Editor’s
note: > indicates progression to another crop;
/ indicates a mixture of crops growing at the same time.
One of the biggest challenges of cover cropping is to fit
cover crops into your current rotations, or to develop new
rotations that take full advantage of their benefits. This
section will explore some of the systems used successfully
by farmers in different regions of the U.S. One might be easily
adapted to fit your existing crops, equipment and management.
Other examples may point out ways that you can modify your
rotation to make the addition of cover crops more profitable
and practical.
Whether you add covers to your existing rotations or totally
revamp your farming system, it is crucial that you devote
as much planning and attention to your cover crops as you
do to your cash crops. Failure to do so can lead to failure
of the cover crop and cause problems in other parts of your
system. Also remember that there is likely no single cover
crop that is right for your farm. Ultimately, rotating cover
crops might be your best strategy. Like any other crop, pest
pressures may build up if a cover crop is grown too often
in the same field.
Before you start:
- Check out two additional short articles: Benefits Of
Cover Crops and Selecting the Best Cover Crops For Your
Farm
- Decide which benefits are most important to you
- Read the examples below, then consider how these cover
crop rotations might be adapted to your particular conditions
- Talk to your neighbors and the other "experts"
in your area
- Start small on an easily accessible plot that you will
see often
Be an opportunist--and an optimist. If your cropping plans
for a field are disrupted by weather or other conditions outside
of your control, this may be the ideal window for establishing
a cover crop. Consider using an early-maturing cash crop to
allow for timely planting of the cover crop. The ideas below
will help you see cover crop opportunities in what used to
look like problems.
Cover Crops for Corn Belt Grain
and Oilseed Production
In addition to providing winter cover and building soil structure,
nitrogen (N) management will probably be a major factor in
your cover crop decisions for the corn>soybean rotation.
A fall-planted grass or small grain will scavenge leftover
N from the previous corn or soybean crop. Legumes are much
less efficient at scavenging N, but will add N to the system
for the following crop. Legume/grass mixtures are quite good
at both.
Corn>Soybean Systems
Keep in mind that: corn is a heavy nitrogen feeder; soybeans
benefit little, if at all, from cover crop N; and that you
have a shorter time for spring legume growth before corn than
before soybeans.
Cover crop features: rye
provides winter cover, scavenges N after corn, becomes a long-lasting
(6-week) residue for your beans to suppress weeds and hold
moisture; hairy vetch provides
spring ground cover, abundant N and a moderate-term (3 to
4 week) mulch for the next corn crop; field
peas are similar to vetch, but residue breaks down
faster; red clover is also similar,
but produces slightly less N and has less vigorous spring
regrowth; berseem clover grows
quickly to provide several cuttings for high-N green manure,
then winterkills.
Here are some options to consider adapting to your system:
Corn>Rye>Soybeans>Hairy Vetch.
In Zone 7 and warmer, you can grow a cover crop every year
between your corn and full-season beans. Also, you can use
wheat or another small grain to replace the cover crop before
beans, in a three-crop, two-year rotation (corn>wheat>doublecrop
beans). In all cases, another legume or a grass/legume mixture
can be used instead of a single species cover crop. Where
it is adapted, you can use crimson clover or a crimson/grass
mixture instead of vetch.
In cooler areas, plant rye as soon as possible after corn
harvest. If you need more time in the fall, try overseeding
in rowed beans at drydown "yellow leaf" stage in
early fall, or in early summer at the last cultivation of
corn. Seeding options include aerial application where the
service is economical, using a specialty high-body tractor
with narrow tires, or attaching a broadcast seeder, air seeder
or seed boxes to a cultivator.
Kill the rye once it is about knee-high, or let it go a bit
longer, killing it a couple of weeks before planting beans.
Killing the rye with herbicides and no-tilling beans in narrow
rows allows more time for cover crop growth, since you don't
have to work the ground. If soil moisture is low, consider
killing the rye earlier. Follow the beans with hairy vetch
or a vetch/small grain mixture. Legumes must be seeded at
least 6 weeks before hard frost to ensure winter survival.
Seed by drilling after soybean harvest, or by overseeding
before leaf drop. Allow the vetch (or mixture) to grow as
long as possible in spring for maximum N fixation.
Worried about planting your corn a bit late because you're
waiting for your cover crop to mature? Research in Maryland,
Illinois and elsewhere suggests that planting corn towards
the end of the usual window when using a legume cover crop
has its rewards. The delay can result in greater yields than
earlier planting, due to greater moisture conservation and
more N produced by the cover crop, or due to the timing of
summer drought (62, 64, 243, 338). Check your state variety
trial data for a shorter season corn hybrid that yields nearly
as well as slightly longer season corn. The cover crop benefit
should overcome many yield differences.
Worried about soil moisture? There's no
question that growing cover crops may consume soil moisture
needed by the next crop. In humid regions, this is a problem
only in an unusually dry spring. Time permitting, allow 2
to 3 weeks after killing the cover crop to alleviate this
problem. While spring rainfall may compensate for the moisture
demand of most cover crops by normal planting dates, rye can
quickly dry out a field. Later in the season, killed cover
crop residues in minimum tillage systems can conserve moisture
and increase yields.
In dryland areas of the Southern Great Plains, lack of water
limits cover crop use.
In any system where you are using accumulated soil moisture
to grow your cash crop, you need to be extra careful. However,
as noted in this section and elsewhere in the book, farmers
and researchers are finding that water-thrifty cover crops
may be able to replace even a fallow year without adversely
affecting the cash crop.
Corn>Rye>Soybeans>SmallGrain>Hairy Vetch.
This rotation is similar to the corn>rye>soybeans rotation
described above, except you add a year of small grains following
the beans. This is the standard rotation in the grain-growing
regions of Paraguay and Brazil, where it is critical to maintain
soil organic matter. In crop rotation research from different
areas, many benefits accrue as the rotation becomes longer.
This is because weed, disease and insect pest problems generally
decrease with an increase in years between repeat plantings
of the same crop.
Residue from small grains provides good organic matter for
soil building, and in the case of winter grains, the plants
help to prevent erosion over winter after soybeans loosen
up the soil. If seeding with small grains, select cover crops
that will stand shade and some traffic.
The length of the growing season will determine how you fit
in cover crops after full-season soybeans in the rotation.
Consider using a short-season bean if needed in order to achieve
timely planting after soybean harvest. Calculate whether cover
crop benefits will compensate for a possible yield loss on
the shorter season beans. If there is not enough time to seed
a legume after harvest, use a small grain rather than no cover
crop at all.
The small grain scavenges leftover N following beans. Legume
cover crops reduce fertilizer N needed for following corn,
a heavy N feeder. If you cannot seed the legume at least six
weeks before a hard frost, consider overseeding before leaf
drop or at last cultivation.
An alternate rotation for the lower mid-South is corn>crimson
clover (allowed to go to seed) > soybeans > crimson
clover (reseeded) > corn. Allow the crimson clover to go
to seed before planting beans. The clover germinates in late
summer under the beans. Kill the cover crop before corn the
next spring. If possible, choose a different cover crop following
the corn this time to avoid potential pest and disease problems
with the crimson clover.
Precaution. In selecting a cover crop to
interseed, do not jeopardize your cash crop if soil moisture
is usually limiting during the rest of the corn season! Banding
cover crop seed in row middles by using insecticide boxes
or other devices can reduce cover crop competition with the
cash crop.
3 Year: Corn>Soybean>Wheat/Red Clover.
This well-tested Wisconsin sequence provides N for corn as
well as general rotation effects in weed suppression and natural
controls of disease and insect pests. It was more profitable
in recent years as the cost of synthetic N increased. Corn
benefits from legume-fixed N, and from the improved cation
exchange capacity in the soil that comes with increasing organic
matter levels.
With the changes in base acreage requirements in the 1996
Farm Bill, growers in the upper Midwest are looking to add
a small grain to their corn>bean rotation. The small grain,
seeded after soybeans, can be used as a cover crop, or it
can be grown to maturity for grain. When growing wheat or
oats for grain, frost-seed red clover or sweetclover in March,
harvest the grain, then let the clover grow until it goes
dormant in late fall. Follow with corn the next spring. Some
secondary tillage can be done in the fall, if conditions allow.
One option is to attach sweeps to your chisel plow and run
them about 2 inches deep, cutting the clover crowns .
Alternatively, grow the small grain to maturity, harvest,
then immediately plant a legume cover crop such as hairy vetch
or red clover in August or early September. Soil moisture
is critical for quick germination and good growth before frost.
For much of the northern U.S., there is not time to plant
a legume after soybean harvest, unless it can be seeded aerially
or at the last cultivation. If growing spring grains, seed
red clover or sweetclover directly with the small grain.
Adding the small grain to the rotation helps control white
mold on soybeans, since two years out of beans are needed
to reduce pathogen populations. Using a grain/legume mix will
scavenge available N from the bean crop, hold soil over winter
and begin fixing N for the corn. Clovers or vetch can be harvested
for seed, and red or yellow clover can be left for the second
year as a green manure crop.
Using a spring seeding of oats and berseem clover has proved
effective on Iowa farms that also have livestock. The mix
tends to favor oat grain production in dry years and berseem
production in wetter years. Either way the mixture provides
biomass to increase organic matter and build soil. The berseem
can be clipped several times for green manure.
Precaution. Planting hairy vetch with small
grains may make it difficult to harvest a clean grain crop.
Instead, seed vetch after small grain harvest.
Cover Crops for Vegetable Production
Vegetable systems often have many windows for including cover
crops. Periods of one to two months between harvest of early
planted spring crops and planting of fall crops can be filled
using fast-growing warm-season cover crops such as buckwheat,
cowpeas, sorghum-sudangrass hybrid, or another crop adapted
to your conditions. As with other cropping systems, plant
a winter annual cover crop on fields that otherwise would
lie fallow.
Where moisture is sufficient, many vegetable crops can be
overseeded with a cover crop, which will then be established
and growing after vegetable harvest. Select cover crops that
tolerate shade and harvest traffic, especially where there
will be multiple pickings, such as in tomatoes or peppers.
Cover crop features: Oats
add lots of biomass, are a good nurse crop for spring-seeded
legumes, and winterkill, doing away with the need for spring
killing and tilling. Sorghum-sudangrass
hybrid produces deep roots and tall, leafy stalks that
die with the first frost. Yellow sweetclover
is a deep rooting legume that provides cuttings of green manure
in its second year. White clover
is a persistent perennial and good N source.
In Zone 5 and cooler, plant rye, oats or a summer annual
(in August) after snap bean or sweet corn harvest for organic
matter production and erosion control, especially on sandy
soils. Incorporate the following spring, or leave untilled
strips for continued control of wind erosion.
If you have the option of a full year of cover crops in the
East or Midwest, plant hairy vetch in the spring, allow to
grow all year, and it will die back in the fall. Come back
with no-till sweet or field corn or another N-demanding crop
the following spring. Or, hairy vetch planted after about
August 1 will overwinter in most zones with adequate snow
coverage. Allow it to grow until early flower the following
spring to achieve full N value. Kill for use as an organic
mulch for no-till transplants or incorporate and plant a summer
crop.
You can sow annual ryegrass right after harvesting an early-spring
vegetable crop, allow it to grow for a month or two, then
kill, incorporate and plant a fall vegetable.
Some farmers maximize the complementary weed-suppressing
effects of various cover crop species by orchestrating peak
growth periods, rooting depth and shape, topgrowth differences
and species mixes. See the profile on page 3 for a good example
of this..
3 Year: Winter Wheat/Legume Interseed> Legume>Potatoes.
This eastern Idaho rotation conditions soil, helps
fight soil disease and provides N. Sufficient N for standard
potatoes depends on rainfall being average or lower to prevent
leaching that would put the soil N below the shallow-rooted
cash crop.
1 Year: Lettuce>Buckwheat>Buckwheat> Broccoli>White
Clover/Annual Ryegrass. The Northeast's early spring
vegetable crops often leave little residue after their early
summer harvest. Sequential buckwheat plantings suppress weeds,
loosen topsoil and attract beneficial insects. Buckwheat is
easy to kill by mowing in preparation for fall transplants.
With light tillage to incorporate the relatively small amount
of fast-degrading buckwheat residue, you can then sow a winter
grass/legume cover mix to hold soil throughout the fall and
over winter. Planted at least 40 days before frost, the white
clover should overwinter and provide green manure or a living
mulch the next year.
California Vegetable Crop Systems
Innovative work in California includes rotating cover crops
as well as cash crops, adding diversity to the system. This
was done in response to an increase in Alternaria blight in
Lana vetch if planted year after year.
4 Year: Lana Vetch>Corn>Oats/Vetch>Dry Beans>Common
Vetch>Tomatoes>S-S Hybrid/ Cowpea>Safflower. The
N needs of the cash crops of sweet corn, dry beans, safflower
and canning tomatoes determine, in part, which covers to grow.
Corn, with the highest N demand, is preceded by Lana vetch,
which produces more N than other covers. Before tomatoes,
common vetch works best. A mixture of purple vetch and oats
is grown before dry beans, and a mix of sorghum-sudangrass
and cowpeas precedes safflower.
In order to get maximum biomass and N production by April
1, Lana vetch is best planted early enough (6 to 8 weeks before
frost) to have good growth before "winter."
Disked in early April, Lana provides all but about 40 lb.
N/A to the sweet corn crop. Common vetch, seeded after the
corn, can fix most of the N required by the subsequent tomato
crop, with about 30 to 40 lb. N/A added as starter.
A mixture of sorghum-sudangrass and cowpeas is planted following
tomato harvest. The mixture responds to residual N levels
with N-scavenging by the grass component to prevent winter
leaching. The cowpeas fix enough N for early growth of the
subsequent safflower cash crop, which has relatively low initial
N demands. The cover crop breaks down fast enough to supply
safflower's later-season N demand.
Precaution. If you are not using any herbicides,
vetch could become a problem in the California system. Earlier
kill sacrifices N, but does not allow for the production of
hard seed that stays viable for several seasons.
Excerpted from Managing Cover Crops Profitably,
Second Edition, by the Sustainable Agriculture Network, reprinted
2000, SAN, Beltsville, MD, pp. 34-42.
Complete text: http://www.sare.org/handbook/mccp2/index.htm
To order: http://www.sare.org/htdocs/pubs/ToOrder.html
To contact SAN off the Web: PHONE: (301)
504-6425; FAX: (301) 504-5207; EMAIL: san@sare.org.
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