Dalgaard et al (2011) has shown
that Danish agriculture by 2050 can halve the GHG emission compared to 1990 and in the same period
increased the bioenergy production, resulting in a positive energy balance from the agricultural sector. An
important assumption is that the total production of livestock products are constant, but with the same
trend in efficiency as in the period 1990 to 2010. This means that the area for production of livestock feed
is reduced and the redundant area can be used for energy crops, figure 6. This together with improved
technology especially in relation to manure handling is the main sources to the estimated reductions
potential.
Figure 6. Contribution from different areas to halve the GHG from Danish agriculture by 2050. From
Dalgaard, et al. (2011).
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