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Research and Science
A UERNATIVE AGRICULTURE iS a systems approach to farming that is more
responsive to natural cycles and biological interactions than conven-
tional farming methods. For example, in alternative farming systems, farm-
ers try to integrate the beneficial aspects of biological interaction among
crops, pests, and their predators into profitable agricultural systems. Alter-
native farming is based on a number of accepted scientific principles and a
wealth of empirical evidence. Some of both are presented in this chapter.
The specific mechanisms of many of these phenomena and interactions
need further study, however. In general, much is known about some of the
components of alternative systems, but not nearly enough is known about
how these systems work as a whole.
Examples of practices or components of alternative systems that the com-
mittee has considered are listed below. Some of these practices are already
part of conventional farming enterprises. These practices include:
Crop rotations that mitigate weed, disease, and insect problems; in-
crease available soil nitrogen and reduce the need for synthetic fertiliz-
ers; and, in conjunction with conservation tillage practices, reduce soil
erosion.
Integrated pest management (IPM), which reduces the need for pesti-
cides by crop rotations, scouting, weather monitoring, use of resistant
cultivars, timing of planting, and biological pest controls.
Management systems to improve plant health and crops' abilities to
resist pests and disease.
Soil-conserving tillage.
Animal production systems that emphasize preventative disease man-
agement and reduce reliance on high-density confinement, costs asso-
ciated with disease, and need for use of subtherapeutic levels of antibi-
otics.
135
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136
ALTERNATIVE AGRICULTURE
ADVOCATES AND PRACTITIONERS OF ALTERNATIVE FARMING
SYSTEMS
Individuals who adhere to philosophies that advocate nonconventional
farming practices. Some farmers never changed to the chemically intensive,
specialized approach to crop and animal production that currently domi-
nates U.S. agriculture. These farmers include followers of traditional organic
farming movements, such as biodynamic agriculture and the systems ad-
vocated by Albert Howard and Eve Balfour [Balfour, 1976; Howard, 1943~.
These individuals also include farmers who farm organically because of
religious beliefs, such as some Amish and Mennonite farmers of Pennsyl-
vania and the Midwest. Others have practiced a generic form of organic
farming not associated with any of the established organic movements
tHarwood, 1983~.
Farmers looking for new ways to reduce production costs. Throughout
the United States, individual farmers have recognized that heavy purchases
of off-farm inputs can put them in a less competitive economic position.
These farmers have modified their farming practices, often in innovative
ways, to reduce production costs. Examples include a wide variety of
conservation tillage systems; the use of legume-fixed nitrogen through ro-
tations; interplanting; the substitution of manures, sewage sludges, or other
organic waste materials for purchased inorganic fertilizers; and the use of
IPM systems and biological pest control.
Farmers responding to consumer interest in chemical-free organic pro-
duce. Many enterprising farmers producing agronomic and horticultural
crops, milk, eggs, poultry, beef, and pork without synthetic chemical inputs
have taken advantage of the fact that many consumers and businesses are
willing to pay higher prices for these sorts of products. In response to
market demand, several commercial supermarket chains have recently be-
gun to market produce grown with no or very low levels of certain syn-
thetic chemical pesticides at prices roughly comparable to those of conven-
tionally grown produce.
Farmers responding to concerns about the adverse impact of many con-
ventional farming practices on the environment. Environmental groups and
soil conservation organizations have raised public awareness of the envi-
ronmental hazards of conventional agricultural practices. As a result of these
hazards and personal concern for the environment, some farmers have
adopted alternative farming practices that are helping to reduce the deteri-
oration of our nation's soil and water resources.
University research scientists. Critics have attacked the colleges and
schools of agriculture in the land-grant universities and the U.S. Department
of Agriculture (USDA} for not researching farming systems that protect the
environment and reduce dependence on synthetic chemical inputs. But
many individuals at these institutions have been investigating for years
practices and systems that have alternative agricultural applications. Exam-
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RESEARCH AND SCIENCE
pies include integrated pest management [IPM], biological controls of pests,
rotations, nitrogen fixation, timing of fertilizer applications, disease- and
stress-resistant plant cultivars, conservation tillage, and use of green manure
crops. These research efforts have fostered some important changes in U.S.
agriculture. As greater effort is made toward implementing the results of
this research, more progress can be expected in the future. Much of the
scientific knowledge of alternative practices summarized below is the result
of research at the land-grant universities and the USDA.
Alternative agriculture organizations. Groups such as Practical Farmers of
Iowa, the Land Stewardship Project, the Institute for Alternative Agriculture,
the Regenerative Agriculture Association, the Center for Rural Affairs, the
Land Institute, and many others have worked to provide farmers with
information on alternatives. They have organized research and demonstra-
tion projects, lobbied state legislatures and Congress for research and dem-
onstration support, and produced numerous technical publications and
reports with information designed to help and encourage farmers to adopt
alternatives.
137
Genetic improvement of crops to resist pests and diseases and to use
nutrients more effectively.
Many alternative agricultural systems developed by farmers are highly
productive (see the boxed article, "Advocates and Practitioners of Alterna-
tive Farming Systems," and Part Two). They typically share much in com-
mon, such as greater diversity of crops grown, use of legume rotations,
integration of livestock and crop operations, and reduced synthetic chemi-
cal use. Although many practices show great promise, the scientific bases
for many of them are often incompletely understood.
During the last four decades, agricultural research at the land-grant uni-
versities and the USDA has been extensive and very productive. Most of
the new knowledge has been generated through an intradisciplinary ap-
proach to research. Scientists in individual disciplines have focused their
expertise on one aspect of a particular disease, pest, or other agronomic
facet of a particular crop. Solving on-farm problems, however, requires more
than an intradisciplinary approach. Broadly trained individuals or interdis-
ciplinary teams must implement the knowledge gained from those in indi-
vidual disciplines with the objective of providing solutions to problems at
the whole-farm level. This interdisciplinary problem-solving team approach
is essential to understanding alternative farming practices.
Agricultural research has not been organized to address this need except
in a few areas, such as IPM, the use of organic residues as an alternative
nutrient source, and the use of leguminous green manure crops and rota-
tions for erosion control and as a nitrogen source. Even this research has
not significantly contributed to the adoption of alternative agricultural sys-
tems for two principal reasons. First, most research has focused on individ-
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ALTERNATIVE AGRICULTURE
ual farming practices in isolation and not on the development of agricultural
systems. This is because of the high expense of farming systems research,
the intradisciplinary nature of university research, and lack of resources.
Second, most research results have been implemented under policies that
encouraged ever-increasing per acre yields as the best way to increase farm
profits and the world food supply.
In contrast, alternative farming research must include the interaction and
integration of all farm operations and must consider the more comprehen-
sive goals of resource management, productivity, environmental quality,
and profitability with minimal government support. Only a limited amount
of research has taken this comprehensive approach. Nevertheless, the sci-
entific literature about specific farm practices and the empirical evidence
from individual operators illustrate the efficacy and potential of alternative
farming methods and provide the foundation on which to build a program
of alternative farming research.
Important elements of the scientific knowledge base relevant to further
development of alternative agricultural systems are briefly reviewed in the
following sections. Knowledge of biological systems and the management
of their interactions throughout agricultural ecosystems are emphasized.
CROP ROTATION
Crop rotation is the successive planting of different crops in the same
field. A typical example would be corn followed by soybeans, followed by
oats, followed by alfalfa. Rotations are the opposite of continuous cropping,
which involves successively planting the same field with the same crop.
Rotations may range between 2 and 5 years (sometimes more) in length and
generally involve a farmer planting a part of his or her land to each crop in
the rotation. Rotations provide many well-documented economic and envi-
ronmental benefits to agricultural producers (Baker and Cook, 1982; Heady,
1948; Heady and Jensen, 1951; Heichel, 1987; Kilkenny, 1984; Power, 1987;
Shrader and Voss, 1980; Voss and Shrader, 1984~. Some of these benefits
are inherent to all rotations; others depend on the crops planted and length
of the rotation; and others depend on the types of tillage, cultivation,
fertilization, and pest control practices used in the rotation. When rotations
involve hay crops, on-farm livestock or a local hay market are generally
required to make the hay crop profitable.
Much of the literature on crop rotations refers to the rotational effect
(Heichel, 1987; Power, 1987~. This term is used to describe the fact that in
most cases rotations will increase yields of a grain crop beyond yields
achieved with continuous cropping under similar conditions. This rota-
tional effect has been shown to exist whether rotations include nonlegumi-
nous or leguminous crops. Corn following wheat, which is not a legume,
produces greater yields than continuous corn when the same amount of
fertilizer is applied (Power, 1987~. The increase in crop yields following a
leguminous crop is usually greater than expected from the estimated quan-
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RESEARCH AND SCIENCE
139
Between 40 and 45 percent of the ld.S. corn
crop is grown in continuous monoculture.
Corn grown continuously generally requires
greater use of fertilizers and pesticides than
corn grown in rotation. This corn field is 10
miles from Kearney, Nebraska, which can
be seen on the horizon. Credit: U.S.
Department of Agriculture.
tity of nitrogen supplied (Cook, 1984; Goldstein and Young, 1987; Heichel,
1987; Pimente] et al., 1984; Voss and Shrader, 1984~. In fact, yields of grains
following legumes are often 10 to 20 percent greater than continuous grain
regardless of the amount of fertilizer applied.
Many factors are thought to contribute to the rotational effect, including
increased soil moisture, pest control, and the availability of nutrients. It is
generally agreed, however, that the most important component of this effect
is the insect and disease control benefits of rotations (Cook, 1984, 1986~.
The increase in soil organic matter, particularly in socI-based rotations, may
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ALTERNATIVE AGRICULTURE
Contour strip cropping can reduce erosion
and pest infestation. When a legume is
included in a rotation, such as the corn-
wheat-alfalfa rotation shown here, nitrogen
fertilizer needs can be decreased. Credit: Grant
Heilman.
be the basis for the improved physical characteristics of soil observed in
rotations. This may account for some yield increase. Certain deep-rooted
leguminous and nonTeguminous crops in rotations may use soil nutrients
from deep in the soil profile. In the process, these plants may bring the
nutrients to the surface, making them available to a subsequent shallow-
rooted crop if crop residue is not removed.
Another benefit common to ah rotations is the control of weeds, insects,
and diseases, particularly insects and diseases that attack the plant roots
(Cook, 1986~. This pest control is achieved primarily through the seasonal
change in food source (the crop), which usually prevents the establishment
of destructive levels of pests. As root disease and insect damage are re-
duced, the healthy root system is better able to absorb nutrients in the soil,
which can reduce the rates of fertilizers needed (Cook, 1984~. Healthy root
systems also take up nutrients more effectively, thus reducing the likelihood
of nutrient leaching out of the root zone.
Rotations with particular crops or crop combinations can provide addi-
tional benefits. Legumes in rotations will fix nitrogen from the atmosphere
into the soil. The amount of nitrogen fixed depends on the legume and the
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RESEARCH AND SCIENCE
141
management system; however, without any additional nitrogen fertilizers,
leguminous nitrogen can support high grain yields (Heichel, 1987; Voss and
Shrader, 1984~. The length of the rotation and yield expectations of the
farmers, however, influence the level and acceptability of these yields.
Hay and forage crops and closely sown field grain crops, such as wheat,
barley, and oats, can provide some soil erosion control benefits in rotations.
In some eroding areas with steep terrains, the practice of strip cropping
corn (a row crop) with wheat (a closely sown crop) or a hay crop, such as
alfalfa, is a common use of rotations to slow erosion. It must be stressed,
however, that tilIage practices greatly influence the erosion control benefits
of crops planted in rotations (Elliott et al., 1987~. For example, a rotation of
corn, soybeans, and wheat is excellent for disease control but not for erosion
control unless no tilIage or reduced tilIage is used.
An indirect but important benefit of all rotations is that they involve
diversification. The benefits of diversification are described in more detail
later in this chapter. In general, however, diversification provides an eco-
nomic buffer against price fluctuations for crops and production inputs as
well as the vagaries of pest infestations and the weather.
Rotations may have their disadvantages, however, particularly in the con-
text of current government subsidies and requirements for federal program
participation (see Chapters 1 and 4~. Rotations that involve diversifying
from cash grains to crops such as leguminous hays with less market value
involve economic tradeoffs (see Chapter 4~. Adopting the use of rotations
may also require purchasing new equipment. As with all sound manage-
ment practices, rotations must be tailored to local soil, water, economic,
and agronomic conditions.
PLANT NUTRIENTS
Soil, water, and air supply the chemical elements needed for plant growth.
Photosynthesis captures energy from the sun and converts it into stored
chemical energy by transforming carbon dioxide from the air into simple
carbohydrates. This stored chemical energy becomes the fuel for all life on
earth. Water is also needed to provide essential elements, transport nutri-
ents and sugars within plants, serve as a medium for essential chemical
reactions, and provide structural form and strength by exerting turgor pres-
sure from inside plant cells. Nutrient elements essential to the chemical
reactions that occur within the plant are taken up from the soil through the
roots. If nutrient elements or water are not adequately available at the time
they are needed, plant growth and development will be affected. Growth
and yield will be reduced or the plant may die.
Plants need three soil-derived nutrient elements in large amounts nitro-
gen, phosphorus, and potassium. These elements are frequently not avail-
able in adequate amounts from soil. Nitrogen is a constituent of all proteins
and a part of chlorophyll, the pigment that reacts to light energy. Nitrogen
is a component of nucleic acids and the coenzymes that facilitate cell reac-
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ALTERNATIVE AGRICULTURE
lions. Phosphorus, as a component of adenosine triphosphate (ATP), is
critical to the development and use of chemical energy within the cell.
Phosphorus is also a constituent of many proteins, coenzymes, metabolic
substrates, and nucleic acids. Unlike nitrogen and phosphorus, potassium
does not have a clear function as a constituent of chemical compounds
within the plant. It is important in regulatory mechanisms affecting funda-
mental plant processes, such as photosynthesis and carbohydrate translo-
cation. In addition to these three nutrients, other soil-supplied nutrients
are essential to plant growth and development: boron, calcium, chlorine,
cobalt, copper, iron, magnesium, manganese, molybdenum, sulfur, and
zinc. These elements are needed in small amounts that are often available
· .
In SOI ,.
Soil Properties anti Plant Nutrients
Soil Texture
The mineral particles that make up the soil are classified on the basis of
their size. Clay particles are the smallest, silt is intermediate, and sand
particles are the largest. The relative proportions of clay, silt, and sand
determine soil texture. Soil texture has a critical influence on water and
nutrient retention and movement through the soil. The large pores among
grains of sand in a sandy soil allow water to pass through with relative
ease, whereas the small pores formed in clay soils slow the flow and retain
water.
Soil particles can exist separately or they can be bound together in larger
aggregates. Organic colloids and clays play a critical role in binding soil
particles into soil aggregates, which increase pore space and water and air
movement.
Cation Exchange
The molecular surfaces of clays and organic colloids have a net negative
charge that interacts with the polar charge of surrounding water molecules.
This causes the colloids to bind with positively charged ions of elements
(cations). Because cations have differing abilities to bind with soil colloids,
one cation may displace another; this is referred to as cation exchange.
Displacement depends on relative bond strength and relative concentration.
The cation exchange capacity of a soil is an expression of the number of
cation-binding sites available per unit weight of soil (Foth, 1978~. This ca-
pacity has a significant effect on nutrient movement and availability and
binding of pesticides in different soils. Because hydrogen ions are cations
that compete with nutrient cations for exchange sites, soil acidity, which is
a measure of hydrogen ion concentration, has a marked effect on which
nutrient elements are bound and which are displaced.
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RESEARCH AND SCIENCE
Soil Quality
143
The quality of agricultural soils is derived from their effectiveness as a
medium that provides essential nutrients and water. Mineral elements in
soil required for plant growth exist in soluble and insoluble forms, which
affects their availability for plant uptake. For example, under acidic or alka-
line soil conditions, phosphorus fertilizer is rapidly converted into less
soluble compounds that may be nearly unavailable for plant nutrition. Even
available forms of phosphorus are bound to clay, and organic soil com-
pounds and are relatively immobile in the soil profile except as a passenger
during soil erosion. In contrast, potassium ant! the ammonium and nitrate
forms of nitrogen are more soluble than phosphorus. Nitrate ions are not
held by negatively charged soil ant! are readily leached. Because of their
positive charges, potassium and ammonium nitrogen are held on the cation
exchange and will not leach appreciably except through sandy soils.
Organic matter in soils influences plant growth in a number of ways. The
greatest benefits of organic matter in soil are its water-holding capacity; the
manner in which it alters soil structure to improve soil filth; its high ex-
change capacity for binding and releasing some mineral nutrients; its pres-
ence as a food source for soil microbiota that recycle soil nutrients; and its
mineralization to nitrogen, phosphorus, and sulfur. The cycling of mineral
nutrients between living organisms and dead organic components of the
soil system provides an important reservoir of the elements needed in plant
growth.
Nutrients are lost from soil through removal by crops, leaching, and soil
erosion. Nitrate nitrogen can also be lost from the soil by conversion to
nitrogen gases (denitrification) or by volatilization of ammonia. Gaseous
loss of sulfur can also occur. Some farming practices help to mitigate the
loss of nutrients and in some cases replace nutrients. For example, crop
rotations that include nitrogen-fixing legumes benefit the soil in several
ways. Legumes, in symbiotic relationships with microbes, fix atmospheric
nitrogen into nitrogen compounds available for plant nutrition. When le-
gumes are plowed under as green manures, they add nitrogen and organic
matter to the soil. Cover crops help hold nitrogen in the root zone during
the winter.
The accumulated scientific knowledge on the role and fate of mineral
elements, organic matter, ant! water in crop growth provides some indica-
tion of why some alternative farming practices succeed and others fail.
Characteristics of a particular crop or farming system that yield maximum
efficiency are not well understood, however. The task remains to assemble
the interdisciplinary expertise needed to analyze and understand the com-
plex relationships that contribute to the relative efficiencies of different
farming systems.
Nutrient Management
The adequate supply of nutrients particularly nitrogen, phosphorus, and
potassium and maintenance of proper soil pH are essential to crop growth.
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ALTERNATIVE AGRICULTURE
Ideally, soil nutrients should be available in the proper amounts at the time
the plant can use them; this avoids supplying an excess that cannot be used
by plants and may become a potential source of environmental contamina-
tion. The current conventional approach is to apply nutrients in the form of
fertilizers at levels needed for maximum profitability. Profitability in the
context of current government programs has generally been achieved, how-
ever, through maximum yield per acre, often in continuous cropping or
short rotations that require significant amounts of fertilizer. The nutrients
in any excess fertilizer or high levels of decomposing organic matter are
subject to leaching or runoff.
An alternative, more environmentally benign approach to nutrient man-
agement is to reduce the need for fertilizer through more efficient manage-
ment of nutrient cycles and precise applications of fertilizer. Such practices
include application of organic waste residues from animals and crops, crop
rotations with legumes, improved crop health that may result in better use
of nutrients, and banded or split applications of fertilizers. In mixed crop
and livestock operations, for example, many of the nutrients contained in
the grain and residue from crops grown on the farm can be returned to the
soil if the manure and crop residues are incorporated into the soil. Crop
rotations that include legumes can also play an essential role in nutrient
cycling, particularly for replenishing the nitrogen supply. Plant residues and
manure can release nitrogen more continuously throughout the growing
season than can common commercial fertilizers. However, nitrogen from
organic sources may be released when crops are not actively absorbing it.
In contrast, inorganic fertilizer nitrogen is relatively quickly converted to
the soluble and leachable nitrate form.
Efforts to provide adequate nutrition to crops continue to be hindered by
inadequate understanding and forecasting of factors that influence nutrient
storage, cycling, accessibility, uptake, and use by crops during the growing
seasons. Soil testing and plant tissue analysis can provide the farmer with
information to assure adequate nutrition for all agronomic and horticultural
crops. But variable soil and climatic conditions that influence nutrient up-
take and Toss make it difficult to predict the most profitable and environ-
mentally safe levels of nutrients. As a result, farmers often follow broad
guidelines that lead to insufficient or excessive fertilization. For example,
~ . ~ ~ .~ . ~ . . ~ ~ . ~ . ~ _ 1 _ _ _1
studies of tertlllzer recommendations revealed tnat some commercial So
testing services consistently recommended the use of far more fertilizer
than was needed (Olson et al., 1981; Randall and Kelly, 19871. Additionally,
some farmers apply more nitrogen than is recommended.
Nitrogen
Nitrogen is the soil-derived plant nutrient most frequently limiting grain
production in the United States. This is ironic because the atmosphere is 79
percent nitrogen by volume. Atmospheric nitrogen is in the form of inert
nitrogen gas, however, which higher-order plants cannot use. Converting
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RESEARCH AND SCIENCE
145
atmospheric nitrogen to ammonia and other forms that plants can use
requires a high energy input. This is true for biological nitrogen fixation as
wed as industrial synthesis. The biological process is fueled by photosyn-
thates; the synthetic industrial process is fueled by natural gas, petroleum,
coal, or hydroelectric power. The predominant process for producing syn-
thetic nitrogen fertilizers involves combining hydrogen from methane gas
and atmospheric nitrogen at high temperature and pressure to form am-
monia. Ammonia can then be converted to nitric acid or combined with
other elements to form a number of nitrogen fertilizers, including ammo-
nium nitrate, ammonium sulfate, ammonium phosphate, and urea. A sig-
nificant amount of energy is required to synthesize ammonia. Conse-
quently, energy and methane gas costs can affect the availability and cost of
synthetic nitrogen fertilizers.
Neutral ammonia molecules gain a hydrogen ion when added to moist
soil and become stable ammonium ions with a net positive charge. Most of
the ammonium ions in soil undergo biological Vitrification, in which oxida-
tion results in the formation of a nitrate ion as well as hydrogen ions that
acidify the soil. Because ammonium ions have a positive charge, they are
adsorbed and held on the soil cation exchange. Nitrate ions, because of
their negative charge, are not adsorbed on the soil exchange complex. While
readily available for plant use, the nitrate freely moves through soil in water
unless it is absorbed by the plant.
Although these basic processes are understood, there is a need to know
much more about nutrient cycling and the behavior of nitrogen under
various environmental conditions. To accomplish this, progress is needed
in estimating the rates of biological reactions that control nitrogen transfor-
mation in soil.
.. . . , . . . . - _ - ~ . ~ .
Legumes as a Source of Nitrogen
Nitrogen can be provided by growing legumes in rotation with grains.
For alternative farming, legumes are an effective and often profitable way
to supply nitrogen. Leguminous nitrogen is consistently released through-
out the growing season when temperatures are high enough to permit
microbial decomposition. Combiner! with the rotational effect, leguminous
nitrogen can support high yields of corn and wheat (Holben, 1956; Koerner
and Power, 1987; Voss anct Shrader, 1984~. The overall contribution of leg-
umes, however, depends on the management system and climate. For ex-
ample, forage legumes are most effective in humid and subhumid regions
(Meisenbach, 1983; U.S. Department of Agriculture, 1980~. In regions with
less than 20 inches of rain a year, deep-rooted, nonirrigated legumes may
decrease subsoil moisture and lead to reduced corn yields the following year
(Meisenbach, 1983~. The profitability of leguminous hay crops is strongly
influenced by the presence of on-farm livestock or a local hay market.
Legumes supply substantial nitrogen to the soil, but the amount of nitro-
gen fixed is highly variable. Different species and cultivars fix different
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ALTERNATIVE AGRICULTURE
For a variety of reasons, most plant diseases cannot be directly controlled.
For example, many of the fungi that infect plant roots have not been fully
investigated, nor has their importance in affecting yield been quantified. In
most cases, farmers cope with diseases by using good farm management
practices and planting resistant varieties. When combined with the existing
natural level of biological control, management and resistant varieties keep
the majority of diseases in check. Nonetheless, disease can still cause sig-
nificant yield loss. Moreover, germplasm for desirable resistance has not
been identified for many of the worId's important crops. More research is
needed to better characterize available germplasm for genetic resistance to
disease and plant transformation.
The development and durability of resistant varieties have been a chal-
lenge to plant pathologists and plant breeders. Genetic strategies to im-
prove the durability of resistance include use of multilines and cultivar
mixtures as well as multigenic or horizontal resistance. Modern genetic
technology will speed the development of resistant crops. It should be
possible to identify genes that confer resistance to a specific pathogen.
These genes would then be introduced to the appropriate plant, without
incorporating other genes that may confer detrimental characteristics. This
gene transfer has been achieved to produce resistance to several plant vi-
ruses in tobacco plants (National Research Council, 1987a). Advances in
understanding the genetic and molecular bases of disease in plants promise
major improvements in plant disease control using genetic rather than
chemical methods (Goodman, 1988~.
Cultural practices such as crop rotations, alteration of soil pH, sanitation,
and adjustment of the timing of planting and harvest to avoid peak periods
of the pathogens complement genetic resistance in many situations. For
example, raising soil pH with lime from 6.5 to 7.5 reduces the severity of
fusarium wilts on tomato and potato crops in Florida (Jones and Woltz,
1981~. Lowering soil pH with sulfur to 5.0 controls potato scab caused by
Streptomyces scabies (Oswald and Wright, 19501. Forms of nitrogen also can
play a significant role in disease severity. For example, ammonium nitrogen
suppresses the disease take-all in wheat but nitrate favors it (Huber et al.,
1968~.
Tillage practices can have effects on pathogen populations and resultant
diseases. Ecofallow is a form of conservation tillage that can reduce stalk
rot of sorghum but permits increases in other diseases (Cook and Baker,
1983~.
Harvesting and processing practices can also influence the inception of
disease. The hydrostatic pressure from tank-washing potatoes causes water
infiltration of pathogens into the lenticels of the tubers, predisposing them
to attack by bacterial soft rot (Bartz and Kelman, 1985~. Potatoes are then
generally treated with a fungicide. There is enormous need and potential
to control diseases by nonchemical methods (Cook and Baker, 1983~. But
there remains a lack of understanding of the underlying mechanisms that
affect disease incidence and severity.
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185
Synthetic chemical control of plant pathogens has become an increasingly
important pest control tactic as agriculture has shifted toward intense cul-
tivation of monocultures (Delp, 1983~. Practices previously used to control
pathogens, such as crop rotations, are not compatible with current crop
specialization (Tweedy, 1983~. Because commercial cultivars are genetically
related, the loss of resistance to pathogens could cause serious problems if
fungicides were not available. In California, the use of methyl bromide and
chloropicrin soil fumigation resulted in huge increases in yield and quality
in several crops (Wilhelm and Paulus, 1980~. This combination is widely
credited with saving the strawberry industry from high production costs
and foreign competition.
Although the total amount of fungicides used in the United States is
much less than the amount of herbicides or insecticides, the potential
chronic health risk to humans is significant. Ninety percent (by weight) of
fungicides applied are known to cause tumors in laboratory animals. Fun-
gicicle residues in food are responsible for the largest share of potential
dietary oncogenic risk from pesticides. Developing fungicides not toxic to
nontarget organisms, including humans, is difficult; very few new fungi-
cides have reached the market. Only four nononcogenic fungicides have
been introduced in the past 15 years that have captured greater than 5
percent of any food crop market (National Research Council, 1987b). In
recent years, there has been a movement toward the development of highly
specific systemic fungicides, but this has accelerated evolutionary selection
of fungicide-resistant plant pathogens. Research to understanc! the mecha-
nisms of resistance could aid the development of chemicals with new modes
of action ant! better-targeted effects.
The introduction or application of biological control agents has not been
very successful with plant pathogens because of the great complexity in
microbial communities. Although many of the management practices that
indirectly control diseases strike a balance between beneficial and deleteri-
ous microorganisms, there is insufficient knowledge to effectively develop
and use biological control agents commercially (Schroth and Hancock, 19851.
Little is known concerning the ecology, classification, and physiology of
biological control organisms or the underlying mechanisms affecting the
interactions among beneficial microorganisms, pathogens, and plants.
The potential to use microorganisms against microorganisms has stirred
the interest of many investigators. A number of companies have pioneered
efforts to develop biological control agents for plant pathogens. Several
products have already reached) the market. An avirulent, antibiotic-produc-
ing strain of Agrobacterium is available to control crown gall tumors of orna-
mental plants and orchard trees caused by Agrobacterium tumefaciens
(National Research Council, 1987a). Plans are underway to market a root-
colonizing Pseudomonas bacterium as a control for Rhizoctonia and Pythium
fungi in cotton.
Another interesting disease control possibility is to stimulate a plant's
own defense system with chemicals or by inoculation with an avirulent
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ALTERNATIVE AGRICULTURE
form of a pathogen. The citrus tristeza virus from Africa entered Brazil in
the 1920s and nearly decimated the citrus industry. In the 1950s, researchers
found a mild strain of the virus that protected trees from the severe strain.
Commercial inoculation with the mild virus began in the late 1960s and has
been very successful so far (National Research Council, 1987a). It remains
unlikely, however, that disease control in continuous crop monocultures in
certain regions, such as fruit and vegetable production in the East and
Southeast, will be possible without use of synthetic chemical fungicides
and fumigants. Disease pressures in areas with high temperatures and
humid*y and long growing seasons are so severe that only dramatic changes
in production systems will enable widespread adoption of alternative dis-
ease control measures.
Alternative Nematode Control
Nematode control is particularly difficult. Strategies include genetic resis-
tance, chemical control, and cultural methods such as rotations (see the
BreDahT, Kutztown, Thompson, and Kitamura case studies). Genetic resis-
tance is successful in only a few cases. Chemical control, which is feasible
only in certain situations, relies on broad-spectrum, highly toxic, and often
volatile materials. It is expensive and hazardous. The decline of basic cul-
tural practices such as rotations, particularly in the Midwest, has led to an
increase in nematodes in soybeans. Rotating corn with soybeans will control
most nematode problems. Current research for nematode control is focusing
on the development of effective cultural practices such as those traditionally
practiced before the advent of broad-spectrum nematocides.
Genetic research to develop nematode-resistant cultivars has been suc-
cessfuT in sugar beets and tomatoes (Goodman et al., 19871. More research
is necessary to determine how various nematodes damage different crops
and how to modify practices if a combination of nematode species is pres-
ent. Similarly, the accuracy and efficiency of techniques for estimating nem-
atode populations needs to be improved.
The biological antagonism level of the soil must be determined if manage-
ment decisions are to be based on an understanding of the relationship
between yield and population density of nematodes. A given number of
nematodes will affect the same crop differently in soils of differing biota.
More basic studies in biological control and interactions in the rhizosphere
are required. Improved assay techniques for assessing the biological antag-
onism coefficients of various soils must be developed.
One promising biological control agent is the pathogenic bacterium Pas-
teuria penetrans, which is effective against several economically important
nematodes. It is expensive to produce on a large scale, however. A less
expensive, but also less effective, biological control option is the use of
plants such as CrotaZaria spectabilis that prevent the nematode from repro-
ducing. Coastal Bermuda grass (Cynodon daclylon) incorporated before
planting lespedeza, tobacco, or vegetable transplants protects against root-
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RESEARCH AND SCIENCE
187
knot nematodes (Meloidogyne spp.) (Burton and Johnson, 1987). Coastal
Bermuda grass will also reestablish itself after the annual crop is harvested.
These plants could be even more effective if they could be genetically
engineered to produce nematode attractants or pheromones.
The selection and development of varieties for resistance and tolerance to
nematode stress will continue. This may involve incorporation of appropri-
ate genetic material into varieties already selected for production, economic,
and marketing qualities. It is still important to develop biological and chem-
ical nematocides that are systemic, easily associated with the root system,
target organism specific, or a combination of these factors. These pesticides
will allow flexibility in management decisions and compensation for man-
agement errors that have promoted or amplified nematode stress problems
in a particular production system.
Alternative Weed Control
Farmers in the United States depend greatly on herbicides to control
weeds. Nearly two-thirds of U.S. pesticide purchases are for herbicides. But
a variety of other means, such as crop rotations, mechanical cultivations,
competition with other plants, and biological control through natural ene-
mies can control weeds (see Spray, BreDahI, Sabot Hill, Kutztown, Thomp-
son, Pavich, and Lundberg case studies). In fact, growers are often unaware
of the forces naturally controlling weeds. The purslane sawfly and the
leafmining weevil, for example, help control pursTane in California. These
insects would be even more effective if their populations were not reduced
by insecticide use. The moth Bactra verutana suppresses the weed Cyperus
rotundus that infests cotton in Mississippi. More than 70 plant-feeding in-
sects and plant pathogens have been introduced to control weeds in the
United States; 14 weed species are now controlled in this way (National
Research Council, 1987a; Osteen et al., 1981). Few weeds are controlled
biologically in agriculture, however, although future opportunities are nu-
merous. For example, many of the hundreds of species of carabid beetles
are seed eaters and could play a role in weed control (Andres and Clement,
1984).
Cultural practices are currently the most effective alternative to herbi-
cides. Cultivating, rotary hoeing, increasing the density of the crop plant
to crowd out weeds, intercropping, timing of planting to give the crop a
competitive advantage, and transplanting seedling crop plants to give them
a head start on weeds are currently practiced and effective measures. Trans-
planting tomatoes to a high density has successfully controlled the growth
of shade-intolerant redroot pigweed. Clover planted as an understory or
living mulch reduces weed growth in corn. Several combinations of cover
crops and tillage practices are effective in controlling weeds in corn and
soybeans.
Weed-tolerant crops and crops that produce substances toxic to weeds are
potentially promising approaches that have received little research atten-
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ALTERNATIVE AGRICULTURE
lion. Naturally occurring phytotoxic allelopathic chemicals, however, may
not always be safer than some of the more undesirable synthetic herbicides.
Introducing weed diseases is also a possibility. The rust Puccinia chondriZZina
controls the rush skeleton weed, which is a problem in wheat and pasture
areas. AZternaria macrospora can inhibit the growth of spurred anode, a
damaging weed in cotton production that is resistant to several cotton
herbicides.
The development of herbicide-resistant crops may offer opportunities to
substitute safer herbicides for more dangerous herbicides. For example,
efforts are being made to develop crops resistant to the herbicide glyphos-
ate, a compound with very low mammalian toxicity. Like other broad-
spectrum herbicides, glyphosate has limited use in crop production because
it destroys crops as well as weeds and therefore must be used before crop
germination or with special application methods and equipment. In re-
sponse to this problem, researchers have isolated glyphosate-resistant genes
and successfully transferred them to poplar trees, tobacco, and tomatoes
(Della-Croppa et al., 1987; Stalker et al., 1985~. If the plants tolerate gly-
phosate, the herbicide could then be used as a postemergent treatment. In
certain cases, this strategy could reduce weed control costs, improve weed
control quality, and reduce human health hazards.
SUMMARY
Alternative farming encompasses a range of farming practices, including
the use of crop rotations, IPM, biological and cultural pest control, use of
organic materials to enhance soil quality, different tilIage methods, and
animal rearing techniques that involve less reliance on antibiotics and con-
finement. The unifying premises of alternative systems are to enhance and
use biological interactions rather than reduce and suppress them and to
exercise prudence in the use of external inputs.
Research has not fully addressed the integration of study results essential
to the adoption of a number of alternative farming methods as unified
systems. Although some components of alternative systems have been ex-
amined, they have been generally studied in isolation. Lack of systems
research is a key obstacle to the adoption of a number of alternative farming
practices. On the whole, land-grant universities and the USDA have not
adequately integrated the results of this research into production systems.
Nonetheless, a significant amount of scientific evidence exists that sup-
ports the effectiveness of a range of alternative practices. There is a large
body of information about the value of legumes in fixing nitrogen, improv-
ing soil quality, reducing erosion, and increasing yields of subsequent crops.
{PM programs are effective, profitable, and increasingly adopted. Although
biological and natural controls are underused, they have been demonstrated
to be effective and warrant increased research support. Genetic engineering
techniques should enhance this aspect of IPM. The integration of livestock
OCR for page 189
RESEARCH AND SCIENCE
189
into farming systems provides additional means for nutrient cycling. Im-
proving forage digestibility needs further research, however.
The scientific basis for some of these practices and their interaction in
agricultural systems is not always understood, but they work. Many farmers
have adopted them and are using them profitably. The economics of these
and other alternative farming practices and systems are discussed in the
following chapter.
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Representative terms from entire chapter:
biological control
