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Cover crops offer effective
tools for protecting your soil from erosion, building tilth
and organic matter (and therefore increasing the microbial
community), and delivering key nutrients to your planting
beds. Specific cover crops are suitable for a range of applications
and for different times of the year. Some are planted in spring,
some in the fall, some will overwinter, and others are winter-killed.
Some are very easy to work with, while others can be very
tenacious, especially if you let them go to seed.
To determine which cover crop species work best overall on
your farm and in your crop rotation, establish two or more
10 feet by 50 feet long (or longer) test strips with different
cover crop species or mixtures for one season. (A 10 foot
by foot strip equals 500 square feet; an acre is 43,560 square
feet).
Next, you need to determine the seeding rate for the small
plot, based on your per-acre seeding rate required for the
cover crop. For example, if the seeding rate of cereal rye
is 3 bushels per acre, and a bushel of rye weighs 56 pounds,
3 bushels totals 168 pounds per acre. Take the square-foot
measurement of your plot (500ft²) and divide that by
the number of square feet in an acre (43,560 ft²). In
this example, 500 / 43560 = 0.011468. Use this decimal amount
to multiply the number of pounds of cover crop seed required
on a per-acre basis. In this example, 0.011468 x 168 pounds
rye = 1.926624, which you round off to 1.93 pounds of rye
seed for your 500 ft² plot. Therefore, 1.93 lbs of rye
seed per your 500ft² plot will be equivalent to the 168
pounds per acre seeding rate.
Use a resource such Managing
Cover Crops Profitably (Sustainable Agriculture
Network, 1998) to help determine the optimal seeding time
and rate for each cover crop. Refer to local extension for
bulletins on cover crops commonly grown in your area. Look
at other farming systems in your region and refer to local
farmers. Many of the crops grown as cover crops are also grown
in other systems for other uses, such as the small grains
as a cash grain or the legumes and grasses in forage mixtures.
When planting cover crops in fall, look at the seven day or
longer weather forecasts for your area. Does the seed have
enough time to germinate and establish itself before winter
in your particular USDA Cold Hardiness Zone? If rain is in
the forecast, try to plant beforehand.
For no-till rolling of cover crops, consider the amount of
biomass that the cover crop can potentially produce and its
typical flowering date in the spring to determine the kill
date that will be most effective for rolling mechanically.
If you roll too soon— before at least 50 percent of
flowering has occurred—not only may you get less biomass,
but you won’t get an effective kill because the crop
is still in the vegetative state.
Try new cover crop species. Some seed companies’ suppliers
will often give a grower a 1- to 5-pound trial sample bag
of seed. Rye will produce more biomass than wheat or barley.
Sorghum-Sudan grass hybrids, being warm season annuals, produce
the most biomass when they are utilized as a summer cover
crop. Sorghum-Sudan has also been used in mixes with sesbania,
sunn hemp, cow pea, and buckwheat. Hairy vetch, a winter annual
legume, produces a natural heavy mat of biomass with a nitrogen
content of 3.5 to 4 percent.
Rye: The gold standard of cold-climate
cover crops
Of all the small grains, cereal rye has the lowest seed germination
temperature and thus can be seeded later in the fall. For
this reason, cereal rye offers the most flexibility in rotation
systems. Rye is the most winter hardy of all the small cereal
grains; its cold tolerance exceeding even that of the hardiest
winter wheats. Rye grows taller and, as a result, produces
more biomass than wheat or barley.
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Popular cultivated varieties of rye include ‘Merced’,
‘Aroostook’ (very cold tolerant), ‘Albion’
(which repels nematodes), ‘Abruzzi’ (the earliest
southern variety), ‘Elbon’ (slightly later to bloom)
and ‘Rymor’ (a late bloomer). ‘Wheeler’
is known to have allelopathic properties and is typically more
expensive. Preliminary research from North Carolina State University
suggested that ‘Vita Graze’ and ‘Athens Abruzzi’
may be the varieties of choice for weed suppression in no-till
corn.
Few varieties of rye maintain distinct characteristics because
they comprise a mixture of types produced by cross pollination.
Cereal rye is largely cross pollinated; most plants are self-sterile.
A variety grown in a different environment may quickly adapt
itself to the new conditions. Changes in characteristics such
as cold resistance, early maturation or development, and color
are the results of segregation as well as outcrossing with
local varieties. Winter rye varieties adapted to the northern
states are usually wholly unsuited to conditions in the South,
where mild temperatures fail to provide sufficient cold to
force such true winter varieties into early heading (vernalization).
The non-hardy varieties grown in the south and southwest actually
have a partial or total spring habit of growth, flowering
and maturing before the hot summer weather.
After planting the cover crops, take notes on emergence,
overwintering capability and stand establishment. Now consider
the “spatial niches” on your farm. Strip cropping
provides a simple method for rotating a cover crop with a
vegetable planting. Alternate field strips or beds of a fall-
or spring-planted cover crop with strips of early-planted
vegetables like potato, onion, cabbage, lettuce or peas. Adjust
the width of your fields to accommodate easier cover crop
seeding (using your seeding equipment as the standard of measure).
Strip cropping is a low-cost, low-input way of getting the
benefits of a cover crop.
For agronomic crops planted on a larger acreage, the strip-cropping
concept can still apply, but the strips will be large, field-sized
strips with the width adjusted for your particular farm planting
and harvesting equipment. Rotating cover crops in field strips
arranged parallel to the contour slope of a hill makes good
agronomic sense; these rotated cover crop strips will prevent
soil erosion over time. Even where the topography is largely
flat, cover crop strips provide other added benefits—such
as attracting beneficial insects, providing crop diversification,
and buffering effects—which can all help break the rapid
spread of disease and insect epidemics that often plague large
monoculture cropping systems.
Cover crop systems do require time, money and management.
If not properly managed, cover crops can interfere with vegetable
or field crops. Find the least expensive cover that meets
your goals. Low cost cover crops like cereal rye can be seeded
for around $21 per acre while higher value cover crops such
as vetch can cost upwards of $60 per acre. To save time and
money, prepare the soil minimally prior to seeding. Conventional
seedbeds that are plowed, disked and harrowed require time
and labor. Consider no-till planting of the cover crop itself,
as this practice will keep higher amounts of crop residue
on the soil surface and save time, labor and fuel.
Know the characteristics and organic matter content of your
soil. In some soils, cover crops allow for earlier field entry
and planting in the spring. Cover crops such as alfalfa, barley
and white clover require good soil drainage (additionally, barley
requires higher nitrogen levels in the fall for tillering.
(Tillers are branches or shoots of the main stem of
grasses. When the small grains (Rye, Barley or Wheat) are grown
as cover crops, more tillers means more potential biomass produced
from the plant.)
For wet sites, use alsike clovers or winter rye. For low-fertility
sites with low pH, use pearl millet, barley, alsike clovers
or birdsfoot trefoil.
Think about the big picture. How will you get your cover
crop established? What type and size of drill is available?
What are the speed-ratio drive settings of the gearing for
the seeders? Do you need to make adjustments? Check the drill
manual to determine the proper fluted feed opening required
for your particular cover crop seed. As you try new cover
crops some drills may not have a “notch” setting
for all the cover crop seed you’re using. For example,
the John Deere grain drill on our farm does not have a setting
for rapeseed. So we planted rapeseed using the grass seed
box of the drill, calibrating it to the crimson clover or
pearl millet setting (both have a seed size similar to rapeseed).
With a little tinkering, we’ve found that there’s
usually a solution close at hand.
The biodiversity you bring into your system with good crop
rotation delivers many benefits, including improved yields,
reduced and often prevented disease transmission, insect control,
weed suppression, soil nitrogen management, improving soil
tilth and structure, improved water utilization and reduced
soil erosion. Cover crops are key in any rotation and, in
the case of organic farming, are often the underlying drivers
of the system.
Dave Wilson is a research agronomist with The Rodale Institute.
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