Agriculture Reference
In-Depth Information
4.5 total animals (slaughtered animals plus the
supporting population required to produce calves
for rearing) are required to produce 363 kg of
hot carcass weight beef in a grass-finished sys-
tem compared with 2.6 total animals in a con-
ventional system. When combined with the
increased time required for animals to grow to
slaughter weight, this increases the carbon foot-
print per unit of grass-finished beef by 67.5%.
The increased land required for grass-finished
production renders whole-scale conversion of
the US beef production system to grass-finished
production practically impossible. However, if
we assume it would somehow be achievable
and that beef production was maintained at
11.8 billion kg as in 2010 (USDA/NASS, 2011), the
increase in carbon emissions would be equal to
adding 25.2 million cars to the road on an annual
basis (Capper, 2012).
Proponents of pasture-based beef finishing
systems may argue that increased GHG emis-
sions from grass-finished cattle are compensated
for by the quantity of carbon sequestered by pas-
tureland. However, pasture does not sequester
carbon indefinitely, nor does it occur at a con-
stant rate. Over time, soil carbon concentrations
reach an equilibrium point, beyond which no
further sequestration occurs unless land is sub-
jected to significant management change (Post
and Kwon, 2000; Schlesinger, 2000). The pre-
sent body of knowledge indicates that the degree
to which carbon may be sequestered by crop or
pastureland is infinitely variable between sys-
tems and is dependent on a myriad of factors
including land use change, tillage, organic mat-
ter input, soil type and crop/pasture species.
Reliable data on carbon sequestration under a
range of environmental conditions and global
regions are notably lacking from environmental
literature and this is one area where future
research would pay dividends in bridging the
current knowledge gap. Although the majority
of US beef animals are finished within feedlot
systems, pasture and forage-based diets are fed
to these animals for half to two-thirds of their
life, and diets for the supporting beef herd (cows,
heifers and bulls) are predominantly based on
forage. Any potential effect of carbon sequestra-
tion in mitigating GHG could hence only be
attributed to the finishing period. In the afore-
mentioned comparison between conventional
and grass-finishing, sequestration would need to
exceed 1.3 t of carbon per ha annually (Capper,
2012). This is a considerable target, given that
Bruce
et al
. (1999) suggest that the potential for
carbon sequestration in well-managed pasture-
land is 200 kg ha
−1
, whereas Conant
et al
. (2001)
report 540 kg ha
−1
. Furthermore, although car-
bon emissions from the finishing population
would be mitigated by this degree of sequestra-
tion, the greater supporting population (cows,
calves, heifers and bulls) conferred by reduced
slaughter weights in the grass-finished system
would also need to be accounted for.
In the event that productivity in grass-
based finishing beef systems could be improved
to that commonly exhibited by maize-based fin-
ishing, sequestration would still need to occur
in order to compensate for the propensity for
pasture-based diets to increase ruminal metha-
nogenesis and thus enteric GHG emissions
(Johnson and Johnson, 1995; Pinares-Patiño
et al
., 2009). Methane production from enteric
fermentation is not a new phenomenon within
the scientific community, yet the link between
climate change and livestock production is a
relatively recent notion. Consumers therefore
often perceive that modern livestock production
causes climate change, whereas historical live-
stock populations were far more environmen-
tally friendly. To put this historical supposition
into context, Capper (2011b) noted that the
60 million American bison that roamed the US
plains until mass extinction in 1880 had total
GHG emissions (based on enteric methane pro-
duction and GHG emissions from manure)
approximately double the carbon emissions from
the US dairy industry in 2007 (Fig. 11.4).
Results from studies comparing resource
use and carbon emissions from organic or pasture-
based dairy systems vary considerably accord-
ing to methodology and region (de Boer, 2003).
Cederberg and Mattsson (2000) reported reduced
total GHG emissions per functional unit of
milk from organic versus conventional dairy
farms despite a reduction in milk yields in the
organic system. A computerized simulation of
European dairy production by Olesen
et al
.
(2006) demonstrated a tendency for organic
farms to have higher GHG emissions per unit of
milk than conventional farms, yet in this example,
milk yields were assumed equivalent between
systems. Given the considerable contribution of
enteric methane to total GHG emissions per unit
