The Energetics of Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 260

energy input in agriculture, and they need food even when
at rest. On this basis, one could reduce the energy cost of
human labor by considering only the extra food energy
needed to perform agricultural work (Figure 18.8).
In many agroecosystems that rely mainly on biological
cultural energy, animals play an important role in cultivating
the soil, transporting materials, converting biomass into
manure, and producing protein-rich foods such as milk
and meat. Animal use increased considerably in agricul-
ture when the transition from shifting cultivation to per-
manent agriculture began to occur.
Although the use of animal labor increases the total
biological cultural energy input and lowers the ratio of
harvested energy to invested energy to the neighborhood
of 3:1, it allows for permanent instead of shifting agricul-
ture, increases the area that can be planted, produces
manures for enriching the soil, and allows for the harvest
of meat, milk, and animal products. In addition, animals
consume biomass that cannot be consumed directly by
humans, which lowers their relative energy cost. An exam-
ple of the energy efficiency of corn production using
animal traction is presented in Figure 18.9.
Biological cultural energy is an important component
of sustainable agriculture. Energy inputs from humans and
their animals are generally renewable, providing energy that
helps transform a greater proportion of solar energy into
harvestable food energy. The use of human and animal labor
takes advantage of the first law of thermodynamics by alter-
ing natural ecosystem processes in ways that concentrate
energy in useful products, but still obeys the second law by
always returning to ecological inputs of energy from the
sun in order to maintain the agroecosystem over the long
term. When doing an energetic analysis of biological cul-
tural energy, it must be remembered that this form of energy
is more than an economic cost for agriculture — it is an
integral part of a sustainable production process.

TA B L E 1 8 . 2
Energy Content of Several Types of Biological
Cultural Energy Inputs to Agriculture

Input Type Energy Value

Human labor, heavy (clearing with
a machete)

400-500 kcal/hr

Human labor, light (driving a
tractor)

175-200 kcal/hr

Large draft animal labor 2400 kcal/hr

Locally produced seed 4000 kcal/kg

Cow manure 1611 kcal/kg

Pig manure 2403 kcal/kg

Commercial compost 2000 kcal/kg

Biogas slurry 1730 Kcal/kg

Source: Cox, G.W. and M.D. Atkins. 1979. Agricultural Ecology .
Freeman: San Francisco.; Pimentel, D. and M. Pimentel (eds.), 1997.
Food, Energy, and Society. 2nd ed. University Press of Colorado:
Niwot, Colorado.; Zhengfang, L. 1994. Energetic and ecological
analysis of farming systems in Jiangsu Province, China. Presented
at the 10th International Conference of the International Federation
of Organic Agriculture Movements (IFOAM), Lincoln University,
Lincoln, New Zealand, December 9-16.

Production
of seeds
5.7%

Production
of ax and
hoe
2.6%

Human labor
91.7%

Energy content of harvest:
6,901,200 kcal/ha

Total cultural energy input: 642,338 kcal/ha

FIGURE 18.7 Cultural energy inputs to a traditional shifting
cultivation corn crop in Mexico. The ratio of the food energy
output to the cultural energy input for this system is 10.7:1.
Only the axe and hoe (used for clearing and seed planting)
required an input of industrial cultural energy. (Pimentel, D.
and M. Pimentel (eds.), 1997. Food, Energy, and Society. 2nd
ed. University Press of Colorado: Niwot, Colorado.)

U SE OF I NDUSTRIAL C ULTURAL E NERGY

Once agriculture began to mechanize, the use of energy
from industrial cultural sources increased dramatically.
Mechanization and industrial cultural energy greatly
increased productivity, but they also changed the nature
of agricultural production. Human and animal labor was
displaced, and farming became tied to fossil fuel produc-
tion and consumption.
Present-day conventional agroecosystems have come
to rely heavily on industrial cultural energy inputs. Corn
production in the U.S. is a good example of an agroeco-
system where almost all of the energy inputs to the system
come from industrial sources. Figure 18.10 shows the total
energy inputs per hectare in corn production, and how this
energy is distributed among the various input types. Bio-
logical cultural energy in the form of human labor is a
minimal part of this system.

Asia, for example, are able to gain up to 38 cal of food
energy for every calorie of cultural energy invested.
The energy value of the human labor in these systems
is calculated by determining how many food calories a
person burns while working. Although this technique pro-
vides good baseline data, it does not take into account a
variety of other factors. One could also consider the energy
required to grow the food that is metabolized while work-
ing, and the energy needed to provide for all the other
basic needs of the human workers when they aren't work-
ing. Such additions would increase the energy value of
human labor. On the other hand, people's basic needs must
be provided for whether or not their labor serves as an