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8/14/2019 Organic - Weed Management for Organic Farmers
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F O R O R G A N I C F A R M E R SWeed Management
PM 1883 August 2003
OMechanical tillage
is an importantcomponent of organic
weed management.
Organic Farming Requires Weed Management
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.)
Organic farmers
use a wide variety
of tools and
strategies to control
weeds without
synthetic chemicals.
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F O R O R G A N I C F A R M E R S
Weed Management
A
2P A G E
E
Natural mulches can regulatethe soil temperatures and moisture,
improve soil quality, and suppressweeds in organic crops.
Weed Productivity and Ecology
A weed is simply a plant that takes advantage of unused resources made available by croppingpractices. Nutrient availability is usually high at thebeginning of the season and decreases during theseason. Water availability depends on the weatherpattern of the particular year, but it is typicallyhigher in the beginning and lower at the end of theseason. Light under the crop canopy that isavailable for weeds is high at beginning, low in themiddle, and generally high again at the end of theseason when crop leaves are senescing. Knowingthe ecology (germination and growth period) of the weeds on your farm is key to organic weedmanagement.
Why Organic?
Environmental, economic,and food safety concernsare among the many rea-sons why some farmerschoose organic production.For example, certifiedorganic soybeans in Iowaaveraged a 200300 per-cent premium price overconventional soybeans. Onthe environmental front,organic farmers hope toreduce the 240 millionpounds of herbicidesapplied each year in theMidwest. Organic farmersalso express concernsabout weed resistance to herbicides and thetransfer of herbicide-resistant genes to wild plantsthat may occur with increasing reliance onherbicide-resistant crops.
According to the federal Organic FarmingProduction Act (OFPA) of 1990, and the USDANational Organic Program (NOP), anyone sellingproducts as organic must follow a set of pre-scribed practices that includes avoiding syntheticchemicals in crop and livestock production and inthe manufacturing of processed products. Organiccertification agencies, serving as the required thirdparty certification, include private agencies, such asthe Organic Crop Improvement Association (OCIA),and public programs, such as the State of IowaDepartment of Agriculture and Land Stewardship(IDALS) organic certification program. For moreinformation, see Organic Agriculture (PM 1880).
Environmental,
economic, and
food safety concerns
are among the many
reasons why
some farmers choose
organic production.
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Organic crops are required to be grown in rotations, asdemonstrated by the corn-soybean-oat-alfalfa rotation,shown at the ISU Neely-Kinyon Farm.
under any remaining rye plant residues to avoidcompetition with the germinating crop.
There is a strong correlation between biomass,tillering (multiple stems), and weed competitive-ness. Barley, for example, has a more extensivetillering system and is more competitive withweeds than wheat. Because small grains also areplanted in narrower rows, these crops are morecompetitive with weeds than corn or soybeans.Many organic farmers opt to fallow a field to a ryefor an entire season if weeds have presented apersistent problem in the past.
Maintaining soil fertility through crop rotations,cover crops, intercrops, and biologically-basedfertilizers will enhance the competitiveness of thecrop plant and inhibit weed growth. Reportsindicate that humic and fulvic acids in compost maymitigate weed seed germination. Small-seededweeds also may be more susceptible to pathogensassociated with high organic matter in compost.Compost placed close to the crop plant reduces theamount of nutrients available to weeds betweencrop rows. Mulch also is effective in suppressingweed establishment.
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MEcological Weed Managemnet
Most organic farmers relyon multiple tactics for theirweed management. Eco-logical weed managementpromotes weed suppres-sion, rather than weedelimination, by enhancingcrop competition and phy-totoxic effects on weeds.Specific methods includethe following:
Crop RotationsCrop rotations are thefoundation of organicfarming. Organic certifica-tion requires that a smallgrain and/or legume beplanted after row crops tomaintain soil health andbiologically based pest management. As anexample, if the legume is plowed under as a covercrop in the fifth year, four years of row crops couldbe grown prior to the green manure crop year. Thesame crop cannot be grown in sequential years;thus, soybeans cannot be grown in the same fieldyear after year. The ideal crop preceding soybeansis winter rye. Soybean fields are rotated to a smallgrain (oats, barley, wheat, or rye) or corn.
A typical crop rotation in Iowa is corn followedby a winter cover of rye, soybeans, and oats with anunderseeding of alfalfa or red clover in the thirdyear. Rye, with its allelopathic properties, will helpprevent weed establishment. In the spring, rye thatis less than 8" in height can be killed with a fieldcultivator. Taller rye plants should be mowed or cutwith a stalk chopper before cultivating. A secondcultivation or disking may be necessary to turn
Ecological weed
management
promotes
weed suppression,
rather than
weed elimination,
by enhancing
crop competition and
phytotoxic effects
on weeds.
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Row cultivators are used two to three times to controlweeds between rows.
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Production Practices
Cultivar or variety selection. Organic farmersselect crop varieties that compete well withweeds. Quick canopy-forming varieties assistthe crop competitiveness over weeds withinand between rows.
Crop density. Planting at the maximum adaptedpopulation will provide the crop an increasedcompetitive edge over weeds.
Row spacing. Closer row spacing generallyleads to greater crop competition with weeds inrow middles. Some organic farmers have drilledsoybeans with rye and obtained excellent weedcontrol but lower yields.
Seed grade. High germination rates are criticalfor 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 isadequately warmed (usually later than May 10)to facilitate rapid germination. Warmer soilsallow quicker emergence and a more competi-
tive crop without major losses in yield.
Physical Tactics forOrganic Weed ManagementPhysical 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 CultivationDepending on the crop, cultivation offers the least
labor-intensive weed control method. Althoughmore than 95 percent of Iowas row crop acres aretreated with herbicides, cultivation remains a viableweed management strategy, in conjunction withother controls. Timely cultivation is critical inorganic weed management. While no prescription isavailable for managing weeds in an organic system,certain practices have been used successfully bythe majority of Iowas organic farmers. Each farmer
F O R O R G A N I C F A R M E R S
Weed Management
must develop his/her own
management plan basedon specific farm/field con-ditions. A harrow or fieldcultivator with a drag- orspring-tooth harrow attach-ment can be used in thespring to kill weeds beforeplanting. Cultivation thenis timed with the pulses of weed seed germinationand growth. The initial
cultivation occurs when the weeds are at the mostvulnerable stage. Fields are rotary hoed at a slowspeed (5 mph) three to five days after planting to killweeds in their initial development or white-threadstage. A harrow also can be used at this stage. Oneweek later, after plants have emerged, fields arehoed again but at a slightly faster speed (79 mph).To avoid killing soybean seedlings, it is critical thatsoybeans are not hoed in the crook stage when thesoybean hypocotyl is just at the soil surface.Soybeans also should not be hoed when plants are
greater than 8" in height. For vegetable croppingsystems, various in-row weeding tool sets, includ-ing finger weeders, basket weeders, Bezzerides
torsion weeders, Spyders , Weed Badgers , andbrush weeders, can be used alone or in combinationon 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.
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Table 1Costs 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% greaterSouthwest Iowa as conventional conventional as conventional than conventional
($103/acre) (48 bu/acre)Organic corn: Statistically equal Similar to Statistically similar 228% greaterSouthwest 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 2Effect 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
Sustainable Agriculture and ResearchEducation [SARE] Sustainable Agricul-ture Network [SAN]. (See page 8 forordering information.)
Two to three row cultivations aretypical for Midwestern organic farmers.The first cultivation occurs at a slowspeed as soon as rotary hoeing ends. Thesecond cultivation usually is completedat mid-season at a faster speed to throwabout 1" of soil around the base of plants.The third cultivation is again performedat a slow speed (5 mph). Various attachments, suchas guidance systems, and plant protection devices,such as shields, can be added to cultivators (seeSteel in the Field ).
The number of acres covered when cultivatingwill depend on the size of the cultivator and thespeed of cultivation. A 6-row cultivator, at speeds of 67 mph, can cover 100 acres in 11 hours.Cultivators with open-top shields and low-profilesingle sweeps throw less than 1" of soil into the rowat this speed. Higher speeds may throw excess soil
onto the plant row and damage small crops. Diskhillers can be used to pull soil away from the plant
when plants are small and turnedoutward on later passes to throw soilaround the base of the plant when theyare taller.
The Long-Term AgroecologicalResearch (LTAR) projects funded by IowaState Universitys Leopold Center forSustainable Agriculture have comparedweed populations and managementcosts in organic and conventional graincrops (Delate et al., 1999). In general,soybeans with a quicker closing canopy
had less weed pressure than organic corn. Soybeanyields were the same as conventional soybeans.Once the fields were in their third year of rotation,corn weed pressure was diminished. Despite lowercorn yields in one year, organic corn provided agreater return overall than conventional corndue to organic premium prices. The results aresummarized in Table 1.
Another study in Chariton, Iowa, comparedweed populations and yields in organic systemsunder different primary tillage methods (Table 2).
Treatments included fall moldboard plowing, fallKverneland plowing, spring moldboard plowing,
Two to three row
cultivations are
typical for Midwestern
organic farmers.
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are usually run at 3840 psi with a tractor speed of 4 mph, but specific rates will be based on thecrop stage, weed species present, and the manu-facturers recommendations.
MulchingMany organic horticultural operations rely on man-ual labor and a combination of mulching/cultivationfor adequate weed control. Natural mulches andsynthetic mulches, including polyethylene film and
polypropylene landscape fabric, are used in organicoperations. Synthetic mulches have proven theirdurability in numerous vegetable productionsystems. According to certified organic regulationsin Iowa, however (IDALS, 2000), synthetic mulchesmust remain intact during the growing season andmust be removed from the field at the end of theseason. Despite the labor-saving aspect of syntheticmulches, most organic growers in Iowa prefernatural mulches for weed control. Natural mulchesare used to regulate surface soil temperatures
and moisture, improve soil quality, and suppressweed activity. Organic mulches add organic matterto the soil during decomposition, thus increasingnutrients, 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, andherb farms, to control weeds and protect againstharsh environments.
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F O R O R G A N I C F A R M E R S
Weed Management
Propane flame burners can be used on organic farms tocontrol weeds between and within rows.
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and fall and spring Rotavator plowing. Kverneland
plows are reported to disturb less of the soil profileand are very effective on CRP land. Rotavators areequally effective on CRP land and provide a morefriable seed bed. Results of this two-year study(soybeanstwo years and cornone year only)showed greater yields in spring-plowed fields over-all. Weed populations were greater in theRotavator treatment over two seasons and threeweed-scouting periods.
Propane Flame BurningMany organic farmershave included propane(LP) flame-burners as anadditional tool in theirweed management tool-box. Flaming is usedparticularly during timesof high field moisturewhen tillage with largemachinery is not feasible.In drier weather, flaming is
used in conjunction withcultivation. Flame weedingof corn can be accom-plished when corn is lessthan 2" in height andwhen corn is 8" to 1.5' in height. Other growthstages are considered potentially damaging to thecorn meristem (growing point).
According to several Iowa farmers, soybeanscan be flamed prior to emergence and at the 1' to1.5' height stage. Caution must be used in flaming
soybeans, however. Mixed results have beenreported, including damaged soybeans anddecreased yields. Because the growing terminal of corn is more protected within the whorl, corn canwithstand greater damage from flaming, comparedto soybeans with their exposed growing points.Several flame burners are commercially available,including complete units and individual burner kitsthat can be attached to tractor tool bars. Flamers
Many organic farmers
have included
propane (LP)
flame-burners as
an additional tool
in their weed
management toolbox.
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Mulch can be made from small grain, soybeanstraw, or baled corn stalks. A Bale Chopper , pulledby a tractor along a raised vegetable bed, can beused to shred small square straw bales for rapidmulch application. Other methods of straw mulchapplication include hand-rolling round hay balesacross 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 freeof synthetic substances, such as preservatives inwood or synthetic dyes in paper products. Yourorganic certification could be jeopardized if synthetic chemicals are introduced through mulchmaterials. Mulch made from materials grown onorganic farms prevents any possibility of pesticideresidues from conventionally grown materials.Wood chips should not be applied to a rapidlygrowing crop, such as an annual plant, to avoidcompetition with the crops nitrogen source as thewood chips degrade.
Other mulches include living and mowedresidues of cover crops (SAN, 1998). Under covercrop residue mulches, weed emergence is decreasedby reductions in both light transmittance and dailysoil temperatures. Hairy vetch ( Vicia villosa Roth)and rye ( Secale cereale L.) residue mulches areused in many organic operations. Mulches shouldbe maintained at a depth of 6 inches to preventstimulation of weed growth. Results with livingmulches have ranged from unwanted competitionwith the crop and delaying fruit set, to having noeffect on yields. Much of the effectiveness of livingmulches depends on the availability of fieldmoisture for both crop and mulch.
Many horticultural crops, such as medicinal andculinary herbs, produce greater yields whenmulched, rather than cultivated for weed manage-ment. Two natural fiber mulches that are currentlyin the research stage for organic horticultural cropproduction are a flax straw mat and a wool mat.These mulches may prove effective for organicoperations, provided the mats are approved for thecertified organic label.
O
I
Effects on Soil Quality
Organic farmers are concerned about the effect of multiple tillage passes on soil quality, includingerosion. In our studies to date, no differences in soilquality (physical and chemical properties) wereobserved among the different tillage treatments. Inresearch at the Neely-Kinyon Farm in SouthwestIowa, after one growing season under organic man-agement, microbial biomass carbon was 228 percentgreater in the organic system, maggroaggregatestability was 15 percent greater, organic carbon was6 percent greater, particulate organic matter carbonwas 8 percent greater, and N mineralization poten-tial was 7 percent greater. Nitrate-N was 44 percentgreater in the conventional system, as reflected inthe excess corn stalk nitrate detected in the firstyear. Potential effects on soil quality will continue tobe monitored, but practices employed by organicfarmers, such as adding organic matter through croprotations, cover crops, green manures, manure andcompost applications, and other biological fertiliz-ers, will help mitigate the erosive effects of tillage.
Conclusion
Integrated weed manage-ment on organic farms re-quires extensive planningand management. Croprotations are the basis forsuccessful organic farmingand are necessary forbreaking weed, insect, anddisease cycles. Cultivationmust be completed withproperly set equipment under soil conditions thatare not conducive to compaction. Monitoring weedgrowth stages also is critical in determining idealcultivation times. Trial and error will govern manydecisions in the first years of organic farming. Learn-ing from other organic farmers and following advicefrom Iowa State University may help in the transition.
Trial and error will
govern many decisions
in the first years of
organic farming.
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Publications mentioned in this booklet can be ordered bycontacting any ISU Extension county office or the ISUExtension Distribution Center at (515) 294-5247. There is acharge, plus shipping and handling, for some publications.
File: Agriculture 2 [A]
. . . and justice for all
The U.S. Department of Agriculture (USDA) prohibits discrimination in all its pro-grams and activities on the basis of race, color, national origin, gender, religion,age, disability, political beliefs, sexual orientation, and marital or family status.(Not all prohibited bases apply to all programs.) Many materials can be madeavailable in alternative formats for ADA clients. To file a complaint of discrimina-tion, write USDA, Office of Civil Rights, Room 326-W, Whitten Building, 14th andIndependence Avenue SW, Washington, DC 20250-9410 or call 202-720-5964.
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30,1914, in cooperation with the U.S. Department of Agriculture. Stanley R. Johnson,director, Cooperative Extension Service, Iowa State University of Science andTechnology, Ames, Iowa.
This research project was partially funded bythe Leopold Center for Sustainable Agricultureat Iowa State University and the USDA-SARE(Sustainable Agriculture Research & Education)Program.
Prepared by Kathleen Delate and Robert Hartzler,Iowa State University.
Edited by Jean McGuire, ISU ExtensionContinuing Education & Communication Services.
Illustrated by Jane Lenahan.
Designed by Mary Sailer, Spring Valley Studio.
Some photos on the cover were provided courtesy of IowaState University College of Agriculture and the USDA-NRCS.
For the latest on organic agriculture from Iowa State Universitygo to http://extension.agron.iastate.edu/organicag/.
L E O P O L D C E N T E R
F O R O R G A N I C F A R M E R S
Weed Management
ReferencesBowman, G. (ed.). 1997. Steel in the FieldA farmers
guide to weed management tools. SustainableAgriculture Network, USDA, National AgricultureLibrary, Beltsville, MD.
Delate, K., and C. Cambardella, 2000. Integrating organic soybeans following CRP land. USDA-SARE AnnualReport, N.C. SARE, University of Nebraska, Lincoln, NE.
Delate, K., C. Cambardella, K. Taylor, and B. Burcham.1999. Comparison of organic and conventional rota- tions at the Neely-Kinyon Long-Term Agroecological
Research (LTAR) site: First year results. LeopoldCenter for Sustainable Agriculture Annual Report,Iowa State University, Ames, IA.
IDALS (Iowa Department of Agriculture and LandStewardship). 2000. Iowa Organic Certification and Organic Standards. Des Moines, IA.
Rynk, R. 1992. On-Farm Composting Handbook. NRAES-
54. Northeast Regional Agricultural EngineeringService. Ithaca, NY.
Sustainable Agriculture Network (SAN). 1998. Managing cover crops profitably. 2nd Edition. SustainableAgriculture Network, USDA National AgricultureLibrary, Beltsville, MD.
