Agriculture Reference
In-Depth Information
now used for food crops and were the harvested biomass used to make biofuel that
offsets fossil fuel use (Gelfand et al. 2013).
Energy balance can also provide a common basis for comparing the GWIs of
different cropping systems because of the interconnection between GHG emissions
and energy use (West and Marland 2002). An accurate estimate of agricultural
energy efficiency—the ratio of useable energy in the end products to the energy
used for production (Gelfand et al. 2010)—can, in addition to illustrating GWI dif-
ferences, provide insights into how society can meet food and fuel security needs
most energy efficiently. Energy efficiency can be calculated using energy balance
tools (e.g., Kim and Dale 2003), and becomes especially useful when assessing the
potential for bioenergy crops to offset fossil fuel use.
Table 12.4 shows the large range in annual energy inputs and food energy
outputs for the MCSE annual cropping systems (Gelfand et al. 2010). While the
Conventional system produced more than 10 times the energy in food than was
used in farming (72.7 vs. 7.1 GJ ha
−1
yr
−1
), the No-till system produced even more
energy (78.5 GJ ha
−1
yr
−1
) and at two-thirds of the energy input (4.9 GJ ha
−1
yr
−1
), for
a net energy efficiency (energy output:input ratio) of 16, far higher than that of the
Conventional (10). High energy costs of tillage account for most of the difference.
Gelfand et al. (2010) also showed that the energy efficiency for food production
was always higher than for liquid fuel production from the same crops, even when
crop residues were to be used for fuel. However, this analysis assumes that food
is produced for direct human consumption; allocating a portion of food crops to
support livestock for meat and dairy production would change the energy balance
because of the inherent inefficiency of energy transfer through food chains.
The Importance of System Boundaries for GWI Comparisons
A full accounting of the GWI or energy balance of an agricultural ecosystem
requires a clear definition of the boundaries that meet the purpose and needs of
the analysis. Inclusion of solar energy inputs, for example, would make fossil fuel
Table 12.4. Crop yields and energy balances for the annual cropping systems of
the MCSE from 1989 to 2007.
a
Annual Cropping
System
Crop Yield
(Mg ha
−1
yr
−1
)
Crop Rotation Energy Balance
(GJ ha
−1
yr
−1
)
Corn
Wheat
Soybean
Farming
Energy Inputs
Food Energy
Output
b
Net Energy
Efficiency
c
Conventional
5.9
3.5
2.3
7.1
72.7
10
No-till
6.3
3.7
2.7
4.9
78.5
16
Reduced Input
5.2
3.1
2.6
5.2
66.9
13
Biologically Based
4.1
2.1
2.4
4.8
53.1
11
a
See Table 12.1 for a description of systems.
b
Food produced for direct human consumption (i.e., not via livestock production for human consumption).
c
Net Energy Efficiency calculated as output to input ratio.
Source:
Gelfand et al. (2010).
