Agriculture Reference
In-Depth Information
Sources of energy for
food production
ECOLOGICAL ENERGY
Solar energy: source
of energy for
production of biomass
CULTURAL ENERGY
Energy supplied by humans
to optimize production of
biomass in agroecosystems
BIOLOGICAL CULTURAL ENERGY
Cultural energy derived from
human and animal sources
Examples: Human labor, animal
labor, animal manure
INDUSTRIAL CULTURAL ENERGY
Cultural energy derived from
non-biological sources
Examples: Electricity, gasoline,
diesel fuel, natural gas
FIGURE 18.3 Types of energy inputs in agriculture. Biological cultural energy and industrial cultural energy can either come
from outside a particular agroecosystem (in which case it is a form of external human input) or be derived from sources with the system.
enables higher yields, such systems still have favorable
returns on their investment of cultural energy.
In mechanized agroecosystems, however, very large
inputs of industrial cultural energy replace most of the
biological cultural energy, enabling high levels of yield
but greatly reducing energy-use efficiency. In the produc-
tion of grains such as corn, wheat, and rice, these agroeco-
systems can yield 1 to 3 cal of food energy per calorie of
cultural energy. In mechanized fruit and vegetable produc-
tion, the energy return is at best slightly greater than the
energy investment, and in most cases it is smaller (Pimen-
tel and Pimentel, 1997). For production of animal food,
the ratio is in most cases even less favorable. Beef pro-
duction in the U.S., for example, requires about 5 cal of
cultural energy for each calorie obtained, and pork
requires as much as 10 cal (Pimentel and Pimentel, 1997).
Since animal foods are valued more for protein con-
tent than total energy content, we should also consider the
energy efficiency of their production in terms of the energy
in the protein of these foods compared to the energy in
the feed consumed by the animals. In these terms, each
calorie of protein in milk, pork, and feedlot beef requires
between 30 and 80 cal of energy to produce. By compari-
son, a calorie of plant protein can be produced with as
little as 3 cal of cultural energy (in the case of protein
from grains). Even the production of concentrated plant
protein (e.g., tofu from soybeans) takes no more than 20
cal of energy for each calorie of protein (Figure 18.4).
The data presented in Figure 18.4 reinforce our claim
that the cultural energy requirement in agriculture is
closely related to the level of modification of natural eco-
system processes. The costs are small when humans leave
the basic structure of the ecosystem intact. When certain
minor modifications are made that increase the abundance
of a specific crop species of interest, more cultural energy
is required, but the return is still favorable. But when a
Flood-irrigated rice in Thailand
(non-mechanized)
38:1
Shifting cultivation of
corn in Mexico
10.7:1
10:1
6.5:1
4.3:1
4.2:1
2.5:1
2.2:1
2.1:1
1.4:1
Pastoral production of milk
and meat in Africa
Traditional non-mechanized
cowpea production in Nigeria
Corn production in Mexico
with oxen
Conventional soybean
production in US
Mechanized corn
production in US
Mechanized wheat
production in US
Mechanized large-scale
rice production in US
Conventional peanut
production in US
Conventional tomato
production in US
1:1.7
Conventional meat production
in US
1:2.5
1:4
Conventional spinach
production in US
Pork meat production
in US
1:10
40:1
30:1
20:1
10:1
1:1
1:10
Calories of food energy : Calories of cultural energy invested
FIGURE 18.4 Comparison of the returns on energy investment
for various agroecosystems. Bars extending to the left indicate
systems in which the realized output is greater than the input; bars
extending to the right indicate systems in which the energy input
is greater than the energy value of the resulting food. (Cox, G. W.
and M. D. Atkins. 1979. Agricultural Ecology . Freeman: San
Francisco.; Pimentel, D. and M. Pimentel (eds.), 1997. Fo o d ,
Energy, and Society. 2nd ed. University Press of Colorado: Niwot,
Colorado.; Pimentel, D., M. Pimentel, and M. Karpenstein-
Machan. 1998. International Commission of Agricultural
Engineering Ejournal (cigr-ejournal.tamu.edu. With permission.)
complex natural ecosystem is replaced by a crop mono-
culture with a life form very different from that of the native
species — as is the case with irrigated cotton in the former
arid scrub lands of the western San Joaquin Valley of
California — cultural energy costs rise steeply. When the
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