Abstract
Alternating strips of alfalfa with corn in northeast Iowa.
Photo by: Tim McCabe, USDA-NRCS |
Intercropping offers farmers the opportunity to engage nature's
principle of diversity on their farms. Spatial arrangements of plants,
planting rates, and maturity dates must be considered when planning
intercrops. Intercrops can be more productive than growing pure
stands. Many different intercrop systems are discussed, including
mixed intercropping, strip cropping, and traditional intercropping
arrangements. Pest management benefits can also be realized from
intercropping due to increased diversity. Harvesting options for
intercrops include hand harvest, machine harvest for on-farm feed,
and animal harvest of the standing crop.
Table of Contents
Principles
Sustainable agriculture seeks, at least in principle, to use nature
as the model for designing agricultural systems. Since nature consistently
integrates her plants and animals into a diverse landscape, a major
tenet of sustainable agriculture is to create and maintain diversity.
Nature is also efficient. There are no waste products in nature.
Outputs from one organism become inputs for another. One organism
dies and becomes food for other organisms. Since we are modeling
nature, let us first look at some of the principles by which nature
functions. By understanding these principles we can use them to
reduce costs and increase profitability, while at the same time
sustaining our land resource base.
Diversity is nature's design
When early humans replaced hunting and gathering of food with domestication
of crops and animals, the landscape changed accordingly. By producing
a limited selection of crop plants and animals, humankind has greatly
reduced the level of biological diversity over much of the earth.
Annual crop monocultures represent a classic example. In response
to this biological simplification, nature has struggled to restore
diversity to these landscapes—that is her tendency. Our "war"
with nature over the tendency to diversity is what we call "weed
control" and "pest management." Of course we could
hardly produce any crops if we simply allowed our fields to return
to natural vegetation, but we can realize some of the benefits of
diversity by planting mixtures of different crops.
Cooperation is more apparent than competition
There is far more cooperation in nature than competition. Cooperation
is typified by mutually beneficial relationships that occur between
species within communities. In The Redesigned Forest, ecologist
Chris Maser offers a glimpse of the cooperation inherent in a northern
temperate forest when he describes a relationship that exists among
squirrels, fungi, and trees. (1) The squirrels
feed on the fungus, then assist in its reproduction by dropping
fecal pellets containing viable fungal spores onto the forest floor.
There new fungal colonies establish. Tree feeder roots search out
the fungi and form a symbiotic association that enables the tree
roots to increase their nutrient uptake. The fungi, in turn, derive
food from the tree roots. Each benefits from the other's presence
or actions.
If we view competition as the driving force in nature, we fail
to see the complex relationships and feel compelled to take actions
that may have unforeseen impacts. The rancher who views coyotes
as competitors (for calves and lambs) and kills them out may later
find the predator helped keep rodent populations in check. With
the predator gone, rodent numbers explode and cause more problems
than ever before. The same is true with many insect pests of crops.
When the only food for insects is crops, that is what they will
eat. With no predator or parasite habitat present in a pure stand
of crop, the pest insect could not possibly have it better. If we
can shift our view of nature from a theme of competition to one
of collaboration, we can act in ways that yield fewer negative consequences.
(2)
Stability tends to increase with increasing diversity
If left undisturbed and unplanted, an abandoned crop field will
first be colonized by just a few species of plants, insects, bacteria,
and fungi. After several years, a complex community made up of many
wild species develops. Once a wild plant and animal community has
reached a high level of diversity, it remains stable for many years.
When wild communities are in the early stages of development, or
when they have lost diversity due to natural catastrophe or human
actions, they are prone to major fluctuations, both in types of
species present and in their numbers. Disease outbreaks in plants
and animals occur more frequently—as do outbreaks of weed,
insect, bird, or rodent pests. One good example is the grasshopper
plagues that follow regional weather shifts. Another is the shift
in weed species dominance following a soil disturbance.
The more complex and diverse communities become, the fewer the
fluctuations in numbers of a given species, and the more stable
the communities tend to be. As the number of species increases,
so does the web of interdependencies. In both higher and lower rainfall
years, there are fewer increases in any one species and fewer fluctuations
in the community as a whole. (2)
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Pursuing Diversity on the Farm
So, then, how can we begin to model our agricultural pursuits after
some of these natural principles? Can we look for patterns in nature
and imitate them? Some pioneering farmers have been able to utilize
nature's principle of diversity to their advantage. Results
of their efforts include lower cost of production and higher profits.
Among the practices that promote diversity and stability are:
Enterprise diversification—Risk reduction through
stability of income and yield are two of the reasons people diversify
their crop and livestock systems. Increasing diversity on-farm also
reduces costs of pest control and fertilizer, because these costs
can be spread out over several crop or animal enterprises.
Crop Rotation—Moving from simple monoculture to
a higher level of diversity begins with viable crop rotations, which
break weed and pest life cycles and provide complementary fertilization
to crops in sequence with each other.
Farmscaping—Diversity can be increased by providing more
habitat for beneficial organisms, habitats such as borders, windbreaks,
and special plantings for natural enemies of pests. See the
ATTRA publication Farmscaping
to Enhance Biological Control for more information on special
plantings for beneficial insects.
Intercropping—Intercropping is the growing of two
or more crops in proximity to promote interaction between them.
Much of this publication focuses on the principles and strategies
of intercropping field crops. A related ATTRA publication, Companion
Planting, provides more information on intercropping of
vegetable crops.
Integration—On-farm diversity can be carried to
an even higher level by integrating animals with intercropping. With
each increase in the level of diversity comes an increase in stability.
This publication focuses on intercropping and provides a section
on integrating livestock with crops.
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Intercropping Concepts
Most grain-crop mixtures with similar ripening times cannot be
machine-harvested to produce a marketable commodity, since few buyers
purchase mixed grains. Because of limited harvest options with that
type of intercropping, farmers are left with the options of hand
harvesting, grazing crops in the field with animals, or harvesting
the mixture for on-farm animal feed. However, some intercropping
schemes allow for staggered harvest dates that keep crop species
separated. One example would be harvesting wheat that has been interplanted
with soybeans, which are harvested later in the season. Another
example is planting harvestable strips, also known as strip cropping.
When two or more crops are growing together, each must have adequate
space to maximize cooperation and minimize competition between them.
To accomplish this, four things need to be considered: 1) spatial
arrangement, 2) plant density, 3) maturity dates of the crops being
grown, and 4) plant architecture.
Spatial Arrangement
There are at least four basic spatial arrangements used in intercropping.
Most practical systems are variations of these. (3)
- Row intercropping—growing two or more crops at
the same time with at least one crop planted in rows.
- Strip intercropping—growing two or more crops
together in strips wide enough to permit separate crop production
using machines but close enough for the crops to interact.
- Mixed intercropping—growing two or more crops
together in no distinct row arrangement.
- Relay intercropping—planting a second crop into
a standing crop at a time when the standing crop is at its reproductive
stage but before harvesting.
Plant Density
To optimize plant density, the seeding rate of each crop in the
mixture is adjusted below its full rate. If full rates of each crop
were planted, neither would yield well because of intense overcrowding.
By reducing the seeding rates of each, the crops have a chance to
yield well within the mixture. The challenge comes in knowing how
much to reduce the seeding rates. For example, if you are planning
to grow corn and cowpeas, and you want mostly peas and only a little
corn, it would be easy to achieve this. The corn-seeding rate would
be drastically cut (by 80% or more), and the pea rate would be near
normal. The field should produce near top yields of peas even from
the lower planting rate and offer the advantage of corn plants for
the pea vines to run on. If you wanted equal yields from both peas
and corn, then the seeding rates would be adjusted to produce those
equal yields.
Maturity Dates
Planting intercrops that feature staggered maturity dates or development
periods takes advantage of variations in peak resource demands for
nutrients, water, and sunlight. Having one crop mature before its
companion crop lessens the competition between the two crops. An
aggressive climbing bean may pull down corn or sorghum growing with
it and lower the grain yield. Timing the planting of the aggressive
bean may fix the problem if the corn can be harvested before the
bean begins to climb. A common practice in the old southern U.S.
cotton culture was to plant velvet beans or cowpeas into standing
corn at last corn cultivation. The corn was planted on wide 40-inch
rows at a low plant population, allowing enough sunlight to reach
the peas or beans. The corn was close enough to maturity that the
young legumes did not compete. When the corn was mature, the beans
or peas had corn stalks to climb on. The end result was corn and
beans that would be hand harvested together in the fall. Following
corn and pea harvest, cattle and hogs would be turned into the field
to consume the crop fodder.
Selecting crops or varieties with different maturity dates can
also assist staggered harvesting and separation of grain commodities.
In the traditional sorghum/pigeonpea intercrop, common in India,
the sorghum dominates the early stages of growth and matures in
about four months. Following harvest of the sorghum, the pigeonpea
flowers and ripens. The slow-growing pigeonpea has virtually no
effect on the sorghum yield. (4)
Plant Architecture
Plant architecture is a commonly used strategy to allow one member
of the mix to capture sunlight that would not otherwise be available
to the others. Widely spaced corn plants growing above an understory
of beans and pumpkins is a classic example.
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Intercrop Productivity
One of the most important reasons to grow two or more crops together
is the increase in productivity per unit of land. Researchers have
designed a method for assessing intercrop performance as compared
to pure stand yields. In research trials, they grow mixtures and
pure stands in separate plots. Yields from the pure stands, and
from each separate crop from within the mixture, are measured.
From these yields, an assessment of the land requirements per unit
of yield can be determined. This information tells them the yield
advantage the intercrop has over the pure stand, if any. They then
know how much additional yield is required in the pure stand to
equal the amount of yield achieved in the intercrop. The calculated
figure is called the Land Equivalency Ratio (LER). To calculate
an LER, the intercrop yields are divided by the pure stand yields
for each component crop in the intercrop. Then, these two figures
are added together. Here's the equation for a corn/pea intercrop
where the yields from pure corn, pure peas, and the yields from
both corn and peas growing together in an intercrop are measured.
(intercrop corn / pure corn) + (intercrop pea / pure pea) = LER
When an LER measures 1.0, it tells us that the amount of land required
for peas and corn grown together is the same as that for peas and
corn grown in pure stand (i.e., there was no advantage to intercropping
over pure stands). LERs above 1.0 show an advantage to intercropping,
while numbers below 1.0 show a disadvantage to intercropping. For
example, an LER of 1.25 tells us that the yield produced in the
total intercrop would have required 25% more land if planted in
pure stands. If the LER was 0.75, then we know the intercrop yield
was only 75% of that of the same amount of land that grew pure stands.
In a South Carolina study, researchers planted intercrops of southern
peas and sweet corn at three different corn plant densities. (5)
The plantings were on raised beds with flat and wide crowns on six-foot
centers. In the center of each bed was a corn row, with two rows
of peas planted 18 inches to either side of the corn row (see Figure
1). The low corn-seeding rate was 6,700 plants per acre, medium
corn was 9,500 per acre, and high was 11,900 plants per acre. Peas
were established at a rate of 31,800 plants per acre in all intercrop
plots. In the pure pea stand, each bed had two rows of peas spaced
24 inches apart. Yields of the intercrops and pure stands are shown
in Table 1.
In this trial there was a yield advantage from intercropping over
growing the two crops in pure stands. Pea yields suffered from the
increased competition in the higher densities of corn. Some practical
on-farm guidelines can be drawn to guide seeding-rate choices for
a two-crop intercrop. To test seeding rates, experiment with three
small plantings of two crops at the following percentages of their
full seeding rates: 1/3 + 2/3, 1/2 + 1/2, and 2/3 + 1/3. From there,
make adjustments for future plantings based on the results and your
expectations.
Table
1. Yields of sweet corn and southern peas from intercrops
(5) |
Seed Rates |
Corn
(pounds/acre) |
Peas (pounds/acre) |
LER |
Full corn |
5600 |
*** |
*** |
Full peas |
*** |
1200 |
*** |
Low corn |
4200 |
800 |
1.41 |
Medium corn |
4600 |
800 |
1.48 |
High corn |
5000 |
500 |
1.30 |
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Managing Intercrops
Figure 1. Sweetcorn and southern pea planting pattern |
Many combinations of crops have been grown or experimented with
as mixed or relay intercrops. Some of these include sunflowers grown
with black lentils, wheat with flax, and canola with flax. Other
combinations include cucumbers, beans, celery, and chives in China;
upland rice, corn, and cassava in Indonesia, and in various parts
of the tropics corn and cassava, corn and peanut, sorghum and millet,
and sorghum and pigeonpeas.
Frequently these cropping combinations involve a short and a tall
crop both planted at the same time. In many cases the tall crop
is harvested first. For example, corn grown with a shorter plant
would be harvested first, then peanut or sweet potato would be harvested
later. Another pattern would be planting two tall crops with different
growth rates. In relay intercrops, different planting dates are
used so that one crop might mature sooner. Corn or sorghum, requiring
three months to mature, can be grown with pigeonpea, requiring 10
months to maturation.
John Bowen and Bernard Kratky, researchers and instructors at the
University of Hawaii, tell us that there are five distinct aspects
to successful multiple cropping. These are 1) detailed planning,
2) timely planting of each crop, 3) adequate fertilization at the
optimal times, 4) effective weed and pest control, 5) efficient
harvesting. (6) Before any fieldwork is begun,
adequate planning should be done. Planning covers selection of crop
species and appropriate cultivars, water availability, plant populations
and spacing, labor requirements throughout the season, tillage requirements,
and predicted profitability of the intercrop. These and other parameters
need to be evaluated before spending money on inputs.
With any crop, seed germination and seedling establishment are
the most critical phases of the entire season. A good seedbed is
needed to get a good stand. Delayed planting may reduce yield, since
crop development may not coincide with the optimal growth periods.
Planning fertilization for intercrops can be challenging, as the
full needs of both crops must be met. Generally, there is little
information available on how to go about this. One possibility would
be to ask for soil test results for each crop separately, then formulate
a recommendation that will cover the needs of both crops to be grown.
Such recommendations are generally 10% to 30% higher than rates
for individual crops.
As with any crop, also accounting for residual or carryover fertility
from past crops saves money. Carryover fertility from intercrops
may well be lower than that of pure stands because of the two crops
having different root types and feeding habits.
Weed and pest controls need in intercrops will likely be different
from those in pure stands. Some disease incidence, such as soybean
or mung bean rusts, may increase when aggravated with high corn
populations and overfertilization. Any disease or pest that prospers
in shady conditions could increase under a taller crop such as corn
or sunflowers. In many cases, insect pest populations are lower
when two or more crops are grown together. More on pest management
will be found later in this publication.
Harvesting of mixed intercrops has been a major limitation to their
adoption in mechanized farming. As mentioned earlier, if the crops
cannot be harvested by animals, or all together as feed, you're
left with hand harvesting. Some crops such as flax and wheat have
been harvested together and mechanically separated. Any other mechanized
harvest efforts must get one crop without damaging the other. One
example would be harvesting wheat over the top of a young stand
of soybeans growing beneath the grain heads. All intercropping strategies—
especially mixed intercropping—require advanced planning and
keen management. Success will likely be the reward for such efforts.
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Examples of Intercrop Systems
Traditional Corn-Bean-Squash Mixed Intercrops
Farmers throughout Central America traditionally grow an intercrop
of corn, beans and squash. Grown together, these three crops optimize
available resources. The corn towers high over the other two crops,
and the beans climb up the corn stalks. The squash plants sprawl
along the ground, capturing light that filters down through the
canopy and shading the ground. The shading discourages weeds from
growing.
This mixture was compared to the individual crops grown separately
in a study near Tabasco, Mexico. (7) In the
study, corn yields were considerably higher in the mixture than
in a pure stand planted at optimum densities. Bean and squash yields
suffered considerable yield reductions when grown in mixture. In
this example if corn were the most important crop, it was beneficial
to grow it in a mixture with squash and beans. The beans and squash
were just a bonus. The LER for the whole mixture was considerably
higher (1.6) than any of the pure stands. See Table
2 for details.
Table
2. Yields of corn, beans and suash grown alone or in a mixture
(7) |
Crop |
Pure Stand
(pounds/acre) |
Intercrop
(pounds/acre) |
Corn |
1096 |
1533 |
Beans |
544 |
98 |
Squash |
383 |
71 |
Corn and Soybean Mixed Intercrops
Canadian researchers (8) have worked with several
corn-soybean intercrop seeding rates to determine their economic
advantages as silage. Pure stands of corn and soybeans were grown
for comparison at 24,000 corn seed per acre and 200,000 soybean
seed per acre. Results showed that intercrops were more cost effective
than pure stands over both years the study was conducted. The study
featured five experimental intercrop seeding rates with two planting
arrangements (alternate and within the row). The researchers concluded
that a planting rate of 16,000 corn seed per acre (67% of the full
corn rate) with 135,000 soybean seed per acre (67% of the full bean
rate) planted within the same rows along with 53 lbs. of N/acre
gave the highest economic returns. (Note: the planter was set to
drop 151,000 seeds per acre.) This mixture gave an LER of 1.14 over
pure stand yields. The crude protein level of the intercrop silage
was considerably higher than that of pure corn silage. A slightly
higher yield was achieved from full stands of both corn and beans
in alternate rows (LER=1.23), but the cost of production was higher,
thus offsetting the improved yields.
Corn and Sorghum Mixed Intercrops
Frank Cawrse, Jr., of Lebanon, Oregon, intercrops forage sorghum
into his silage corn. He first plants the corn at 28,000 seed per
acre, then goes back over the field with a drill with enough drop
tubes closed off to plant 8 pounds of sorghum on 32-inch rows in
between the corn. He also plants two different maturities of corn,
a 95-day and a 75-day, to even out the silage moisture content.
He harvests a mix of corn in hard dent and soft dent, and sorghum
in the milk stage. (8)
Strip Cropping Corn/Soybeans/Small Grains
South Dakota farmer Tod Intermill plants alternating strips of
corn, soybeans, and spring wheat on his farm. (9)
The strips are six rows wide in a ridge-till system. All the crop
plantings are adapted to existing equipment widths. Regular herbicide
treatments can be applied using a ground sprayer of strip width.
Even the wheat is drilled on ridges, using a drill with individual
depth gauges on each opener. Intermill orients his rows east and
west to minimize the shading effects of taller crops like corn.
The crops are planted in a wheat–corn–soybean pattern,
with soybeans on the north side of the corn (Figure
2). This arrangement reduces the effect of corn shading often
associated with a straight corn-soybean pattern, since the wheat
is mature before the corn has a chance to shade it. Corn gains the
greatest benefit from the additional sunlight interception on the
outside rows of the corn strip.
Figure 2. Corn, soybeans, and wheat strip-cropped |
Iowa farmer Tom Frantzen strip-crops oats, corn, and soybeans on
ridge-till rows. He views his strips as a crop rotation in one field.
His rows are oriented generally east and west on the contour. His
1989 strip-crop corn yields were 166 bushels per acre, compared
to 130 for his farm average. Stripped soybean yields were two bushels
lower than farm average. His oat yields were 109 bushels stripped
and 100-bushel farm average. Tom was not surprised at the increase
in corn yields. The outer strip rows captured more sunlight. His
average corn border row yielded 198 bushels per acre next to the
soybeans and 177 bushels next to oats. The soybean yields were 37
bushels, even with the increased shading on the border rows. This
loss was made up in the middle rows with yields of 44 bushels per
acre. Oats showed a 107-bushel yield on the soybean side, a 103-bushel
yield on the corn side, and 99 bushels in the middle. Tom says the
strip intercropping is no more labor intensive than monocrop fields.
His profits were $76 per acre for the stripped fields and $55 for
the same crops grown in monoculture. (11)
Rick Cruse, an Iowa State University agronomist, has observed several
characteristics that narrow strips (12 to 30 feet wide) offer. The
strips accommodate the pest management and soil building advantages
of rotations and the yield boost of border rows. With proper management
the border effect can pay off; managed improperly, it can cost yield.
With oat and corn strips, the early-maturing oats are nearly mature
before corn can pose much of a shade and competition problem. The
corn can also provide wind protection for the oats. When the oats
are harvested, more sunlight is available to the corn. In times
of low moisture, oats may rob the corn border rows of water. In
his field trials, Cruse found a 5% increase in oat yields on their
borders while corn realized a 12 to 15% increase.
Soybean yields dropped by 10% on their border rows, but the yields
in the soybean middle rows were higher than they would be in a solid
field, possibly representing a windbreak effect. (10)
Some have experimented with a shorter corn variety in the border
row to minimize shading. One farmer tried planting six rows of corn
and doubling his soybean strips to 12 rows to eliminate the impact
of corn shading on the beans. This same farmer found that corn strips
wider than eight rows did not provide adequate results. Using a
12-row planter, it's easy to establish the 6-row strips by filling
the middle six hoppers with corn and the outer three hoppers with
beans. Some farmers plant higher corn populations and add higher
nitrogen rates in the border rows to take advantage of the extra
sunlight exposure. Most farmers agree that strip cropping corn,
soybeans, and oats works best with ridge-till or no-till. When the
field is tilled, it's difficult to gauge where the rows should
go in order to get the strips even.
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Escalating Diversity and Stability
to a Higher Level
Ecologists tell us that stable natural systems are typically diverse,
containing many different types of plants, arthropods, mammals,
birds, and microorganisms. In stable systems, serious pest outbreaks
are rare, because natural controls exist to automatically bring
populations back into balance. Planting crop mixtures, which increase
farmscape biodiversity, can make crop ecosystems more stable, and
thereby reduce pest problems.
There is overwhelming evidence that plant mixtures support lower
numbers of pests than do pure stands (11) and
there are two schools of thought on why this occurs. One suggests
that higher natural enemy populations persist in diverse mixtures
due to more continuous food sources (nectar, pollen, and prey) and
favorable habitat.
The other thought is that pest insects that feed on only one type
of plant have greater opportunity to feed, move around in, and breed
in pure crop stands because their resources are more concentrated
than they would be in a crop mixture. (12)
Regardless of which reason you accept, the crops growing together
in the mixture complement one another, resulting in lower pest levels.
Intercropping also aids pest control efforts by reducing the ability
of the pest insects to recognize their host plants. For example,
thrips and white flies are attracted to green plants with a brown
(soil) background, ignoring areas where vegetation cover is complete—including
mulched soil. (13) Some intercrops have a spatial
arrangement that produces the complete vegetation cover that would
be recognized as unfavorable to thrips and whiteflies. Other insects
recognize their host plants by smell. Onions planted with carrots
mask the smell of carrots from carrot flies. For more information
on companion planting for insect management, see the ATTRA publications
Farmscaping
to Enhance Biological Control and Companion
Planting.
Innovative farmers are paving the way with intercrops and realizing
pest management benefits as a result. Georgia cotton farmers Wayne
Parramore and sons reduced their insecticide and fertilizer use
by growing a lupine cover crop ahead of their spring-planted cotton.
(14) They started experimenting with lupines
on 100 acres in 1993, and by 1995 were growing 1,100 acres of lupines.
Ground preparation for cotton planting is begun about 10 days prior
to planting by tilling 14-inch wide strips into the lupines (Figure
3). Herbicides are applied to the strips at that time, and row
middles remain untouched. The remaining lupines provides beneficial
insect habitat and also serve as a smother crop to curtail weeds
and grasses. The lupines in the row middles can be tilled in with
the cultivator later in the season to release more legume nitrogen.
Figure 3. Young cotton planted into lupines. |
In the Parramores' system, all the nitrogen needs of the
cotton crop are met with cover crops except for 10 units per acre
of starter nitrogen and another 15 units applied while spraying
herbicides. Petiole samples taken every week to monitor plant nitrogen
show that cotton grown with lupines maintains a normal range of
tissue nitrogen throughout the growing season. The nitrogen level
in cotton grown solely with fertilizer is very high initially, then
subsequently falls back to a lower level. In one comparative year,
the cotton grown following lupine produced 96 more pounds of lint,
with only 25 units of commercial nitrogen, compared to a field with
125 units of nitrogen and no lupines. Additionally, the lupine field
required less spraying for insects—only twice compared to
five sprays for the commercial nitrogen field. This reduction saved
60% on insecticides, amounting to $35 per acre. The reduction in
need for pesticides is attributed to the large population of beneficial
insects generated and sustained in this system. The lupines provide
food for aphids and thrips, which attract ladybugs, big-eyed bugs,
and fire ants as predators. When the cotton gets big enough to shade
out the lupines, the beneficial insects move to the cotton rather
than migrating from the field. The Parramores estimate that improved
yields, combined with cost reductions, are netting them an additional
$184 per acre with the strip tillage lupine system when compared
to the conventional management system.
Alfalfa is one of the best crops for attracting and retaining beneficial
insects. This characteristic can be enhanced further. Strip-cutting
alfalfa (i.e., cutting only half of the crop at any one time, in
alternating strips) maintains two growth stages in the crop; consequently,
some beneficial habitat is available at all times. In some cases
alfalfa is mixed with another legume and a grass. Auburn University
researcher Mike Gayler is just starting research projects using
alfalfa as an attractant crop for beneficials. He speculates that
it will work in the Southeast with proper management. Other main-season
strip crops that research suggests will benefit cotton crop pest
management include cowpeas, sorghum, corn, and crotalaria. (15)
Dr. Sharad Phatak of the University of Georgia has been working
with cotton growers in Georgia testing a strip-cropping method using
annual winter cover crops. (16) Planting cotton
into strip-killed crimson clover improves soil health, cuts tillage
costs, and allows him to grow cotton with no insecticides and only
30 pounds of nitrogen fertilizer. Working with Phatak, farmer Benny
Johnson reportedly saved at least $120/acre on his 16-acre test
plot with the clover system. There were no insect problems in the
test plot, while beet armyworms and whiteflies were infesting nearby
cotton and requiring 8 to 12 sprayings to control. Cotton intercropped
with crimson clover yielded more than three bales of lint per acre
compared to 1.2 bales of lint per acre in the rest of the field.
(16) Boll counts were 30 per plant with crimson
clover and 11 without it. Phatak identified up to 15 different kinds
of beneficial insects in these strip-planted plots.
Phatak finds that planting crimson clover seed at 15 pounds per
acre in the fall produces around 60 pounds of nitrogen per acre
by spring. By late spring, beneficial insects are active in the
clover. At that time, 6- to 12-inch planting strips of clover are
killed with Roundup™ herbicide. Fifteen to 20 days later the
strips are lightly tilled and cotton is planted. The clover in the
row-middles is left growing to maintain beneficial insect habitat.
When the clover is past the bloom stage and less desirable for beneficials,
they move readily onto the cotton. Even early-season thrips, which
can be a problem following cover crops, are limited or prevented
by beneficial insects in this system. The timing coincides with
a period when cotton is most vulnerable to insect pests. Following
cotton defoliation, the beneficials hibernate in adjacent non-crop
areas.
Phatak points out that switching to a whole-farm focus while reducing
off-farm inputs is not simple. It requires planning, management,
and several years to implement on a large scale. It is just as important
to increase and maintain organic matter, which stimulates beneficial
soil microorganisms. Eventually a "living soil" will
keep harmful nematodes and soilborne fungi under control. (16)
For more information on management of soil-borne diseases, see
the ATTRA publication Sustainable
Management of Soilborne Plant Diseases.
Texas dryland farmer Ron Gobel intercrops 8-row strips of sesame
and cotton for insect control benefits. The sesame harbors many
beneficial insects, including high populations of lace-wings, assassin
bugs, and lady beetles. Ron's 1995 crop was planted late due
to prolonged spring rains. He did not use a Bt cotton variety. Early
frost terminated the crop two weeks earlier than normal yet he still
produced 0.8 bales per acre under dryland conditions. His sesame
produced 800 pounds per acre. The 1996 cotton rows were planted
where the sesame rows were the previous year, and sesame planted
where cotton was before.
Since Ron sells his cotton for a premium price in the organic market,
he cannot spray any synthetic insecticides. Consequently, he must
rely on beneficial insects attracted to his fields by cultural practices
and a handful of natural insecticides.
Following the fall harvest Ron plants annual rye at a low rate
of 20 to 40 pounds per acre. The rye is tilled in prior to crop
planting in the spring. Ron believes the rye helps with soil moisture
retention and weed control. During the 1997 crop year his fields
suffered only minimal boll weevil damage. Ron noticed lots of adult
bollworm moths but no worms. The eggs were eaten or parasitized
by the beneficials.
Ron's fields were scouted as part of a boll weevil eradication
program. The scouts were amazed at the lack of worms and the high
numbers of beneficial insects. The cotton crop was sprayed one time
with diatomaceous earth impregnated with natural pyrethrum, which
was acceptable under the organic standards. The insect scouts noticed
a 70% reduction in adult boll weevil population three days after
the spray. They were so surprised, that they placed cages of 20
live weevils in the field to see whether the spray was working.
The next day, 45% of those weevils were dead. The entomologists
speculated that the weevils were getting enough of the diatomaceous
earth on their leg joints to cut their exoskeletons, allowing the
pyrethrum to kill them.
In a scientific study, Mississippi researchers interplanted 24
rows of cotton with 4 rows of sesame to study the intercrop's
effects on tobacco budworms and bollworms (Heliothis spp.). Throughout
the growing season, larvae numbers were much higher in the sesame
than on the cotton until late August, indicating the worm's
preference for sesame. Following a large summer rain at a time when
the sesame was reaching maturity, the Heliothis adults became more
attracted to the cotton. The researchers noted that sesame's
attractiveness to Heliothis and sesame's ability to harbor
high numbers of beneficial insects made it useful in a cotton pest
management program. (17)
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Escalating Diversity and
Stability to an Even Higher Level
The diversity created by intercropping can be enhanced even further
by integrating livestock (single or mixed species) into the cropping
plan as harvesters. Allowing animals to harvest feed crops in the
field puts gain on animals at the cost of crop production—considerably
less than the purchase price of the grain. If you think about it,
feed grains cost a lot less when they're not run through a
$150,000 combine or hauled 1000 miles across the country.
Grazing animals and other livestock can be managed on croplands
to reduce costs, increase income, and increase diversity. There
are ways of incorporating animals into cropping without the farmer
getting into animal husbandry or ownership directly. Collaboration
with neighbors who own animals will benefit both croppers and livestock
owners. Grazing or hogging-off of corn residue is one example where
a cost can be turned into a profit. The animals replace the $6 per
acre stalk mowing cost and produce income in animal gains.
Shasta College provides a unique demonstration of integrating livestock
with intercrops. Shasta is a two-year community college located
in Redding, California, that offers associate degrees in several
branches of agriculture. Stan Gorden (18) heads
the college's holistic resource laboratory, where students
get hands-on experience with ranching and farming. (19)
Stan and his students have taken intercropping to a high level of
efficiency. They run hogs, sheep, cattle, and chickens together
over 42 small paddocks of various forages and crops growing on 100
acres of college-owned land. One paddock is a pumpkin patch, another
a garlic and carrot patch. Some are planted in alfalfa or mixes
of grasses and clover. Not all the pastures have water sources for
the animals, so water is moved on a trailer tank when necessary.
The animals are moved daily in a planned grazing system during rapid
plant growth and much more slowly, up to five days on a paddock,
during slow plant growth.
Some of the paddocks are planted with mixtures of either winter
or summer forage or grain crops. An intercrop of cereal grain, fava
beans, and Canadian field peas is planted for winter grain, each
crop at 1/3 normal seeding rate. The grain mixture is combine-harvested
to make energy and protein supplement feed as needed. After harvest,
the animals are turned into the paddock to glean what's left.
For summer feed, a mixture of milo planted on 18-inch rows is intercropped
with a row of black-eyed peas planted six inches to either side
of each sorghum row, using a drill with partitions in the seedbox.
The milo provides a trellis for the pea vines to run on (Figure
4). The milo/black-eyed mixture requires no herbicide. Before
peas and milo were grown together, the milo pure stand would be
plagued with whiteflies and green bugs. Mixing the two crops together
ended the pest problem. Cowpeas have extrafloral nectaries that
attract lots of beneficial insects. This could explain the absence
of pest insects in the mixture. The milo/pea mixture is harvested
by setting the combine to cut at the height of the milo heads. This
yields a milo to bean ratio of 2:1—ideal for feed.
Figure 4. Cowpeas and milo growing together |
The college animal herd consists of 20 sows that farrow on pasture,
35 head of cattle, 50 sheep, and 30 laying hens that all range together.
The hens are with the herd during the day and roost in a nearby
eggmobile at night. Gorden selects breeds and genetics to fit this
system, as opposed to selecting breeds for maximum production, and
adapting a system to match the animal. The animals benefit one another.
The sheep learn to stay close to the middle of the herd to avoid
predators, which are fended off by the hogs. The cattle learn that
the hogs know how to break the pumpkins open, so they stick close
and get some too. The hogs eat the cow and sheep droppings and benefit
from the predigestion. The hens scavenge wasted seeds from the various
crops. There are three different kinds of hens, each of which lays
eggs of a different color. The eggs are marketed as rainbow eggs,
with each dozen containing four white, four blue, and four brown
eggs. The chickens also scratch apart cattle dung pats searching
for insects, thus destroying cattle parasites.
Gorden says that developing and maintaining this high level of
diversity has required creativity, selection criteria, constant
monitoring, and re-examining traditional beliefs. By challenging
long-held beliefs, he and his students discovered that hogs do not
need standard farrowing crates and that sheep and cattle are compatible
grazers. Animals and crops are selected and culled according to
their ability to adapt to this complex system. Shasta College has
one of the largest heritage hog herds in the country. The hogs have
been fitted with humane nose rings to prevent rooting. Also, hog
breeds are selected that don't root up the ground nor eat
the baby lambs when they are born. The sows farrow on pasture with
only a single bale of hay for bedding. Hogs are not vaccinated,
nor are needle teeth removed or other detailing done. Sows generally
wean 12 pigs with no supplemental feed. The only purchased input
is some nitrogen and phosphorus fertilizer applied to the pastures.
The pigs are only touched twice; once to castrate and once to wean.
As with the hogs, the cattle and sheep are selected to prosper on
grass. Predators are not controlled in any way. Any animal that
gets killed by wandering off is naturally selected out of the herd.
The sheep/hog/cow mix provides much better utilization of forage
than single species grazing. Since the animals do most of the harvesting,
less fossil fuel and labor-hours are expended. There are no pens
to wash and no manure to deal with. The herd is controlled using
an electric fence charged up to 8,000 volts to hold the sheep.
Before the 100-acre crop/animal integration project began in 1987,
the College's agriculture resource laboratory was costing
$8,000 per year. That was the first year the resource laboratory
started managing holistically. By 1996, the resource lab's
income was up $12,000, and expenses were down $10,000—rendering
a $14,000 profit over the 1987 figure. During that same time the
soil organic matter has increased from 1.7% to 3.2 %. (18)
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Intercropping for Disease Control
Under direction of an international team of scientists, farmers
in China's Yunnan province made some simple changes in their
rice production methods. (20) They changed
from planting their typical pure stand of a single rice variety
to planting a mixture of two different rice varieties. Their primary
reason for trying this new technique was to reduce the incidence
of rice blast, the main disease of rice. The technique was so successful
at reducing blast disease that the farmers were able to abandon
chemical fungicides they had been using. The biodiversity effect
is apparent here in that if one variety of a crop is susceptible
to a disease, the denser the stand, the worse the disease can spread.
If susceptible plants are separated by non-host plants that can
act as a physical barrier to the disease, the susceptible variety
will suffer less disease infection. Rice blast moves from plant
to plant via airborne spores. These spores can be blocked by a row
of a resistant variety. In this on-farm study, the rice was harvested
by hand. Separating the varieties was easily done during harvest,
since one variety towered above the other.
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Adapting Intercropping to Your Farm
Intercropping has been important in the U.S. and other countries
and continues to be an important practice in developing nations.
In traditional systems, intercropping evolved through many centuries
of trial and error. To have persisted, intercropping had to have
merit biologically, environmentally, economically, and sociologically.
To gain acceptance, any agricultural practice must provide advantages
over other available options in the eyes of the practitioner . Many
of the impediments to adoption of new strategies or practices of
diversification are sociological (Will I look foolish to my neighbors?
Will I fail?) and financial (What are the risks? What is the profit
potential?) rather than technological.
Farmers have generally regarded intercropping as a technique that
reduces risks in crop production; if one member of an intercrop
fails, the other survives and compensates in yield to some extent,
allowing the farmer an acceptable harvest. Pest levels are often
lowered in intercrops, as the diversity of plants hampers movement
of certain pest insects and in some cases encourages beneficial
insect populations.
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References
- Maser, Chris. 1990. The Redesigned
Forest. Stoddart, Toronto, Canada. 224 p.
- Savory, Allan. 1998. Holistic
Management—A New Framework for Decision-Making. 2nd edition.
Island Press. Covelo, CA. 550 p.
- Grossman, Joel, and William Quarles.
1993. Strip intercropping for biological control. IPM Practitioner.
April. p. 1–11.
- Willy, R.W., et al. 1983. Intercropping
studies with annual crops. In: Better Crops for Food, CIBA Foundation
Symposium 97. Pitman, London, UK.
- Francis, R., and D.R. Decoteau.
1993. Developing an effective southernpea and sweet corn intercrop
system. Hort Technology. Vol. 3, No. 2. p. 178–184.
- Bowen, John F., and Bernard A.
Kratky. 1986. Successful multiple cropping requires superior management
skills. Agribusiness Worldwide. November/December. p. 22–30.
- Amador, M.F. 1980. Behavior of
three species (corn, beans, squash) in polyculture in Chontalpa,
Tabasco, Mexico. CSAT, Cardenas, Tabasco, Mexico.
- Martin, Ralph, Don Smith, and
Harvey Voldeng. 1987. Intercropping corn and soybeans. Sustainable
Farming. REAP Canada. McGill University, Macdonald Campus. www.eap.mcgill.ca
- Anon. 1987. Intercropping bolsters
silage yields. Hay and Forage Grower. August. p. 29.
- Tonneson, Lon, and Jim Houtsma.
1991. Adding new wrinkles to alternate strips. The Farmer. September
7. p. 8–9.
- Anon. 1990. Strip intercropping
offers low-input way to boost yields. Sensible Agriculture. May.
p. 7–8.
- Altieri, M.A., and M. Leibman.
1994. Insect, weed, and plant disease management in multiple cropping
systems. In Francis, C.A. (ed.). Multiple Cropping Systems. Macmillan
Company, New York. 383 p.
- Ecological Agriculture Projects.
Mixing Crop Species. McGill University, Macdonald Campus. www.eap.mcgill.ca/CSI_2.htm
- Dirnerger, J.M. 1995. The bottom
line matters—you can laugh at him on the way to the bank.
National Conservation Tillage Digest. October–November.
p. 20–23.
- Rincon-Vitova. No date. Product
Information: Biological Control Solutions for Cotton Pests. Rincon-Vitova
Insectaries, Inc. Oak View, CA. 6 p.
- Yancey, Cecil Jr. 1994. Covers
challenge cotton chemicals. The New Farm. February. p. 20–23.
- Laster, M.L., and R.E. Furr.
1972. Heliothis populations in cotton-sesame interplantings. Journal
of Economic Entomology. Vol. 65, No. 5. p. 1524–1525.
- Stan Gorden
Department of Agriculture and Natural Resources
Shasta College
P.O. Box 496006
Redding, CA 96049-6006
530-225-4687
E-mail: sgorden@shastacollege.edu
- Richardson, Pat. 1997. Polyculture
makes the most of biodiversity. HRM of Texas Newsletter. Summer.
p. 5, 7.
- Wolfe, Martin S. 2000. Crop strength
through diversity. Nature. August. p. 681–682.
Intercropping Principles and Production Practices
By Preston Sullivan
NCAT Agriculture Specialist
Cole Loeffler, HTML Production
IP 135
Slot 8
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