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Crop Rotations
... with methods of farming in which grasses
form an important part of the rotation, especially those that
leave a large residue of roots and culms, the decline of the productive
power is much
slower than when crops like wheat, cotton, or potatoes, which leave
little
residue on the soil, are grown continuously.
Henry Snyder, 1896
There are very good reasons to rotate crops. Rotating
crops usually means fewer problems with insects, parasitic nematodes,
weeds, and diseases caused by bacteria, viruses, and fungi. Rotations
are effective for controlling insects like the corn rootworm, nematodes
like the soybean cyst nematode, and diseases like root rot of field
peas. In addition, rotations that include legumes supply nitrogen
to succeeding crops. Growing sod-type forage grasses, legumes, and
grass-legume mixes as part of the rotation also increases soil organic
matter. When you alternate two crops, such as corn and soybeans,
you have a very simple rotation. More complex rotations require
three or more crops and a five- to 10-year (or more) cycle to complete.
Rotations are an important part of any sustainable
agricultural system. Yields of crops grown in rotations are frequently
about 10 percent higher than when grown in monoculture. When you
grow a grain or vegetable crop following a legume, the extra supply
of nitrogen certainly helps. However, yields of crops grown in rotation
are often higher than in monoculture, even when both are supplied
with plentiful amounts of nitrogen. In addition, following a non-legume
crop with another nonlegume also produces higher yields than a monoculture.
For example, when you grow corn following grass hay, or cotton following
corn, you get higher yields than when corn or cotton are grown year
after year. This yield benefit from rotations is sometimes called
a rotation effect. Another important benefit of rotations is that
growing a variety of crops in a given year spreads out labor needs
and reduces risk caused by climate or market conditions.
Rotations Influence Soil
Organic Matter Levels
You might think you're doing pretty well if soil organic
matter remains the same under a particular cropping system. However,
if you are working soils with depleted organic matter, you need
to build up levels to counter the effects of previous practices.
Maintaining an inadequately low level of organic matter won't do!
The types of crops you grow, their yields, the amount
of roots produced, the portion of the crop that is harvested, and
how you treat crop residues will all affect soil organic matter.
Soil fertility itself influences the amount of organic residues
returned, because more fertile soils grow higher-yielding crops,
with more residues.
The decrease in organic matter levels when row crops
are planted on a virgin forest or prairie soil is very rapid for
the first five to 10 years, but eventually, a plateau or equilibrium
is reached. After that, soil organic matter levels remain stable,
as long as production practices aren't changed. An example of what
can occur during 25 years of continuously grown corn is given in
figure 11.1. Soil organic matter levels increase when the cropping
system is changed from a cultivated crop to a grass or mixed grass-legume
sod. However, the increase is usually much slower than the decrease
that occurred under continuous tillage.
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Figure 11.1 Organic matter changes
in the plow layer during long-term cultivation followed by hay-crop
establishment. |
A long-term cropping experiment in Missouri compared
continuous corn to continuous sod and various rotations. More than
9 inches of topsoil was lost during 60 years of continuous corn.
The amount of soil lost each year from the continuous corn plots
was equivalent to 21 tons per acre. After 60 years, soil under continuous
corn had only 44 percent as much topsoil as that under continuous
timothy sod. A six-year rotation consisting of corn, oats, wheat,
clover, and two years of timothy resulted in about 70 percent as
much topsoil as found in the timothy soil, a much better result
than with continuous corn. Differences in erosion and organic matter
decomposition resulted in soil organic matter levels of 2.2 percent
for the unfertilized timothy and 1.2 percent for the continuous
corn plots.
Two things happen when perennial forages (hay-type
crops) are part of the rotation and remain in place for some years
during a rotation. First, the rate of decomposition of soil organic
matter decreases, because the soil is not continually being disturbed.
(This also happens when using no-till planting, even for non-sod-type
crops, such as corn.) Second, grass and legume sods develop extensive
root systems, part of which will naturally die each year, adding
new organic matter to the soil. Crops with extensive root systems
stimulate high levels of soil biological activity. The roots of
a healthy grass or legume-grass sod return more organic matter to
the soil than roots of most other crops. Older roots of grasses
die, even during the growing season, and provide sources of fresh,
active organic matter. Roots of plants also continually give off,
or exude, a variety of chemicals that nourish nearby microorganisms.
We are not only interested in total soil organic matter
we want a wide variety of different types of organisms living in
the soil. We also want to have a good amount of active organic matter
and high levels of well decomposed soil organic matter, or humus,
in the soil. Although most experiments have compared soil organic
matter changes under different cropping systems, few experiments
have looked at the effects of rotations on soil ecology. The more
residues your crops leave in the field, the greater the populations
of soil microorganisms. Experiments in a semiarid region in Oregon
found that the total amount of microorganisms in a two-year wheat-fallow
system was only about 25 percent of the amount found under pasture.
Conventional moldboard plow tillage systems are known to decrease
the populations of earthworms, as well as other soil organisms.
More complex rotations increase soil biological diversity. Including
perennial forages in the rotation enhances this effect.
Residue Availability
As pointed out in chapters 3, 5, and 8, more residues
are left in the field after some crops than others. High residue-producing
crops should be incorporated into rotations whenever possible.
Species Richness and Active Rooting Periods
In addition to the quantity of residues remaining
following harvest, a variety of types of residues is also important.
The goal should be a minimum of three different species in a rotation,
with more if possible. The percent of the time that living roots
are present during a rotation is also important. The period that
active roots are present varies considerably, ranging from 32 percent
of a corn-soybeans rotation to 57 percent of the time for a beans-wheat
rotation to 76 percent of the time for a 3-year beans-wheat-corn
rotation (table 11.1).
Farm Labor and Economics
Before discussing appropriate rotations, let's consider
some of the possible effects on farm labor and finances. If you
grow only one or two row crops, you must work incredibly long hours
during planting and harvesting seasons. Including forage hay crops
and early harvested crops, along with those that are traditionally
harvested in the fall, allows farmers to spread their labor over
the growing season, making the farm more easily managed by family
labor alone. In addition, when you grow a more diversified group
of crops, you are less affected by price fluctuations of one or
two crops. This may provide more year-to-year financial stability.
Although, as pointed out above, there are many possible
benefits of rotations, there are also some costs or complicating
factors. It is critically important to carefully consider the farm
family's labor and management capacity when exploring diversification
opportunities. You may need more equipment to grow a number of different
crops. There may be conflicts between labor needs for different
crops; cultivation and sidedressing nitrogen fertilizer for corn
in some locations might occur at the same time as harvesting hay.
In addition, the more diversified the farm, the less chance for
time to relax.
General Principles
Try to consider the following principles when you're
thinking about a new rotation:
1. Follow a legume forage crop, such as clover or
alfalfa, with a high nitrogen-demanding crop, such as corn, to
take advantage of the nitrogen supply.
2. Grow less nitrogen-demanding crops, such as oats,
barley, or wheat, in the second or third year after a legume sod.
3. Grow the same annual crop for only one year,
if possible, to decrease the likelihood of insects, diseases,
and nematodes becoming a problem.
4. Don't follow one crop with another closely related
species, since insect, disease, and nematode problems are frequently
shared by members of closely related crops.
5. Use crop sequences that promote healthier crops.
Some crops seem to do well following a particular crop (for example,
cabbage family crops following onions, or potatoes following corn).
Other crop sequences may have adverse effects, as when potatoes
have more scab following peas or oats.
6. Use crop sequences that aid in controlling weeds.
Small grains compete strongly against weeds and may inhibit germination
of weed seeds, row crops permit mid-season cultivation, and sod
crops that are mowed regularly or intensively grazed help control
annual weeds.
7. Use longer periods of perennial crops, such as
a forage legume, on sloping land and on highly erosive soils.
Using sound conservation practices, such as no-till planting,
extensive cover cropping, or strip-cropping (a practice that combines
the benefits of rotations and erosion control), may lessen the
need to follow this guideline.
8. Try to grow a deep-rooted crop, such as alfalfa,
safflower, or sunflower, as part of the rotation. These crops
scavenge the subsoil for nutrients and water, and channels left
from decayed roots can promote water infiltration.
9. Grow some crops that will leave a significant
amount of residue, like sorghum or corn harvested for grain, to
help maintain organic matter levels.
10. When growing a wide mix of crops as is done
on many direct marketing vegetable farms try grouping into blocks
according to plant family, timing of crops (all early season crops
together, for example), type of crop (root vs. fruit vs. leaf),
or crops with similar cultural practices (irrigated, using plastic
mulch).
Rotation Examples
It's impossible to recommend specific rotations for
a wide variety of situations. Every farm has its own unique combination
of soil and climate and of human, animal, and machine resources.
The economic conditions and needs are also different on each farm.
You may get useful ideas by considering a number of rotations with
historical or current importance.
A five- to seven- year rotation was common in the
mixed livestock-crop farms of the northern Midwest and Northeast
during the first half of the 20th century. An example of this rotation
is the following:
Year 1. Corn
Year 2. Oats (mixed legume/grass hay seeded)
Years 3, 4, and 5. Mixed grass-legume hay
Years 7 and 8. Pasture
The most nitrogen-demanding crop, corn, followed the
pasture, and grain was harvested only two of every five to seven
years. A less nitrogen-demanding crop, oats, was planted in the
second year as a "nurse crop" when the grass-legume hay
was seeded. The grain was harvested as animal feed and oat straw
was harvested to be used as cattle bedding; both eventually were
returned to the soil as animal manure. This rotation maintained
soil organic matter in many situations, or at least didn't cause
it to decrease too much. On prairie soils, with their very high
original contents of organic matter, levels still probably decreased
with this rotation.
For many years, the western corn rootworm was
effectively controlled by alternating between corn and soybeans.
Recently, populations of the rootworm with a longer resting
period have developed and they are able to survive the very
simple rotation. |
In the corn belt region of the Midwest, a change in
rotations occurred as pesticides and fertilizers became readily
available and animals were fed in large feedlots, instead of on
crop-producing farms. Once the mixed livestock farms became grain-crop
farms or crop-hog farms, there was little reason to grow sod crops.
In addition, government commodity price support programs unintentionally
encouraged farmers to narrow production to just two feed grains.
The two-year corn-soybean rotation is better than monoculture, but
it has a number of problems, including erosion, groundwater pollution
with nitrate and herbicides, depletion of soil organic matter, and
increased insect problems (see box). Research indicates that with
high yields of corn grain there may be sufficient residues to maintain
organic matter. With soybeans, residues are minimal.
The Thompson mixed crop-livestock (hogs and beef)
farm in Iowa practices an alternate seven-year corn belt rotation
similar to the first rotation we described. For fields that are
convenient for pasturing beef cows, the Thompson rotation is as
follows:
Year 1. Corn
Year 2. Soybeans
Year 3. Corn
Year 4. Oats (mixed legume/grass hay seeded)
Years 5, 6, and 7. Mixed grass-legume hay
Organic matter is maintained through a combination
of practices that include the use of manures and municipal sewage
sludge, green manure crops (oats and rye following soybeans and
hairy vetch between corn and soybeans), crop residues, and sod crops.
These practices have resulted in a porous soil that has significantly
lower erosion, higher organic matter content, and more earthworms
than neighbors' fields
A four-year rotation under investigation in Virginia
uses mainly no-till practices as follows:
Year 1. Corn, winter wheat no-till planted
into corn stubble.
Year 2. Winter wheat grazed by cattle, foxtail millet no-till
planted into wheat stubble and hayed or grazed, alfalfa no-till
planted in fall.
Year 3. Alfalfa harvested and/or grazed.
Year 4. Alfalfa is harvested and/or grazed as usual until fall,
then heavily stocked with animals to weaken it so that corn can
be planted the next year.
This rotation follows many of the principles discussed
earlier in this chapter. It was designed by researchers, extension
specialists, and farmers and is very similar to the older rotation
described earlier. A few differences exist this rotation is shorter,
alfalfa is used instead of clover or clover-grass mixtures, and
there is a special effort to minimize pesticide use under no-till
practices. Weed-control problems occurred when going from alfalfa
(fourth year) back to corn. This caused the investigators to use
fall tillage followed by a cover crop mixture of winter rye and
hairy vetch. Some success was achieved suppressing the cover crop
in the spring by just rolling over it with a disk harrow and planting
corn through the surface residues with a modified no-till planter.
The heavy cover crop residues on the surface provided excellent
weed control for the corn.
Traditional wheat-cropping patterns for the semi-arid
regions of the Great Plains and the Northwest commonly include a
fallow year to allow storage of water and more mineralization of
nitrogen from organic matter for use by the next wheat crop. However,
the wheat-fallow system has several problems. Because no crop residues
are returned during the fallow year, soil organic matter decreases
unless manure or other organic materials are provided from off the
field. Water infiltrating below the root zone during the fallow
year moves salts through the soil to the low parts of fields. Shallow
groundwater can come to the surface in these low spots and create
"saline seeps," where yields will be decreased. Increased
soil erosion, caused by either wind or water, commonly occurs during
fallow years and organic matter decreases (at about 2 percent per
year, in one experiment). In this wheat monoculture system, the
build-up of grassy weed populations, such as jointed goat grass
and downy brome, also indicates that crop diversification is essential.
Farmers in this region who are trying to develop more
sustainable cropping systems should consider using a number of species,
including deeper-rooted crops, in a more diversified rotation. This
would increase the amount of residues returned to the soil, reduce
tillage, and lessen or eliminate the fallow period.
A four-year wheat-corn-millet-fallow rotation under
evaluation in Colorado was found to be better than the traditional
wheat-fallow system. Wheat yields have been higher in this rotation
than wheat grown in monoculture. The extra residues from the corn
and millet also are helping to increase soil organic matter. Many
producers are also including sunflower, a deep-rooting crop, in
a wheat-corn-sunflower-fallow rotation. Sunflower is also being
evaluated in Oregon as part of a wheat cropping sequence.
Vegetable farmers who grow a large selection of crops
find it best to rotate in large blocks with each containing crops
from the same families or having similar production schedules or
cultural practices. Many farmers are now using cover crops to help
"grow their own nitrogen," utilize extra nitrogen that
might be there at the end of the season, and add organic matter
to the soil. A four- to five-year vegetable rotation might be as
follows:
Year 1. Sweet corn followed by a hairy vetch/winter
rye cover crop.
Year 2. Pumpkins, winter squash, summer squash followed by
a rye or oats cover crop.
Year 3. Tomatoes, potatoes, peppers followed by a vetch/rye
cover crop.
Year 4. Crucifers, greens, legumes, carrots, onions, and
miscellaneous vegetables followed by a rye cover crop.
Year 5. (If land is available) Oats and red clover or buckwheat
followed by a vetch/rye cover crop.
Another rotation for vegetable growers uses a two-
to three-year alfalfa sod as part of a six- to eight-year cycle.
In this case, the crops following the alfalfa are high-nitrogen-demanding
crops, such as corn or squash, followed by cabbage or tomatoes,
and, in the last two years, crops needing a fine seedbed, such as
lettuce, onions, or carrots. Annual weeds in this rotation are controlled
by the harvesting of alfalfa a number of times each year. Perennial
weed populations can be decreased by cultivation during the row-crop
phase of the rotation.
Most vegetable farmers do not have enough land or
the markets to have a multi-year hay crop on a significant portion
of their land. Aggressive use of cover crops will help to maintain
organic matter in this situation. Manures, composts, or other sources
of organic materials, such as leaves, should also be applied every
year or two to help maintain soil organic matter.
Sources
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properties after 100 years of continuous cultivation. Journal
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Baldock, J.0., and R.B. Musgrave. 1980. Manure and
mineral fertilizer effects in continuous and rotational crop sequences
in central New York. Agronomy Journal 72:511518.
Barber, S.A. 1979. Corn residue management and soil
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Processes for Productivity and Environmental Quality. 1998.
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(eds.). Michigan State University Extension Bulletin E-2646. East
Lansing, MI.
Coleman, E. 1989. The New Organic Grower. Chelsea
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carbon and nitrogen. Soil Science Society of America Journal
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Luna, J.M., V.G. Allen, W.L. Daniels, J.F. Fontenot,
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Washington, D.C.: National Academy Press. This is the reference
for the rotation experiment in Virginia.
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National Academy Press. Washington, D.C. This is the reference for
the rotation used on the Thompson farm.
Peterson, G.A., and D.G. Westfall. 1990. Sustainable
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2123, 1990, Bismark, ND. Great Plains Agricultural Council
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sequence.
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