Weed Management For Organic Farmers.pdf
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Organic Ag Bulletin 1883
Weed Management
FOR ORGANIC FARMERS
Organic farmers
O
Organic Farming Requires Weed Management
use a wide variety
of tools and
Organic farmers use a wide variety of tools and strategies to control weeds without
strategies to control
synthetic chemicals. Successful organic farmers continually adapt their weed
weeds without
management practices as weed populations shift. Producers should have a good
synthetic chemicals.
understanding of the philosophies and legalities of organic farming before they plan
their weed management strategies. A brief overview of organic agriculture follows;
for further details, see Iowa State University Extension publication
Organic
Agriculture
(PM 1880). (See page 8 for ordering instructions.)
Mechanical tillage
is an important
component of organic
weed management.
PM 1883 August 2003
Weed Management
FOR ORGANIC FARMERS
E
A
Weed Productivity and Ecology
Environmental, economic,
and food safety concerns
are among the many rea-
sons why some farmers
choose organic production.
For example, certified
organic soybeans in Iowa
averaged a 200–300 per-
cent premium price over
conventional soybeans. On
the environmental front,
organic farmers hope to
reduce the 240 million
pounds of herbicides
applied each year in the
Midwest. Organic farmers
also express concerns
about weed resistance to herbicides and the
transfer of herbicide-resistant genes to wild plants
that may occur with increasing reliance on
herbicide-resistant crops.
According to the federal Organic Farming
Production Act (OFPA) of 1990, and the USDA
National Organic Program (NOP), anyone selling
products as “organic” must follow a set of pre-
scribed practices that includes avoiding synthetic
chemicals in crop and livestock production and in
the manufacturing of processed products. Organic
certification agencies, serving as the required third
party certification, include private agencies, such as
the Organic Crop Improvement Association (OCIA),
and public programs, such as the State of Iowa
Department of Agriculture and Land Stewardship
(IDALS) organic certification program. For more
information, see
Organic Agriculture
(PM 1880).
Environmental,
A weed is simply a plant that takes advantage of
unused resources made available by cropping
practices. Nutrient availability is usually high at the
beginning of the season and decreases during the
season. Water availability depends on the weather
pattern of the particular year, but it is typically
higher in the beginning and lower at the end of the
season. Light under the crop canopy that is
available for weeds is high at beginning, low in the
middle, and generally high again at the end of the
season when crop leaves are senescing. Knowing
the ecology (germination and growth period) of
the weeds on your farm is key to organic weed
management.
economic, and
food safety concerns
are among the many
reasons why
some farmers choose
organic production.
Natural mulches can regulate
the soil temperatures and moisture,
improve soil quality, and suppress
weeds in organic crops.
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Why Organic?
M
under any remaining rye plant residues to avoid
competition with the germinating crop.
There is a strong correlation between biomass,
tillering (multiple stems), and weed competitive-
ness. Barley, for example, has a more extensive
tillering system and is more competitive with
weeds than wheat. Because small grains also are
planted in narrower rows, these crops are more
competitive with weeds than corn or soybeans.
Many organic farmers opt to fallow a field to a rye
for an entire season if weeds have presented a
persistent problem in the past.
Maintaining soil fertility through crop rotations,
cover crops, intercrops, and biologically-based
fertilizers will enhance the competitiveness of the
crop plant and inhibit weed growth. Reports
indicate that humic and fulvic acids in compost may
mitigate weed seed germination. Small-seeded
weeds also may be more susceptible to pathogens
associated with high organic matter in compost.
Compost placed close to the crop plant reduces the
amount of nutrients available to weeds between
crop rows. Mulch also is effective in suppressing
weed establishment.
Most organic farmers rely
on multiple tactics for their
weed management. Eco-
logical weed management
promotes weed suppres-
sion, rather than weed
elimination, by enhancing
crop competition and phy-
totoxic effects on weeds.
Specific methods include
the following:
Ecological weed
management
promotes
weed suppression,
rather than
weed elimination,
Crop Rotations
Crop rotations are the
foundation of organic
farming. Organic certifica-
tion requires that a small
grain and/or legume be
planted after row crops to
maintain soil health and
biologically based pest management. As an
example, if the legume is plowed under as a cover
crop in the fifth year, four years of row crops could
be grown prior to the green manure crop year. The
same crop cannot be grown in sequential years;
thus, soybeans cannot be grown in the same field
year after year. The ideal crop preceding soybeans
is winter rye. Soybean fields are rotated to a small
grain (oats, barley, wheat, or rye) or corn.
A typical crop rotation in Iowa is corn followed
by a winter cover of rye, soybeans, and oats with an
underseeding of alfalfa or red clover in the third
year. Rye, with its allelopathic properties, will help
prevent weed establishment. In the spring, rye that
is less than 8" in height can be killed with a field
cultivator. Taller rye plants should be mowed or cut
with a stalk chopper before cultivating. A second
cultivation or disking may be necessary to turn
by enhancing
crop competition and
phytotoxic effects
on weeds.
Organic crops are required to be grown in rotations, as
demonstrated by the corn-soybean-oat-alfalfa rotation,
shown at the ISU Neely-Kinyon Farm.
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Ecological Weed Managemnet
Weed Management
FOR ORGANIC FARMERS
Production Practices
•
Cultivar or variety selection.
Organic farmers
select crop varieties that compete well with
weeds. Quick canopy-forming varieties assist
the crop competitiveness over weeds within
and between rows.
•
Crop density.
Planting at the maximum adapted
population will provide the crop an increased
competitive edge over weeds.
•
Row spacing.
Closer row spacing generally
leads to greater crop competition with weeds in
row middles. Some organic farmers have drilled
soybeans with rye and obtained excellent weed
control but lower yields.
•
Seed grade.
High germination rates are critical
for a rapid canopy.
•
Sowing date.
To provide a competitive advan-
tage, warm season crops (e.g., corn, soybeans,
and dry beans) are planted when the soil is
adequately warmed (usually later than May 10)
to facilitate rapid germination. Warmer soils
allow quicker emergence and a more competi-
tive crop without major losses in yield.
must develop his/her own
management plan based
on specific farm/field con-
ditions. A harrow or field
cultivator with a drag- or
spring-tooth harrow attach-
ment can be used in the
spring to kill weeds before
planting. Cultivation then
is timed with the pulses of
weed seed germination
and growth. The initial
cultivation occurs when the weeds are at the most
vulnerable stage. Fields are rotary hoed at a slow
speed (5 mph) three to five days after planting to kill
weeds in their initial development or white-thread
stage. A harrow also can be used at this stage. One
week later, after plants have emerged, fields are
hoed again but at a slightly faster speed (7–9 mph).
To avoid killing soybean seedlings, it is critical that
soybeans are not hoed in the crook stage when the
soybean hypocotyl is just at the soil surface.
Soybeans also should not be hoed when plants are
greater than 8" in height. For vegetable cropping
systems, various in-row weeding tool sets, includ-
ing finger weeders, basket weeders, Bezzerides
®
torsion weeders, Spyders
®
, Weed Badgers
®
, and
brush weeders, can be used alone or in combination
on a multiple component weeding frame (See
Steel
in the Field
(EDC 125), produced by the USDA
Organic farmers
select crop varieties
that compete well
with weeds.
Physical Tactics for
Organic Weed Management
Physical controls are a key factor for weed manage-
ment on all organic farms. Physical methods of
control include cultivation, propane flame burning,
and mulching.
Mechanical Cultivation
Depending on the crop, cultivation offers the least
labor-intensive weed control method. Although
more than 95 percent of Iowa’s row crop acres are
treated with herbicides, cultivation remains a viable
weed management strategy, in conjunction with
other controls. Timely cultivation is critical in
organic weed management. While no prescription is
available for managing weeds in an organic system,
certain practices have been used successfully by
the majority of Iowa’s organic farmers. Each farmer
Row cultivators are used two to three times to control
weeds between rows.
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Sustainable Agriculture and Research
Education [SARE] Sustainable Agricul-
ture Network [SAN]. (See page 8 for
ordering information.)
Two to three row cultivations are
typical for Midwestern organic farmers.
The first cultivation occurs at a slow
speed as soon as rotary hoeing ends. The
second cultivation usually is completed
at mid-season at a faster speed to throw
about 1" of soil around the base of plants.
The third cultivation is again performed
at a slow speed (5 mph). Various attachments, such
as guidance systems, and plant protection devices,
such as shields, can be added to cultivators (see
Steel in the Field
).
The number of acres covered when cultivating
will depend on the size of the cultivator and the
speed of cultivation. A 6-row cultivator, at speeds of
6–7 mph, can cover 100 acres in 11 hours.
Cultivators with open-top shields and low-profile
single sweeps throw less than 1" of soil into the row
at this speed. Higher speeds may throw excess soil
onto the plant row and damage small crops. Disk
hillers can be used to pull soil away from the plant
Two to three row
when plants are small and turned
outward on later passes to throw soil
around the base of the plant when they
are taller.
The Long-Term Agroecological
Research (LTAR) projects funded by Iowa
State University’s Leopold Center for
Sustainable Agriculture have compared
weed populations and management
costs in organic and conventional grain
crops (Delate et al., 1999). In general,
soybeans with a quicker closing canopy
had less weed pressure than organic corn. Soybean
yields were the same as conventional soybeans.
Once the fields were in their third year of rotation,
corn weed pressure was diminished. Despite lower
corn yields in one year, organic corn provided a
greater return overall than conventional corn
due to organic premium prices. The results are
summarized in Table 1.
Another study in Chariton, Iowa, compared
weed populations and yields in organic systems
under different primary tillage methods (Table 2).
Treatments included fall moldboard plowing, fall
Kverneland
®
cultivations are
typical for Midwestern
organic farmers.
plowing, spring moldboard plowing,
Table 1
Costs and Effectiveness of Weed Control Measures in Organic vs. Conventional Systems
System
Weed Populations
Management Costs
Yields
Returns
Organic soybeans:
Statistically the same
Less than
Statistically the same
368% greater
Southwest Iowa
as conventional
conventional
as conventional
than conventional
($103/acre)
(48 bu/acre)
Organic corn:
Statistically equal
Similar to
Statistically similar
228% greater
Southwest Iowa
to conventional
conventional
in 2 out of 3 years
than conventional
in 2 out of 3 years
($172/acre)
(average: 130 bu/acre)
Table 2
Effect of Tillage on Weed Management and Yields
System
Weed Populations
Yields
Organic soybeans:
Statistically greater grasses in Rotavator
®
in Year 1;
No statistical differences in Year 1;
Southeast Iowa
no statistical differences in Year 2
spring-plowed greater in Year 2
(59 bu/acre)
Organic corn:
Statistically greater broadleaves in spring-plowed;
Spring-plowed greater (227 bu/acre),
Southeast Iowa
statistically greater grasses in Rotavator
®
but no statistical differences between
spring and fall moldboard plowed
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