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1
Dimensions of Precision Agriculture
The management of agricultural production is undergoing a change, both in
philosophy and technology. Until recently, agricultural managers have generally
made decisions regarding fields based on average conditions within those fields,
with data that was often sparse and qualitative in nature. Soil fertility was deter-
mined by compositing soil cores into a single sample that was intended to best
describe conditions across a field. Field scouting for crop condition or pest infes-
tations was done at a few locations within the field, and observations often have
been more qualitative than quantitative. For the most part, whole fields have been
considered to be the basic agricultural production units, and have been managed
for the mean condition or, in the case of pest management, managed intensively
to overcome variability within that field.
Historically, a desire to improve production efficiency and farm income has
stimulated interest in innovative technologies. Advances in technology, as well
as other factors such as farm policy have contributed to increases in the size of
individual farmsteads and fields within a farmstead. With this larger scale of
operation, the potential for the individual to effectively manage variability by
observation and experience has declined precipitously. In addition, as individual
farm fields increased in size, within-field variability has generally increased. A
major feature of today’s precision agriculture is that it allows producers to man-
age previously unmanaged variability as well as the increased variability result-
ing from increased field size. In other words, precision agriculture will allow
several geographic units currently being managed as a single entity (a field) to be
addressed as individual decision-making units. Managers will be able to respond
to the distinctive agronomic characteristics that exist within the subunits, in con-
trast to today’s approach of addressing the average needs of several units or ex-
treme conditions in parts of the field, such as pest outbreaks in small patches.
16
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DIMENSIONS OF PRECISION AGRICULTURE 17
The incorporation of information technologies into agricultural production
practices began in the mid-1980s and has increased sharply in recent years. While
the use of information in agricultural decision making is not new, agriculture is
experiencing a vast increase in the amount of information available, and in the
timeliness and means by which information can be collected, analyzed, and used
to manage inputs and outcomes of agricultural practices. The application of new
information technologies in agriculture is known by several terms, including pre-
cision agriculture, precision farming, and site-specific management. A variety of
definitions have been offered for the concept of integrating information technolo-
gies with agronomic practices. Most authors have focused on the ability to obtain
data and to vary production inputs on a subfield basis. While this is an important
aspect, there are other geographic scales at which information can be obtained
and used to facilitate site-specific management. The committee chose to view
precision agriculture broadly, adopting the following definition:
Precision agriculture is a management strategy that uses information tech-
nologies to bring data from multiple sources to bear on decisions associated
with crop production.
A key difference between conventional management and precision agricul-
ture is the application of modern information technologies to provide, process,
and analyze multisource data of high spatial and temporal resolution for decision-
making and operations in the management of crop production. Advances in the
technologies will be an evolutionary process and they will continue to be adapted
for agricultural decision making.
Precision agriculture has three components: capture of data at an appropriate
scale, interpretation and analysis of that data, and implementation of a manage-
ment response at an appropriate scale and time. Each particular manageable fac-
tor has its own scale of variability. Area-wide management of insects and weather
forecasting for crop management decisions are examples of variables that are
managed at a scale larger than the individual field. Other factors like soil fertility
and pest distributions can vary significantly at the subfield level and over the
growing season. Therefore, it is natural and important to perceive precision agri-
culture in terms of finer spatial or temporal units of decision making.
PRECISION AGRICULTURE AND
AGRICULTURAL MANAGEMENT
Advances in information technology and their application in crop produc-
tion, which are labeled as precision agriculture in this report, are creating the
potential for substantial change in management and decision making in agricul-
ture. The word potential in the previous sentence is critically important. The vari-
ous technologies and practices that will make up tomorrow’s precision agricul-
ture are only emerging, being tested and refined, and implemented or rejected
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18 PRECISION AGRICULTURE IN THE 21ST CENTURY
today. This process is further enhanced by the dynamic nature of advances in
information technology. A capability that is technically or economically unfea-
sible can become feasible as a result of a technological innovation occurring well
outside the arena of agricultural technology development or agricultural research.
Thus the process by which precision agriculture is adopted could be fragmented
and discontinuous. Therefore, it is impossible to specify the precise dimensions
and characteristics of the precision agriculture of the future.
Precision agriculture could materially affect on-farm decision-making pro-
cesses that depend on implied knowledge gained by observation and experience.
While its precise dimensions continue to evolve, the following features character-
ize most precision agriculture applications in use or under development:
• Data capture tends to be electronic, automated, and relatively inexpensive.
• Data capture can occur more frequently and in more detail.
• Information, either captured as a part of field operations or purchased
externally, can be considered separate input into the production operation.
(It is also a feature of integrated pest management and sustainable agricul-
ture concepts.)
• Data interpretation and analysis can be more formal and analytical.
• Scientific decision rules are applicable to actual farming operations.
• Implementation of the response can be more timely and more site specific.
• Performance of alternative management systems can be quantitatively
evaluated.
The long time lags between input decision making, application of inputs, and
observation of yields in crop production systems make it difficult to evaluate
decision-making effectiveness. The chance for misinterpreting results is further
heightened when inputs and outcomes are observed rather than measured. The
difficulty of learning in such settings is not constrained or unique to farmers.
Considerable research has documented that human decision making is more likely
to suffer bias and misinterpretation when (1) feedback loops are long between the
time the decision is made and the outcome occurs and (2) cause/effect linkages
are not simple (Einhorn, 1980; Hogarth and Makridakis, 1981). These two char-
acteristics apply to traditional crop production settings.
The uncertainties associated with the rapid evolution of information tech-
nologies and the dynamics of the process of adopting precision agriculture repre-
sented a significant challenge in the preparation of this report. However, these
same uncertainties provided considerable excitement and a sense of mission for
the project. Tomorrow’s precision agriculture will be significantly affected by
actions in the public and private sectors today.
The focus of this committee, therefore, was not on predicting a single future.
Rather, members chose to recognize the uncertainties inherent in the future evo-
lution of precision agriculture and to emphasize possible paths and the implica-
tions of those paths. Further, the study recommendations define key actions that
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DIMENSIONS OF PRECISION AGRICULTURE 19
society can undertake to extend the dimensions of precision agriculture where
they are deemed most desirable.
GEOGRAPHIC CONTEXT: SCALES IN THE SPATIAL SPIRAL
Agricultural production systems vary in many ways, including scale of op-
eration, commodities produced, and philosophical approaches to management.
Current production systems draw on diverse approaches and knowledge bases.
For any approach, information technologies will play an increasingly important
role in agricultural production and natural resource management. This impact
will be felt directly through the coupling of newly acquired information with
recently developed tools for agricultural production, on-demand products and
services, and increased access to information and services.
A number of scales characterize crop production systems of today. These
scales might be viewed as a continuum ranging from individual plants in a field to
plant populations, fields, farmsteads, and regions. Others have used this Lewin-
Kolb model of hierarchies as an organizational structure to study complex issues
such as pesticide regulation and diversity in agroecosystems (Olson et al., 1995).
Consider this continuum in the form of a spatial spiral ascending from the sub-
field to national geographical levels (Figure 1-1). As we move up the spiral we
Communication Threads FIGURE 1-1 Scales in a spiral.
A number of scales characterize
crop production systems of to-
day. In precision agriculture, an
unprecedented amount of spatial
Country and temporal data may become
available at the individual plant,
Ecoregion
farm, and regional scales. At
each scale various processes will
influence crop production. A
County goal is to determine an optimal
scale for data collection and
Farmstead management response. Commu-
nication technologies will pro-
vide connecting threads up and
Field
down the spatial spiral.
Population
of plants
Individual
plant
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20 PRECISION AGRICULTURE IN THE 21ST CENTURY
move from individual plants to fields and regions. Fresco (1995) underscores the
need to relate phenomena or outcomes to processes occurring at both higher and
lower level scales. The goal is to determine an optimal scale at which each pro-
cess is to be studied, one in which variability is minimal. For example, if plant
population is dependent upon small-scale variation in soil physical and chemical
properties, then varying seeding rates may require information and hardware ca-
pable of rate changes every few centimeters. Such information may reside locally
in a nearby computer in a farmhouse. At a wider scale, real-time weather infor-
mation collected from locally placed weather stations may provide irrigation or
area-wide pest management information in a timely manner to improve decision
making for a field, farmstead, or county.
Communication technologies will provide connecting threads up and down
the spatial spiral. Telephone, high-speed digital lines, and wireless communica-
tions are needed to link the various levels together. For example, digital data
could be collected by an on-the-go yield monitor in a combine, sent via a wire-
less cellular link to the operator’s home computer, and retrieved via a high-
speed Internet connection by an agricultural chemical dealer. The dealer may
then add the yield data to a nutrient management analysis and send recommended
fertilizer application rates for various subfield units back to the farm operator’s
computer.
Different scales of assessment are being used to investigate aspects of crop-
environment systems. Scale can be considered for both information sources and
management actions. Depending on the situation, data from different scales may
be combined and used to determine management actions at another scale. For
example, a producer deciding what crop and variety to plant (field scale) may
consider the available forward contracting prices (national or global scale), the
availability of custom field operations (farm scale), and a field map of soil wa-
ter-holding capacity (subfield scale). With precision agriculture methods, such
decisions can be made with more objective data. Some of the uncertainty factors
can be reduced with the information technologies of precision agriculture, al-
though the extent to which this will be feasible and of value to the grower is not
clear.
Information technologies permit the modern producer to obtain detailed spa-
tially explicit information at the scale of entire farms but with information suffi-
cient for efficiently managing the land at the fine scale. Most of the new precision
agriculture technologies can be used to disaggregate information—for example,
to characterize soil, yield, nutrients, and water variation within fields—as well as
to assemble regional information. Perhaps the ultimate disaggregation would be
to look at agricultural fields as a collection of individual plants. The extent to
which data are disaggregated or reassembled for different spatial units depends
on the nature of the management problem and the resolution of the data gathering
techniques. Decision makers will need to consider the spatial heterogeneity of the
area being managed and the relative value of the information. (A brief review of
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DIMENSIONS OF PRECISION AGRICULTURE 21
the impact of information technologies on current management decision processes
can be found in Chapter 2.)
Subfield Management
The potential for individually managing small areas, whose size is deter-
mined by local characteristics and crop value, is one of the most enticing aspects
of precision agriculture. The ability to repeatedly locate a specific site and mea-
sure agronomic characteristics provides an opportunity to optimize management
throughout the production area. Subdividing a field into small management units
may improve both the economic and environmental sustainability of crop pro-
duction systems.
The earliest advocates of precision agriculture took the approach that man-
agement decisions should be based on soil characteristics, assuming that similar
soil series could be managed as homogeneous units. Subsequent research showed
that for many soils, nearly the same nutrient variability exists within the mapped
soil series as among them (Sadler et al., 1995). Even precise management based
on variability of the physical and chemical properties within soil types may or
may not be sufficient for optimal management of crop production activities.
As producers try to manage smaller areas, the law of limits comes into play
more strongly. For any given site, from year to year, the most limiting factors to
crop growth can change from nutrient or moisture availability (deficit or excess),
to disease or insect pests, to weather factors. In fact, the limiting factor may
change within the growing season as the crop matures and its needs change. For
improved decision making, managers must be aware of the limiting factors for
each subfield unit and be able to modify management at that scale. The determi-
nation of the most limiting factors is currently both difficult and expensive, and
these costs are considered by decision makers. All of these concerns point to the
need for analytical systems and technologies that can determine the important
factors and decision-support systems that can use available data.
Some management factors exhibit a relatively small amount of variability. For
example, levels of less mobile soil nutrients (i.e., potassium and phosphorus) may
exhibit little variation in crop response within some fields that have received heavy
fertilizer applications for many years. These crops may be subject to greater variabil-
ity from other influences—such as weather, nitrogen, diseases, and insects—particu-
larly if the time frame for assessing the performance of a method is short (i.e., a
single growing season). Similarly, technologies that work well for one cropping
system or biophysical setting may not work in another. Efficacy testing should be
done for a variety of settings and systems and over several growing seasons.
Beyond Subfield Management
It is unarguable that an individual grower’s precision agriculture data has
substantial additional value when combined above the subfield level with similar
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22 PRECISION AGRICULTURE IN THE 21ST CENTURY
data from other production operations. Management strategies consistent with
our definition of precision agriculture are currently practiced, and new strategies
will be developed that address spatial and temporal variability at the scale of the
whole field and larger. While this report focuses on subfield level precision agri-
cultural practices, a discussion of two key larger-scale strategies follows.
Data Warehousing
Large amounts of spatially referenced data on individual fields are, or soon
will be, generated by yield monitors, real-time and remote sensors, on-the-ground
sampling and observation by producers and consultants. This site-specific infor-
mation will have value for use within individual fields in ways discussed in Chap-
ter 2, but will also have value when combined with data on the same variables
collected for nearby fields. Seed, chemical, and machinery agribusiness, among
others, are assisting growers in data collection and interpretation. In a number of
cases, agribusiness is providing financial assistance so growers will share data
with the agribusiness itself. Several companies have promoted a concept of data
aggregation which permits growers as well as an agribusiness free access to
participant’s data. Still others have promoted concepts of data collection in which
data could be purchased by third parties. Many growers have expressed opposi-
tion to any of their data being shared with others. However, most growers do
agree that there is economic value in the learning that results from data sharing
and that may increase the likelihood of vertical integration of agricultural opera-
tions. Though it is unlikely that a commercial interest will freely share informa-
tion to which they have purchased rights and made further investments, other
groups may see benefits from voluntary sharing. Grower clubs such as Practical
Farmers of Iowa have been successful models of farmer-directed research in
which land grant or private sector consultants act as facilitators in planning and
implementing research trials. The idea is for a number of growers to implement
similar practices of interest in their farm operation (i.e., row-spacing, herbicide
dose and timing, cultivar selection) in statistically sound on-farm experiments
(Stroup et al., 1993). In these clubs, data are openly shared to identify desirable
practices in local growing areas. Imagine the same grower clubs now sharing
spatially referenced data from experiments where growers agree to apply similar
agronomic practices. The potential to create locally derived recommendations
from locally collected data is a fascinating prospect. In effect, a version of this
vision is in practice today with the private crop consultant. By working with
numerous growers, the consultant is afforded the opportunity to observe how
diverse recommendations can affect crop fitness, yield, and production efficiency
in farming enterprises as small as several acres to those that extend over thou-
sands of acres. Such an approach would require growers to openly share data with
fellow producers.
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DIMENSIONS OF PRECISION AGRICULTURE 23
Landscape Analysis
There are opportunities to link management decisions at various levels to
improve soil and water quality. The National Research Council’s report on Soil
and Water Quality (National Research Council, 1993) described the inherent links
between farming systems and the landscape. Management practices to improve
input use efficiency and reduce erosion can improve the quality of the surround-
ing watershed. The Committee on Long-Range Soil and Water Conservation rec-
ommended use of landscape buffer zones that connect farms and fields, provide
widespread protection to waterways, and prevent soil degradation. Focusing on
the impact of within-field production practices on adjacent ecosystems changes
the unit of analysis to the landscape scale for studies on agricultural nonpoint
sources of pollution. Landscape analysis considers effects of farming practices on
larger areas than a specific crop field. Coordinating information at various levels
could enhance protection of the environment. For instance, tracking production
practices across a watershed could be useful in targeting areas with soil and water
quality problems (National Research Council, 1993).
Regional Management
The appropriate scale for management will vary according to the factor most
limiting to productivity. Manageable factors such as soil fertility or weed compe-
tition may vary significantly at a subfield level, thus input use can be based on
subfield units. However, there may be more utility in managing other factors at a
field or farm level. For example, because insects migrate over areas larger than a
field, monitoring their movements on a regional scale may be appropriate. Ac-
quiring other regional data also may improve the accuracy of the decision-mak-
ing process.
Information provided to producers that is regional in nature, can have a di-
rect impact on local management decisions. Evapotranspiration is typically moni-
tored using networks of weather stations that cover large areas. Regional data
also interacts with more site-specific data that producers can incorporate into
their decision making. The California Irrigation Management Information Sys-
tem (CIMIS) is a computerized crop weather information system that producers
can access by modem or the Internet to obtain hourly and daily weather condi-
tions. Producers combine regional evapotranspiration data and local soil- and
crop-specific coefficients for their fields to determine the daily water use and
water demand of their farms (see Box 1-1).
It is unclear how to appropriately use data collected at different spatial scales
together to help make better decisions. There are significant statistical and mod-
eling issues to be addressed. Precision agriculture will greatly increase the amount
and perhaps the availability of geographic data snapshots for many cropping
fields, which will increase the demand for these analytical techniques.
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24 PRECISION AGRICULTURE IN THE 21ST CENTURY
BOX 1-1
California Irrigation Management Information System
California has more than 10 years of experience operating the Califor-
nia Irrigation Management Information System (CIMIS), a computerized
crop weather information system that growers can access by modem or
the Internet to obtain hourly and daily weather conditions. The first five
weather stations went on-line for research from May 30th to June 7th,
1982. Stations number one and two were installed in Fresno County,
numbers three and four were installed in Santa Cruz County, and num-
ber five was installed in Kern County. By the end of 1982, 27 stations
were operating. After three years of research and testing, CIMIS was
made available to the general public on July 1, 1985 (Eching et al., 1995;
Eching and Moellenberndt, in press). Ninety CIMIS weather stations are
now in use throughout California, with information generated from a num-
ber of sensors at each site which are directly linked to a computer. The
stations are ground referenced with latitude, longitude, and elevation
readings.
CIMIS is an excellent example of current technology that provides
information on crop water requirements. Growers use the CIMIS weather
system and soil- and crop-specific coefficients for their fields to deter-
mine the daily water use and water demand of their farms. Vendors may
combine these data with data from other sources and provide specialty
products tailored to weather information needs for specific crops.
CIMIS is operated by the State Department of Water Resources in
cooperation with the University of California, local water districts, and
various agencies. The information gathered at each site includes maxi-
mum, minimum, and average air temperatures and relative humidity read-
ings. Data are also collected on precipitation, evapotranspiration, dew
point, vapor pressure, average soil temperature, wind speed and run,
and solar radiation.
Evapotranspiration data represent water loss from soil evaporation
and crop transpiration and referenced to water use for a healthy grass;
values must be multiplied by a crop coefficient developed for various
growth stages. Evapotranspiration data are used as an aid in irrigation
scheduling. Growers and consultants use the information to maintain crop
water-use budgets by comparing how much water has been applied to a
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DIMENSIONS OF PRECISION AGRICULTURE 25
BOX 1-1 Continued
field with how much water the crop is using each day. Water use can be
projected and water can then be ordered from the local irrigation district
for delivery to the field before the crop depletes the available water in the
soil. The crop water-use information does not take into account the appli-
cation efficiency of various irrigation systems, however, nor does it calcu-
late the leaching requirement for salt-affected soils.
Information from CIMIS weather stations used for assessing crop
water requirements is widely disseminated through various means of
communication. Farmers in the San Joaquin Valley can listen to the radio
for daily early morning agricultural reports that include evapotranspira-
tion values and crop coefficients for numerous crops. The information is
supplied to the radio station by agricultural consultants as a service to the
industry.
A CIMIS report is part of a weekly newspaper (Ag Alert) published by
the California Farm Bureau Federation in Sacramento. The weekly refer-
ence evapotranspiration information is shown in a histogram, along with
comparison data from the corresponding week of the previous year and
an average year. Growers with computers and modems can access daily
and weekly evapotranspiration data directly from CIMIS, through several
sites via the Internet, or from the Agri-Tech Information Network main-
tained at California State University-Fresno. Growers can call a contact
at the University of California-Davis for crop coefficient information.
Growers and businesses that subscribe to the Data Transmission
Network (DTN) satellite information service on-line can access daily and
monthly CIMIS weather data for all 90 operating stations in the state. The
computer hardware and satellite dish are owned by the company provid-
ing the service, so there is no need for individuals to invest in expensive
computer equipment.
All levels of producers, regardless of farm size, have many ways to
access the CIMIS weather information. Crop water-use data are avail-
able for the current season and from historical databases, some of which
go back to 1982. The major efforts made by the California agricultural
industry in disseminating CIMIS evapotranspiration data should be used
as an example of how to saturate a production region with important
information which has been shown to aid decision making.
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26 PRECISION AGRICULTURE IN THE 21ST CENTURY
BOX 1-2
The Crop Consultant of Tomorrow
It’s early Friday morning in late June. John pours his first cup of cof-
fee, turns on his computer, and reviews the list of fields he will visit today.
With the click of his mouse, he opens a client list and downloads weed,
insect, and nutrient application maps created by his farmer clients as
they cultivated their corn fields late yesterday afternoon. At the same
time, satellite images of crop greenness are downloaded for 12 fields.
These images complement others collected earlier this year and in pre-
ceding years. When John reads these images into his geographic infor-
mation system, he extracts information about pest risk with several deci-
sion tools for pest management and nutrient use efficiency. John transfers
the information from his kitchen computer to the lap-top in his pickup
truck. Before heading out the door, he reviews the maps of each of his
fields to determine how to best use his three crop scouts that day. On-
the-go sensing supplemented by smart or directed sampling is a very
important part of John’s management efficiency plan and has resulted in
timely crop management decisions which would otherwise have been
missed. After visiting each of the 12 fields, John sits with his farmer client
and reviews summary maps of variability in crop moisture, canopy clo-
sure, and pest pressure. John knows the best decisions are made when
their collective wisdom—his and the farmer’s—is aided by the new types
of information. John knows his clients have diverse opinions and man-
agement philosophies. Some want little help from advanced information
technologies whereas others value the added information.
ENABLING TECHNOLOGIES
A fascinating aspect of precision agriculture is that a single technology is not
being undertaken to improve a single practice. Instead, across the crop-produc-
tion sector of the United States, precision agriculture is emerging as the conver-
gence of several technologies with application to several management practices.
However, every technology is not necessarily required or applicable for every
practice on all crops, and development and enhancement of several of the poten-
tially relevant basic technologies are being driven by forces outside of the agri-
cultural sector. Thus it is difficult to develop a generally accepted view of the
dimensions of precision agriculture. Every area of information technology—mi-
croelectronics, sensors, computers, telecommunications—is in an evolutionary
process of continuous improvement. As these introductions take place, some prod-
ucts will become economically feasible for agricultural applications. In Box 1-2,
describing a vision of tomorrow’s crop consultant is considered. According to
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DIMENSIONS OF PRECISION AGRICULTURE 33
thus allowing the user to reduce the amount of unsampled area in a given applica-
tion. In map-based applications, maps are based on a limited number of samples
thus creating the potential for errors in estimating conditions between sample
points. An additional uncertainty is associated with GIS due to the temporal dis-
connection that occurs when samples are mapped at some point in time and a
response is made at some later time. In the case of dynamic variables such as soil
nitrogen content or pest distributions, significant change in the amount and distri-
bution of the attribute of interest can take place during that time (Sudduth et al.,
1997; Wollenhaupt et al., 1997).
Sensor based VRT is employed on Midwest farm equipment to:
• Vary anhydrous ammonia application in response to soil type variations.
• Vary planting population in response to soil CEC and topsoil depth
variations.
• Vary herbicide rates in response to soil organic matter variations.
• Vary starter fertilizer in response to soil CEC variance.
• Vary nitrogen fertilizer at side-dress time in response to soil CEC, topsoil
depth, and soil nitrate levels.
Map-based VRT is employed in the high-volume commercial (contracted)
application of phosphorus and potassium fertilizers and lime using high-flotation
applicators. Map-based variable-rate application systems for farm tractor use are
widely available for liquid fertilizers, anhydrous ammonia, herbicides, and seeds.
Map-based VRT controls for water and fertilizer are also available for center
pivot irrigation systems.
Because of the additional capital and maintenance expense for high volume,
pneumatic or liquid material control systems in high-flotation VRT, application
costs are higher than for conventional floater application technology. Floater VRT
application of granular fertilizers is typically $2 to $3 per acre higher than non-
VRT applications.
Costs for upgrading tractor-mounted application controllers to add VRT ca-
pability are often nominal. Upgrading a controller to allow for automated adjust-
ment of application rates is a minor technical departure, representing only a soft-
ware/hardware interface. However, the producer must also have a computer that
manages GIS data and sends rate change commands to the controller, and a GPS/
DGPS receiver. Such a system can be assembled by more technologically sophis-
ticated producers. In other cases, a VRT system may be more complex and costly,
incorporating multiple chemical injection hardware and GIS/GPS/DGPS systems
as an integrated, dealer-installed unit. Regardless of the type of VRT system uti-
lized by a grower, implementation of a map-based VRT system requires full con-
sideration of all related costs, including data acquisition, the GIS and GPS/DGPS
to create and execute application maps, and the often time-consuming intellectual
capital investment in learning how to successfully use all components of the tech-
nology.
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34 PRECISION AGRICULTURE IN THE 21ST CENTURY
The cost of obtaining and interpreting soil test information on which to base
floater or tractor-based application rates is a limiting factor in the site specificity
of map-based VRT. Soil samples normally are acquired at a rate of one sample
per 2.5 acres to reduce costs for collection and analysis. In an Illinois test, fertil-
izer requirements based on 2 grid sizes were compared to uniform application
rates. With a grid size of 0.156 acre, recommended fertilizer rates decreased
dramatically resulting in a fertilizer savings of $18.00 per acre compared to $0.25
per acre savings with a 2.5 acre sized grid. The cost to collect the samples on the
more detailed grid, however, far exceeded any savings in fertilizer costs (Illinois
Agri-News, 1996). One key to improving the efficacy of map-based VRT is the
development of additional cost-saving, higher sampling density sensor method-
ologies.
Groundbased Sensors
Basic research is needed to investigate soil and crop processes applicable to
development of ground-based sensing systems. Sensors offer the opportunity to
automate collection of soil, crop, and pest data at a level of intensity not economi-
cally feasible with manual sampling and laboratory methods. Fields are highly
heterogeneous. Increased sampling will result in accurate characterization of
within-field variability. Improvements to VRT and crop modeling are expected to
advance rapidly with a higher spatial density of measured soil and crop param-
eters. Sensors are needed that are fast, efficient, and can assess factors important
to crop production.
Moran et al. (1996) concluded that the information from ground-based sen-
sors is needed for soil organic matter, soil moisture, cation exchange capacity,
nitrate nitrogen, compaction, soil texture, salinity level, weed detection, and crop
residue coverage. These parameters as well as soil pH, and availability of phos-
phorus and potassium cannot be ascertained by remote-sensing technology. More-
over, the use of real-time ground-based sensors provides the grower control over
timing of data acquisition not possible with satellite or aircraft sensing techniques.
Sensors have been developed or are under way to measure soil and crop
conditions including soil organic matter, soil moisture content, electrical conduc-
tivity, soil nutrient level, and crop and weed reflectance (Sudduth et al., 1997).
Continuous, real-time electrochemical soil chemical constituent sensors are cur-
rently available for nitrate measurement and are dedicated to specific application
in corn side-dress applications. A real-time acoustic soil texture sensor and a real-
time soil compaction tester are also under development at Purdue University (Liu
et al., 1993; Morgan and Ess, 1996).
Some important real-time indexes may be determined by their relationships
to other variables rather than by direct determination. Soil conductivity is appro-
priate for concurrent real-time assays of salinity, soil moisture, organic matter,
cation exchange capacity, soil type and soil texture. Recently, this work was ex-
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DIMENSIONS OF PRECISION AGRICULTURE 35
tended to non-saline soil methods in combination with electrochemical constitu-
ent sensing which separates components of direct contact conductivity (Colburn,
1997). Conductivity component analysis is employed for georeferenced data gath-
ering and analysis by several commercial companies as well as for VRT in
midwest crops. Apparent soil conductivity using electromagnetic methods is an
indicator of clay content, depth to claypan, soil water content, hydraulic charac-
teristics, productivity (Kitchen et al., 1996), and as a promising substitute for
yield monitoring (Jaynes et al., 1995).
For immobile constituents (i.e., phosphorus and potassium), industry has not
yet chosen to introduce real-time sensors. In some cases, phosphorus and potas-
sium levels in the corn belt states where VRT was first used, are very high, and
field availability has been found to exceed producer needs for the current crop
year and the near future. In other regions, such as western states, lower availabil-
ity of immobile nutrients is common. For these nutrients, discontinuous nutrient
sensor mapping methods have the potential for gathering and analyzing soil
samples in separate field operations. Three systems are under development by
government and academia which automatically extract and analyze soil samples
for phosphorus, potassium, and nitrates (Adsett and Zoerb, 1991; Birrell, 1995;
Morgan and Ess, 1996).
There exists the potential for a vast increase in the timeliness and amount of
information if additional means of data collection and analysis become avail-
able. Sensors will play an important role in supporting technology for precise
applications of nutrients, pesticides, and other inputs. Only a few commercial
sensors are available today. Efforts continue by both private companies and the
public sector to develop real-time sensors for additional agricultural indexes.
Basic research in the sensors arena is fundamental to an improved understanding
of the variations in site-specific crop production in a wide variety of regional
production systems.
Remote Sensing
Remote sensing—the acquisition of information from remote locations such
as an airplane or satellite—is a potentially important source of data for precision
agriculture. In the long term, remote sensing could provide numerous forms of
information, both spatially and temporally. However, improvements are needed
in the analytical products and delivery systems if remote sensing is to meet its
promise for precision agriculture.
For more than 30 years remote sensing has been envisioned as a valuable
source of information for crop management. The pioneering research of Colwell
(1956) showed that infrared aerial photography could be used to detect loss of
vigor of wheat and other small grains resulting from disease. Although much
research and development was directed at large-area crop inventory applications
of satellite data in the 1970s (MacDonald and Hall, 1980), much less attention
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36 PRECISION AGRICULTURE IN THE 21ST CENTURY
BOX 1-3
Remote Sensing Vegetation Indexes
One of the earliest digital remote sensing analysis procedures devel-
oped to identify and enhance the vegetation contribution in an image was
the vegetation index (VI), a ratio created by dividing the red by the near-
infrared spectral bands (Tucker, 1979). The basis of this relationship is
the strong absorption (low reflectance) of red light by chlorophyll and low
absorption (high reflectance) in the near-infrared by green leaves. A form
of this ratio, in digital and map formats, is one of the principal data prod-
ucts that will be provided to producers for crop assessment. Dense green
vegetation produces a high ratio, while soil, plant litter, and geologic min-
erals have low ratio values, thus yielding a maximum contrast (Baret and
Guyot, 1991; Huete et al., 1994; Verstraete and Pinty, 1996; Verstraete
et al., 1996).
A number of related indexes have been developed that minimize the
effects of atmospheric and/or soil variation. The Normalized Difference
Vegetation Index (NDVI), the ratio of the difference between the red and
near-infrared bands divided by their sum, is the most widely used VI
(Huete and Tucker, 1991; Kaufman and Tanre, 1992). Although, these
indexes correlate to various plant parameters linked to the leaf area, it
has been hard to determine precisely what plant property is being sensed
(Baret and Guyot, 1991; Myneni et al., 1995; Pinty et al., 1993). The
ratios correlate most closely with the fraction of absorbed incident photo-
synthetically active radiation, and for this reason the indexes can be in-
puts to models for estimating evapotranspiration and crop growth (Asrar
et al., 1984; Myneni and Williams, 1994; Sellers, 1985). Although many
other band combinations and analyses could provide important additional
information for agriculture, these VIs will be the most widely used be-
cause they are easy to produce and closely associated with particular
crop processes.
has been directed at crop management applications. Satellite data have not had
spatial resolution, temporal frequency, and delivery times sufficient for the needs
of production agriculture. In addition, supporting technologies and infrastructure
have not been available. Nevertheless, the understanding of crop spectral and
radiometric relationships gained from past research is relevant to crop manage-
ment applications (Bauer, 1985).
Jackson (1984) described the potential for remote sensing in crop manage-
ment, and stressed that it is critical to provide frequent coverage, rapid data deliv-
ery, spatial resolution of 5 to 20 meters, and integration with agronomic and
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DIMENSIONS OF PRECISION AGRICULTURE 37
meteorological data into expert systems. These points were reiterated by Moran
et al. (1997) in a recent review of the potential of remote sensing to acquire
information for identifying and analyzing site-soil spatial and temporal variabil-
ity within fields.
In the past 10 years there have been rapid advances in acquiring and process-
ing multispectral imagery with multispectral video by using digital cameras from
aircraft. This approach has the flexibility of aerial photography acquisition and
the advantage of digital multispectral imagery (Moran et al., 1996; Pearson et al.,
1994). Although most planning and effort are going into the development of sat-
ellite systems, aircraft-acquired imagery may continue to be needed when ex-
tremely high resolution imagery is required. Aircraft platforms also provide an
opportunity for developing and testing new sensors (i.e., thermal infrared and
hyperspectral sensors) for future satellite systems.
A sequence of remotely sensed images over time can provide information
about crop growth and the spatial variation within fields. Detailed spatially dis-
tributed multitemporal information, in visual form, is not readily obtainable from
conventional crop management systems or from site-specific crop management
methods. Remotely sensed images (i.e., color infrared aerial photographs or mul-
tispectral images acquired from satellites or airplanes) show spatial and spectral
variation resulting from soil and crop characteristics. These images show the state
or condition of fields when the images were acquired. One of the most useful
aspects of remote sensing is its ability to generate images showing the spatial
variation in fields caused by natural and cultural factors. This information is not
limited by sampling interval or geostatistical interpolations (Moran et al., 1997).
Images acquired at different times during a season can be used to determine
changes such as growth rates and condition. These data, in turn, can be compared
with data from previous years and may be helpful in predicting yield.
Commercial interest is growing in the potential of remote sensing to contrib-
ute to site-specific crop management, particularly as precision agriculture tech-
niques are being developed and the possibility of routine, frequent acquisition of
remote sensing data by satellites seems likely. Several earth-observing satellites
are scheduled for launch over the next decade by governments and private indus-
tries. By 2005, 40 or more land observation satellites are expected be available
(Stoney, 1996). Many of these satellites will acquire imagery with spatial resolu-
tions ranging from 1-3 meters for panchromatic images to 3-15 meters for multi-
spectral imagery. Others will have resolutions of 10-30 meters but with addi-
tional spectral bands, including thermal infrared on LANDSAT-7. Still other
systems will collect radar data at varying resolutions. These sensors have promise
for many types of measurements beyond identifying crop type, including moni-
toring crop stresses and condition, soil properties, and moisture. A major research
challenge is the development of robust image analysis methods for agriculture,
and a major educational need is training satellite data providers to meet agricul-
ture needs.
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38 PRECISION AGRICULTURE IN THE 21ST CENTURY
BOX 1-4
Contemporary Remote Sensing Technology
The technologies that can contribute to site-specific crop manage-
ment—remote sensing, the global positioning system, yield monitors
and mapping, geographic information systems, variable-rate applica-
tion technology, computers, and electronic communication—are cur-
rently converging. Rapid growth in precision agriculture is stimulating
renewed interest in developing remote sensing, especially from satel-
lites, for crop management applications. Imagery acquired from con-
tinuously orbiting satellites operated by commercial companies will
enhance the possible applications and utility of remote sensing, and
farmers will not have to contend with the challenges of collecting photo-
graphs. Fritz (1996) suggests that despite high development costs, sat-
ellite systems will be cost competitive with aerial imaging systems. He
indicates that per unit of coverage, satellite imagery may be only one-
half the cost of aerial imaging.
The changes in U.S. policy resulting from the 1992 Land Remote
Sensing Policy Act and the 1994 Presidential Directive on LANDSAT
Remote Sensing Strategy specifically encourage commercial system de-
velopment and operation and have led to several companies developing
plans to launch satellite systems in 1997 through 1999. The new imaging
satellites will acquire panchromatic (1- to 3-meter spatial resolution) and
multispectral (4- to 15-meter resolution) imagery over swaths of 6 to 30
kilometers. At least two companies are targeting agriculture and preci-
sion farming as either the primary application or as a major target of their
planned marketing and sales efforts.
Remote sensing products could play an important role in site-specific
crop management, and there is also excellent market potential for the
acquisition, processing, and delivery of remote sensing information. Per-
haps no other application of remote sensing requires data so often over
such large geographic areas. However, infrastructure to meet this re-
quirement is not currently in place. Widespread application and success-
ful adoption of remote sensing data products are not likely until such an
infrastructure is developed; cadres of people who understand the rela-
tionships between crop-soil properties and remote sensing are especially
important. Similarly, more information and study on integration and use
of spatial information in crop management is needed as well as opportu-
nities for training in the use of spatial information. It will be very important
for systems and data products to be based on crop producer needs, and
for provisions to be made for farmers and others to develop an under-
standing of remote sensing.
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DIMENSIONS OF PRECISION AGRICULTURE 39
Crop Production Modeling
A broad range of spatially explicit crop response models is needed to evalu-
ate the efficacy of precision agriculture methods and provide the basis for precise
recommendations. Many models for predicting how crops respond to climate,
nutrients, water, light, and other conditions already exist, yet most of these do not
include a spatial component appropriate to precision agriculture applications
(Sadler and Russell, 1997). GIS can provide the means to run the model continu-
ously across an extensive area using data that reflect continually varying condi-
tions. Time series and other temporal analyses can aid in predicting final crop
yield. Current models may be extended to account for spatial effects, such as edge
effects along field boundaries. In the ecological and biometeorological literature,
however, several spatially explicit models have been developed to predict hourly,
daily, and annual rates of evapotranspiration and photosynthesis, and several spa-
tially distributed hydrologic models predict surface and subsurface flows. Meso-
scale climate models can resolve cells as small as 5 to 10 kilometers for predict-
ing weather conditions.
Pests are not dispersed evenly throughout the environment. To the extent that
the factors influencing their spatial distribution are understood, their dispersion
and potential for damage can be modeled. GIS can be used for spatially variable
data for these factors. As with crop response models, a distinct pest model can be
run continuously across a landscape, using GIS to input data to the model and
display results (loosely coupled model), or a spatially explicit model can be cre-
ated within the GIS software (tightly coupled model). GIS can provide the basis
for multiscalar effects, for example, incorporating results of a regional pest pres-
sure model into a system for generating within-field recommendations based on
locally variable conditions.
A crop growth model could be used as a decision aid for determining differ-
ent yields based on varying plant populations, which could help a producer de-
cide when to plant or replant areas within a field based on plant population data
and risk factors for various soil types. Having to make a decision to replant a field
that is in a questionable condition is perhaps the hardest decision a producer
faces. Any information to aid such decisions and reduce risk would be valuable.
In many crop production areas, landscape factors can cause dramatic varia-
tions in yield. Landscape elements affect many properties relevant to plant growth,
including soil texture, soil organic matter, and temperature. Landscape morphol-
ogy affects soil moisture available to crops by its influence on drainage and catch-
ment area. Soil surveys typically do not have sufficient resolution to capture this
variability in enough detail to support precision recommendations; even field-
based sampling on a regular grid may miss relevant soil-landscape features. Strati-
fying sampling density on the basis of landscape features may be more cost effec-
tive and informative than a simple grid. GIS allow users to create and manage
digital elevation or digital terrain models created by photogrammetric methods
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40 PRECISION AGRICULTURE IN THE 21ST CENTURY
(analysis of stereo pairs of aerial photographs) with new techniques using
interoferometric radar or by continuous three-dimensional coordinate measure-
ments with in-field equipment. Precise recommendations can be made to the ex-
tent that the relationships are understood between soil properties and surface
morphology (i.e., slope, slope length, aspect, curvature, landscape position, catch-
ment area, and drainage) derived from digital elevation or digital terrain models.
Crop models do not offer a panacea for problem solving; they are limited in
their ability to simulate various parts of a biological system. Most of the crop and
pest models available or developed to date were not designed to be used for man-
aging spatial and temporal variation. It is not clear whether a predictive model, an
explanatory model, or a hybrid approach will be more appropriate for precision
agriculture. Alternatively, data mining and other techniques may be used to ex-
tract valuable information from large amounts of stored data. However, crop
modeling is currently an important tool for gaining a theoretical understanding of
a crop production system.
Decision Support Systems
Decision support systems (DSS) are used in agriculture for tactical, strategic,
and policy-level decision support. Because producers are continually faced with
making tactical decisions, such tools are becoming increasingly useful on the
farm. However, few DSS are in general use by agricultural producers today, in
part due to difficulty in use and limited information provided—from their point
of view. They have been used to aid in decisions that are complicated by large
amounts of information and data. A simple conceptual diagram of a DSS is shown
in Figure 1-2 (Petersen et al., 1993). Data collected by a consultant, obtained
through a weather forecasting service, or acquired through a sensing operation
are analyzed and linked with appropriate decision rules that identify actions to
assist in producer decision making.
DSS rules are not developed to make a single recommendation but rather to
provide decision makers with choices; decision support systems should be seen
as sources of valuable tactical information. As is the case for crop modeling and
current management recommendations, DSS have been developed for whole
fields, and subfield variation has been largely ignored. Although subfield tactical
decisions have been practiced by producers for many years (i.e., rouging, spot-
spraying or rope-wicking residual weeds, or spot-treating chinch bugs in sor-
ghum), most management practices are implemented for whole fields.
The relationship between the scale of an operation and the resolution and
variability of sample data used in a DSS is important. To demonstrate this point,
consider the appropriateness of using DSS in two sites with widely differing char-
acteristics. The variation in the assessed attribute used in the DSS is high at one
site and low at the other. The DSS may be adequate for whole-field decisions at
the site with low variability but not appropriate for the site with high variability.
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DIMENSIONS OF PRECISION AGRICULTURE 41
FIGURE 1-2 Conceptual diagram of a decision support system. Tracing the steps in the
figure, information can be viewed as flowing from the environment via instrumented or
human sensors as data to a database. The information as data is analyzed and manipulated
for storage or transmission to a user as part of a decision process. The information pro-
cessed for a decision results in an action to be executed within the environment. After the
action is carried out, the environment is again monitored to begin a new cycle of informa-
tion flow. Thus, information flows to and from the environment in an endless loop that
begins with sensing and ends with action. A DSS integrates expert knowledge, manage-
ment models, and timely data to assist producers with daily operational and long-range
strategic decisions. SOURCE: Petersen et al., 1993. Reprinted with permission; copyright
1993, Agronomy Society of America, Crop Science Society of America, and Soil Science
Society of America.
The site with high variability may require a DSS in which other attributes are
assessed or the whole field is subdivided to overcome the variation. Assessing the
relationship between attribute variation and DSS performance has been largely
ignored in relation to pest management and only superficially addressed regard-
ing soil fertility.
Similarly, decision support systems do not address the problem of spatial
heterogeneity. This is true for weed management DSS such as HERB,
WeedSOFT, and PC-Plant Protection, and for insect and disease management
programs; irrigation and crop selection programs are all whole-field based. Re-
searchers recently combined weed management DSS with spatial weed infesta-
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42 PRECISION AGRICULTURE IN THE 21ST CENTURY
tion maps to determine the value of spatial information in pest management. In
these simulations, pest density at individual locations in fields was used for the
infestation level input to the DSS. Lindquist et al. (in press) found that a treatment
map based on spatial information (800 observations) was a great improvement
over use of the mean field population. The simulation indicated that, on average,
herbicide use would be reduced by 30 percent to 40 percent with such an ap-
proach. Christensen et al. (1996) also found herbicide reductions of 30 percent to
40 percent when they mapped weed populations in several cereal grains in Den-
mark. In each case spatial data were used to run an economic-threshold-based
DSS.
Although such simulations show that subfield management could lead to sig-
nificant changes in management practices, numerous questions remain unan-
swered. First, the issue of risk of improper decisions is a real concern to consult-
ants and producers. DSS have only recently begun to be used for many large
acreage crops. Their slow adoption has partly resulted from concern over risk of
nonperformance. Consultants are providing a service to a client and are concerned
that the client be pleased with the outcome of their service, and the producer is
concerned about the real agronomic impact of uncontrolled pests and the social
implications of infested fields. Another concern is that the long-term effect of
spatial management on infestation level and distribution is largely unknown. Seed
production by uncontrolled plants and egg or cyst production by insects and nema-
todes may result in infestations growing or in spatial orientations changing in
ways that make GIS maps less valuable. Such concerns require studies to assess
these longer-term impacts on precision agriculture.
There is also the question of the extent to which a knowledge base exists for
subfield decisions. For example, relatively little is known about the suitability of
crop cultivars for specific soil types or cultivar-fertility-pesticide interactions.
Little is known about the interactions between agronomic practices and their en-
vironment at the subfield scale. A solid knowledge base will become more impor-
tant as a foundation for more information-intensive practices. Additionally, as the
complexity of databases in DSS grows, the inputs needed to initiate these applica-
tions will also grow. For example, two years ago, the University of Nebraska
released a weed management DSS that required little information on soil type. In
the most recent release, the user can determine the potential risk to ground and
surface water contamination from pesticide use, but the user must be familiar
with the specific soil type in that field. Also program developers will be chal-
lenged to make these decision aids easy to use. In the example, county soil maps
are being incorporated in the new version of WeedSOFT; the user will find the
field on the county map and click on the location and the DSS will do the rest.
To develop the needed database, researchers will need to approach param-
eterization used to aid decision making in a new way. Rather than restricting data
collection to a handful of research station field trials, researchers will have to find
a way to use producers’ fields as laboratories. Harnessing spatially referenced
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DIMENSIONS OF PRECISION AGRICULTURE 43
data collected on individual farmsteads makes it possible to set parameters for
data sets within localized areas. Such an approach would allow DSS to incorpo-
rate local parameters, which has not been possible due to the cost of parameter-
ization and of programming expertise. It is likely that future development and
maintenance of decision support systems will be accomplished through land grant,
Agricultural Research Service, consultant, producer, and other information ser-
vice provider consortia.
LOOKING TO TOMORROW
Information technologies have the potential to provide considerable amounts
of useful information for decision making in precision agriculture. A suite of
tools will be used to assess and manage agronomic factors important to crop
production. For these new tools to function properly, however, they will need to
be user friendly for producers and consultants. Information technologies will pro-
duce enormous data sets on crops and their interactions with their environment.
The challenge remains as to how to convert these data into useful suggestions to
aid in the decision-making process for the producer.
Representative terms from entire chapter:
crop production