Weed Management For Organic Farmers.pdf

Weed Management

FOR ORGANIC FARMERS

Organic farmers

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

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?

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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