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OCR for page 5952
Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 5952-5959, May 1999
Colloquium Paper
This paper was presented at the National Academy of Sciences colloquium "Plants and Population: Is There Time?"
held December 5-6, 1998, at the Arnold and Mabel Beckman Center in Irvine, CA.
Ecological intensification of cereal production systems: Yield
potential' soil quality' and precision agriculture
KENNETH G. CASSMAN
Department of Agronomy, University of Nebraska, Lincoln, NE 68583-0915
ABSTRACT Wheat (Triticum aestivam L.), rice (Oryza
saliva L.), and maize (Zea mays L.) provide about two-thirds
of all energy in human diets, and four major cropping systems
in which these cereals are grown represent the foundation of
human food supply. Yield per unit time and land has increased
markedly during the past 30 years in these systems, a result
of intensified crop management Involving Improved germ-
plasm, greater inputs of fertilizer, production of two or more
crops per year on the same piece of land, and irrigation.
Meeting future food demand while minimizing expansion of
cultivated area primarily will depend on continued intensifi-
cation of these same four systems. The manner in which
further intensification is achieved, however, will differ mark-
edly from the past because the exploitable gap between
average farm yields and genetic yield potential is closing. At
present, the rate of increase in yield potential is much less than
the expected increase in demand. Hence, average farm yields
must reach 70-80% of the yield potential ceiling within 30
years in each of these major cereal systems. Achieving con-
sistent production at these high levels without causing envi-
ronmental damage requires improvements in soil quality and
precise management of all production factors in time and
space. The scope of the scientific challenge related to these
objectives is discussed. It is concluded that major scientific
breakthroughs must occur in basic plant physiology, ecophysi-
ology, agroecology, and soil science to achieve the ecological
intensification that is needed to meet the expected increase in
food demand.
This paper explores biophysical constraints to global food
security in the next century. Emphasis is placed on wheat, rice,
and maize, because these three plant species account for the
majority of calories in human diets (http://apps.fao.org/,
agricultural production, November 1998), and they are likely
to remain the mainstay of human nutrition in the foreseeable
future. Intensification of wheat, rice, and maize cropping
systems was largely responsible for averting a shortfall in food
supply during the past three decades a 30-year period mark-
ing the advent of the so-called "green revolution." The most
salient feature of these intensified systems was greater yield
per unit land and time.
Although appropriate government policies and social con-
ditions also were required to promote intensification, three
production factors were largely responsible for the increased
production achieved by farmers. These factors were: (i) new
"miracle" varieties of wheat and rice released in the mid to late
1960s, which had a higher harvest index (HI; the ratio of grain
to total crop biomass), shorter stature, and increased stalk
strength that reduced susceptibility to lodging, as well as steady
improvement in maize hybrids; (ii) increased application of N
fertilizer, which allowed greater net primary production with
PNAS is available online at www.puas.org.
out fear of lodging; and (~iii) massive investments in irrigation
infrastructure, which were justified by the greater yield po-
tential, fertilizer responsiveness, and increased cropping in-
tensity made possible with new varieties and hybrids. In
addition, the reduction in time, from planting to maturity of
the new varieties, also permitted an increase in cropping
intensity. While only one crop harvest per year was possible
with landrace genotypes, earlier maturity allowed two and
sometimes three cereal crop harvests per year on the same
piece of land. Annual double-crop systems with rice, wheat,
and maize are now the dominant cropping system where soil,
climate, and water allow intensified cropping. Hence, the scope
for a further increase in cropping intensity is limited. Contin-
ued expansion of irrigated area has slowed markedly in recent
years, and future prospects for increasing irrigated land are
limited by both water supplies and environmental concerns (1~.
The foundation of global food security now is built on four
major cereal production systems in which modern farming
practices are used. These systems include: (i) irrigated annual
double- and triple-crop continuous rice systems in the tropical
and subtropical lowlands of Asia, which account for about 25%
of global rice production, (ii) irrigated annual rice-wheat
double-crop system, which is the primary cereal production
system in northern India, Pakistan, Nepal, and southern China,
(iii) temperate maize-based, rain-fed cropping systems of the
North American plains, which contribute more than 40% of
global maize supply, and (iv) the favorable rain-fed wheat
systems of northwest and central Europe, which account for
more than 20% of global wheat supply. Cropping systems of
smaller extent comparable to each of the major systems listed
above also are found in other regions of the world that have
similar natural resource endowments.
The Need for Ecological Intensification
Increased yield from intensification of wheat, rice, and maize
systems contributed 79-96% of the total increase in the global
supply of wheat, rice, and maize since 1967 (Table 13. Although
wheat area has remained relatively constant, total maize area
increased by 30 million hectares (ha), which is 12% greater
than the total USA maize area in 1997 (http://apps.fao.org/~.
An additional 446 million ha of land would be required to
achieve 1997 levels of wheat, rice, and maize production at
1967 yield levels, which represents 3-fold greater more area
than the present total area of wheat, rice, and maize in the USA
and China combined. Hence, intensification of cereal produc-
tion systems has spared expansion of agriculture into natural
ecosystems and marginal land prone to degradation from
intensive cropping (2~.
Although intensification has spared natural ecosystems
from conversion to agricultural uses, greater use of applied
inputs and inefficient farming practices have contributed to
Abbreviations: HI, harvest index; ha, hectare.
5952
OCR for page 5953
Colloquium Paper: Cassman
Table 1. Worldwide harvested area of the major cereals in 1967
and 1997, average growth rates in yield and production since 1967,
and the projected growth rate in cereal demand from 1993 to 2020
81
~ 6
1967-1997 Grain demand, s
~5
~ 4
._
co
.o 2
Annual growth rate, No
1967 1997 Yield Production 1993-2020
Wheat 219 229
Rice 128 150
Maize 112 142
Sources: http://apps.fao.org/ and ref. 5.
2.3 2.5 1.22
1.9 2.5 1.19
1.8 2.6 1.49
non-point-source pollution problems, such as ground and
surface water pollution, and a reduction in biodiversity in
agroecosystems as well as other ecosystems that are affected by
outputs from food production systems (3~. In addition, both
intensification and expansion of agricultural area contribute to
anthropogenic effects on the Earth's biogeochemical cycles
(4~. At issue, then, is whether further intensification of cereal
production systems can be achieved that satisfy the anticipated
increase in food demand while meeting acceptable standards
of environmental quality. This goal can be described as an
ecological intensification of agriculture. Success will depend
on sustaining yield increases in the existing major irrigated and
favorable rain-fed cereal systems because of limited opportu-
nities for greater cropping intensity and expansion of irrigated
area.
An important question is how much additional cereal pro-
duction will be required. The International Model for Policy
Analysis of Agricultural Commodities and Trade (IMPACT) is
one of several econometric models used to estimate future
food demand based on population and economic growth rates
as well as other factors that influence supply and demand. A
recent update of this model predicts a rate of increase in
demand for the major cereals between 1993 and 2020 of about
1.2% per annum for wheat and rice, and 1.5% for maize (Table
1) (5~. Extrapolating these rates of increase from 1997 to 2027
gives estimates for wheat, rice, and maize demand of 882, 827,
and 916 million metric tons, respectively. Hence, cereal pro-
duction increases during the next 30 years of 44% for wheat,
43% for rice, and 56% for maize provide reasonable targets for
researchers concerned with the factors governing global food
supply capacity. The goal of achieving this increase without a
net expansion of cultivated area also seems appropriate to
avoid further losses of the Earth's remnant natural ecosystems.
Intensification in Favorable and Unfavorable Environments
Rice yield trends since 1967 in three Asian countries illustrate
the relationship between intensification, natural resource en-
dowments, and the potential impact on global food supply (Fig.
1~. In South Korea, all rice is produced with irrigation. Yields
increased rapidly until 1980 because of adoption of modern
farming practices. Thereafter, yields have stagnated because
the average yield of about 6.3 tons (t) ha-i is approaching the
existing yield potential that can be achieved with the best
available technology. South Korea has a summer monsoon
climate with low solar radiation during the rice growing season
that is similar to Kyoto, Japan where rice yield potential
averages about 7.8 t ha-i (6~. Assuming a similar yield poten-
tial in South Korea, average rice yields achieved by South
Korean farmers are about 80% of the yield potential ceiling.
Indonesian rice yields have increased steadily because of
expansion of irrigated area, adoption of improved varieties,
and use of fertilizers. At present, about 70% of rice area is
irrigated (Fig. 1~. The yield trajectory suggests that further
increases in Indonesian rice yields are possible. In contrast, rice
yields in Thailand have barely increased because 75% of rice
area is produced without irrigation (i.e., rain-fed systems) on
Proc. Natl. Acad. Sci. USA 96 (1999J 5953
1 1
· Indonesia x = 6.3 t hay
- Thailand
~Korea /
f' -
/~- Y= 3.2+0.216X
- hi- ~ r2 = 0.84 ._
Y. .~
Y = 1.9 + 0.094X
~ ~ r2 = 0.96
=-~e ·~-~-~-~.
`\Y=1.7+0.016X
0 ~ 1
1960 1970 1980 1990 2000
Year
r2=0.67
FIG. 1. National average rice yields from 1967 to 1997 in three
Asian countries (http://apps.fao.org).
poor quality soils. Drought, flooding, and infertile soils impose
severe limits on the ability to increase yield even though
farmers have access to improved varieties and fertilizer inputs.
Despite these constraints, it is noteworthy that Thailand is
presently the world's largest exporter of rice, which reflects an
extensive rice production area rather than high productivity,
and the scope for increased rice production is very limited.
Similar tendencies in relation to natural resource endow-
ments are seen in the yield trends of wheat and maize. Modern
crop management practices have had the greatest impact on
yields in irrigated systems, such as the high-production rice and
wheat systems in Asia, and in rain-fed environments where
both climate and soil quality are favorable for crop growth,
such as the wheat systems of northwest and central Europe and
maize-based systems in North America. In unfavorable rain-
fed environments with poor soils and harsh climate, wheat and
maize yields have risen slowly during the past 30 years.
Ecological intensification of cropping systems in unfavor-
able rain-fed environments mostly depends on reducing the
reliance on subsistence cereal production, integration with
livestock enterprises, greater crop diversification, and agro-
forestry systems that provide higher economic value and also
foster soil conservation (7~. The magnitude of increase in the
food supply from such advances will be relatively small,
however, because present yields are very low and the primary
constraint is lack of water. All crop yields are directly related
to the amount of water transpired (8~. Hence, the potential to
increase the amount of transpiration in water-limited environ-
ments by genetic improvement is relatively small. Instead,
increasing the amount of plant-available water per unit of
rainfall by improvements in soil and residue management that
increase infiltration and reduce runoff will have much greater
impact on yield and yield stability than can be expected from
genetic improvement. No-till and reduced tillage systems
developed in the USA are examples of such practices (9~. In
tandem with research on integrated nutrient management,
applied research to adapt conservation tillage technologies for
use in unfavorable rain-fed systems in developing countries
would have a large positive impact on local food security and
increased standards of living, but they will have little impact on
the global food-supply balance.
At the other extreme are high-production systems in which
average farm yields are presently above 70% of yield potential.
Rice production in Korea, Japan, and parts of the USA and
China, and wheat production in some areas of northwestern
Europe have reached this level. Further increases in yield will
be difficult to achieve without an increase in the genetic yield
potential of crop varieties and hybrids.
OCR for page 5954
5954 Colloquium Paper: Cassman
The greatest opportunities for sustained yield increases from
further intensification are found in irrigated and favorable
rain-fed systems where present average farm yields are less
than 70% of yield potential. Rice systems in Indonesia are an
example of such systems (Fig. 1), and most irrigated rice and
wheat systems in the developing countries of Asia fall into this
category. For example, the mean climate-adjusted rice yield
potential in this region is estimated to be about 8.5 t ha-i (10)
while average irrigated rice yields are presently 5.0 t ham (7),
or about 60% of the climate-adjusted yield potential. Other
major food production systems in this category are the favor-
able rain-fed maize-based systems in North America, the
irrigated rice-wheat systems of Pakistan, northern India, Ne-
pal, and China, rain-fed wheat in central Europe, and the
cereal production areas in favorable rain-fed regions of Ar-
gentina and Brazil.
Yield Potential
The gap between average yields presently achieved by farmers
and yield potential is determined by the yielding ability of
available crop varieties or hybrids and the degree to which crop
and soil management practices allow expression of this genetic
potential. Maintaining a sizable yield gap is crucial for sus-
taining steady increases in average crop yields as can be seen
in the example of rice production in South Korea (Fig. 14.
Knowing the rate of gain in yield potential and the physiolog-
ical basis for these gains in the past 30 years provides insight
about future prospects.
Evans (11) defines crop yield potential as the yield of a
cultivar when grown in environments to which it is adapted,
with nutrients and water nonlimiting and with pests, diseases,
weeds, lodging, and other stresses effectively controlled. Al-
though this definition seems straightforward, it is difficult to
measure yield potential under actual field conditions because
it is impossible to eliminate all abiotic and biotic stresses.
Hence, a more functional definition of yield potential is the
yield obtained when an adapted cultivar is grown with the
minimal possible stress that can be achieved with best man-
agement practices. Although there is some imprecision in the
specification of minimal possible stress and best management
practices, crop simulation models can provide reasonable
estimates of functional yield potential in a given environment
based on the physiological relationships that govern plant
growth and development (6, 12~. In irrigated systems, yield
potential primarily is determined by solar radiation and ther-
mal regime during crop growth. A water-limited yield potential
also can be simulated for rain-fed systems by accounting for the
water balance of the system.
The largest contribution to the increased yield potential of
modern wheat and rice varieties came from the increase in HI.
In both crops, there was a quantum leap in HI from introduc-
tion of dwarfing genes into the new varieties developed the
1960s. There has been little further increase in HI of rice,
which is about 0.50-0.55 in recently released varieties (13), and
the scope for continued increases is limited by the need to
maintain sufficient leaf area and stem biomass for interception
of solar radiation, physical support, and storage of assimilates
and N used in grain filling. Recent wheat cultivars appear to
have a relatively low HI of 0.41-0.47 when grown with
irrigation in California and Mexico (14, 15), and a further
increase in HI might be feasible. In maize, increased HI has
contributed little to the genetic yield gains of modern hybrids
(164.
With relatively little possibility for increases in HI, greater
yield potential must come from increases in net primary
productivity. Heterotic vigor has been heavily exploited during
the past 50 years of maize breeding. Hybrid rice provides about
a 7-10% yield advantage compared with the best inbred
varieties when grown at yield potential levels (17~. Although it
Proc. Natl. Acad. Sci. USA 96 (1999)
has been widely adopted in China, hybrid rice technology is in
the early stages of testing and commercialization in other
Asian countries. Development of hybrid wheats also may
deliver an increase in yield potential, but it remains in the
experimental phase because of high seed production costs.
Switching from inbreds to hybrids provides a one-time boost
to yield potential on the order of 10%. Thereafter, further
increases in yield potential depend on an increase in canopy
photosynthesis per unit of intercepted light or a decrease in the
metabolic costs of synthesis and maintenance of carbohy-
drates, proteins, and lipids. There is little compelling evidence,
however, that plant physiologists or breeders have been suc-
cessful at increasing the assimilatory or metabolic efficiencies
of the major cereal crops (11~. Some argue that the processes
governing radiation use efficiency, a parameter that integrates
both photosynthetic capacity and metabolic costs, are conser-
vative and therefore offer little opportunity for improvement
through genetic manipulation (12~.
A growing body of evidence suggests that much of the
observed genetic gain in yield during the past 30 years can be
attributed to greater stress resistance rather than an increase
in yield potential. In large part, this change in perspective
results from greater recognition of factors that confound
interpretation of side-by-side comparisons of old and new
cultivars or hybrids. Such studies have provided most of the
estimates of genetic gain in yield potential. Table 2 summarizes
recent reports in which rates of gain in both yield potential and
resistance to stress were evaluated (13, 15, 17, 18~. For tropical
rice and temperate maize, these reports suggest that there has
been no detectable increase in yield potential although steady
progress has been made toward improving stress tolerance.
Tollenaar (16) also concluded that resistance to multiple
stresses has contributed most to genetic yield gain of temperate
maize hybrids used in southern Canada. Only wheat has shown
a genetic gain in both yield potential and stress resistance. It
also should be noted that the rate of genetic gain in yields has
been mostly linear for each of these cereals, regardless of
whether it results from an increase in yield potential or stress
resistance. Hence, the relative rate of increase has decreased
with time.
In summary, breeders have greatly improved stress resis-
tance in each of the major cereals but have been less successful
in pushing out the yield frontier. For tropical rice and tem-
perate maize, the exploitable yield gap appears to be closing.
During the past 30 years, breeders have relied on empirical
selection for yield as their primary selection criteria by using
a brute-force numbers approach. While the use of molecular
markers should improve breeding efficiency for increased
Table 2. Annual rate of genetic gain in yield of wheat and rice
varieties and maize hybrids in relation to year of release when
grown without full control of biotic and abiotic stresses (with stress)
or at yield potential levels (minimal stress)
Rate of genetic yield gain,
annual percentage rate
With stress Minimal stress
Crop and region Period Oldest Newest Oldest Newest
Bread wheat, 1962-88 2.6
NW Mexico
Tropical rice, Philippines 1966-95
Temperate maize
Iowa, USA
Nebraska, USA
1.6 1.0 0.8
1.0 0.8
O O
1967-91 1.2 0.9
1983-97 0
o
Rates of gain are linear in relation to year of release and therefore
are computed separately in relation to the yield levels of the oldest and
most recent (newest) releases. Yield potential with minimal stress has
not changed in rice and maize during the period of study (13, 15, 17,
18~.
OCR for page 5955
Colloquium Paper: Cassman
resistance against diseases and insect pests and for improving
the fit of genotype to the environment, it is not clear whether
marker-assisted breeding can contribute to raising the yield
potential ceiling in nonstress environments. Seed yield is under
complex genetic control involving thousands of genes. The
notion that "yield genes" can be concentrated by marker
selection to achieve greater yield potential remains to be
proven. In contrast, breakthroughs in molecular biology now
allow new approaches for manipulating and investigating the
genetic controls on key assimilatory and metabolic processes
that govern radiation use efficiency. While these possibilities
are exciting, such work is in the early stages of experimenta-
tion. Therefore, the most likely scenario for yield potential of
the major cereal crops is one of small, incremental increases
during the next three decades, and these modest improvements
will require considerable research investment. Nonetheless,
even small increases in yield potential represent important
contributions to global food security.
Soil Quality
Soil quality, like yield potential, is an elusive concept that is
difficult to define and measure. Definitions of soil quality in
recent literature stress the capacity to support biological
productivity, maintain environmental quality, and promote
plant and animal health (19~. Despite this broad definition, it
can be argued that the specific soil properties that support crop
productivity, such as nutrient reserves, water holding capacity,
and favorable structure for root growth, are the same prop-
erties that contribute to the environmental services that soils
furnish. These soil properties include: physical attributes such
as the size and continuity of pores, aggregate stability, imped-
ance, and texture, which together determine soil structure;
chemical properties such as organic matter content and com-
position, nutrient stocks and availability, mineralogy, and the
amount of elements and compounds that are deleterious to
plant growth; biological attributes such as the quantity, activ-
ity, and diversity of microbial biomass and soil fauna.
A reduction in soil quality as a result of human activities can
be defined as soil degradation. Water erosion, wind erosion,
chemical degradation (including nutrient depletion and loss of
organic matter, salinization, acidification, and chemical pol-
lution), and deterioration of physical properties are the four
major types of soil degradation. One recent study, which
involved 250 scientists from 21 regions, estimated the global
extent, severity, and causes of soil degradation (20~. Total area
with some form of soil degradation was estimated to be about
2,000 million ha. Inappropriate farming methods, deforesta-
tion, and overgrazing were identified as the primary causes.
Water or wind erosion was estimated to have affected 84% of
the total degraded area. More than 80% of all degraded land
was located in Africa. Asia, and South and Central America.
About 60% was found in dry-land regions poorly suited for
intensive agriculture. Because the production practices and
physical processes that cause erosion are well understood,
technical solutions to prevent this kind of degradation are
available. Barriers to adoption often involve issues of land
tenure, access to credit and inputs, and other socio-economic
factors. Efforts to encourage adoption of erosion control
practices are crucial to improve the local food security and
welfare of people who live in erosion-effected areas. Likewise,
prevention of erosion in upland watersheds that feed major
irrigation systems can have an impact on food production
capacity in highly productive lowland areas as a result of
reduced sedimentation in reservoirs and irrigation systems.
This sediment load increases maintenance costs of irrigation
infrastructure and reduces reservoir storage capacity, which
can result in water shortages in highly productive irrigated
areas.
Proc. Natl. Acad. Sci. USA 96 (1999) 5955
Erosion also can be a problem in favorable rain-fed regions
with good soils and adequate rainfall for crop production.
Much of the crop land in the north-central USA falls into this
category but soil conservation methods have been developed
to prevent erosion. No-till and reduced tillage systems main-
tain crop residues on the soil surface and protect against the
direct impact of raindrops and increase infiltration rates. As a
result, a greater proportion of incident rainfall is stored in the
soil profile while both runoff and erosion are reduced. Long-
term experiments also indicate that these soil conservation
practices help maintain soil quality by stabilizing soil organic
matter content at higher levels than with conventional plowing
(21~.
In addition to erosion, the Oldeman study (20) estimates 555
million ha have undergone various forms of chemical and
physical degradation not directly associated with erosion. For
most forms of chemical degradation, the governing processes
and methods of prevention and restoration are well under-
stood. Salinization (22) and acidification (23) fall into this
category, as do human-induced soil toxicities which are side
effects of salinization, acidification, or pollution. Although
these kinds of degradation can be remedied, the cost can be
prohibitive as degradation becomes severe. Prevention is the
key. Nutrient depletion and loss of soil organic matter in
cropping systems that receive little or no nutrient inputs as
fertilizers or manure are also straightforward to diagnose and
correct given access to nutrient sources, purchasing power, or
credit. For example, soil degradation by nutrient depletion
occurs in traditional slash and burn systems practiced by
subsistence farmers in the forests and savannas of the humid
and subhumid tropics where the fallow period is decreasing
because of population pressure (24~. Here again, technical
solutions are available and the major constraints to adoption
are mostly social, political, and economic in nature.
Although blatant forms of degradation occur largely in areas
with poor soils or unfavorable climate and there are technical
solutions to prevent these problems, it would be a mistake to
conclude that soil degradation is not a major threat to food
security. Instead, subtle and complex forms of soil degradation
can occur in some of the world's most productive agricultural
systems, and it is argued here that these less obvious forms of
degradation may become an increasingly important constraint
to food production capacity in the next century. Moreover, it
is unlikely that the previous estimates of soil degradation (20)
account for subtle forms of degradation in high-production
systems because the extent and causes of such degradation
have only recently been recognized.
The yield decline phenomenon that occurs in a number of
long-term experiments with annual double- and triple-crop
irrigated rice systems is an example of faint changes in soil
properties that can have a large impact on productivity (25~. It
appears to result from a cascade of effects associated with a
subtle change in soil organic matter chemistry. These effects
have been studied in several long-term experiments in the
Philippines that were initiated in the 1960s concurrent with the
release of IR8, the first widely grown modern indica rice
variety in Asia. In one study, for example, six rice varieties were
grown each season in replicated treatment plots with different
amounts of applied N. All other nutrients are supplied as
required. Weeds and insects were controlled by using recom-
mended practices. From 1968 to 1991, IR8 was one of the six
varieties grown while the other five entries were continually
changed to represent the best available International Rice
Research Institute germplasm at each point in time. A yield
decline occurred in all three cropping seasons of the year and
at all levels of N addition (26~. In the dry season when solar
radiation and temperature provide conditions for greatest
yield potential, yield of the highest-yielding varietal entries
decreased by 130 kg ha- ~ yr- ~ in the treatment receiving the
OCR for page 5956
5956 Colloquium Paper: Cassman
greatest amount of N fertilizer, which was also the highest
yielding N treatment (Fig. 2~.
The yield of IR8 decreased more rapidly and displayed
considerably more variability than the highest-yielding variet-
ies. Greater yield stability of the newer varieties resulted from
greater resistance to fungal and bacterial diseases and to insect
vectors that transmit viral diseases. Eventually IR8 could not
be adequately protected from viral infection despite the use of
recommended insecticide control measures for the insect
vectors. In contrast, this same level of pest control was
adequate to protect the newer varieties from viral infection.
Hence, the difference in yield and the greater yield stability of
the highest-yielding varieties compared with IR8 provides an
estimate of the genetic gain breeders have made in resistance
to pests as discussed in the previous section.
Even with greater disease and insect resistance, however,
yield of the newest varieties also declined with time although
yield potential of the more recent varieties was no different
from IR8 (ref. 13; Table 2~. Subsequent studies demonstrated
that yields could be restored to yield-potential levels when
additional N fertilizer was added and the crop was protected
against sheath blight (Rhizoctonia solani) (26, 27~. Both the
incidence and severity of this disease increase when plants are
supplied with sufficient N to achieve yield-potential levels in
the humid tropics (28~. These results indicated that soil N
supply capacity had decreased because yields could be restored
by application of additional N in combination with fungicide
but not with fungicide alone. This reduction in soil N supply
was not associated with a decrease in soil organic matter or soil
N content, both of which increased in the treatments that
received the greatest amount of applied N (26~. In fact, the
conservation or increase in soil organic matter appears to be
a common feature of continuous, irrigated double- and triple-
crop rice systems although the chemical structure of the most
labile soil organic matter fractions become enriched with
phenol subunits (29, 30~. It is hypothesized that the decrease
in soil N supply was caused by the accumulation of phenol C
in these youngest humic fractions, and that phenol enrichment
10
8
· ~
~0 0
_ ~ ~o
O
6
He 4
ci:
car
DRY SEA SO/I/
Y= 8.2-0.13X
r= 0.82***
~. . ~
o
1 _ ~e
Y =7.8 - 0.21 X
r = 0.66***
.
a
~ O
O ~
.
~ ~ -.
· · ·~0
2 - . · HIGHEST YIELDING
CULTIVARS
o--o IR8
r, 1 1 1 1 1 1 1
~, .
o
o
~O
''A
o
1 1 1
68 70 72 74 76 78 80 82 84 86 88 90
FIG. 2. Yield trends from 1968 to 1990 of the highest yielding rice
cultivars and IR8 in a long-term continuous cropping experiment at
the International Rice Research Institute in the Philippines. Repro-
duced from ref. 25 with permission of John Wiley & Sons, Ltd.,
London.
Proc. Natl. Acad. Sci. USA 96 (1999)
results in a reduced rate of N mineralization from these organic
matter pools (27, 29~.
Companion studies have found that phenol accumulation in
young humus appears to be a characteristic feature of irrigated
rice systems in the tropics, and it appears to reduce the N use
efficiency of the system. A recent field study demonstrated that
incorporation of rice stubble during the fallow period when soil
is aerated, instead of the standard practice of incorporation in
flooded soil, can increase the soil N supply to a following rice
crop (31~. This finding is consistent with the phenol accumu-
lation hypothesis and suggests that tillage and residue man-
agement practices can be modified to increase soil N supply by
allowing phenol oxidation and thereby reducing requirements
for applied N.
Further work is required to fully elucidate the factors
responsible for the yield decline in long-term experiments on
intensive rice systems. More important, however, is to deter-
mine the extent of this phenomenon in farmers' fields. Making
this assessment is difficult because soil N supply could decrease
in a farmer's field, just as it does in the long-term experiments,
but it would be not be detected if farmers applied increasing
amounts of N to maintain yields (324. Although this situation
would mean a decrease in yield per unit of applied N. many
other production practices and environmental conditions also
affect N fertilizer efficiency and farmers are not likely to notice
the reduction in N fertilizer efficiency. Macronutrients other
than N and perhaps micronutrients also may become limiting
at high yield levels once the N constraint is alleviated (33~.
These non-N nutrients may not be limiting at the reduced yield
levels achieved when the crop was N deficient, but they become
limiting at higher yield levels made possible by improved plant
N nutrition. Nutrient balance studies clearly indicate that soil
potassium is being depleted in most irrigated rice systems in
Asia at present levels of K inputs and outputs.
There is also evidence of subtle forms of soil degradation
occurring in other major high-production cereal production
systems, including rice yield declines in long-term experiments
on rice-wheat systems in India (34, 35) and in a no-till
continuous corn system in the USA (36~. In both systems,
declining yield trends also were associated with an increase in
soil organic matter although present understanding asserts
that soil organic matter content is positively correlated with
soil quality. It is also noteworthy that there is no evidence of
a positive maize yield trend during the past 25 years in
long-term experiments conducted in the north-central USA
despite regular replacement of hybrids. Each of these exper-
iments include both irrigated and rain-fed maize systems, with
and without crop rotation, and at least one treatment receives
recommended nutrient inputs and crop management practices.
In all of these experiments, however, mean yield is well below
yield potential levels, which indicates the crop is exposed to
stress of some kind during the growing season. This notable
lack of a positive yield trend is inconsistent with the steady
genetic improvement in stress tolerance of maize hybrids (refs.
16 and 17; Table 2~. Subtle forms of soil degradation could
account for the lack of a positive yield trend in these long-term
experiments, although, to date, there is no direct evidence to
support this hypothesis and there has been little effort to
investigate it. In contrast to rice and maize, positive yield
trends in wheat yields during the past 25-30 years can be found
in a long-term experiment conducted in India and another in
northwest USA (37~. These trends are consistent with the
steady increase in wheat yield potential (Table 24.
Subtle changes in soil properties and subsequent effects on
yield and input requirements illustrate the complexity of the
relationships between soil quality and cropping system per-
formance. It is postulated that subtle forms of soil degradation
are occurring in some of the most important cereal production
systems in the world. It is further argued that an increase in soil
quality will be required to achieve sustained yield increases of
OCR for page 5957
Colloquium Paper: Cassman
1.2-1.5% annually for the next 30 years because increased
inputs of energy, nutrients, water, and pest control measures
are required to offset a decrease in soil quality (Fig. 3~. In
addition, more sophisticated management practices are
needed to apply the additional inputs properly because soil
degradation reduces the resource buffer provided by good soil
quality and decreases the margin of error for nearly all crop
management practices, especially in high-yield systems. Con-
sequently, identifying the critical thresholds for specific soil
properties that have the greatest influence on productivity is
an important, but neglected, scientific quest. Although there is
considerable research interest in the assessment of soil quality
(38), most of this work is descriptive and does not attempt to
quantify relationships between specific properties and crop
productivity. A thorough understanding of the rate and causes
of change in soil quality and subsequent effects on yields and
input requirements will be required to sustain yield increases
in the major high-production cereal systems.
Precision Agriculture
The gap between average farm yields and the yield potential
ceiling must shrink during the next 30 years because the yield
potential of tropical rice and maize appears to be stagnant and
wheat yield potential is increasing more slowly than the
expected increase in demand (Tables 1 and 2~. Hence, achiev-
ing consistent cereal yields that exceed 70~o of the yield
potential barrier depends on sophisticated management of soil
and water resources and applied inputs. A precision agriculture
approach is required to insure that the requisite resources for
crop growth are available and crop protection needs are met
without deficiency or excess at each point in time during the
growing season. Precision management can be applied uni-
formly to an entire field by exact timing and placement of a
particular field operation, or it can involve site-specific man-
agement within a field to account for variation in soil prop-
erties, crop resource requirements, pests, and disease.
Site-specific management that relies on variable application
of an input or management operation is most relevant to
large-scale agriculture in which field size and within-field
variation are great enough to justify the cost of needed
equipment. In most developed countries, this technology is
presently available and allows application of seed, nutrients,
water, and pest control measures to meet the specific require
YA
YE
Ye
YD
Original soil
\ A
~' ,,_
_.~D
Decreased soil
quality
Ix
INPUTS Increasing
FIG. 3. Conceptual model illustrating the relationship between
crop yields and input requirements as influenced by soil quality. A
decrease in soil quality from an initial state (curve A) can result in the
need for greater inputs of energy, nutrients, water, seed, and pest
control measures to achieve the same yield. The slope and asymptote
of the shifted response (shown by curves B. C, and D) depend on the
type of soil degradation and can result in a reduction in input use
efficiency, yield potential, or both.
Proc. Natl. Acad. Sci. USA 96 (1999) 5957
meets at each location within a field. Theory predicts increas-
ing fertilizer use efficiency from site-specific versus uniform
nutrient application as the magnitude and negative skewness of
variation in native soil nutrient supply increases and as yield
levels approach the yield potential ceiling (39~. Simulations
also predict a reduction in nitrate leaching from improved N
fertilizer efficiency with site-specific management (40~.
In practice, validation of theory has been difficult to achieve.
One study, which compared uniform versus site-specific N
application to irrigated maize, found a significant yield in-
crease from site-specific application in one of 12 site-year
comparisons and a negative yield response in another (41~. No
yield difference was observed in the other 10 comparisons, and
the amount of N applied was similar using both methods.
Despite detailed soil sampling to develop the site-specific N
application guidelines, the authors attributed the lack of
response to the inaccuracy in prediction of N fertilizer re-
quirements by present methods of soil testing that do not
account for the dynamic controls on soil and fertilizer N
availability or crop N requirements.
Although theoretical estimates of economic and environ-
mental benefits from site-specific deployment of variety or
hybrid, plant density, nutrients, pest control measures, and
irrigation are large, successful implementation by farmers will
require accurate data about the spatial variability in soil
properties, pest and disease incidence, and crop physiological
status, as well as exact knowledge of crop response to this
variability. Remote sensing capabilities are under development
that may improve the accuracy and reduce the cost of real-time
measurements of spatial variability in crop physiological status
and pest pressure. In contrast, detailed knowledge of the
ecophysiological processes governing crop response to inter-
acting environmental factors is not sufficiently robust to make
accurate predictions of site-specific input requirements or the
expected outcome from their application. This knowledge gap
is the key limiting factor to adoption of site-specific manage-
ment in large-scale agricultural systems in developed coun-
tries.
In developing countries, the need for precision agriculture
also will be crucial to achieve cereal yield increases that must
approach yield potential levels in the major production sys-
tems. Because field size is typically less than 0.5 ha, precision
agriculture will involve field-specific management practices.
Recent on-farm studies of double-crop rice systems in several
Asian countries document tremendous field-to-field variation
in native soil N supply within small production domains in
which soil properties are similar. For example, grain yields
without applied N ranged from 2,400 to 6,000 kg ha-i in 42
different rice fields surrounding one village in the Philippines,
and the variation was attributed to differences in soil N supply
(Fig. 4~. Similar results have been obtained in other major rice
production centers in southern India, Indonesia, Thailand, and
Vietnam where double- and triple-crop rice systems are the
dominant food production system (42~. In all cases, the large
variation in soil N supply was not associated with differences
in soil organic matter content, total N. or other measures of
soil N availability (32), which is consistent with results from the
long-term experiments on double- and triple-crop rice systems.
Hence, the same processes that account for the subtle changes
in soil organic matter composition in the long-term experi-
ments also may influence soil N supply in areas where con-
tinuous irrigated rice cropping systems are the dominant cereal
production system.
Given the tremendous variation in soil N supply among
fields with similar soil types and crop management practices,
field-specific N fertilizer requirements will be needed to
optimize yield and profit, and to minimize N fertilizer losses.
Related studies have identified large field-to-field variability in
soil P and K supply. Field-specific management also will be
needed for these nutrients. As is the case for soil N supply,
OCR for page 5958
5958 CO110qUiUm Paper: CaSSman
121 1
11
O
1992 Dry Season
n = 42
2400 3000 3600 4200 4800 5400 6000
Grain yield without applied N (kg ha-1)
2.
3.
FIG. 4. Frequency distribution of rice grain yield without nitrogen
fertilizer application in 42 different farmer's fields in the Guimba 4.
Municipality, Central Luzon, Philippines. Reprinted from ref. 32 with
permission from Elsevier Science.
5.
present soil-test methods do not accurately predict soil P or K
availability, or the yield response from P or K application (33~.
One would expect a similar degree of field-to-field variation in
soil N. P. and K supply will be found in the irrigated rice-wheat
systems of Asia because rice also is grown in flooded soil in
these systems.
Most government and regional extension systems in Asia
presently provide blanket fertilizer recommendations for large
production domains of wheat and rice. Given the magnitude of
field-to-field variation within small areas with similar soil type,
this approach results in inefficient use of nutrients and will
limit efforts to increase average farm yields because of nutrient
deficiencies, excesses, and imbalances. On the other hand, the
need to achieve field-specific nutrient management in hun-
dreds of millions of small rice and wheat fields in low-income
countries is a daunting challenge. Field-specific pest manage-
ment also will be needed to protect crops from insect pests and
diseases while minimizing the use pesticides. In addition, the
incidence and severity of a number of major diseases and insect
pests increase when cereal crops are provided sufficient N to
achieve the lush canopy required for high yield levels. Devel-
oping the scientific capacity, technology transfer mechanisms,
and farmer education to allow diagnosis of limiting factors,
prediction of expected yields and input requirements, and
implementation of field-specific management will be a crucial
component of food security in these low-income countries that
rely on high-production cereal systems.
Conclusions
The projected increase in food demand and the goal of
minimizing expansion of cropped area dictates that cereal
yields will approach the yield potential ceiling in many of the
world's most productive cropping systems within the next three
decades. Ecological intensification of these high-production
cropping systems is fundamental to achieving food security
under this scenario and raises several questions: What is the
prognosis for increasing the yield potential of wheat, rice, and
maize and by how much? What is the direction of change in soil
quality in the major cereal production systems and how will
these changes affect crop productivity? Can crop and soil
Proc. Natl. Acad. Sci. USA 96 (1999J
management practices be improved to achieve consistently
high yields while meeting acceptable environmental stan-
dards? It is argued that the present state of knowledge is far
from sufficient to answer these questions despite the need for
answers and widespread application of this knowledge within
a relatively short timeline.
It is concluded that global food security 30 years hence will
depend on rapid scientific advances in understanding the
physiological basis of crop yield potential, the processes gov-
erning the relationship between soil quality and crop produc-
tivity, and plant ecology related to the many interacting
environmental factors that determine crop yields. Achieving
these scientific advances is possible, but present levels of
investment in these specific research areas, both in the USA
and elsewhere, are not adequate to meet the challenge.
I am grateful to Daniel T. Walters for his insightful comments and
suggestions on an earlier draft of this manuscript.
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Representative terms from entire chapter:
soil quality
