Agriculture Reference
In-Depth Information
land, its soil C gain more than offset all GHG emis-
sions from a beef production system. Similarly,
Liebig
et al
. (2010) reported that even moderately
and heavily grazed long-term native grasslands
were net CO
2
e sinks, even after accounting for the
enteric CH
4
produced by the cattle.
Estimating soil C stocks in agricultural land
is not easy because soils vary considerably and
changes in soil C occur very slowly (over dec-
ades), making changes difficult to measure. The
IPCC (2006) determines changes in soil C stocks
from soil geographical databases and land-use
and management for a given country. Tier 1 uses
defaults and reference C stocks, tier 2 replaces
default values with country-specific estimates
and tier 3 estimates changes in soil C from com-
plex models (McGill, 1996).
have been reported by Rotz
et al
. (2010; reported
as 0.69 but converted to 0.99 CO
2
e kg
−1
ECM
after removing biogenic CO
2
and soil C seques-
tration) and by McGeough
et al
. (2012;
0.83 CO
2
e kg
−1
FPCM, no changes in soil C
stocks) with about 90% of the total emissions
allocated to milk in both studies. Differences in
methodology, especially method of allocation to
milk versus beef, FU (kg CO
2
e kg
−1
milk, ECM or
FPCM) and changes in C stocks, make it very dif-
ficult to compare GHG intensities across studies.
Methane is the largest contributor to the
GHG intensity of milk (45-70%; Beukes
et al
.,
2010; Castanheira
et al
., 2010; Bell
et al
., 2011;
Kristensen
et al
., 2011; McGeough
et al
., 2012).
Of this, >75% is derived directly from enteric fer-
mentation (Rotz
et al
., 2010; McGeough
et al
.,
2012). It is therefore apparent that abatement of
enteric CH
4
would result in the most significant
reduction in GHG emissions from both confine-
ment and pasture-based dairy production sys-
tems. Emissions of N
2
O account for about
20-40% of the GHG intensity of milk (Rotz
et al
.,
2010; Flysjö
et al
., 2011b; Kristensen
et al
., 2011;
McGeough
et al
., 2012). The relatively large con-
tribution of N
2
O indicates that there is consid-
erable opportunity to reduce the GHG intensity of
milk production through diet formulation to
reduce the amount of N excreted by animals and
by adopting alternative manure management
practices or reducing the use of inorganic fertiliz-
ers (Olesen
et al
., 2006; Rotz
et al
., 2010).
A number of strategies to reduce the GHG
intensity of milk production have been pro-
posed, with some of the more promising
approaches discussed below.
Greenhouse gas life cycle assessment
of dairy production
A number of farm-based LCAs of GHG emissions
from dairy production have been published in
recent years (Cederberg and Mattsson, 2000;
Haas
et al
., 2001; Phetteplace
et al
., 2001;
Cederberg and Stadig, 2003; Casey and Holden,
2005a, b; Schils
et al
., 2005; Lovett
et al
., 2006,
2008; Olesen
et al
., 2006; Williams
et al
., 2006;
Vergé
et al
., 2007; Thomassen
et al
., 2008b;
Basset-Mens
et al
., 2009; Beukes
et al
., 2010;
Castanheira
et al
., 2010; Gerber
et al
., 2010, 2011;
O'Brien
et al
., 2010; Rotz
et al
., 2010; Bell
et al
.,
2011; Kristensen
et al
., 2011; Vellinga
et al
., 2011;
McGeough
et al
., 2012) with most of the earlier
ones summarized by Crosson
et al
. (2011).
In an industry-wide analysis of global dairy
production, Gerber
et al
. (2010) reported an
average GHG intensity (cradle to farm gate;
kg CO
2
e kg
−1
FPCM) of 2.4, ranging from 1 to 2
for the industrialized regions of the world to the
highest value of 7.5 for sub-Saharan Africa. In
this analysis, the allocation of GHG from culled
stock was done on the basis of protein, emissions
from surplus calves fattened for meat were
entirely attributed to meat production, and
emissions during pregnancy were entirely attrib-
uted to milk production. Changes in C stocks as
a result of land use change in the previous
20 years were also accounted for using IPCC
(2006) methodology. Slightly lower intensities
of milk production for North American systems
Production Efficiency
Productivity of individual dairy cows has
increased tremendously over the years in many
parts of the world due to genetics, nutrition and
management (see Chapter 2, this volume, for fur-
ther details). These improvements in animal pro-
ductivity and efficiency have helped to reduce the
GHG intensity of milk production, because emis-
sion intensity is inversely related to productivity
(Gerber
et al
., 2011). For example, Capper
et al
.
(2009) reported that modern US dairy farms pro-
duce the same quantity of milk as dairy farms in
