|
||||||||||||||||
|
||||||||||||||||
The following HTML text is provided to enhance online readability. Many aspects of typography translate only awkwardly to HTML. Please use the page image as the authoritative form to ensure accuracy. 2
|
||||||||||||||||
 |
 |
Page 44 |
 |
 |
 |
|
| ||||||||||||||||||||||||||||||||
|
|
|||||||||||||||||||||||||||||||
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 44
2
A New Way to Practice Agriculture
Modern U.S. production systems have evolved to unprecedented levels of
production efficiency. These advances have resulted from germ plasm improve-
ment, use of synthetic fertilizers and pesticides, and use of advanced agricultural
machinery. These production inputs have resulted in greater efficiency despite
the fact that detailed management data regarding the particular crops was gener-
ally unavailable.
The availability of new kinds of information about production fields, farm-
steads, products, and markets opens the door to new ways of practicing agricul-
ture. Information technologies offer an unparalleled ability to characterize the
nature and extent of variation occurring in agricultural fields and to develop opti-
mized management strategies for these conditions. At the field and subfield scale,
information about the spatial heterogeneity of site characteristics makes it pos-
sible to manage the variation rather than attempting to overcome the variation
with sufficiently high uniform rates of agricultural inputs. Because of the com-
plex interactions of factors affecting agricultural production, uniform and spa-
tially variable management will result in different inputs and outputs.
CHANGES IN FARM MANAGEMENT RESEARCH
Precision agriculture changes a farm manager’s philosophy, because focus
of attention changes from average field conditions to the variation of those condi-
tions. The research methods that have contributed to today’s production efficiency
may not be the most appropriate for the future. Agricultural research has focused
mainly on identifying robust strategies such as input use recommendations or
farming practices that can be generalized and applied across a diverse set of envi-
44
OCR for page 45
A NEW WAY TO PRACTICE AGRICULTURE 45
A
B
FIGURE 2-1 Crop yield and profit maps. A. Raw yield as measured on average over
25 × 25 meter cells. B. Profit variability within the field. Profit for each cell is calculated
using the yield for that cell and production cost estimates for irrigated corn in North Texas.
Conventional management was used over the entire field. The images above were col-
lected from an irrigated corn field in the panhandle of Texas. The yield variability is ex-
treme, compared to other fields in the area. However, it does document the yield and profit
variability that can be found in the producer’s fields. SOURCE: Maps developed by
Stephen Searcy, Texas A&M University.
OCR for page 46
46 PRECISION AGRICULTURE IN THE 21ST CENTURY
BOX 2-1
Linking Crops, Information Technology,
and Decision Making
INTEGRATING PRODUCTION AND MARKETING DECISIONS
In the future, producers may use crop status data and predictive crop
growth models to make more precise input and marketing decisions. Pro-
ducers would like to monitor crop growth to more accurately determine
crop irrigation, pesticide application, and harvesting schedules. Crop as-
sessments have the potential to increase accuracy of yield estimates in
advance of harvest. More precise harvest date and yield information could
be useful at producer cooling facilities, processing plants, and in the
marketplace. Processors want to optimize production and maintain effi-
ciency by controlling the flow of raw commodities entering their plants.
Many grocery stores need to arrange purchases of produce three weeks
in advance of harvest and release advertisements prior to the harvest
date. More accurate information on crop yields and harvest dates is im-
portant in markets where a consistent supply of commodities is neces-
sary to meet consumer demand. It is likely that increased crop status
information will impact decision making not only in a producer’s opera-
tion, but throughout the food and delivery system.
VEGETATIVE GROWTH TO FRUIT DEVELOPMENT
By monitoring trends in vegetative growth, a producer may more ac-
curately match production inputs to crop needs. Observed shifts in crop
Computer en-
hanced vegetation
map of a canta-
loupe field using
aerial imaging
technology.
SOURCE: Data
acquired by
RESOURCE21 for
Fordel, Inc.,
Mendota, California.
OCR for page 47
A NEW WAY TO PRACTICE AGRICULTURE 47
BOX 2-1 Continued
growth patterns may provide important clues to critical plant physiologi-
cal changes. In the vegetative stage of plant growth, foliage acts as a
sink for carbohydrates. As a plant shifts from vegetative to reproductive
growth, plant resources are allocated to fruit development. Crops have
been studied in some production areas to determine timing of fruit devel-
opment. For instance, cotton shifts toward foliar senescence and crop
maturity six weeks after initiation of first flower. The shift in carbohydrate
allocation of cotton helps growers determine the last dates for irrigation
and application of defoliation pesticides. After fruit development begins,
the cotton plant is sensitive to overwatering; additional water could stimu-
late a surge of vegetative growth that would interfere with cotton boll
maturation. Producers have observed that cantaloupe changes from veg-
etative to fruit growth three and a half weeks after bloom stage. A more
accurate prediction of this shift would enable melon growers to schedule
irrigation and harvesting schedules more precisely.
MONITORING CHANGES IN CROP GROWTH
Remote sensing images acquired over the growing season allow a
producer to monitor crop condition and to compare performance among
field sites having different cover densities. Development of crop growth
continued on next page
GREEN VEGETATION INDEX PERCENT
100
1st
WATER
80
BLOOM 3rd
60 DATE WATER
THIN 2nd FIRST
40 WATER HARVEST
DATE
75% PICK
20 EMERGENCE
0
7/6
8/2
8/9
6/15
6/22
6/29
7/13
7/19
7/26
8/18
8/23
8/30
DATE - 1994
20% Initial Greenness 40% Initial Greenness
50% Initial Greenness 60% Initial Greenness
80% Initial Greenness
Changes in crop growth, open pollinated cantaloupe. SOURCE: Data pro-
cessed by Jack Paris, California State University, Fresno, GeoInformation
Technology Center; Enhanced by John LeBoeuf, Fordel, Inc.
OCR for page 48
48 PRECISION AGRICULTURE IN THE 21ST CENTURY
BOX 2-1 Continued
graphs from remotely sensed vegetation indexes has the potential to
better inform growers of the approaching harvest date. Within the field,
relative differences in the vegetation index can show whether a crop is
developing uniformly at any one time or over the growing season. The
aerial image shown is a computer enhanced green vegetation map of a
140-acre cantaloupe field. Ten meter resolution aerial images of the field
have been acquired eight times over the summer. Data collected from
sites with varying levels of crop growth have been extracted from the
images to show the pattern of their development (see graphs Changes in
Crop Growth). The graphs illustrate how seasonal progression in crop
canopy growth can be tracked for five sites that initially are at different
growth performance levels (i.e., 20 percent initial greenness). Changes
in the growth rates of each site are seen clearly as are changes in the
relative ranking late in the season of sites at 80 percent and 60 percent
initial greenness. The crop relative growth rate declines when the growth
shifts to fruit production. Superimposed on the crop development graphs
are key dates showing the relationship between crop condition and man-
agement decisions. Seasonal changes in plant cover and biomass can
be linked to predictions of future crop growth, harvest timing, and yield
estimates. When these kinds of data are used in a crop production model
it can assist in farm management decisions. This capability will be impor-
tant in irrigated agriculture as producers could manipulate water inputs or
fertilizers to advance or slow down crop maturity. The ability to follow
changes in crop development for specific field locations is an emerging
area of precision agriculture.
ronments. Precision agriculture strategies attempt to adjust field practices to ac-
commodate known variability of important factors. As practiced today, precision
agriculture is primarily based on a few parameters, such as soil nutrients or weed
maps. Understanding the impact of multivariate interactions is a challenge to
both producers, consultants, and scientists. The amount and complexity of avail-
able information has increased at a phenomenal rate. Growers will have access to
large databases, but the ability to extract useful information will have to be devel-
oped. Agriculturists may find themselves uncertain about what information to use
and how it can add value to production systems.
Systems Approach
Crops are integrators of the biophysical environment within a field. Crops
express their genetic potential and reaction to local soil, pest, and climatic condi-
OCR for page 49
A NEW WAY TO PRACTICE AGRICULTURE 49
tions through the quantity and quality of harvested product. Interactions occur-
ring among these agronomic factors affect crop performance; for instance,
weather conditions favoring growth of an insect vector can lead to outbreaks of
viral diseases, resulting in damaged crops. A systems approach to agronomic
management considers multiple interactions occurring in an agroecosystem (Na-
tional Research Council, 1989, 1996b). Manipulating agroecosystems to achieve
greater productivity depends on an understanding of the relationships among ag-
ronomic factors. The potential for generating large, detailed data sets presents a
challenge for agricultural scientists who are developing tools to further the under-
standing of those interactions and their effects on yield. Agricultural scientists
can use data sets generated under experimental conditions, as well as those gener-
ated by producers in attempting to understand the interactions in cropping sys-
tems. The adoption of precision agricultural management is most likely to occur
for those factors and interactions for which there is enough understanding to ac-
curately predict the outcomes and economic value of actions to manipulate the
crop in its agroecosystem.
Mapping spatial yield patterns is a logical step in visualizing field variability.
However, a yield measurement in itself cannot explain the cause of variation.
Information is more valuable when causal relationships can be determined be-
tween various data sets describing a field. Yield maps can be superimposed on maps
of other data collected from the same location. The analysis of these data layers
with a GIS and other analysis tools may reveal spatial relationships among agro-
nomic components contributing to yield variation (Skotnikov and Robert, 1996).
Spatial and Temporal Variation
The most significant impact of precision agriculture on crop production sys-
tems is likely to be on how management decisions are made and on the time-
space scales that are addressed, not on actual production practices. Precision ag-
riculture techniques may increase efficiency of input use by allowing the producer
to manage the crop on both a spatial and temporal basis with prescriptive rather
than prophylactic treatments. The management of a crop production system in-
volves many decisions, all of which are interrelated and ultimately affect profit.
Crop production is subject to uncertainty due both to stochastic processes (prima-
rily weather) and to unmeasured variability in agronomic conditions (i.e., soil
fertility). Precision management tools may improve decisions related to site con-
ditions, thereby reducing this aspect of uncertainty in the management system.
However, the performance of precision agriculture depends on the interaction
between site conditions and stochastic factors. Stochastic factors such as weather
often have a greater impact on yield variability than variations in soil productiv-
ity. For example, a study comparing variable-rate and uniform application of su-
perphosphate on narrow leafed lupine (Cook et al., 1996) found that variable-rate
application based on nutrient response curves estimated using data from a single
OCR for page 50
50 PRECISION AGRICULTURE IN THE 21ST CENTURY
year performed poorly because the estimated nutrient response function did not
take variability in weather conditions into account adequately. Other studies have
similarly shown low correlation between yield and applied fertilizer when weather
conditions and other important factors are not included in nutrient response mod-
eling (Huggins and Alderfer, 1995). If subfield management is to be successful, it
must be based on techniques that encompass the simultaneous effects of the most
important factors influencing yield, rather than individual factors taken in isola-
tion. For example, successful variable-rate application strategies will likely be
based on nutrient response curves that incorporate soil characteristics, weather
conditions, and other factors in addition to applied fertilizer.
The ability to respond to changing production conditions is likely to be as
important as understanding variability in beginning-of-the-season production
conditions (i.e., in soil productivity). Farmers make judgments on input use con-
ditional on likely yields for a field given anticipated weather conditions. This
balance between anticipated income and expenditure on inputs is subject to con-
siderable uncertainty. Precision agriculture techniques that allow the producer to
manage initially for a lower yield goal and still respond to seasons in which yield
potential is greater than normal could substantially ameliorate the effects of that
uncertainty. Such an ability to respond to temporal variability would be particu-
larly desirable for handling weather-related risk. For non-irrigated production
areas, perhaps the greatest threat to crop production is lack of sufficient moisture
to mature the crop. Low soil moisture often causes farm managers to opt for
lower initial chemical and fertilizer application rates and thus forego additional
crop production should weather conditions turn out more favorable than antici-
pated. Similar situations occur when the potential for leaching in light sandy soils
limits the amount of nitrogen that can be applied in a single application (Booltink
et al., 1996).
Precision management systems can be envisioned that could respond to the
yield potential of the crop as it varies within the growing season. This more reac-
tive approach to spatially and temporally variable conditions will depend on the
ability to economically assess the need for and delivery of production inputs.
Crop status data and predictive models that could accurately estimate yield sev-
eral weeks or months before harvest could improve marketing decisions. A cur-
rent example of this capability is the use of cotton growth modeling. Cotton
phenological development assessments and historical weather data have been suc-
cessfully used to predict lint yield during the growing season (Landivar and
Hickey, 1997; Plant et al., 1997). An indeterminate crop such as cotton lends
itself to in-season management, as the plant will put on fruit that will never result
in mature lint. Early knowledge of yield potential could affect financial decision
making related to inputs (whether to apply crop protection chemicals) and mar-
keting (adjustments of forward contracts). While cotton growth modeling has
been done on a whole-field basis, its predictive capability may be improved with
the intensive data sets that result when using subfield management (Landivar and
OCR for page 51
A NEW WAY TO PRACTICE AGRICULTURE 51
Searcy, 1997). Producers face many factors that can be managed or will affect
management decisions. The complete realization of precision agriculture’s po-
tential may depend on development of predictive crop models that can vary and
co-vary the manageable factors on a specific farmstead. It is unclear whether such
an approach will rely on detailed mechanistic models, models based on a wealth
of data generated for a given farmstead, or a hybrid of these two approaches.
MANAGEMENT FACTORS
The management factors in a precision agriculture system are essentially the
same as those in a conventional crop production system. Producers can control
some of these factors and can only react to others. Factors such as nutrient man-
agement have been emphasized in the development of precision agriculture
whereas others, such as pest management, have received relatively little atten-
tion. Management factors can be subdivided into two groups: those that can be
managed with today’s technologies and those for which new management meth-
ods are promising but require further development. However, because precision
agriculture methods are still developing, all factors will likely experience ad-
vancements and improvements.
Precision agriculture is based on the availability of intensive data about im-
portant agronomic indexes. The process of obtaining these data has a cost and, at
least for some factors, the greater the data requirements, the greater the cost. As a
result, producers and their advisors must decide how detailed the required data
should be. The practicality of the data often depends on how long the information
has value in management decisions. Indexes such as soil type and topography
have long-term usefulness. The investment in obtaining this information will have
returns for many growing seasons. Factors such as nutrient availability (except
nitrogen), soil-borne pathogens, and perennial weed infestations may exhibit in-
termediate usefulness because they change slowly. Available soil moisture, nitro-
gen availability, and insect pressure are examples of short-term dynamic indexes.
The accuracy of the information about any of these factors degrades over time
and may be thought of as having a half-life. Cost-benefit analysis could be used
in precision agriculture by considering the half-life of the information, the poten-
tial returns from its use, and the cost of obtaining and analyzing it. However, the
half-life of agronomic indexes can only be estimated.
Methods of adding value to data sets also have potential, and are in need of
study. For example, spatial data on soil variation could be used to regulate fertil-
izer requirements, but also may be useful in regulation of herbicide rate and for
directed sampling of pest distributions (Fleischer et al., 1997; Johnson et al., 1997).
Recent studies have shown that targeting of sample sites based on other known
characteristics of the field (typically topology and soil type changes) can result in
more accurate maps, often with fewer samples (Hollands, 1996; Wang et al.,
1995).
OCR for page 52
52 PRECISION AGRICULTURE IN THE 21ST CENTURY
A precision agriculture approach to crop management requires producers to
consider information about production units in the same manner as production
inputs such as fertilizer or irrigation. As the technologies for measurement of
important agronomic indexes improve, producers may have to evaluate precision
management for all aspects of production systems. The following sections at-
tempt to briefly address the potential for precision management strategies for
factors that impact crop production. Each section deserves a detailed examina-
tion, but the intent here is to highlight possibilities, while leaving the in-depth
examination to others.
Crop Genetics
Crop productivity depends on the genetic makeup of the plant variety and the
response of the variety to its environment. Varieties are selected for particular
genetic traits (i.e., drought tolerance, resistance to diseases and insects, and yield).
In a precision agriculture scenario, producers try to match crop variety and popu-
lations with various conditions that exist in a field. The introduction and mainte-
nance of transgenic varieties requires sophisticated management techniques. For
example, the use of Bt-enhanced seed varieties (cultivars engineered to contain
Bacillus thuringiensis, a bacterium that produces a protein toxic for insect pests)
requires the planting of nontransgenic varieties in refuge areas to avoid the devel-
opment of resistance. The requirements for refuge areas vary with the intended
management practices, and precision agriculture techniques may predict the opti-
mum location for each variety.
Similar potential exists for the use of herbicide-resistant varieties. If there is
a yield penalty associated with the varieties containing the herbicide-resistance
gene, the producer may wish to plant that variety only where weed problems
exist. Variable-rate technology planters can change the variety being planted in
each portion of the field. However, if resistant and non-resistant varieties are
mixed in the same field, the planting sites for each would have to be recorded and
used in any subsequent herbicide applications in order to ensure that susceptible
plants are not sprayed. Changing varieties to manage for drought-prone soils has
also been proposed.
Plant Population
Knowing when to change the plant population density for optimum yield in
fields with known variability would benefit a producer and could be done by
varying the seeding rate on a planter or grain drill. Preliminary data indicate that
a positive net return can be achieved by varying plant population according to
depth of topsoil (Barnhisel et al., 1996). If a producer expected fair-to-poor con-
ditions for germination and emergence of seeds on productive soils, the seeding
OCR for page 53
A NEW WAY TO PRACTICE AGRICULTURE 53
rate could be increased. Competition from weed pressure may be reduced by
increasing the seeding rate at planting (Mortensen et al., in press). Increasing the
seeding rate in small-grain crops in California can help control Johnson grass and
smart weed. Plant population data could also be used to check on the effective-
ness of precision planters that drop seeds at a set spacing up the row, which is an
important manageable factor when expensive hybrid seed is used. Improved
knowledge of field conditions and pest pressure can help a producer make plant-
ing decisions.
Although seeding rates can be adjusted at planting, many factors can affect
the plants throughout the season. Varying seeding rates may not necessarily re-
sult in an expected plant population distribution. Knowing the actual plant popu-
lation at harvest time is important in interpreting yield maps and for management
decisions made throughout the growing season. Technology currently under de-
velopment for corn in the Midwest will measure variability of a plant population
and plant spacing (Birrell and Sudduth, 1995; Easton, 1996; Plattner and Hummel,
1996). Information gathered across a field will generate a data layer that can be
compared with yield maps, desired seeding rates, or weed maps. These devices
are still in the developmental stage but illustrate the potential for using sensing
techniques to gather useful information as a part of normal field operations such
as cultivation or harvesting.
Soil Variability
Soils vary significantly as a result of regional geological origins and past and
present cultural practices. At the highest level of resolution, soil physical, bio-
logical, and chemical properties vary vertically, horizontally, with treatment, and
with time. For example, variable distributions of soil nutrients in fields may result
from improperly adjusted mechanical application equipment (Bashford et al.,
1996; Olieslagers et al., 1995). In other cases, past practices, such as an old feed-
lot, can generate local pockets of higher organic matter producing healthier plants
than surrounding areas. Thus, natural variability patterns and management prac-
tices need to be considered in assessments of soil spatial variability.
Soil layers that restrict rooting depth are a major concern in many areas.
Electromagnetic induction techniques have been used to assess the presence of
and depth to claypan layers (Doolittle et al., 1994). Limited work on assessing
soil compaction has indicated a potential profitable return to site-specific tillage
operations instead of whole-field subsoiling (Fulton et al., 1996). Soil physical,
chemical, and biological properties have dramatic effects on crop production.
However, only a few commercially available sensors can assess these properties
in the field. Practitioners are limited to sampling and laboratory analysis for de-
termination of in-field variability, which is costly and time consuming. The num-
ber of commercially available sensors will be a limiting factor for precision agri-
culture in the immediate future.
OCR for page 54
54 PRECISION AGRICULTURE IN THE 21ST CENTURY
FIGURE 2-2 Soil and crop variability observed in remote sensing. This image was
acquired over fields in the Sacramento Valleys near Davis, California by a NASA airborne
sensor on August 20, 1992. The area has diverse crops from fruit and nut orchards, toma-
toes, corn, alfalfa, and safflower growing on deep loam soils. At this time many of the
summer crops have been harvested and soil variation within and between soil units is
evident. Despite the low spatial resolution needed for many precision agriculture applica-
tions (about 20 m by 20 m, or 400 m2), relative to new spaceborne sensors that can provide
1-5 m resolution, the connection between some soil patterns and apparent crop growth
differences is evident in the fields. Non-uniform growth conditions within fields are com-
mon. SOURCE: Data acquired by NASA Advanced Visible Infrared Imaging Spectrom-
eter (AVIRIS) and processed by University of California, Davis Center for Spatial Analy-
sis and Remote Sensing (CSTARS).
OCR for page 55
A NEW WAY TO PRACTICE AGRICULTURE 55
Soil Fertility
The concept of using information about variability to manage specific sites
in a field is not new. Farmers of ancient times were keen observers of crop perfor-
mance and recognized benefits from spreading different amounts of manure and
liming materials on different kinds of soils (Kellogg, 1957). In the 1620s, colo-
nists observed site-specific fertilizer practices of Indian farmers who placed fish
directly at the roots of each corn plant. In 1929, researchers Bauer and Linsley
suggested marking a field in 3-foot pace intervals in the north-south and east-
west directions to determine field position for variable application of limestone
materials (Goering, 1993). Today’s information technologies have the potential
to generate more sophisticated assessments and responses to within-field hetero-
geneity and variation in soil fertility.
Uniformity trials have been used to study soil heterogeneity by simply plant-
ing a crop that was uniformly managed throughout the growing season. The field
was divided into small segments and crop yield was measured on each segment.
Crop yield variability among segments was the measure of varying levels of soil
fertility in the field. Crop yields obtained from a uniformity trial were plotted on
a map, and field segments having similar yields were connected by smooth lines.
These yield maps were interpreted as soil fertility contour maps. LeClerg et al.
(1962) made two general conclusions from these early uniformity trials:
• Soil fertility variations are not distributed randomly but are to some de-
gree systematic; that is, contiguous field segments are more likely to be
alike than are segments separated by some distance.
• Soil fertility is seldom distributed so systematically that it can be described
by a mathematical formula.
A common strategy in soil fertility management is to match fertilizer inputs
with crop needs. The goals of this mass balance approach are to increase nutrient
uptake efficiency and minimize fertilizer losses. Fertilizer rate recommendations
for immobile nutrients (i.e., phosphorus, potassium, and zinc) are based almost
entirely on soil test levels calibrated for a specific crop, soil type, and climate.
Nitrogen fertilizer rates are based on estimates of yield potential (average or spa-
tial) with corrections or credits for nitrogen in soil profile, legume, manure, and
soil organic matter sources. Recently, producers have been encouraged to adjust
timing of fertilizer applications to reduce environmental risks. For example, ni-
trogen losses due to leaching can be reduced by minimizing the time between
application and plant uptake (Killorn et al., 1995). Conventional approaches to
soil testing based on averages are inadequate for characterizing temporal and
spatial variation of soil properties.
The most widely used precision agriculture technique is probably the man-
agement of soil nutrients and pH. Precision management of soil nutrients can
increase profit in two ways. The first is improved crediting of residual nutrients
OCR for page 56
56 PRECISION AGRICULTURE IN THE 21ST CENTURY
BOX 2-2
Site-Specific Forestry Management
Modern forestry practices require extensive harvest planning to maxi-
mize or optimize harvesting while maintaining yields from forests over
many decades. Forest management today must consider forest ascetics
and scenic vistas, historic and archeological sites, competing uses for
recreation, grazing, and other extractive uses like harvesting for mush-
rooms, medicinal plants, ornamentals, and mining of mineral deposits.
Many forestry companies have adopted the use of models within a
GIS to aid in site-specific management of forest resources. The eco-
nomic value of forest products is sufficiently high to justify extensive use
of site-specific technologies.
Management plans include decisions about modes of logging, such
as by helicopter, tower, line, or chain, which depend on topography, stand
condition, and distance to roads. GPS is used by timber cruisers to iden-
tify specific trees for harvest and locations of harvested trees are entered
into the GIS database. GIS is used to predict the potential for soil erosion
after logging, especially if the site is close to a stream, and to develop
mitigation strategies. This allows erosion models based on actual soil
characteristics, topography, and site conditions (i.e., cover type) to be
used in developing spatially explicit erosion hazard estimates over the
site rather than arbitrary rules like distance to roads or streams.
Evaluation of off-site nutrient and herbicide transport are other con-
cerns. Fire hazard is another risk factor that can be minimized using
spatially explicit models for site management and mitigation. Fire risk
and fire hazard models require a digital terrain model, information about
the fuel load and its vertical and horizontal distribution, and weather infor-
mation. Foresters may also use site-specific forest mensuration models
to predict tree growth that consider site-specific soil fertility and moisture
remaining in the soil after a crop is harvested. This works best for less mobile soil
chemical properties such as phosphorus and potassium concentrations or pH. Ni-
trogen is more mobile and requires more frequent sampling to assess the appro-
priate credit levels. Nitrogen remaining in the soil after harvest may be available
to the next crop, unless temperature and rainfall conditions result in leaching or
volatilization. More accurate crediting of residuals can reduce costs and environ-
mental load where overapplications would have occurred, and can improve yields
for locations that would have been undertreated. Second, precision management
of soil nutrients allows the producer to set variable yield goals for fields that do
not have a uniform productive potential. With variable yield goals, inputs for a
OCR for page 57
A NEW WAY TO PRACTICE AGRICULTURE 57
BOX 2-2 Continued
conditions, and microclimatic differences. Dynamic growth models of
varying complexity aid in developing a long-range yield plan for a site.
Annual remote sensing images, like LANDSAT Thematic Mapper and
aerial photography are frequently used to assess reforestation success,
erosion, competition from shrubs, and tree mortality. This use is pre-
dicted to increase as the next generation of satellites becomes available.
The new satellites will permit information extraction about canopy condi-
tion beyond properties related to total foliage display. High spatial resolu-
tion radar, LIDAR (an acronym for light detection and ranging), and opti-
cal sensors will obtain information about forest structure and biomass
distribution. More frequent temporal coverage will permit earlier detec-
tion of insects and other environmental stresses.
Many charismatic, endangered, and sensitive wildlife species require
forests with late successional structural features to provide forage, nest-
ing, and perching locations. Forests with standing and down dead trees,
open gaps and spatially distributed forest patches, corridors for migra-
tion, vertical and horizontal crown complexity, and other pattern fea-
tures may be mapped and tracked in a GIS. In addition, GIS based
models using remotely sensed information provide a mechanism for
evaluating the impact of site-specific logging on wildlife habitat condi-
tions necessary for protection of these species. Other forestry applica-
tions for site-specific methods include mapping the spread of insects
and fungal pathogens, to regional impacts of air pollution, like acid depo-
sition and ozone, on forest health and species specific mortality. As
competing demands for conservation, recreational use, and economic
extraction increase, GIS databases, using GPS linked site data, and
remote sensing monitoring, offer the hope that site-specific methods
can be used to optimize management decisions.
specific area of the field can be matched with the expected yield, and supplied at
a more economically optimal level (Hergert et al., 1997). Additional on-farm
research is necessary to determine the economic returns from different approaches
to soil fertility management in precision agriculture.
The evaluation of soil nutrient levels across a field is typically performed by
taking soil samples, analyzing them for nutrient content, and interpolating values
between the sampling points (Wollenhaupt et al., 1997). Figure 2-3 shows a field
with the sites where soil samples were taken, and a resulting interpolated phos-
phorus map. The actual values for soil phosphorus concentration are known only
at the sampled points; all other values are estimated. Both the method used for
OCR for page 58
58 PRECISION AGRICULTURE IN THE 21ST CENTURY
Sampling Point
FIGURE 2-3 Map of soil test phosphorus. This figure shows a field with the sites where
soil samples were taken and a resulting interpolated phosphorus map. The actual values for
soil phosphorus concentration are known only at the sampled points. All other values are
estimated. Both the method used for locating the sample points and the interpolation meth-
ods are important to the accuracy of application maps that might be derived from such a
layer. SOURCE: Stephen Searcy, Texas A&M University.
locating the sample points and the interpolation methods are important to the
accuracy of application maps that might be derived from such a layer.
Although procedures vary with the supplier, many fields have been grid
sampled to determine nutrient levels. Grid sampling involves overlaying a grid
on the boundaries of a field; the grid spacing used may be uniform or may have a
wide range of resolutions. Soil properties can vary at any level of resolution and
with sampling date. Because inherent field variability is not well understood,
determination of grid resolution has been based mostly on costs. Obtaining and
analyzing soil samples is expensive; thus, the number of samples included in the
grid is based on the potential return from improved nutrient management. For
example, one sample may be collected from a 10-acre field of grain, whereas two
or more samples may be collected from each acre of a higher value crop such as
potatoes.
The sampling techniques used are determined by the type of data analysis to
be performed. Suggested sampling techniques include taking samples at the cen-
OCR for page 59
A NEW WAY TO PRACTICE AGRICULTURE 59
ter point of a uniform grid, randomizing sample locations within each grid cell,
and targeting sampling based on nonuniform field characteristics such as slopes
or changes in soil type (Wollenhaupt et al., 1994; Wollenhaupt et al., 1997). No
single sampling methodology has been determined to be optimum, and this con-
tinues to be a problem due to differential field characteristics, crop response, and
profit potential. Grid sampling and nutrient mapping are most useful for nutrients
considered to be relatively stable over time. The information gained from sam-
pling and mapping soil phosphorus, potassium, various micronutrients, and pH is
often used for three to five years, allowing sampling and analysis costs to be
amortized over several growing seasons. Grid sampling has also been used for
nitrogen, primarily on higher value crops (Schneider et al., 1996). Because vari-
able-rate fertilization is still a relatively new technique, little information is avail-
able to describe the long-term effects on soil productivity.
A variety of techniques are used to analyze data from samples collected in a
grid pattern. The scientific community frequently uses a geostatistical technique
known as kriging to interpolate between widely dispersed data points. This in-
volves calculating variances between multiple data pairs and predicting values at
unknown points as a function of distance and expected variance from known
sample points. This computational technique is intensive and has not been widely
adopted by agricultural software vendors. Commercially available agricultural
GIS programs often use a simpler form of an inverse distance-weighted calcula-
tion that requires fewer computations and less judgment by the user. These meth-
ods of interpolation generate new point values based on the distance to the vari-
ous neighboring sample and a weighting scheme. Wollenhaupt et al. (1997)
describe several different interpolation techniques, and suggest that no one tech-
nique is clearly superior. They point out that a proper strategy for sampling is of
much greater importance than interpolation method. A promising nonparametric
surface generation technique that uses an averaged shifted histogram method
could improve efficiency in analyzing point data sets (Scott and Whittaker, 1996;
Whittaker and Scott, 1994). An increased knowledge base in geostatistical meth-
ods should improve interpretation of precision agriculture data.
Sampling for soil characteristics has inherent problems with resolution and
accuracy at non-sampled locations. With appropriate sensors available, real-time
techniques can give data on a much finer scale, eliminate the need to estimate
values, and contribute to enhanced VRT methods. Although a few sensors are
currently available, more capability, including fine scale resolution, for sensing
important crop and soil parameters are needed.
Pest Management
The spatial and temporal complexity of pest behavior provides an opportu-
nity to integrate numerous strategies to manage pests in agroecosystems. The
ability to integrate information from a variety of sources will be necessary for the
OCR for page 60
60 PRECISION AGRICULTURE IN THE 21ST CENTURY
pest management decision-making process. With the state of precision agricul-
ture technologies today, site-specific pest management is limited to less mobile
pests, such as weeds. Insects are difficult to characterize in a timely and economic
manner. Because insect populations can change rapidly in a few days, the half-
life of data on insect densities would be short.
The complex nature of pesticides and their use in the environment creates a
need for information and guidance in the decision-making process. The safety
and regulatory aspects of pesticide use have placed a monitoring and reporting
burden on producers in some agricultural production regions that is most easily
managed with a GIS database and automated recording of applications. Improved
pest management is likely to increasingly use precision agriculture technologies,
with potential benefits to farm workers, the environment, and our food supply.
Pesticide Management
Producers are interested in saving on input costs and applications. Environ-
mental improvements may be derived by reducing pesticide use through preci-
sion application of chemicals only to areas with pest infestations instead of treat-
ing entire fields at uniform rates. Results of analyses of weed populations in 12
Nebraska fields show that postemergence herbicide applications could be re-
duced 71 percent for broadleaf weeds and 94 percent for grass weeds if only
infested areas were treated (Johnson et al., 1997). Weed seedling density varied
from 10 to 41 seedlings per meter of row length on fields with low to severe
weed infestations. Associated crop losses varied from 20 to 43 percent. The
authors estimate that herbicide use could be reduced 30 to 72 percent if real-time
sensing and discrimination of weed species could be accomplished. Such site-
specific determinations for spraying could also increase crop yields when whole-
field spraying is not justified because average weed infestation is below the
economic threshold.
Precision agriculture techniques can aid in making decisions on the rate of
pesticides applied across a field. Detailed field maps can identify soils that are
prone to leaching problems. Many agricultural chemicals, especially pre-emer-
gence herbicides, are labeled for different application rates based on soil condi-
tions. Soil texture and percentage organic matter are important for identifying the
correct application rate of these materials. An example of this is trifluralin
(Treflan), which has different rates for three different soil texture classes: coarse
sand (light soils), silt (medium soils), and fine clay (heavy soils). The lowest rates
are for the coarse sandy soils that are more apt to leach materials.
Pest control is an area for which both the spatial and temporal aspects of
precision agriculture could contribute to environmental improvement. Pests
could be managed not only by specific areas but also by timing treatment ac-
cording to damage thresholds arrived at by integrating scouting reports, remote
sensing, and on-the-go sensor input; rather than by using fixed prophylactic treat-
OCR for page 61
A NEW WAY TO PRACTICE AGRICULTURE 61
ments. Although spot treatments to target pests may reduce pesticide use, there
is little evidence that this management strategy is profitable after the cost of
obtaining the pest density data is included. An increased understanding is needed
of the cost effectiveness and social benefits from managing pests with precision
technologies.
Farmworker Safety
The use of differentially corrected GPS (DGPS) may reduce pesticide han-
dler exposure to toxic pesticides. For aerial applications of pesticides, pilots for-
merly depended on pesticide handlers on the ground to guide them across fields
to be sprayed. With DGPS technologies, pilots can substitute instrumentation for
human labor to maintain proper patterns of application. Pesticide handlers do not
need to be exposed to the agrichemicals.
Weather
Weather is perhaps the dominant factor in crop production and is certainly
one of the greatest sources of uncertainty. However, it is not a manageable factor.
At best, producers can react to recent weather conditions and predict the effect of
future weather patterns suggested by historical probabilities. Some meteorologi-
cal indexes, such as temperature, humidity, and solar radiation are relatively con-
stant over large areas or regions. Rainfall can be highly variable, even on the
subfield scale. Efforts to incorporate weather data into precision agriculture tech-
niques, especially decision-support tools, will be extremely important in attempts
to understand the interactions of the many factors influencing a crop.
Suppliers
Newer satellites and Doppler radar are key components of spatially distrib-
uted weather information. The current sensors will be expanded by the addition
of several new systems to be launched over the next decade, which will provide
unprecedented weather information in terms of spatial detail and temporal fre-
quency. For example, the new National Oceanic and Atmospheric Administra-
tion GOES-8 satellite obtains weather data every half-hour at a spatial resolution
of one kilometer, providing an important link in making detailed, spatially dis-
tributed weather information available at the farm level. The GOES-8 satellite
acquires vertical sounder data on humidity, temperature, and other properties at a
spatial resolution of four kilometers. These data can be used to model regional
and mesoscale atmospheric conditions to provide the agricultural community
high-resolution information on current and predicted conditions. Doppler radar
images are updated every 15 minutes. Many additional satellites, from both pub-
OCR for page 62
62 PRECISION AGRICULTURE IN THE 21ST CENTURY
lic and private sectors, are expected to be launched over the next decade for
weather observation.
The private sector has become increasingly important in the supply of
weather service information to the agricultural community. Weather service pro-
viders offer satellite dishes and computer hardware and software that bring de-
tailed, timely weather information to farms. Satellite receivers, modems, and
Internet connections have brought weather data to the agricultural community in
various formats, from short-range hourly and daily weather data to predictions
of weather conditions spanning time periods from immediate hourly predictions
through weekly, monthly, quarterly, and annual forecasts. These services are
mainly focused on regional weather summaries, but customization may be pos-
sible if a market exists for field- or subfield-scale weather data. That market is
not likely to develop unless precision agriculture tools are designed to include
site-specific meteorological data. These services may include data inputs from
weather stations established at or around farms. Automated portable weather
stations with data loggers, modems, and software are commercially available for
direct weather observation within the farm, but these are generally limited to one
per farm.
The rapid proliferation of weather service information providers in the agri-
cultural sector, many with on-line access, has set the stage for accessing other
types of information for farm management. Providers already bundle information
from public and private sources and combine weather data with crop models to
provide specialized data and services to their agricultural customers. Access to
real-time information via satellite links allows producers to use the information
without significant investments in expensive computer hardware and software
and without substantial time investments in processing raw data to obtain desired
information. Use of these services may spur the adaptation of other site-specific
technologies.
Monitoring Precipitation
Precipitation is the primary weather index watched by most producers. Satel-
lite images, Doppler radar, and interpolated weather station data are being used to
create county- and field-level rainfall maps with updates every 15 to 30 minutes;
the updates allow a producer to watch an approaching storm front as it moves into
and across a region. For precision agriculture, estimates of subfield rainfall
amounts would be of great value.
Relative Humidity
Humidity readings are especially important for forecasting the infestation
and spread of fungal pathogens, such as downy and powdery mildew. Humidity
is also important in biological control programs for monitoring conditions favor-
OCR for page 63
A NEW WAY TO PRACTICE AGRICULTURE 63
able for introduction of beneficial insects and mites. Daily humidity maps are
among the products offered by weather information suppliers.
Harvest
Many management factors associated with harvest affect both the quality
and quantity of marketable product. Precision agriculture harvesting techniques
have been focused on quantitative measurements (yield and moisture content).
The yield data obtained during the harvest operation are a critical input to any
precision management system. However, data sets that describe subfield varia-
tion of crop development also have other implications. Producers must be con-
cerned with the quality aspects of their products. Product quality can be as impor-
tant a determinant of profitability as quantity.
In the future it may be possible to map product quality as well as yield. Real-
time quality sensors do not exist, but predictive crop models may be a substitute.
If crop growth models can use subfield-scale data to predict product quality, a
producer may be able to avoid harvesting a portion of the field, for example, that
has a high probability of containing aflatoxin. If a cotton field has areas with
significant differences in lint quality, the producer may choose to operate the
pickers so that the cotton would be placed in modules with more uniform quali-
ties. With sufficient knowledge of the product quality, GIS and differential global
positioning system technologies could be used to schedule harvest operations to
optimize marketing opportunities.
Marketing
Marketing is often considered to be the most important factor in the profit-
ability of agricultural production. Precision agriculture techniques have not yet
been developed to the point of significantly affecting crop marketing decisions.
However, the availability of a detailed data set that describes the growing condi-
tions and all chemical applications used in the production of a crop can have
economic value. Product identity and documentation can be particularly impor-
tant for products intended for human consumption. The use of detailed data sets
to predict yields several weeks in advance of harvest would be of great value in a
marketing plan that uses forward contracting or options. The communication tech-
nologies associated with precision agriculture could potentially provide market-
ers of agronomic products more complete information on market trends.
SUMMARY: EFFECT ON MANAGEMENT
The previous sections have described the various ways in which precision
agriculture will affect crop management. Some of the practices described are
based on documented experiences with these new technologies. Others are the
OCR for page 64
64 PRECISION AGRICULTURE IN THE 21ST CENTURY
committee’s best guess at future developments and their results. It is likely that
some predictions may be wrong, others may be technically possible but will never
become economically feasible, and unforeseen tools, techniques, and applica-
tions will be developed. Regardless of the accuracy of our vision, the crop man-
agement practices of the twenty-first century will be significantly affected by
these information technologies.
Crop yields typically represent only a small portion of the genetic potential
of the plants. The loss of yield potential is attributed to the many limiting factors
that can affect a plant during its life. The previous sections have described some
of those factors, and the decisions that producers can make over the course of a
cropping season. Precision agriculture has the potential to affect crop manage-
ment practices by reducing or removing the effects of limiting factors. It seems
clear from the evidence to date that precision agriculture technologies will be
used in the management of some factors for some crops in some regions. Major
limitations to adoption in a broader range of cropping systems include an incom-
plete understanding of agronomic parameters and their interactions, the cost of
obtaining site-specific data, and a limited ability to integrate information from
sources with varying resolutions and timing. There are many possibilities for
incorporating detailed information into management decisions. The realization of
those possibilities will depend on creative scientists and engineers inventing and
improving the tools of precision agriculture.
Representative terms from entire chapter:
crop production