Agriculture Reference
In-Depth Information
The conversion efficiency of sugarcane in tropical areas —
about 4.0% — is one of the highest known.
Even though these efficiencies are relatively low, they
are still several times greater than the average conversion
efficiency of mature natural vegetation, which is estimated
to be about 0.1% (Pimentel et al., 1978). We must also
take into consideration the fact that little of the biomass
in natural vegetation is available for human consumption,
whereas a large portion of the stored energy in agricultural
species is consumable (Figure 18.1).
Since much of the food consumed in developed coun-
tries is not plant biomass but animal biomass, we should
also examine the efficiency of the conversion from plant-
matter energy to meat, milk, and eggs. The production of
animal biomass from plant biomass is inefficient because
animals lose so much metabolic energy to maintenance
and respiration.
Analysis of this conversion is normally done in terms
of the energy content of the protein in the animal biomass,
since meat, milk, and eggs are produced mainly for their
protein. Feedlot or confined livestock need 20 to 120 units
of plant food energy to produce each unit of protein
energy, depending on the animal and the production sys-
tem. This is equivalent to an efficiency of 0.8% at the low
end and 5% at the high end. If these conversion efficien-
cies are combined with those for the production of the
animals' feed, the inefficiency of animal production sys-
tems becomes evident. As an example, the plant products
fed to feedlot cattle contain about 0.5% of the solar energy
that reached the plants, and the protein in the consumed
meat of the cattle contains 0.8% of the energy that was in
TABLE 18.1
Units of Energy Measure
Unit
Definition
Equivalents
Calorie (cal)
The amount of heat
necessary to raise 1 g
(1 ml) of water 1°C at 15°C
0.001 kcal
4.187 J
Kilocalorie (kcal)
The amount of heat needed
to raise 1 kg (l L) of water
1°C at 15°C
1000 cal; 4187 J;
3.968 Btu
British thermal unit
(Btu)
The amount of heat needed
to raise 1 lb of water 1°F
252 cal;
0.252 kcal
Joule (J)
The amount of work done
in moving an object a
distance of 1 m against a
force of 1 N
0.252 cal;
0.000252 kcal
CAPTURE OF SOLAR ENERGY
The starting point in the flow of energy through ecosys-
tems and agroecosystems is the sun. The energy emitted
by the sun is captured by plants and converted to stored
chemical energy through the photosynthetic process dis-
cussed in Chapter 3 and Chapter 4. The energy accumu-
lated by plants through photosynthesis is called primary
production because it is the first and most basic form of
energy storage in an ecosystem. Energy left after the
respiration needed to maintain plants is net primary pro-
duction (NPP) and remains as stored biomass. Through
agriculture, we concentrate this stored energy in biomass
that can be harvested and utilized, either by consuming it
directly or by feeding it to animals that we can either
consume or use to do work for us.
Plants vary in how efficiently they can capture solar
energy and convert it to stored biomass. This variation is
the result of differences in plant morphology (e.g., leaf
area), photosynthetic efficiency, and physiology. It also
depends on the conditions under which the plant is grown.
Agricultural plants are some of the most efficient plants,
but even in their case the efficiency of their conversion of
sunlight to biomass rarely exceeds 1% (a 1% efficiency
means that 1% of the solar energy reaching the plant is
converted to biomass).
Corn, considered one of the most productive food and
feed crops per unit of area of land, can produce as much
as 15,000 kg/ha/season of dry biomass, divided fairly
equally between grain and stover. This biomass represents
about 0.5% of the solar energy reaching the cornfield
during the year (or about 1% of the sunlight reaching the
field during the growing season). A potato crop that yields
40,000 kg/ha of fresh tubers (the equivalent of 7000 kg/ha
of dry matter) has a conversion efficiency of about 0.4%.
Wheat, with a grain yield of 2700 kg/ha and a dry matter
yield of 6750 kg/ha, has about 0.2% conversion efficiency.
4.0
3.0
1.0
% of solar radiation reaching
the surface annually that is
converted into biomass
0.5
0.5
0.4
0.3
0.2
0.1
Natural
vegetation
(average)
Wheat
Rice
Potatoes
Corn
Forage
grass
(average)
Sugar
cane
FIGURE 18.1 Efficiency of solar energy-to-biomass conver-
sion. (Pimentel, D., D. Nafus, W. Vergara, D. Papaj, L. Jaconetta,
M. Wulfe, L. Olsvig, K. French, M. Loye, and E. Mendoza.
1978. Bioscience 28: 376-382.; Pimentel, D., W. Dahzhong,
and M. Giampietro. 1990. In S. R. Gliessman (ed.), Agroecology:
Researching the Ecological Basis for Sustainable Agriculture .
pp. 305-321. Springer-Verlag: New York.; Ludlow, M. M. 1985.
Australian Journal of Plant Physiology 12: 557-572. With
permission.)
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