Implications
The livestock sector requires a significant amount of natural resources and has an
important role in global greenhouse gas emissions. The most important greenhouse gases
from animal agriculture are methane and nitrous oxide.
Mitigation strategies aimed at reducing the emission intensity of this sector are
needed to meet the increasing demand for livestock products driven by population
growth.
To increase the effectiveness of mitigation strategies, the complex interactions
among the components of livestock production systems must be taken into account to
avoid environmental trade-offs.
Introduction
According to the United Nations (UN, 2017), the
world population increased by approximately 1 billion inhabitants during the last
12 years,
reaching nearly 7.6 billion in 2017. Although this growth is slower than 10 years
ago (1.24%
vs. 1.10% per year), with an average increase of 83 million people annually, global
population will reach about 8.6 billion in 2030 and 9.8 billion in 2050. Population
growth,
urbanization, and income rise in developing countries are the main driver of the increased
demand for livestock products (UN, 2017). The
livestock sector requires a significant amount of natural resources and is responsible
for
about 14.5% of total anthropogenic greenhouse gas emissions (7.1 Gigatonnes of carbon
dioxide equivalents for the year 2005; Gerber et al.,
2013). Mitigation strategies aimed at reducing emissions of this sector are needed
to limit the environmental burden from food production while ensuring a sufficient
supply of
food for a growing world population. The objectives of this manuscript are to 1) discuss
the
main greenhouse gas emissions sources from the livestock sector and 2) summarize the
best
mitigation strategies.
Impact of Livestock on Climate Change
The most important greenhouse gases from animal agriculture are methane and nitrous
oxide.
Methane, mainly produced by enteric fermentation and manure storage, is a gas which
has an
effect on global warming 28 times higher than carbon dioxide. Nitrous oxide, arising
from
manure storage and the use of organic/inorganic fertilizers, is a molecule with a
global
warming potential 265 times higher than carbon dioxide. The carbon dioxide equivalent
is a
standard unit used to account for the global warming potential (IPCC, 2013).
Figure 1 was adapted from the Global Livestock
Environmental Assessment Model (GLEAM) developed by FAO (FAO, 2017) and shows in carbon
dioxide equivalents the greenhouse gas incidences
that enteric fermentation and manure storage have across the main livestock species
raised
worldwide.
Figure 1.
Greenhouse gases incidence of enteric fermentation and manure storage by animal type,
expressed as Gigatonnes of carbon dioxide equivalents. Data referred to 2010 (FAO,
2017).
In addition to greenhouse gases arising from enteric fermentation and manure storage,
feed
production together with the related soil carbon dioxide and nitrous oxide emissions
is
another important hot spot for the livestock sector. Soil carbon dioxide emissions
are due
to soil carbon dynamics (e.g., decomposing plant residues, mineralization of soil
organic
matter, land use change, etc.), the manufacturing of synthetic fertilizers and pesticides,
and from fossil fuel use in on-farm agricultural operations (Goglio et al., 2018).
Nitrous oxide emissions are emitted when
organic and inorganic fertilizers are applied to the soil.
As shown in Figure 2, feed production and processing
contribute about 45% of the whole sector (3.2 Gigatonnes of carbon dioxide equivalents).
Enteric fermentation producing about 2.8 Gigatonnes (39%) is the second largest source
of
emissions. Manure storage with 0.71 Gigatonnes accounts for about 10% of the total.
The
remaining 6% (0.42 Gigatonnes of carbon dioxide equivalents) is attributable to the
processing and transportation of animal products (Gerber
et al., 2013).
Figure 2.
Livestock emissions by source (adapted from Gerber
et al., 2013). Direct livestock emissions are shown in red.
Feed production (Figure 2) includes all the greenhouse
gas emission arising from 1) land use change, 2) manufacturing and use of fertilizers
and
pesticides, 3) manure excreted and applied to fields, 4) agricultural operations,
5) feed
processing, and 6) feed transport. Although these processes result in a large share
of the
livestock supply chain, in this article, we mainly focus on direct livestock emissions
enteric fermentation, manure storage, and manure excreted/applied to the soil. All
other
emissions are outside the scope of this article.
Enteric fermentation
Enteric fermentation is a natural part of the digestive process of ruminants where
bacteria, protozoa, and fungi contained in the fore-stomach of the animal (rumen),
ferment
and break down the plant biomass eaten by the animal. Plant biomass in the rumen is
converted into volatile fatty acids, which pass the rumen wall and go to the liver
through
the circulatory system. This process supplies a major part of the energy needs of
the
animal and enables the high conversion efficiency of cellulose and semi-cellulose,
which
is typical of ruminants. The gaseous waste products of enteric fermentation, carbon
dioxide and methane, are mainly removed from the rumen by eructation. Methane emission
in
the reticulorumen is an evolutionary adaptation that enables the rumen ecosystem to
dispose hydrogen, which may otherwise accumulate and inhibit carbohydrate fermentation
and
fiber degradation (McAllister and Newbold,
2008). The emission rate of enteric methane varies according to feed intake and
digestibility.
Manure storage
Manure acts as an emission source for both methane and nitrous oxide, and the quantity
emitted is linked to environmental conditions, type of management and composition
of the
manure. Organic matter and nitrogen content of excreta are the main characteristics
influencing emission of methane and nitrous oxide, respectively. Under anaerobic
conditions, the organic matter is partially decomposed by bacteria producing methane
and
carbon dioxide. Storage or treatment of liquid manure (slurry) in a lagoon or tank
promotes an anaerobic environment which leads to an increase in methane production.
Long
storage periods and warm and wet conditions can further increase these emissions (EPA,
2010). On the other hand, nitrous oxide
emissions need a combination of aerobic and anaerobic conditions to be produced.
Therefore, when manure is handled as a solid (dung) or deposited on pastures, nitrous
oxide production increases while little or no methane is emitted. Nitrous oxide is
generated through both the nitrification and denitrification processes of the nitrogen
contained in manure, which is mainly present in organic form (e.g., proteins) and
in
inorganic form as ammonium and ammonia. Nitrification occurs aerobically and converts
ammonium and ammonia to nitrites and then nitrates, while denitrification occurs
anaerobically converting nitrates to nitrous oxide and nitrogen gas (Saggar, 2010).
The balance between ammonium and ammonia is highly
affected by pH, with ammonia increasing as pH increases.
Feed production
Almost 60% of the global biomass harvested worldwide enters the livestock subsystem
as
feed or bedding material (Krausmann et al.,
2008). Greenhouse gas emissions from feed production represent 60–80% of the
emission coming from eggs, chicken and pork, and 35–45% of the milk and beef sector
(Sonesson et al., 2009). As shown in Figure 2, emissions from feed production account
for
about 45% of the livestock sector. The application of manure as fertilizer for feed
crops
and the deposition of manure on pastures generates a substantial amount of nitrous
oxide
emissions representing about half of these emissions (Gerber et al., 2013). Although
livestock feed production often involves large
applications of nitrogen to agricultural soils, good manure management can reduce
the need
for manufactured fertilizers.
Livestock Mitigation Strategies
The extreme heterogeneity of the agricultural sector needs to be taken into account
when
defining the overall sustainability of a mitigation strategy, which can vary across
different livestock systems, species, and climates. Generally, no measure in isolation
will
encompass the full emission reduction potential, while a combination selected from
the full
range of existing options will be required to reach the best result (Llonch et al.,
2017). It is also important to consider the “pollution
swapping” effect when evaluating the effectiveness of a mitigation strategy (Hristov
et al., 2013). Reduction of methane emissions
during enteric fermentation might be counteracted by increased greenhouse gas emissions
in
applied manure. Reduction of direct nitrous oxide emissions during storage might result
in
higher nitrate leaching and ammonia volatilization during field application.
Mitigation may occur directly by reducing the amount of greenhouse gases emitted,
or
indirectly through the improvement of production efficiency. The main strategies to
mitigate
greenhouse gas emissions in the livestock sector have been investigated and are summarized
in Table 1.
Table 1.
Mitigation potential of various strategies
Strategies
Category
Potential mitigating effect*
Methane
Nitrous Oxide
Enteric fermentation
Forage quality
Low to medium
†
Feed processing
Low
Low
Concentrate inclusion
Low to medium
†
Dietary lipids
Medium
†
Electrons receptors
High
†
Ionophores
Low
†
Methanogenic inhibitors
Low
†
Manure storage
Solid-liquid separation
High
Low
Anaerobic digestion
High
High
Decreased storage time
High
High
Frequent manure removal
High
High
Phase feeding
‡
Low
Reduced dietary protein
‡
Medium
Nitrification inhibitors
‡
Medium to high
No grazing on wet soil
Low
Medium
Increased productivity
High
High
Animal management
Genetic selection
High
‡
Animal health
Low to medium
Low to medium
Increase reproductive eff.
Low to medium
Low to medium
Reduced animal mortality
Low to medium
Low to medium
Housing systems
Medium to high
Medium to high
*High = ≥30% mitigating effect; Medium = 10–30% mitigating effect; Low = ≤10%
mitigating effect. Mitigating effects refer to percent change over a “standard
practice” according to Newell Price et al.
(2011); Borhan et al. (2012);
Hristov et al. (2013); Montes et al. (2013); Petersen (2013); Battini et
al. (2014); Knapp et al. (2014);
Llonch et al. (2017); Mohankumar Sajeev et al. (2018).
†Inconsistent/variable results.
‡Uncertainty due to limited research or lack of data.
Enteric fermentation
Decreasing methane emissions from ruminants is one pressing challenge facing the ruminant
production sector. Strategies for reducing this source of emissions focus on improving
the
efficiency of rumen fermentation and increasing animal productivity. A large number
of
mitigation options have been proposed (e.g., diet manipulation, vaccines, chemical
additives, animal genetic selection, etc.) with different efficiencies in reducing
enteric
methane as shown in Table 1.
Forage quality and digestibility affect enteric methane production. Lignin content
increases during plant growth, consequently reducing plant digestibility. Therefore,
harvesting forage (especially grass) for ensiling at an earlier stage of maturity
increases its soluble carbohydrate content and reduces lignification. According to
Knapp et al. (2014) practices aimed to increase
forage quality have shown a potential enteric methane reduction of about 5% per unit
of
fat protein corrected milk.
Physical processing of forages, such as chopping, grinding, and steam treatment, also
improves forage digestibility and mitigates enteric methane production in ruminants
(Hristov et al., 2013). However, the reduction
potential of this practice was reported to be less than 2% per unit of fat protein
corrected milk (Knapp et al., 2014).
Improving diet digestibility by increasing concentrate feeding is another effective
mitigation strategy, reducing by 15% methane emissions per unit of fat protein corrected
milk (Knapp et al., 2014). However, the ratio
of forage to concentrate has to be carefully taken into account when applying this
strategy. Indeed, although a marked reduction of enteric methane can be expected with
rates of concentrate inclusion between 35% and 40% (Gerber et al., 2013). A greater
proportion of dietary fermentable carbohydrates
could increase the risk of metabolic diseases (e.g., rumen acidosis).
Addition of fats or fatty acids to the diets of ruminants can decrease enteric methane
emissions by both decreasing the proportion of energy supplied from fermentable
carbohydrates and changes in the microbial population of the rumen (Llonch et al.,
2017). Although some byproducts (e.g., cottonseed,
brewer’s grains, cold-pressed canola meal, etc.) are effective in reducing enteric
fermentation (Moate et al., 2011), the
mitigation potential of high oil byproducts has not been well-established and in some
cases methane production may increase due to increased fiber intake (Hristov et al.,
2013). The inclusion of lipids higher than 10% can
lead to impairment of ruminal function due to changes to the microbial population
which in
turn decreases the ability to digest fiber. Lipid diet supplementation between 5%
and 8%
of the dry matter intake is an effective mitigation strategy (Grainger and Beauchemin,
2011) with a potential enteric methane
reduction of about 15% per unit of fat protein corrected milk (Knapp et al., 2014).
Feed additives (electron receptors, ionophoric antibiotics, chemical inhibitors, etc.)
have also been tested for their ability to decrease methane emissions (Beauchemin
et al., 2009). However, the unknown
toxicity and the health risks associated with the use of some of these compounds may
severely constrain widespread adoption (Herrero et
al., 2016).
Manure storage
Increased animal density together with continuous inflow of nutrients from imported
feeds
is likely to increase volumes of manure to be managed. Stored manure accounts for
a
relatively small amount of direct agricultural greenhouse gases (Figure 2), and it
is technically possible to mitigate a very high
percentage of these emissions (Hristov et al.,
2013). In the following section, some of the most effective mitigation strategies
are discussed.
As methane production increases with the temperature of stored manure, a reduction
of
storage temperature has been reported to drop these emissions by 30–50% (Borhan et
al., 2012). However, the net greenhouse
gas mitigation resulting from this strategy can vary widely, and it is strictly related
to
the energy used and the cooling system adopted.
Frequent removal of manure to an outside storage facility is an effective practice
that
can be accomplished using grooved floors combined with regular scraping of manure,
especially for pigs and some cattle production systems. Indeed, if the channels underneath
the stable are emptied regularly, and the manure/slurry are transported to an outside
storage facility, this practice has the potential to reduce methane and nitrous oxide
emissions by 55% and 41%, respectively (Mohankumar
Sajeev et al., 2018). On poultry farms the litter/manure is usually removed at
the end of the crop; however, advanced layer housing using belt scrapers can efficiently
remove litter/manure continuously and decrease greenhouse gas emissions (Fournel et
al., 2012).
Solid-liquid separation is a processing technology that partially separates the solids
from liquid manure using gravity or mechanical systems such as centrifuges or filter
presses. As shown in Table 1, the greenhouse gas
mitigation potential of this technique has been reported to be higher than 30% compared
with untreated manure (Montes et al., 2013).
The organic component with a larger particle size follows the solid stream during
the
separation process, and it is then stored in stockpiles. The aerated condition of
the
storage can then limit the potential for methane to be emitted; however, ammonia loss
through composting and generating high temperatures can be accelerated. Also, the
remaining liquid fraction is still a potential source of indirect nitrous oxide emissions.
Indeed, once the fibrous and large pieces of organic material are subtracted, it will
not
form a crust during storage, leading to increased volatilization of ammonia by increasing
the mass transfer coefficient at the surface. Although greenhouse gas mitigation of
the
solid-liquid separation process can be partially counterbalanced by ammonia emissions,
it
is important to note that there are many management practices that can overcome these
issues, such as covering slurry storage and the use of injection for land application
(Holly et al., 2017).
Anaerobic digestion is a biological degradation process, which in the absence of oxygen,
produces digestate and biogas (mainly methane and carbon dioxide) from manure. Biogas
collected from the system is often used to generate electricity, to fuel boilers or
furnaces, or to provide combined heat and power. Taking into account the greenhouse
gas
emissions arising from the use of the digestate as fertilizer, and the credit for
the
renewable energy produced, anaerobic digestion has been reported to yield more than
30%
reduction in greenhouse gas emissions when compared with traditional manure handling
systems (Battini et al., 2014). However,
further attention to the management of the digestate leaving the anaerobic digestion
is
needed. Indeed, mineralization of the organic nitrogen occurring during biological
degradation increases the inorganic nitrogen content and pH of the effluent, which
in turn
may increase ammonia volatilization (Petersen and
Sommer, 2011). Combining anaerobic digestion and solid-liquid separation could
reduce the amount of ammonia lost following digestion (Holly et al., 2017).
Diet severely affects excretion of nitrogen in most farm animals, therefore grouping
livestock on the basis of their feed requirements can help in reducing this source
of
nitrous oxide in the excreta. Although a low-protein diet could effectively mitigate
nitrous oxide emissions from cattle manure storage (Table
1), some attention must be given to manipulating dietary nitrogen (Montes et al.,
2013). For example, decreasing
protein could lead to an increase of fermentable carbohydrates, which in turn will
likely
increase methane production.
The diet for all animal species should be balanced for amino acids to avoid a depression
in feed intake and a decrease in animal productivity. Manufactured amino acids are
routinely used to balance the diet of monogastrics (pigs and poultry), but the
environmental impact associated with the manufacturing of these supplements must be
considered when including amino acids as a greenhouse gas mitigation strategy. In
ruminants, supplementation of free amino acids results in fast degradation in the
rumen,
without a significant increase in animal productivity. On the contrary, rumen-protected
amino acids resist chemical alterations in the rumen and can reach the intestine where
they are absorbed, improving milk yield in dairy cows. Overall, feeding protein close
to
the animal’s requirement is recommended as an effective mitigation strategy to reduce
ammonia and nitrous oxide emissions from manure (Montes et al., 2013).
Feed production
The timing, quantity, and method of fertilizer applications are important factors
influencing soil nitrous oxide emissions. The nitrogen fertilizer applied is susceptible
to loss by leaching and denitrification before crop uptake. Therefore, ensuring that
appropriate amounts of nitrogen get to the growing crop and avoiding application in
wet
seasons or before major rainfall events, are valuable practices which could help in
optimizing biomass production and reduce soil greenhouse gas emissions.
As lower methane emissions occur after manure land application, decreasing storage
time
can effectively help in reducing greenhouse gas emissions (Table 1). However, the
resulting frequent soil applications can have a variable
effect on nitrous oxide emissions from field and carbon dioxide emissions from fuel
combustion. Avoiding application during prolonged periods with wet soil and periods
of low
plant nitrogen uptake could help in increasing the effectiveness of this practice
(Hristov et al., 2013).
Adequate storage facilities can provide greater flexibility in choosing when to apply
manure to fields, while the use of on-farm manure analysis could help the farmer develop
a
nutrient management plan and minimize environmental impacts (Newell Price et al.,
2011).
The use of nitrification inhibitors has the potential to reduce nitrogen leaching
by
inhibiting the conversion of ammonia to nitrate. However, this beneficial effect is
weakened by a reported increase in indirect nitrous oxide emission that can result
from
increased ammonia volatilization (Lam et al.,
2016). This highlights the importance of considering both gases when evaluating
the use of nitrification inhibitors as an option to mitigate climate change. Overall,
nitrification inhibitors have been demonstrated as an effective practice to reduce
nitrous
oxide emissions (Table 1).
Intensive rotational grazing systems are being promoted as a good way to increase
forage
production and reduce nitrous oxide emissions (Table
1). These systems are characterized by multiple smaller fields called paddocks
for the rotation of livestock. By subdividing pastures and rotating animals, farmers
can
manage stocking densities and grazing duration and thereby manage nitrogen excreta
distribution and vegetation regrowth. A more uniform distribution of urine throughout
the
paddock would reduce the effective nitrogen application rate, which could translate
into a
reduction in nitrous oxide emissions (Eckard et al.,
2010). Keeping animals off the paddocks during wet weather will reduce sward
damage and soil compaction. In addition, avoiding excreta deposition at these times
will
reduce nitrous oxide emissions and nitrogen leaching (Luo et al., 2010).
Animal management
There is a direct link between greenhouse gas emission intensities and animal efficiency.
The more productive the animal is, the lower the environmental impact will be (on
a per
unit of product basis). Both management quality and expression of full genetic potential
are necessary to increase production efficiency.
Breeding for more productive animals can lead to a reduction of the nutrient requirements
needed to reach the same level of production. This is a valuable greenhouse gas mitigation
strategy (Table 1). A more efficient animal will
retain more dietary nitrogen protein and there will less nitrogen in feces and urine
(Gerber et al., 2013). Genetic improvement of
daily gain and feed conversion that has been achieved in broilers over the last 20
years
has reduced substantially the emissions per unit of weight (Williams and Speller,
2016). Nevertheless, strategies that aim to
change animal phenotypes to enhance productivity or efficiency may harm animal health
and
welfare unless these effects are measured and controlled (Llonch et al., 2017). Animals
of a particular genotype selected for
increased production will only be able to realize this potential on a high input system
in
which resources are adequately supplied. In other words, new breeds and crosses can
lead
to substantial greenhouse gas reduction, but they need to fit within production systems
and climates that may be characterized by limited resources and other constraints.
Poor fertility means that more breeding animals are required in the herd to meet
production targets, and more replacements are required to maintain the herd size,
which in
turn increases greenhouse gas emissions. Improved fertility in dairy cattle could
lead to
a reduction in methane emissions by 10–24% and reduced nitrous oxide by 9–17% (Table
1). Nevertheless, increasing reproductive
pressure may increase the metabolic demands associated with pregnancy and lactation
that
could negatively affect animal health and increase the risk of metabolic diseases,
reduce
immune function and in turn reduce fertility (Llonch
et al., 2017).
Poorer livestock health and welfare are associated with behavioral and metabolic changes,
which can effect greenhouse gas emissions in several ways. Animals fighting an infection
will need more energy for maintenance. A recent study in the United Kingdom investigated
cost-effective ways to reduce greenhouse gas emissions by improving cattle health.
These
studies found that cattle diseases can increase greenhouse gas emissions up to 24%
per
unit of milk produced and up to 113% per unit of beef carcass (Williams et al., 2015).
A disease that temporarily reduces feed
intake or the ability to digest feed, leads to a decline in growth rate, which will
result
in more time and energy needed to reach the same end point.
Conclusion
Agriculture in general, and livestock production, in particular, contributes to global
warming through emissions of methane and nitrous oxide. To meet future needs of an
expanding
population, animal productivity will need to increase and greenhouse gas emission
intensity
per unit of product will need to decrease. One of the principal ways to achieve this
environmental standard is to adopt effective mitigation strategies. To increase the
effectiveness of these strategies, complex interactions among the components of livestock
production systems must be taken into account to avoid environmental trade-offs.
Unfortunately, there is not a standard procedure to follow. Mitigation practices should
not
be evaluated individually, but as a component of the entire livestock production system.
The
majority of these strategies aim to increase productivity (unit of product per animal),
which in most cases cannot be achieved without good standards of animal health and
welfare.
Optimizing animal productivity has a powerful mitigating effect in both developed
and
developing countries; however, the size of the effect will also depend on factors
such as
the genetic potential of the animal and adoption of management technologies.