Comparative efficiency of energy use in crop production

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Comparative efficiency of energy use in crop production
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Bulletin - Connecticut Agricultural Experiment Station ; 739
Heichel, G. H.
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New Haven, Conn.
Connecticut Agricultural Experiment Station
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Crops ( jstor )
Corn ( jstor )
Energy efficiency ( jstor )

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Reprint 1974

The Connecticut Agricultural Experiment Station
New Haven Bulletin 739 November 1973


During the past decades, abundant and relatively cheap food has
been available to Americans because the application of technology and
scientific research has fed more people with less labor on fewer acres.
Part of this efficiency has come from substituting machines for manual
labor and from using pesticides and fertilizers.
Fuel energy is consumed in building and running the machines that
till the earth and in producing the agricultural chemicals that increase
yield. But now prices have risen and shortages loom.
While Connecticut residents do not put silage corn, sorghum or soy-
beans directly on their tables, these crops feed the cows and chickens
that provide our beef, milk and poultry. Any increases in the cost of
animal feed soon reach the consumer as increases in the prices of beef,
chicken, eggs, milk, cheese and butter. The same is true for the wheat
and grains that go into bread and cereal.
Since we surely do not want to have to pick up the hoe and return
to primitive digging to feed ourselves, we must know where energy is
consumed in food production and find ways to become more efficient.
This bulletin by Gary Heichel shows how well modern agricultural prac-
tices have kept up with increased fuel consumption and suggests ways to
conserve energy and still maintain food production.

PAUL E. WAGGONER, director

Comparative Efficiency of Energy Use

In Crop Production

G. H. Heichel

Sunlight, fossil fuels, and the labor of man and beast are the major
sources of energy for crop production. Sunlight provides the energy for
the biochemical processes that reduce carbon dioxide in the air to carbo-
hydrate in the crop. But, only a small portion of the energy available
from sunlight is conserved in photosynthesis-the majority escapes as heat.
In addition to sunlight, the production of food, feed and fiber is
supplemented by cultural energy. This energy comes from human and
animal labor, fossil fuels burned by tractors and vehicles during cultiva-
tion and harvesting, and energy used in transportation and in processing.
Cultural energy also includes all energy required to grow seed, construct
buildings, and to produce machinery, chemicals and fertilizers. A small
part of the cultural energy is conserved when the crop utilizes plant nu-
trients from fertilizers, but most is ultimately dissipated into the environ-
ment as heat, not transformed into harvestable energy.
Agriculture, which supposedly consumes more petroleum than any
other industry (9), uses about 10% of the petroleum products marketed
(24). About 2.5% of the annual production of electricity is used by
farmers (9). Tractors consume about 1.5% of the mineral fuels used each
year in the United States (30). Petroleum and electricity consumed on

/ Processing
Fuels Fuels //// Buildings
w Fertilizer
Materials Machines Fertilization YIELD
Crop Varieties Food, Feed,
Labor Lbr \ and Fiber
Labor Labor Seedinoa

Figure 1. The flow of energy from fuels and labor during the manufacturing, dis-
tribution, and culture of food, feed and fiber.


farms, however, are only a small part of the energy required for crop
production. Energy from fuels and labor is used during acquisition of raw
materials, fabrication of machines, construction of buildings, production
of fertilizers, chemicals and seed, and during cultural operations neces-
sary to transport and process food, feed and fiber (Fig. 1). While sun-
light used in photosynthesis is free, cultural energy is growing scarcer
and dearer. Because of this, we seek new ways to utilize cultural energy
more efficiently.
This bulletin describes a method that can be used to evaluate cul-
tural energy efficiency for both developing and advanced agricultural
systems by comparing the input of human and animal labor and supple-
mental sources of energy with the caloric yield of crops.
Efficiency. The efficiency of light utilization by crops is usually as-
sessed by determining the number of calories of dry matter produced by
photosynthesis. This may be computed as Output (calories of yield)
over Input (calories of active sunlight). Although the efficiency of photo-
synthesis can theoretically reach 12% (45), respiration of dry matter
and incomplete capture of sunlight by leaves reduce the observed ef-
ficiency to about 3% in a crop of maize (44). Indeed, the average crop
of maize captures only 1% of the 1012 calories of photosynthetically active
sunlight falling upon an acre of land during a three month growing
Few have attempted to measure the use of cultural energy in crop
production. Cottrell (11) was perhaps the first to compare the energy
requirements of primitive and advanced agricultural systems, but further
interest was lacking for about two decades. The concept of caloric gain
was used to evaluate caloric returns from the expenditure of human labor
in primitive agriculture (32). One calorie of human energy invested in
farming under these primitive conditions yielded 16 to 17 calories of
digestible energy. Thus, caloric gain, or the yield expressed as a multiple
of the energy investment, was about 16. However, a similar concept ap-
plied to advanced agriculture, where energy from fossil fuel replaces
that previously supplied by man and draft animals (13), suggests that
a calorie or more of fuel is expended for a calorie of yield (10, 28, 30,
33), making the caloric gain one or less.
If efficient production maximizes the gain of output per unit of
input, primitive agriculture appears superior to the agriculture of tech-
nologically advanced countries. Because of this, the wisdom of expending
large inputs of fossil fuel in agriculture has been questioned by some
(29, 30). When energy in advanced countries grows scarce, cultural
energy for food production will be more dear, so clearly, a technique for
assessing the efficiency of the use of cultural energy in the production
of food and feed will be valuable.


The usual definition of efficiency is the ratio of useful output of
energy to the inputs of energy. This measures how completely the inputs /
are transformed into the outputs. It was noted earlier, however, that
practically no cultural energy is transformed into plant tissue, thereforeI
the usual definition is not applicable.
Farmers invest calories of energy in tillage, fertilizers, pesticides,
irrigation, harvesting, and processing to help crops convert calories of
sunlight into calories that man or animals can digest. Calculating the
caloric gain, or the ratio of calories of yield to the investment of calories
of cultural energy (cal cal-1), for specific cropping systems reveals wheth-
er the investment has multiplied, remained static, or declined. Caloric
gain will be considered as a measure of the efficiency of utilization of
cultural energy. Thus, by contrasting caloric gain among cropping sys-
tems, the comparative efficiency of the utilization of cultural energy is

Cropping Systems
Fifteen agricultural systems, each comprising a crop with its own
set of cultural practices, were selected for analysis. These systems in-
cluded four levels of the application of technology; primitive swidden
agriculture based upon human labor, a less primitive rice culture relying
upon human and animal labor, corn culture during the transition from
animal labor to the internal combustion engine, and modem agriculture.
Primitive and Developing Agriculture. Two of the systems, the
vegetable garden of New Guinea (32) and the paddy rice culture in the
Philippines, are comparatively primitive. Human labor is the sole cultural
energy for the vegetable garden. Water buffalo supplement human labor
as cultural energy in a developing agriculture exemplified by the paddy
rice culture. Few metal implements made with fossil fuel are employed
in either system. For both primitive and developing agriculture, an ac-
count of the expenditure of energy by man and beast appears to be an
acceptable estimate of cultural energy.
Transition Agriculture. Three agricultural systems are typical of corn
culture in the United States between 1910 and 1920. During this period,
animal labor and metal implements supplemented human energy. Energy
used to construct and maintain buildings, to house draft animals, to make
machinery and implements and to store crops, is all classified as cultural
Both coal and gasoline were used to fuel stationary engines powering
forage cutters to produce corn silage, while the other two systems studied
were the prevalent methods of producing grain at the time. In one of
these, the corn was shocked before removal of the ear, while in the other,
the ear was harvested from the standing plant. Energy expended by man
and draft animals completes the estimate of cultural energy.


Modern Agriculture. The remaining systems studied are the modern
production of corn grain, corn silage, grain sorghum, soybeans, oats,
alfalfa-brome hay, rice, sugarbeets, sugarcane, and peanuts in selected
areas of the United States. These crops represent a range of cultural
requirements and energy yields. Corn grain, corn silage, grain sorghum,
soybeans, oats, and alfalfa-brome hay, are consumed by animals. Rice,
peanuts, sugarbeets, and sugarcane require various amounts of processing
before consumption by humans. Indeed, the cultural energy required to
process sugarbeets and sugarcane is sufficiently large to justify separate
attention. Specifying the consumer is important in assessing requirements
for cultural energy, because food and feed crops clearly demand different
amounts of cultural energy.
In 1970, tractors and other machines were the major source of cul-
tural energy. Human energy was required to operate machines, but
animal power had become obsolete. Thus, in addition to human energy,
the analysis must account for the fossil fuel used in farming and process-
ing operations and energy needed to construct buildings, make imple-
ments and vehicles and to produce fertilizers and pesticides that are char-
acteristic of advanced agriculture.
Since buildings, implements and tractors and other vehicles are
used for many years, estimates of depreciation were used to prorate the
energy needed to produce them for both transitional and modern agricul-
Cultural Energy
Cultural energy was calculated from the expenditure of human and
animal energy in agricultural operations and from crop production costs
itemized in farm records. Estimates of cultural energy for the production
of vegetables in New Guinea were previously published (32). The human
and animal labor requirements for production of paddy rice in the
Philippines were taken from reports of the International Rice Research
Institute (18-20). Cost records for specific crops reveal the variable and
fixed costs associated with crop production. Fixed costs include deprecia-
tion of equipment, insurance, taxes, and interest, while variable costs in-
clude fuel, seed, fertilizer, lime, feed, repairs, rented equipment, and
labor. Insurance, taxes, and interest were not translated into energy in
this study.
Estimates of the expenditure of cultural energy were made from
information about variable and fixed farming costs. Representative costs
of production were sought in areas with yields comparable to the average
national yields of each crop taken from Statistical Abstracts (37).
For primitive agriculture, there were no identifiable fixed costs. The
variable costs were the hours of human and animal labor required to
grow the crop. The costs of producing corn in the United States during
the period from 1910 to 1920 came from USDA Bulletin 1000 (27).


Variable costs of production and the labor requirements in 1970 for corn
grain, corn silage, grain sorghum, soybeans, oats, alfalfa-brome hay, rice,
and peanuts were taken from crop budgets prepared by the Economic
Research Service (23, 26, 35, 43). Crop budgets of regions having yields
near the national average were chosen. Estimates of the fixed costs were
adopted from various sources for corn grain (17), corn silage (17, 21),
grain sorghum (17), soybeans (17), oats (17), alfalfa-brome hay (17),
and peanuts (1). A tentative value was adopted for rice since no recent
fixed costs for rice were available. California sources (7, 42) were adopted
for the fixed and variable costs of producing sugarbeets. The Sugar Divi-
sion of the Agricultural Stabilization and Conservation Service provided
unpublished fixed and variable costs for sugarcane production in Hawaii

For each crop, the human labor expenditure in hrs acre-1 was con-
verted to energy at the rate of 175 kilocalories (Kcal) hr1 (2). When
draft animals such as horses or water buffalo were used, animal labor
was converted to energy at the rate of 2,400 Kcal hr- (4, 18). The heat
of combustion of coal used to fire steam engines in 1910 to 1920 was
about 6,650 Kcal kg-1 (8). Cost accounts list the gallons of fuel required
per acre or the dollar value of fuel and repairs. When the latter was re-

120 I I


U 80 -
L- 80

1900 1910 1920 1930 1940 1950 1960 197060

M 40.
o L

>-product for 1900 to 1970. Derived from data in references 22, 6, 37, and



ported, dollars were converted to gallons of gasoline or diesel fuel at
$0.28 gal-, and to energy at 32,000 Kcal gal-' (8).

Value of Energy
After converting the labor and fuel consumed in production to
energy, the dollar value of seed, fertilizer, lime, herbicides, insecticides,
custom hire of equipment and depreciation of the physical plant remained.
All are sources of cultural energy for crop production, but the energy
and labor required for their production are not easily assessed.
Since production of goods and services in commerce consumes energy,
the following model was used: The annual consumption of energy from
mineral fuels was related to the gross national product in current dollars,
and the consumption of energy accompanying the production of a dollar
of goods and services was calculated for 1900 to 1970 (Fig. 2). By using
the value of energy as dollars of goods and services, dollars were trans-
lated into energy without explicit knowledge of the portion of the cost of
machinery, fertilizer, herbicides, or depreciation that was directly at-
tributable to the consumption of fossil fuel, other sources of energy, or to
labor. About 100.6 megacalories (Meal) were consumed to produce a
dollar of goods and services in 1915, a value which was used to translate
current dollars into energy for the period between 1910 and 1920. In
1970, about 17.4 Meal dollar-' were used to produce goods and services.

Crop Yields
The useful part of the harvested plant, e.g. grain, fruit, or sugar, is the
economic yield. Vegetable garden yield in New Guinea was previously
reported (32, 33). The average yield of corn grain for 1915 in the United
States was taken as representative of 1910 to 1920 (38). The average yield
of corn silage for 1919 in the United States, the earliest record, was
similarly chosen (38). The national average yield for 1970 was used for
all modern crops except corn grain and corn silage. Figures for 1969 were
used for corn because of the leaf blight epidemic in 1970.
The dry matter production or yield of a crop includes the economic
yield plus the portion of the crop like stalks or straw that is discarded.
The total dry matter production excluding roots was calculated from
published values of the ratio of economic to total yield for corn grain
(14), grain sorghum (31), soybeans (15), oats (34), rice (18), sugar-
beets (6), sugarcane (5), and peanuts (16). Data for estimating the total
dry matter production of the vegetable garden were unavailable. Eco-
nomic yield was assumed to equal the total yield for alfalfa-brome hay
and corn silage with no corrections for senescence of leaves during the
season or for height of stubble. Total production of dry matter was not
estimated for corn producing grain between 1910 and 1920 because re-
liable estimates of the ratio of economic to total yield were not available.


Energy in Crops
The energy content of the economic yield of a crop, i.e. the frac-
tion of the dry matter production consumed by man or livestock, was
calculated as digestible energy. Digestible energy values for humans
(2, 25) were used when man was the consumer of crops such as rice,
sugarbeets, sugarcane, and peanuts. Digestible energy values for cattle
(2) were used for crops such as corn grain, corn silage, sorghum, soy-
beans, oats, and hay. Because economic yield varies widely among crops,
the total energy of the crop was also calculated to reveal the yield of
energy if the entire crop could be utilized. The total energy of the crop
was computed from total dry matter production by assuming that the
heat of combustion of dry matter synthesized during photosynthesis is
3,700 Keal kg-' (45). Budgets of cultural energy and yield for all systems
except the vegetable garden in New Guinea are given in Appendices I
and II.


Sources of Cultural Energy. Labor, fuel, fertilizer, pesticides, process-
ing and depreciation of machinery and buildings account for 80 to 90%
of the cultural energy required for the agricultural systems shown in
Table I
Relative contributions of cultural inputs to total cultural energy in several
cropping systems.
Cultural Input
(% of Total Cultural Energy)
Pesti- Pro- Deprecia-
Labor Fuel Fertilizer sides cessing tion Other
Cropping System
Corn grain (Iowa, 1915) 20 67 13
Corn grain (Pennsylvania, 1915) 18 73 9
Corn grain (Illinois, 1969) 0.06 47 19 4 21 9
Corn silage (Iowa, 1915) 12 12 69 7
Corn silage (Iowa, 1969) 0.09 52 10 4 27 7
Rice (Philippines, 1970) approx. 100 -
Rice (Louisiana, 1970) 0.05 41 5 5 33 16
Sorghum (Kansas, 1970) 0.06 37 16 2 37 8
Soybeans (Missouri, 1970) 0.08 56 0.05 7 25 11
Oats (Minnesota, 1970) 0.06 51 10 0.4 32 7
Alfalfa Hay (Missouri, 1970) 0.12 52 4 27 17
Peanuts (No. Carolina, 1970) 0.13 41 6 12 23 18
Sugarbeets (California, 1970) 0.05 29 6 6 33 7 19
Table 1. The investment of cultural energy to grow an acre of crop by
modern methods is about 1% of the 1012 calories of photosynthetically
active sunlight concurrently received by a field (Fig. 3 and the Ap-
pendices), r' about the same amount of digestible energy captured by
photosynthesis. Practically all cultural energy for rice culture in the


Philippines was attributable to labor, a situation similar to that in the
primitive vegetable garden in New Guinea (33). In modern rice produc-
tion, fuel consumption contributes about 41%, depreciation of machines
and buildings contributes about 33%, fertilizer and pesticides each con-
tribute about 5% and labor only contributes 0.05% of the cultural energy

Corn production in the early 20th century derived about 17% of
its cultural energy from labor and 70% from depreciation of machines
and buildings. In comparison, corn production in 1970 derived about
0.07% of its cultural energy from labor, 50% from fuel, 15% from fer-
tilizer, 4% from pesticides, and 24% from depreciation of machines and





0 2 4 6 8 10 12 14
(10 Megocolories acre' yeari)

Figure 3. Expenditure of cultural energy and yield of total energy in 15 agricultural
The following code applies to the 15 agricultural systems in Figures 3, 4, and
5: 1. Paddy rice, Philippines, 1970; 2. Vegetable garden, New Guinea, 1962; 3. Corn
for grain, Iowa, circa 1915; 4. Corn for grain, Pennsylvania, circa 1915; 5. Corn
silage, Iowa, circa 1915; 6. Alfalfa-brome hay, Missouri, 1970; 7. Oats, Minnesota,
1970; 8. Sorghum for grain, Kansas, 1970; 9. Soybeans, Missouri, 1970; 10. Sugarcane,
Hawaii, 1970, cultural energy excludes processing; 10'. Sugarcane, Hawaii, 1970,
cultural energy includes processing; 11. Corn for grain, Illinois, 1970; 12. Corn silage,
Iowa, 1970; 13. Sugarbeets, California, 1970, cultural energy excludes processing; 13'.
Sugarbeets, California, 1970, cultural energy includes processing; 14. Peanuts, North
Carolina, 1970; 15. Irrigated rice, Louisiana, 1970.

_ ol5






buildings. Cultural energy comes from similar sources in other modern
cropping systems except for soybeans, which require little fertilizer;
oats, which require few pesticides; and sugarbeets, which require 33%
of the cultural energy during processing. Sugarcane requires about 44%
of the cultural energy for processing (Appendix II), but costs are un-
known for the remaining inputs. Clearly, the change from primitive to
modern cropping systems was characterized by a replacement of labor
with other forms of energy.
Cultural Energy and Total Energy Yield
A remarkable pattern occurs among the 12 cropping systems for
which yields of total energy were calculated (Fig. 3). The paddy rice
in the Philippines that required about 330 Meal acre-1 yr- of cultural
energy from human and animal labor yielded about 14,000 Meal acre-1
yr-1 of total energy. Compared with this relatively primitive system, sub-
stituting fossil fuel and other energy sources for labor stimulated greater
yield of total energy, both for corn cultured during the early 1900's and
for some of the major crops grown in 1970. Initially, the yield of total
energy from the cropping systems responded dramatically to increasing
expenditure of cultural energy, a pattern exemplified by soybeans, grain
sorghum, sugarcane, and corn for grain (8-11, Fig. 3) but not by hay or
oats (6 and 7, Fig. 3). The maximum energy yield was reached by sugar-
cane, a crop that requires about 2,600 Meal acre-1 yr-', and corn silage,
a crop that requires about 6,000 Meal acre-' yr-'. Yields of total energy
subsequently declined with consumption of cultural energy, and cropping
systems for sugarbeets, peanuts, and irrigated rice exemplify this trend
(13-15, Fig. 3).

Cultural Energy and Digestible Energy Yields
Total energy yields reveal the crop energy that would be available
to a consumer, if all the energy were digestible. In reality this does not
occur because crops display a range of economic yields. For example, 40
to 45% (w/w) of the dry mass of the corn plant is grain, but refined
sugar is only 20 to 30% (w/w) of the dry mass of sugarcane, therefore,
assessing the yield of digestible energy is a more realistic approach (Fig.
Primitive, Developing, and Transition Agriculture. Yields of digest-
ible energy were low in cropping systems like the vegetable garden and
paddy rice that derived cultural energy from man and animals (1 and 2,
Fig. 4). The result was only about 5,000 to 6,000 Meal acre-1 yr-1 from
these systems. Digestible yield rose when machinery was combined with
draft animals to supplement the labor of man. The three cases typical
of corn production during the early 20th century exemplify this response
(3-5, Fig. 4). During this period, expenditure of 50% more cultural
energy to produce corn silage rather than grain more than doubled the


60 i i



a 12

w .* 30

P-. 20
8 14.15
o u/.1

0 I I I I I
10 3 ,3
1 e4 13 1'
2 7

0 2 4 6 8 10 12 14
(103 Megocalories acre"l year"l)

Figure 4. Expenditure of cultural energy and yield of digestible energy in 15 agricul-
tural systems. See Fig. 3 for explanation of number code.
yield of digestible energy. This was because practically all of the plant
was harvested for animal feed.
Modern Agriculture. Among modern cropping systems, yield of
digestible energy generally increased with cultural energy to a maximum,
exemplified by corn for grain and silage, at about 5,000 to 6,000 Meal
acre-' (11 and 12, Fig. 4). Sugarcane was an interesting exception. Al-
though it is an extremely productive crop in terms of dry weight, at
12,000 Meal acre-' yr-1, the digestible yield from sugarcane is little more
than that of hay and is poo'-r than that of sorghum. Yet for this modest
return, 19 to 115% more cultural energy was expended. The specific
value of cultural energy for sugarcane varies with the amount of process-
ing. Since sugarcane is a food and not a feed, over 40% of the cultural
energy is used to process the crop for human consumption (10 and 10',
Fig. 3). This shows that the contribution of processing must be clearly
distinguished when analyzing cultural energy requirements of food and
feed crops.
Digestible energy yields decline with expenditure of cultural energy
exceeding about 6,000 Mcal acre-' yr-'. This is exemplified by sugarbeets,
peanuts, and rice, which consume about twice the cultural energy of


corn but yield about half the digestible energy (13-15, Fig. 4). Sugarbeets,
like sugarcane, require considerable energy for processing, but yield
only twice the digestible energy of oats. This crop consumes only 18 to
26% of the cultural energy of sugarbeets.
Clearly, among these 15 examples, there is a maximum yield of
digestible energy that occurs in cropping systems requiring about 6,000
Meal acre-' yr-. For a surprising number of modern cropping systems,
a 10- to 50-fold increase in the expenditure of cultural energy only doubles
or triples the digestible energy yield compared with the more primitive

0 2 4 6

8 10 12 14

(10 Megacolories acre" year- )
Figure 5. The caloric gain, or ratio of the yield of digestible energy to the invest-
ment of cultural energy, of 15 agricultural systems. This ratio is used as a
measure of the efficiency of energy use. See Fig. 3 for explanation of num-
ber code.


examples. Thus, substantial expenditures of cultural energy often fail to
produce corresponding increases in yields. This response suggests that
progressively larger expenditures of cultural energy are being used less
efficiently in crop production.

Caloric Gain and Efficiency
Now, it is appropriate to consider whether the cultural energy ex-
pended in producing a crop is returned as digestible energy for man or
livestock. Prior to the use of machines and fossil fuels in crop produc-
tion, about 16 calories of digestible energy were realized for each calorie
of cultural energy spent in production. When machines were teamed
with draft animals in transition agriculture, the gain of cropping systems
fell to between 3 and 6 (3-5, Fig. 5). Interestingly, the Iowa corn farmer
of 1915 produced digestible energy about 2.5 times more efficiently than
his contemporary in Pennsylvania. While the Iowa farmer shucked his
corn directly from the stalk into the wagon, the Pennsylvania farmer
bundled the plants into shocks before using more energy to remove the
ear from the stalks. The decrease in caloric gain between primitive and
transition agriculture is largely attributable to the expenditure of fossil
fuels either on the farm or in manufacturing.
Modern cropping systems yield approximately 5 calories or less of
digestible energy per calorie of cultural energy (6-15, Fig. 5). Indeed,
comparing the caloric gain of cropping systems reveals that energy re-
turn generally decreases with increasing investment. The culture of corn
for grain and silage is an interesting exception. Compared with crops
like soybeans (9, Fig. 5) or sugarcane (10-10', Fig. 5), investment of cul-
tural energy in corn production increases energy return (11 and 12, Fig.
5). For crops like rice, peanuts, and sugarbeets with processing included,
the system operates at a gain of one or less. In the production of peanuts
and rice, a calorie of cultural energy was traded for a calorie of digestible
energy, while more calories were spent in production of sugarbeets than
were recovered in yield.
The majority of the modern cropping systems returned several
calories of digestible energy per calorie of cultural energy. Indeed, modem
crop production uses energy more efficiently than once thought (28, 29,
30). There are, however, clearly significant differences in efficiency of
energy use among modern cropping systems, so gains in efficiency may
be realized by using certain systems rather than others to produce di-
gestible energy.
Value of Energy. When the cultural energy for an input was un-
available, e.g., the energy consumption accompanying fertilizer or ma-
chinery production, an estimate of the energy consumption of the goods
and services derived from their retail value was used (Fig. 2). The various


inputs to agricultural production undoubtedly have dissimilar energy
requirements, but calculating the cultural energy needed to manufacture
and distribute each input is beyond the scope of this bulletin. The simple
derivation used in Fig. 2 yields a representative value of energy for a
unit of gross national product, and this value is applied to all inputs ex-
cept fuel and labor.
Some consequences of this approach are revealed by comparing the
energy equivalent of the retail value of fertilizer with independent es-
timates of the energy consumed in producing fertilizer nutrients (Table
2). Values in column (a) logically might exceed those in column (b)
because the retail cost of fertilizer includes blending and distribution to

Table 2
Energy requirements for production of plant nutrients.
Cost' Energy Requirement
Nutrient dollars lb-' Meal lb-'
(a)' (b)"
N 0.06 1.04 2.95
P 0.22 3.83 3.67
K 0.05 0.87 0.50

mean 1.91 2.35
SNutrient prices from Table 31, reference 43.
'Computed assuming 17.4 Mcal dollar- (Fig. 2).
SEstimates of the energy requirements for production prior to blending. Includes costs
of shipping to bulk plant in corn belt. The Tennessee Valley Authority provided
estimates for N produced as NH4NOa, P2O produced as triple superphosphate, and
KsO. The Potash Company of America furnished an estimate for production of K20.

the farm while the estimates provided by manufacturers in (b) do not.
However, estimates in (a) markedly exceed those in (b) for only one
of the three nutrients, and the contrast between the two sets of estimates
implicates other factors. The estimate by the Tennessee Valley Authority
(TVA) suggests that nearly three times as much energy is needed to
produce a pound of N as NHNO, than would be predicted from the
retail value of the nutrient. This apparently reflects the relatively large
energy requirement of ammonia synthesis compared with energy con-
sumption in the overall economy. Conversely, estimates by the TVA and
The Potash Company of America suggest that the energy required for
potash production is 40 to 60% of that calculated from the retail value
of.K,O. This contrast apparently reflects the relatively small energy con-
sumption needed to recover a naturally occurring nutrient that requires
little additional processing before use. The two estimates agree within
about 5% for phosphorus production. The mean energy requirements for
the production of fertilizer nutrients agree within about 20%. This sug-
gests that estimating cultural energy from the energy spent to produce
a dollar of goods and services is reasonable.


Energy Requirements of Corn Production. The efficiency of cultural
energy utilization in producing corn grain or silage has changed little
over the past half century (Fig. 4). A tripling of the yield of digestible
energy from dorn grain accompanied a tripling of the energy spent for
production between 1915 and 1969. Consequently, 3.3 to 4.8 cal cal-1
were harvested in 1915, and 4.4 cal cal-1 were harvested in 1969. A near
doubling of the yield of digestible energy from corn silage accompanied
a similar increase in the energy spent for production between 1915 and
1969. Thus, corn silage returned 5.9 cal cal-1 in 1915, and 5.3 cal cal-i
in 1969. The increase in yield of corn grain exceeded that of silage
because the ratio of grain to total dry mass increased more than total dry
matter production.

Cultural energy invested in corn production increased because
economically inexpensive fossil fuels were substituted for more expensive
human and animal labor. The costs of production both as dollars and as
energy rose substantially between 1915 and 1969 (Appendix I), and the
increase in the value of energy shown in Fig. 2 has contributed to these
increasing costs. Nevertheless, human and animal labor still costs more,
thus substitution of inexpensive cultural energy for expensive muscular
energy continues.

Inflation is one factor in the increasing value of energy shown in
Fig. 2. The dollar currently purchases much less energy than it did a
half-century ago, thus inflation contributes to the increased costs of pro-
duction. Another factor responsible for increased energy consumption is
a decrease of energy efficiency in the production of goods and services
(10). An example is our increasing dependence upon electricity, which
incurs a great loss of energy in transmission, instead of coal to produce
tractors, combines and plows. Nevertheless, the increase in corn yields
has kept the efficiency of production practically constant over the past
50 years. The current trend of more costly energy and less efficient energy
use in manufacturing might decrease the efficiency of corn production if
the rate of yield increase slows down. Clearly, these methods can be
used to examine the historical trend of efficiency involving other crops
as well.

Alternate Cultural Practices. Fuel accounts for about half of the
cultural energy used in modern crop production (Table 1). But, any ef-
fort to conserve fuel and increase efficiency by a partial return to labor-
intensive agricultural systems like Philippine rice culture would likely
be accompanied by an abrupt decline in total productivity. This would
happen because output per worker would decline, and less land would
be cultivated by the limited supply of farm labor. Indeed, the New Guinea
vegetable garden and the Philippine rice culture are the most efficient
in terms of energy utilization, but are the least productive of the 15 sys-
tems compared (Figs. 4 and 5).


Small increases in efficiency might follow use of alternate practices
such as the growth of green manure crops or utilization of more animal
manures in lieu of chemical fertilizers. The energy cost of substituting
animal manure is not clear, however. Utilization of green manure crops as
a substitute for chemical fertilizers might require twice as much crop-
land to assure sufficiently fertile soil to maintain the present acreage of
grain crops. Since fertilizer accounts for only about 9% of the cultural
energy used in the nine modern cropping systems (Table 2), any in-
crease in efficiency from using less fertilizer would be relatively small.
Similarly, efficiency of energy use would hardly change with increased
use of insect and disease resistant varieties or by substitution of biological
controls since pesticides consume only about 5% of the cultural energy
used in the eight modern systems (Table 1).

Crop Productivity. Many energy-efficient modern systems utilize a
crop that is efficient in using sunlight (Fig. 5). Using photosynthetically
efficient corn or sorghum as feed crops instead of inefficient species like
oats or soybeans might increase energy efficiency. Similarly, obtaining
digestible energy from a crop such as sugarcane instead of using inef-
ficient species like sugarbeets, rice, or peanuts might increase energy
efficiency as well. Oats, soybeans and sorghum require similar investments
of cultural energy, but sorghum returns twice the digestible energy (Fig.
5). Corn returns two to three times as much digestible energy as sugar-
beets, rice, and peanuts at half the investment of cultural energy. Similar-
ly, sugarcane returns twice the digestible energy of sugarbeets, peanuts,
and rice, from only half the cultural energy. Thus, selecting a rapidly
growing, productive crop species is clearly one possibility for increasing/
the efficiency of modern agriculture.
Digestible Energy and Quality Products. Investigating the combina-
tion of crop and cultural operations that requires the least amount of cul-
tural energy to produce the caloric needs of a man or an animal might
be tempting. Balanced nutrition, however, requires both protein and
energy, thus the system for producing digestible energy most efficiently
might include a cereal crop that is high in calories but low in protein.
This means that evaluation of the energy cost of producing protein from
different crops should be instructive.

The digestible protein in maize is 7% and in soybeans is 37%, for
example (2). Since modern corn and soybean systems in Appendix I re-
turn 0.09 and 0.28 kg protein Meal-' cultural energy respectively, corn
that returns 69% more calories yields 68% less protein per calorie ot
cultural energy than soybeans. Furthermore, soybean protein is nutri-
tionally superior to that from corn. Clearly then, both yield of energy
and quality of protein should be considered in production of food and
feed. Development of photosynthetically efficient grain crops with yields
of quality protein like high-lysme corn, or an increase in the photo-


synthetic efficiency of low-yield, protein-rich crops like soybeans would
serve this goal.
Efficiency of modern agriculture will increase when more can be
harvested as digestible energy or utilized as an auxiliary energy source
for crop culture. The difference between the total and digestible energies
of various cropping systems illustrates the reservoir of potentially useful
energy, and in many crops, less than half of the energy stored during
photosynthesis is utilized (Fig. 3 and 4). Increasing the grain-stover
ratio, the economic yield, and the food value of crops will stimulate yields
of digestible energy.
Fuel from Crop Wastes. Another possibility for increasing efficiency
is to exploit the energy potential of crop residues as the Hawaiian sugar
industry does by using bagasse for fuel. The net yield of oil from one
metric ton of dry organic wastes is estimated at 1.4 barrels (3), or about
1.26 Meal kg- of crop residue. The net oil potential of crop residues
could not be' utilized with 100% efficiency in crop culture due to loss of
energy during manufacturing. Nevertheless, the oil potential from corn
residues at 1,500 Meal acre-' or from sugarbeet residues at 5,000 Meal
aore-x would substantially lower the cultural energy requirement for
these crops. Because of fuel conversion possibilities, continued atten-
tion must be given to using the fuel potential of the readily accessible
wastes in cropping systems.


Striking differences occur in the efficiency of use of cultural energy,
both between cropping systems of primitive and advanced agriculture,
and among modern cropping systems. In a primitive country, the yield
of digestible energy is small, but the caloric gain of the cropping system
is large. For example, about 16 calories of digestible energy are har-1
vested for each calorie of cultural energy invested in the relatively primi-
tive rice culture of the Philippines.
During the early 20th century, the labor of man and beast was
supplemented by fossil fuels in the United States. The yield increased in
comparison with the more primitive agricultural systems but the caloric
gain decreased from about 15 to 5 because supplemental energy was
utilized less efficiently.
Modern agriculture derives practically all of its cultural energy from
fossil fuels or other energy sources that replace labor. The caloric gain
of modern crop production approaches five in efficient systems like corn
culture. In the less efficient systems of rice, peanut, or sugarbeet, how-
ever, it is one or less. For a surprising number of modern cropping sys-
tems, a 10- to 50-fold increase in the use of cultural energy has only
doubled or tripled the digestible energy yield compared with the more


primitive systems using substantially less technology. This response ex-
emplifies a decline of energy yield with energy investment, and reveals
that progressively larger expenditures of cultural energy are being used
less efficiently in crop production.
The most efficient agricultural systems were the labor-intensive e
swidden agriculture and the Philippine rice culture. However, these
labor-intensive cropping systems were also the least productive in terms
of digestible energy (per acre). Conservation of energy by substitution
of more labor for fossil fuels would probably cause an abrupt decline in
the number of acres in crop production.
Many of the modern agricultural systems that exhibit relatively ef-
ficient use of cultural energy incorporate a photosynthetically efficient
crop. Thus, significant gains in efficiency of cultural energy use might
be obtained by incorporating more efficient species with high yields of
digestible energy.
Other possibilities for increasing the energy efficiency of crop pro-
duction are use of cultural practices that consume less energy, growth
of crops that require little processing before consumption, increases in
the digestibility of crops, and utilization of the energy in crop residues.



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I am indebted to my colleagues Drs. C. R. Frink and J. G. Horsfall
for stimulating discussions and valuable criticism during the preparation
of this manuscript. I am grateful to R. D. Young and P. S. Jack for es-
timates of the energy requirements of fertilizer production. Preliminary
drafts of the manuscript were reviewed by Dr. H. P. Kortschak of the
Hawaiian Sugar Planter's Association and Dr. R. Evenson of Yale Univer-


Appendix I

Energy Input




Other Variable Costs

Fixed costs

Total Cultural Energy

hr acre-
Meal acre-'
hr acre-'
Meal acre-'
gal acre-'

Mcal acre-'
dollars acre"
Meal acre-'
dollars acre-'
Meal acre-'

Corn Corn Corn
Grain Grain Grain
(Iowa) (Pennsylvania) (Illinois)
(1915) (1915) (1969)

17.3 50.3 5.4
3.0 8.8 0.95
43.9 54.0 -
105.4 129.6 -

0.66 0.61 31.10
66.4 61.4 541
3.50 5.50 20.30
352 553.3 353

Meal acre-' 527 753.1 1,695
Meal acre-' yr-' 1,596 2,282 5,136

Growing Season yr 0.33 0.33 0.33
Economic Yield kg acre-' 711 711 2,120
Digestible Energy
Seasonal Meal acre-' 2,503 2,503 7,462
Annual Meal acre' yr-1 7,584 7,584 22,612
Total Yield kg acre' 3,410
Potential Energy
Seasonal Meal acre- 12,617
Annual Meal acre-' yr- 38,233
Digestible Energy/
Cultural Energy 4.8 3.3 4.4

Corn Corn
Silage Silage
(Iowa) (Iowa)
(1915) (1969)

( +14 bs coal)












5.9 5.3


Garden Rice Rice Sorghum Soybeans Oats Alfalfa Hay Peanuts
(New Guinea) (Philippines) (Louisiana) (Kansas) (Missouri) Minnesota) (Missouri) (No. Carolina)
(1962) (1970) (1970) (1970) (1970) (1970) (1970) (1970)



4.24 2.67
0.74 0.47

.7 12

7.65 27.71
1.34 4.8

18 49

1,760 320 544 384 576 1,568
62.94 12.84 11.11 7.80 13.15 78.54
1,095 223 193 136 229 1,367
80.00(estl 18.50 13.84 13.81 17.27 50.54
1,392 322 241 240 300 879

561 118 4,249
321 331 11,936

1.75 0.36 0.36
550 1,658
(brown rice) (brown rice)
9,218 2,035 5,969
5,267 5,716 16,766
1,360 3,417


866 979 760 1,106 3,819
2,623 2,966 2,304 2,213 11,572

0.33 0.33 0.33 0.50
1,300 737 713 2,540


4,576 2,497 1,882 5,588 5,211
13,867 7,567 5,704 11,176 15,792
3,100 2,286 1,550 2,540 3,425

12,643 11,470 8,458 5,735 9,398 12,673
35,514 34,758 25,631 17,379 18,796 38,402

1.4 5.3 2.6 2.5 5.1 1.4


Appendix II

Energy Input


Other Variable Costs

Fixed costs

Total Cultural Energy
(Excluding Processing)

(Including Refining)
Total Cultural Energy
(Including Processing)
Growing Season
Economic Yield

Digestible Energy
Total Yield

hr acre'1
Meal acre-'
gal acre-'
Mcal acre-'
dollars acre-'
Meal acre-'
dollars acre-'
Meal acre-'

Mcal acre-'
Meal acre-' yr-'

Meal acre'

Meal acre-'
Meal acre-' yr-'

kg acre-'

Meal acre-'
Meal acre-' yr-'
kg acre-'

Potential Energy
seasonal Meal acre-'
annual Meal acre-' yr-1
Digestible Energy/
Cultural Energy
1) Including processing cal cal-'
2) Excluding processing cal cal-

(Calif., 1970)

(Hawaii, 1970)

178.20 ( @ $0.045 kg-'
3,101 raw sugar)
37.40 176.99
651 3,080

( @ $0.066 kg-')(



(refined sugar)




@ $0.036 kg-' + labor
@ 0.008 hr kg-')


(refined sugar)







AGR 101

Founded in 1875 First Station in the Nation