[PDF]Weed Management for Organic Farmers

FOR OR G A N KC FARMER S



Organic farmers Organic Farming Requires Weed Management



use a wide variety



of tools and



strategies to control



weeds without



synthetic chemicals.



Organic farmers use a wide variety of tools and strategies to control weeds without
synthetic chemicals. Successful organic farmers continually adapt their weed
management practices as weed populations shift. Producers should have a good
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.)




Iowa State University

University Extension



PM 1883 August 2003



■ V.



Weed Management

FOR O R G A Ml C FARMERS ■ <]



Why Organic?



Weed Productivity and Ecology



Environmental,



economic, and



food safety concerns



are among the many



reasons why



some farmers choose



organic production.



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).

Natural mulches can regulate
the soil temperatures and moisture,
improve soil quality, and suppress
weeds in organic crops.



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.




PAGE



Ecological Weed Managemnet



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:




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



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.




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.



PAGE



Weed Management



FOR ORGANIC FARMERS V v ' '



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.

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



Organic farmers



select crop varieties



that compete well



with weeds.



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




Row cultivators are used two to three times to control
weeds between rows.



PAGE




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
cultivations are
typical for Midwestern
organic farmers.



^ 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® 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 :
Southwest Iowa


Statistically the same
as conventional


Less than

conventional

($103/acre)


Statistically the same
as conventional
(48 bu/acre)


368% greater
than conventional


Organic corn:
Southwest Iowa


Statistically equal
to conventional
in 2 out of 3 years


Similar to

conventional

($172/acre)


Statistically similar
in 2 out of 3 years
(average: 130 bu/acre)


228% greater
than conventional


Table 2

Effect of Tillage on Weed Management and Yields


System


Weed Populations




Yields





Organic soybeans :
Southeast Iowa



Statistically greater grasses in Rotavator® in Year 1 ;
no statistical differences in Year 2



No statistical differences in Year 1 ;
spring-plowed greater in Year 2
(59 bu/acre)



Organic corn: Statistically greater broadleaves in spring-plowed;

Southeast Iowa statistically greater grasses in Rotavator®



Spring-plowed greater (227 bu/acre),
but no statistical differences between
spring and fall moldboard plowed



? A G I



Weed Management

F O R O R G A 1ST I C FARMERS :



and fall and spring Rotavator® plowing. Kverneland®
plows are reported to disturb less of the soil profile
and are very effective on CRP land. Rotavators® are
equally effective on CRP land and provide a more
friable seed bed. Results of this two-year study
(soybeans — two years and corn — one year only)
showed greater yields in spring-plowed fields over-
all. Weed populations were greater in the
Rotavator® treatment over two seasons and three
weed-scouting periods.

Propane Flame Burning

Many organic farmers
have included propane
(LP) flame-burners as an
additional tool in their
weed management tool-
box. Flaming is used
particularly during times
of high field moisture
when tillage with large
machinery is not feasible.
In drier weather, flaming is
used in conjunction with
cultivation. Flame weeding
of corn can be accom-
plished when corn is less
than 2" in height and

when corn is 8" to 1.5' in height. Other growth
stages are considered potentially damaging to the
corn meristem (growing point).

According to several Iowa farmers, soybeans
can be flamed prior to emergence and at the l 1 to
1.5' height stage. Caution must be used in flaming
soybeans, however. Mixed results have been
reported, including damaged soybeans and
decreased yields. Because the growing terminal of
corn is more protected within the whorl, corn can
withstand greater damage from flaming, compared
to soybeans with their exposed growing points.
Several flame burners are commercially available,
including complete units and individual burner kits
that can be attached to tractor tool bars. Flamers





Propane flame burners can be used on organic farms to
control weeds between and within rows.



are usually run at 38-40 psi with a tractor speed of
4 mph, but specific rates will be based on the
crop stage, weed species present, and the manu-
facturer's recommendations.

Mulching

Many organic horticultural operations rely on man-
ual labor and a combination of mulching/cultivation
for adequate weed control. Natural mulches and
synthetic mulches, including polyethylene film and
polypropylene landscape fabric, are used in organic
operations. Synthetic mulches have proven their
durability in numerous vegetable production
systems. According to certified organic regulations
in Iowa, however (IDALS, 2000), synthetic mulches
must remain intact during the growing season and
must be removed from the field at the end of the
season. Despite the labor-saving aspect of synthetic
mulches, most organic growers in Iowa prefer
natural mulches for weed control. Natural mulches
are used to regulate surface soil temperatures
and moisture, improve soil quality, and suppress
weed activity. Organic mulches add organic matter
to the soil during decomposition, thus increasing
nutrients, soil porosity, water holding capacity,
microbial populations, and cation exchange capacity.
Straw mulch is commonly used in organic horticul-
tural operations, such as garlic, strawberry, and
herb farms, to control weeds and protect against
harsh environments.



PAGE



Mulch can be made from small grain, soybean
straw, or baled corn stalks. A Bale Chopper®, pulled
by a tractor along a raised vegetable bed, can be
used to shred small square straw bales for rapid
mulch application. Other methods of straw mulch
application include hand-rolling round hay bales
across vegetable beds before planting. Wood chips,
shredded newspaper, and other plant-based resi-
dues can be used as mulches, typically in tree crops.

Caution must be taken to select materials free
of synthetic substances, such as preservatives in
wood or synthetic dyes in paper products. Your
organic certification could be jeopardized if
synthetic chemicals are introduced through mulch
materials. Mulch made from materials grown on
organic farms prevents any possibility of pesticide
residues from conventionally grown materials.
Wood chips should not be applied to a rapidly
growing crop, such as an annual plant, to avoid
competition with the crop's nitrogen source as the
wood chips degrade.

Other mulches include living and mowed
residues of cover crops (SAN, 1998). Under cover
crop residue mulches, weed emergence is decreased
by reductions in both light transmittance and daily
soil temperatures. Hairy vetch (Vicia villosa Roth)
and rye (Secale cereale L.) residue mulches are
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