Agriculture Reference
In-Depth Information
'greenhouse effect' provided by GHGs the global
mean temperature would be −19°C compared
with the actual global mean temperature of
14°C (IPCC, 2007). However, since the beginning
of the industrial revolution the atmospheric
concentration of GHG has greatly increased
(e.g. CO
2
concentrations have increased from
about 280 parts per million (ppm) in pre-
industrial times to 392 ppm in 2012; IPCC,
2007; NASA, 2012), and many scientists believe
the increased atmospheric carbon concentrations
are leading to global climate change (Oreskes,
2004). Ultimately, increases in atmospheric CO
2
concentration are being driven by the combustion
of fossil fuels and land use change (e.g. clearing
of forests) (Raupach
et al
., 2007). Atmospheric
concentrations of N
2
O have increased from
270 parts per billion (ppb) in pre-industrial
times to 319 ppb in 2005 due largely to increased
fixation of atmospheric N
2
(which constitutes
approximately 78% of the atmosphere) from
human activities (e.g. synthetic fertilizer produc-
tion) (Davidson, 2009). Atmospheric CH
4
con-
centrations have increased from about 700 ppb
in pre-industrial times to around 1745 ppb
today and anthropogenic sources of CH
4
include
ruminant livestock, energy production, landfills
and rice cultivation (Seinfeld and Pandis, 2006).
The main GHG from animal agriculture produc-
tion (CO
2
, CH
4
and N
2
O) have different global
warming potentials (GWP) or abilities to trap
heat based on their absorption spectrum and
lifetimes within the atmosphere, so it is common
to standardize all three to a 100-year CO
2
equiv-
alent (CO
2
-eq). The 100-year GWP of CO
2
is 1,
CH
4
is 25 and N
2
O is 298, which means CH
4
is
25 times more potent and N
2
O 298 times more
potent at trapping heat than CO
2
(IPCC, 2007).
There are many sources of GHG emis-
sions from animal agriculture systems when
considering the total production chain from
the farm to dinner plate, including CO
2
and N
2
O
emissions from the soil, CO
2
emissions from
the burning of fossil fuels (in farm equipment,
transportation and electricity generation) and
CH
4
emissions from manure management and
enteric fermentation. Table 9.1 lists some of
the sources of GHG from animal agriculture
production. Carbon dioxide emissions also
occur from the respiration processes of live-
stock, but this CO
2
is not considered a
net
emission because the animals are consuming
plants that previously sequestered CO
2
from
the atmosphere through photosynthesis
(Pitesky
et al
., 2009). For the purposes of this
chapter, we will focus on the enteric GHG
emissions from ruminant livestock and give
an overview of the GHG from pig production
systems.
Enteric CH
4
emissions refer to those that
arise from the digestive tract of an animal as a
result of microbial activities and represent a loss
of dietary gross energy (GE) (up to 12% of die-
tary GE can be lost as CH
4
emissions in cattle;
Johnson and Johnson, 1995). The majority of
enteric CH
4
emissions result from microbial pro-
cesses inside the rumen (the largest of four com-
partments comprising a ruminant's stomach)
and not the hindgut (e.g. large intestine).
However, though minor in comparison with
those from the rumen, there are CH
4
emissions
arising from the hindgut in ruminants (approxi-
mately 13% of total enteric CH
4
; Ellis
et al
., 2008).
In comparison with ruminants, enteric CH
4
emissions from monogastric animals (e.g. pigs)
are minimal and occur only from the hindgut.
Figure 9.1 shows a simplified model of
the microbial processes that occur in the
rumen that lead to CH
4
formation. When feed
enters the rumen, it undergoes degradation and
Table 9.1
Three most important greenhouse gas emissions from animal agriculture production and
their sources.
Chemical
formula
100 year global warming
potential (IPCC, 2007)
Sources in animal agriculture production
(cattle, small ruminants and pigs)
Greenhouse gas
Carbon dioxide
CO
2
1
On-farm fossil fuel use, soil tillage
Methane
CH
4
25
Enteric fermentation (minor for pigs),
anaerobically stored manure
Nitrous oxide
N
2
O
298
Denitrification processes occurring
in soil amended with manure
