Principles of Sustainable Weed Management for Croplands


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Principles of Sustainable Weed Management for Croplands

  1. 1. PRINCIPLES OF SUS TAINABLE WEED SUSTAINABLE MANAGEMENT FOR CROPL ANDS MANA FOR CROPL OPLANDS AGRONOMY SYSTEMS SERIESAbstract: To some extent, weeds are a result of crop production, but to a larger extent they are a consequence ofmanagement decisions. Managing croplands according to nature’s principles will reduce weed problems. And whilethese principles apply to most crops, this publication focuses on agronomic crops such as corn, soybeans, milo, andsmall grains. The opportunities to address the root causes of weeds are not always readily apparent, and often requiresome imagination to recognize. Creativity is key to taking advantage of these opportunities and devising sustainablecropping systems that prevent weed problems, rather than using quick-fix approaches. Annual monoculture cropproduction generally involves tillage that creates conditions hospitable to many weeds. This publication discussesseveral alternatives to conventional tillage systems, including allelopathy, intercropping, crop rotations, and a weed-free cropping design. A Resources list provides sources of further information.By Preston Sullivan boundaries within which we operate and theNCAT Agriculture Specialist rules for success within those boundaries.September 2003 The “weed control” paradigm is reactive— it addresses weedFirst, Free Your Brain problems by usingAs Iowa farmer Tom Frantzen poetically states: various tools and tech-“Free your brain and your behind will follow.” nologies. “How am IWhat Tom is referring to is discovering new para- gonna get rid of this vel-digms. Joel Barker, author of Paradigms—The vet-leaf?” and “How doBusiness of Discovering the Future (1), defines a I control foxtail?” are re-paradigm as a set of standards that establish the active statements. The conventional tools to “get rid of” or “control” Ta b le of Contents weeds—cultivation and herbicides—are reactive measures for solving the problem. The Successful Weed .................................. 2 The Root Cause of Weeds ........................... 2 Farmers would generally agree that weeds are Weed Seed Banks and Germination ............. 3 not in the field because of a deficiency of her- Proactive Weed Management ..................... 4 Weed-Free by Design .................................. 7 bicides or cultivation. Rather, weeds are the Reactive Measures ..................................... 8 natural result of defying nature’s preference Weed Control Tools and Their Effects .......... 8 for high species diversity and covered ground. Integrated Weed Management .................. 11 Nature is trying to move the system in one Other ATTRA Publications of Interest ........ 12 direction, the farmer in another. We create References ............................................... 12 weed problems through conventional crop Additional Resources ............................... 14 production methods. After we create these problems, we spend huge sums of money and labor trying to “control” them.ATTRA is the national sustainable agriculture information service operated by the National Centerfor Appropriate Technology under a grant from the Rural Business-Cooperative Service, U.S.Department of Agriculture. These organizations do not recommend or endorse products,companies, or individuals. ATTRA is headquartered in Fayetteville, Arkansas (P.O. Box 3657,Fayetteville, AR 72702), with offices in Butte, Montana, and Davis, California.
  2. 2. The opposite of reacive thinking is proactive converted from conventional tillage to no-till,thinking, by which we seek what we want the weed population generally shifts from an-through effective design and planning. A pro- nual to perennial weeds. Perennial weeds pos-active approach to weed management asks, sess many of the characteristics of annual“Why do I have weeds?” This publication will weeds: competitiveness, seed dormancy, andexpose you to some proactive principles of crop- long-lived seed. In addition to these character-land management that can make weeds less of istics, many perennial weeds possess perennat-a problem. It also offers some reactive strate- ing parts such as stolons, bulbs, tubers, and rhi-gies to deal with the weeds that remain bother- zomes. These parts allow the parent plant tosome. regenerate if damaged and to produce new plants from the parent plant without seed.The Successful Weed Additionally, the perennating parts serve as food storage units that also enhance survival.Weeds can be divided into two broad catego- These stored-food reserves allow for the rapidries—annuals and perennials. Annual weeds regrowth perennial weeds are known for.are plants that produce a seed crop in one year,then die. They are well adapted to succeed in The Root Cause of Weedshighly unstable and unpredictable environmentsbrought about by frequent tillage, drought, or When a piece of land is left fallow, it is soonother disturbance. They put much of their life covered over by annual weeds. If the field iscycle into making seed for the next generation. left undisturbed for a second year, briars andThis survival strategy serves plants in disturbed brush start to grow. As the fallow period con-environments well, since their environment is tinues, the weed community shifts increasinglylikely to be disturbed again. The annual plant toward perennial vegetation. By the fifth year,must make a crop of seed as soon as possible the field will host large numbers of young treesbefore the next disturbance comes. Annual in a forest region, or perennial grasses in a prai-plants also yield more seed than do perennial rie region. This natural progression of differentplants, which is why humans prefer annual plant and animal species over time is a cycleover perennial crops for grain production. known as succession. This weed invasion, in allWhen we establish annual crop plants using till- its stages, can be viewed as nature’s means ofage (i.e., disturbance) we also create an envi- restoring stability by protecting bare soils andronment desirable for annual weeds. increasing biodiversity.Perennial weeds prosper in less-disturbed and Weeds are evidence of nature struggling to bringmore stable environments. They are more com- about ecological succession. When we clearmon under no-till cropping systems. Their ob- native vegetation and establish annual crops,jective is to put some energy into preserving the we are holding back natural plant succession,parent plant while producing a modest amount at great cost in weed control. To better under-of seed for future generations. After a field is stand this process, think of succession as a coil spring. Managing cropland as an annual mo- noculture compresses the spring¾leaving it straining to release its energy as a groundcover of weeds. In contrast, a biodiverse perennial grassland or forest is like the coil spring in its uncompressed condition—a state of relative sta- bility with little energy for drastic change (Fig- ure 1) (2). Generally speaking, biodiversity leads to more stability for the ecosystem as a whole. Modern crop agriculture is typified by large acre- ages of a single plant type, accompanied by a high percentage of bare ground—the ideal en- vironment for annual weeds to prosper in thePAGE 2 //PRINCIPLES OF WEED MANAGEMENT
  3. 3. Weed seed distribution and density in agricul- tural soils are influenced by cropping history and the management of adjacent landscapes, and may be highly variable. A study of western Nebraska cropland found 140 seeds per pound of surface soil, equivalent to 200 million seeds per acre (3). Redroot pigweed and common lambsquarter accounted for 86%. Growing without competition from other plants, a single redroot pigweed plant can produce more than 100,000 seeds, while a common lambsquarter plant can produce more than 70,000 seeds (4). New weed species can enter fields by many routes. Equipment moved from one field to the next—especially harvest equipment—spreads weed seeds, as does hay brought from one farm to another. Crop seed is often contaminated with weed seed, and livestock transport weed seeds from one farm to another in their diges- tive tracts and in their hair. Practical actions that can be taken to prevent the introduction and spread of weeds include the use of clean seed (check the seed tag for weed-seed levels), cleaning equipment before moving from one field to the next, and composting manures that contain weed seeds before applying them to the field. Survival and germination of weed seeds in the soil depend on the weed species, depth of seed burial, soil type, and tillage. Seeds at or near the soil surface can easily be eaten by insects, rodents, or birds. Also, they may rot or germi- nate. Buried seeds are more protected from seed- eating animals and buffered from extremes of temperature and moisture. On average, about 4% of broadleaf and 9% of grass weed seeds present in the soil germinate in a given year (5). Results from seed burial experiments demon- strated that seeds of barnyard grass and green foxtail buried at 10 inches showed germination rates of 34 to 38% when dug up and spread on the soil surface. In the same study, seed buried at one inch showed only one to five percent ger- mination. In another study, seeds were buried at different depths for a period of three years. Seed germination was greater with increasing depth of burial (3). These studies show that seeds near the surface face lots of hazards to their survival, while those buried deeply by till- age are more protected. When those deep-bur-//PRINCIPLES OF WEED MANAGEMENT PAGE 3
  4. 4. ied seed are plowed up to the surface again they Manure application may stimulate weed ger-have a good chance of germinating and mination and growth. Studies have shown thatgrowing. poultry manure does not contain viable weed seeds, yet weed levels often increase rapidly inTable 1 shows that viable weed seeds are widely pastures following poultry manure application.distributed in moldboard and ridge-till systems. Since chickens and turkeys have a gizzard ca-A higher percentage of seed remains near the pable of grinding seeds, weed seeds are not likelysoil surface under chisel plow and no-till. The to pass through their digestive systems intact.moldboard plow and ridge-till systems are stir- Additionally, most poultry rations contain fewring the soil more, burying lots of weed seeds, if any weed seeds. The weed germination isand keeping weed seed more evenly distributed probably caused by effects of ammonia on thedown to a six-inch depth. weed-seed bank already present in the soil. The effect varies depending on the source of the lit- ter and the weed species present. Manure from hoofed livestock (e.g., sheep, cattle, and horses), on the other hand, may indeed con- tain weed seed that has passed through their digestive systems. Composted manure con- tains far fewer weed seeds than does raw ma- nure because the heat generated during the composting process kills them. Fertilization practices can also affect weed ger-After a seed is shed from the parent plant, it mination. Where fertilizer is broadcast, the en-can remain dormant or germinate. There are tire weed community is fertilized along with theseveral different types of dormancy. Seeds with crop. Where fertilizer is banded in the row, onlyhard seed coats possess “innate” dormancy. the crop gets fertilized.Several weed species, including pigweed, haveseed coats that require mechanical or chemical Proactive Weed Management Strategiesinjury and high-temperature drying to breakdormancy. Another type of innate dormancy In the preceding sections, we saw how weedscan best be described as after-ripening, mean- are established and maintained by human ac-ing the seed requires further development after tivities. So, how do we begin to manage anit falls off the plant before it will germinate. unnatural system to our best benefit withoutSeveral grass and mustard family weeds require compromising the soil and water? We can startafter-ripening (7). “Induced” dormancy results by putting the principles of ecology to work onwhen seeds are exposed to unfavorable condi- our behalf, while minimizing actions that onlytions, such as high temperatures, after being address symptoms.shed from the parent plant. “Enforced” dor-mancy occurs when conditions favorable to Crops that kill weedsweed germination are absent. The seeds remaindormant until favorable conditions return. Al- Some crops are especially useful because theytogether, multiple types of dormancy ensure that have the ability to suppress other plants thatsome weed seeds will germinate and some will attempt to grow around them. Allelopathy re-remain dormant for later seasons. fers to a plant’s ability to chemically inhibit the growth of other plants. Rye is one of the mostSome weed species are dependent on light for useful allelopathic cover crops because it is win-germination; some germinate in either light or ter-hardy and can be grown almost anywhere.darkness; others germinate only in the dark. Rye residue contains generous amounts of al-Thus, there are no hard and fast rules for man- lelopathic chemicals. When left undisturbed onaging an overall weed population according to the soil surface, these chemicals leach out andlight sensitivity. prevent germination of small-seeded weeds. Weed suppression is effective for about 30 to 60PAGE 4 //PRINCIPLES OF WEED MANAGEMENT
  5. 5. days (8). If the rye is tilled into the soil, the ef- cides on weed control in a tobacco study (11).fect is lost. Just the absence of tillage alone gave 68% grass control and 71% broadleaf control.Table 2 shows the effects of several cereal covercrops on weed production. Note that tillage In other studies, North Carolina researchers in-alone, in the absence of any cover crop, more vestigated combinations of herbicide use andthan doubled the number of weeds. cover crop plantings on weed control (12). Rye and subterranean clover showed the highest weed control without herbicides (Table 4). Nei- ther provided as much control as herbicides, however. Tillage reduced weed control consid- erably where no herbicide was used, as com- pared to no-tillage.A weed scientist in Michigan (9) observed thatsome large-seeded food crops planted into ryemulch had high tolerance to the allelopathiceffects, while smaller-seeded crops had less tol-erance. In the study, corn, cucumber, pea, andsnapbean no-till planted under rye mulch ger-minated and grew as well or better than thesame crops planted no-till without mulch.Smaller-seeded crops, including cabbage and By season’s end the weed control resulting fromlettuce, showed much less germination, growth, cover crops alone had decreased (Table 5). Theand yield. Weeds that were reduced by rye researchers concluded that additional weedmulch included ragweed (by 43%), pigweed control measures must be applied with cover(95%), and common purslane (100%). crops to assure effective weed control and prof- itable yields.Dr. Doug Worsham, a North Carolina weedscientist, concluded that leaving a small grainmulch and not tilling gives 75 to 80% early-sea-son reduction of broadleaf weeds (10). Table 3shows the results of tillage, mulch, and herbi- Other crops that have shown allelopathic effects include sunflowers, sorghum, and rapeseed. Weed control ability varies among varieties and management practices. Sweet potatoes have been shown to inhibit the growth of yellow nut- //PRINCIPLES OF WEED MANAGEMENT PAGE 5
  6. 6. sedge, velvetleaf, and pigweed. Field trials Smother Crops and Mulchshowed a 90% reduction of yellow nutsedgeover two years following sweet potatoes (13). Certain crops can be used to smother weeds. Short-duration plantings of buckwheat and sor-Rapeseed, a type of mustard, has been used to ghum-sudangrass, for example, smother weedscontrol weeds in potatoes and corn under ex- by growing faster and out-competing them. Inperimental conditions. All members of the mus- northern states, oats are commonly planted astard family (Brassicaceae) contain mustard oils a “nurse crop” for alfalfa, clover, and legume-that inhibit plant growth and seed germination grass mixtures—the oats simply take the place(14). The concentration of allelopathic mustard of weeds that would otherwise grow betweenoils varies with species and variety of mustard. the young alfalfa plants.Researchers have begun to study ways to man- With enough mulch, weed numbers can beage mustard’s weed-suppressive abilities in crop greatly reduced. Nebraska scientists appliedproduction. In a Pacific Northwest study, fall- wheat straw in early spring to a field whereplanted ‘Jupiter’ rapeseed and sundangrass wheat had been harvested the previous August.were evaluated for suppression of weeds grow- At the higher straw rates, weed levels were re-ing in spring-planted potatoes. In the spring, duced by more than two thirds (see Figure 2).the researchers either tilled or strip-killed the Wheat, like rye, is also known to possess allelo-rapeseed in preparation for potato planting. The pathic qualities, which may have contributedfirst year of the study, rapeseed reduced mid- to the weed suppression.season weed production 85% more than fallow-ing. By the end of the season, weed production Figure 2: Weed Levels at twowas reduced by 98% with rapeseed, but only Nebraska Locations (16).50% the second year. Potato yields are shown 600in Table 6. Sidney Weed numbers/acre 500 North Platte 400 300 200 100 0 0 0.75 1.5 2.25 3 Mulch rate - tons/acre Crop RotationsIn general, typical levels of cover crop residues,when left on the soil surface, can be expected to Crop rotations limit the buildup of weed popu-reduce weed emergence by 75 to 90% (15). As lations and prevent major weed species shifts.these residues decompose, the weed suppression Weeds tend to prosper in crops that have re-effect will decline also. Residues that are more quirements similar to the weeds. Fields of an-layered and more compressed will be more sup- nual crops favor short-lived annual weeds,pressive (15). Small-seeded weeds that have whereas maintaining land in perennial cropslight requirements for sprouting are most sensi- favors perennial weed species. Two examplestive to cover crop residue. Larger-seeded an- would be shattercane in continuous sorghumnual and perennial weeds are least sensitive to and downy brome in continuous winter wheat.residue. Effective management strategies in- In a crop rotation, the timing of cultivation,clude growing cover crops that produce high mowing, fertilization, herbicide application,amounts of residue, growing slower-decompos- and harvesting changes from year to year. Ro-ing cover crops, packing the mulch down with tation thus changes the growing conditionsimplements that compress it, and using meth- from year to year—a situation to which fewods other than cover crops to control large- weed species easily adapt. Rotations that in-seeded annual and perennial weeds. clude clean-cultivated annual crops, tightlyPAGE 6 //PRINCIPLES OF WEED MANAGEMENT
  7. 7. spaced grain crops, and mowed or grazed pe- drill would be plugged. Tractor tires will fol-rennial sod crops create an unstable environ- low the skips, resulting in no damage to thement for weeds. Additional weed control may obtained by including short-season weed-smothering crops such as sorghum-sudan or Studies in Missouri and Ohio showed that wheatbuckwheat. Crop rotation has long been recog- yields were three to six bushels per acre lessnized for this ability to prevent weeds from de- when intercropped with soybeans than whenveloping to serious levels. solid-drilled and grown alone (18). Generally, soybean yields are higher when intercroppedIn a dryland wheat study, continuous winter into wheat than when double cropped behindwheat was compared to a rotation of winter wheat in the central and northern Midwest,wheat/proso millet/fallow or a winter wheat/ where double cropping is risky due to a shortersunflower/fallow rotation (17). The year be- growing season. For more information on in-fore, at the start of the study, the fields were in tercropping, request the ATTRA publicationwinter wheat and were sprayed with entitled Intercropping Principles and ProductionRoundup™ (glyphosate) and 2,4-D. The sun- were treated with Prowl™(Pendimethalin). All other weed control was Weed-Free by Designby mechanical means, including a sweep and Thus far, we’ve seen that weeds are a symptomrodweeder as needed. During the two-year of land management that defies nature’s, weed levels were 145 plants per square Stirring the soil with tillage creates conditionsyard for the continuous wheat, 0.4 plants per favorable for weed germination and survival.square yard for the winter wheat/proso millet Monocultures of annual crops hold naturalfallow system, and 0.3 for the winter wheat/ plant succession back and minimize biodiversity,sunflower fallow system. inviting weed populations to thrive. When we try to maintain bare ground, weeds grow to Intercropping cover the soil and increase biodiversity.Intercropping (growing two or more crops to-gether) can be used as an effective weed con- If we take a proactive approach to the wholetrol strategy. Having different plant types grow- agricultural system, rather than just looking ating together enhances weed control by increas- the parts, we can use the principles of nature toing shade and increasing crop competition with our advantage instead of fighting them. We willweeds through tighter crop spacing. Where one never win the war against nature, and, she hascrop is relay-intercropped into another stand- much more patience than we do. When we trying crop prior to harvest, the planted crop gets to break the rules of nature, we end up break-off to a weed-free start, having benefited from ing ourselves against the rules.the standing crop’s shading and competitionagainst weeds. Such is the case when soybeans Let’s look at an agronomic system where—byare interplanted into standing green wheat— design—weeds simply are not a problem. Onethe thick wheat stand competes well with weeds of the biggest shortcomings in American agri-while the soybeans are getting started. Plant- culture is the separation of plant and animaling method, planting date, and variety must be production. Commodity crop production ofwell-planned in advance. Though soybeans can corn, milo, and soybeans is really a componentbe directly drilled into the standing green wheat, of animal production because these crops areless wheat damage occurs if the wheat is largely fed to livestock. It seems inefficient toplanted in skiprows. Skiprows are created by grow grains separately and haul them to ani-plugging certain drop tube holes in the grain mal-feeding facilities. At Shasta College indrill. Soybeans can be planted with row units Redding, California, Dr. Bill Burrows has de-set at spacings matched to the skiprows in the veloped a series of complementary crop andwheat. For example, if the wheat is drilled on animal systems. He plants a mixture of milo7½-inch rows, to create a 30-inch row spacing and cowpeas together, with no herbicide. Thefor soybeans, every fourth drill hole in the wheat milo and cowpeas are so vigorous they //PRINCIPLES OF WEED MANAGEMENT PAGE 7
  8. 8. outcompete any weeds present. Here nature’s The various tools available for weed manage-principle of biodiversity is obeyed rather than ment fall into two categories: those that enhancefought with herbicides. Previously, when the biodiversity in the field and those that reduce itmilo was grown separately, he had to spray for (Table 7). This is not to imply a “good vs. bad”greenbugs. After he started with the pea-milo distinction. Rather it is meant to describe themixture, the greenbug problem disappeared. effect of the tool on this important characteris-When the milo and peas are mature, he com- tic of the crop/weed interaction. In general, asbines them. This produces a milo to pea ratio plant diversity increases, weeds become less ofof 2/3 to 1/3, which is ideal for feed. a problem.After grain harvest he turns his animal mixtureof hogs, cattle, sheep, and chickens into thestanding crop stubble, thereby adding more di-versity. All the waste grain is consumed by live-stock, and the stubble trampled into the soil, ata profit in animal gains to the farmer. Whatfew weeds may have grown up with the cropcan be eaten by the livestock. Under typicalsingle-crop scenarios, the waste grain would rotin the field and the farmer might incur a$6/acre stalk mowing cost. In this case, fol- Weed Control Tools and Their Effectslowing the principle of biodiversity increasedprofit by lowering cost. Bill and his team de- Herbicidessigned weeds out of the system. Other oppor- Since herbicide information is abundantly avail-tunities exist to design weeds out of the farm- able from other sources, it is not covered in de-ing operation. These opportunities are limited tail in this guide. Herbicides can be effective inonly by human creativity—the most maintaining ground cover in no-till systems byunderutilized tool in the toolbox. replacing tillage operations that would other- wise create bare ground and stimulate moreReactive Measures weed growth. Until better weed managementThe reactive paradigm of weed management is approaches can be found, herbicides will con-typified by the word control. This word assumes tinue to remain in the toolbox of annual cropthat weeds are already present, or to be ex- production. However, some farmers are realiz-pected, and the task is to solve the problem ing that with continued herbicide use, the weedthrough intervention. Agriculture magazines problems just get worse or at best stay aboutare chock-full of advertisements promising sea- the same. Nature never gives up trying to fillson-long control, complete control, and control the vacuum created by a simplified bare-groundof your toughest weeds. These ads imply that monoculture, and long-term use of the samethe secret is in the proper tank mix of herbi- herbicide leads to resistant weeds, as they adaptcides. Examining these ads from a cause-and- to the selection pressure applied to them. Buteffect standpoint, we might well conclude that compared to tillage systems where bare groundweeds are caused by a deficiency of herbicides is maintained, herbicide use may be consideredin the field. the lesser of two evils. At least where ground cover is maintained, the soil is protected fromWhen selecting a tool for weed management, it erosion for future generations to farm. Therehelps to understand the weed’s growth stages are many approaches to reducing costly herbi-and to attack its weakest growth stage (the seed- cide use, such as banding combined withling stage). Alternatively, management tech- between-row cultivation, reduced rates, andniques that discourage weed seed germination using some of the other methods discussedcould be implemented. In so doing, a farmer earlier.can identify a means of control that requires theleast amount of resources.PAGE 8 //PRINCIPLES OF WEED MANAGEMENT
  9. 9. Least-toxic Herbicides knapweed, Centaurea maculosa, that has strongCorn gluten meal has been used successfully on herbicidal properties. Knapweed uses thelawns and high-value crops as a pre-emergent compound as an allelopathic method ofherbicide. It must be applied just prior to weed competing with other plants. Several companiesseed germination to be effective. A common rate are interested in producing an environmentallyis 40 pounds per 1000 square feet, which friendly natural herbicide from the root exudate.suppresses many common grasses and Since catechin is naturally occuring, newherbaceous weeds (19). Two name brand weed herbicides made from it may be eligible for EPA’scontrol products containing corn gluten meal fast-track approval process (21).are WeedBan™ and Corn Weed Blocker™. Weeder GeeseHerbicidal soaps are available from Ringer Weeder geese have been used successfully bothCorporation and from Mycogen. Scythe™, historically and in more recent times. They areproduced by Mycogen, is made from fatty acids. particularly useful on grass weeds (and someScythe acts fast as a broad-spectrum herbicide, others, too) in a variety of crops. Chinese orand results can often be seen in as little as five African geese are favorite varieties for weedingminutes. It is used as a post-emergent, sprayed purposes. Young geese are usually placed indirectly on the foliage. the fields at six to eight weeks of age. They work well at removing weeds between plants in rowsVinegar is an ingredient in several new that cannot be reached by cultivators or hoes.herbicides on the market today. Burnout™ and If there are no trees in the field, temporary shadeBioganic™ are two available brands. Both of will be needed. Supplemental feed and waterthese are post-emergent burndown herbicides. must be provided as well. Water and feed con-They are sprayed onto the plant to burn off top tainers can be moved to concentrate the geesegrowth—hence the concept “burndown.” As in a certain area. A 24- to 30-inch fence is ad-for any root-killing activity with these two equate to contain geese. Marauding dogs andherbicides, I cannot say. The label on Burnout™ coyotes can be a problem and should be plannedstates that perennials may regenerate after a for with electric fencing or guard animals. Atsingle application and require additional the end of the season, bring geese in for fatten-treatment. ing on grain. Carrying geese over to the next season is not recommended, because older geeseResearchers in Maryland (20) tested 5% and 10% are less active in hot weather than younger birds.acidity vinegar for effectiveness in weed control. Additionally, the cost of overwintering themThey found that older plants required a higher outweighs their worth the next season. Geeseconcentration of vinegar to kill them. At the have been used on the following crops: cotton,higher concentration, they got an 85 to 100% strawberries, tree nurseries, corn (after lay-by),kill rate. A 5% solution burned off the top fruit orchards, tobacco, potatoes, onions, sugargrowth with 100% success. Household vinegar beets, brambles, other small fruits, and orna-is about 5% acetic acid. Burnout™ is 23% acetic mentals. ATTRA can provide more informa-acid. Bioganic™ contains 10% acetic acid plus tion on weeder geese.clove oil, thyme oil, and sodium lauryl sulfate.AllDown Green Chemistry herbicide™ containsacetic acid, citric acid, garlic, and yucca extract. TillageMATRAN™ contains 67% acetic acid and 34% Tillage and cultivation are the most traditionalclove oil. Weed Bye Bye™ contains both vinegar means of weed management in agriculture.and lemon juice. Vinegar is corrosive to metal Both expose bare ground, which is an invitationsprayer parts the higher the acidity, the more for weeds to grow. Bare ground also encouragescorrosive. Plastic equipment is recommended soil erosion, speeds organic matterfor applying vinegar. decomposition, disturbs soil biology, increases water runoff, decreases water infiltration,Dr. Jorge Vivanco of Colorado State University damages soil structure, and costs money tohorticulture department isolated the compound maintain (for fuel and machinery or for hand“catechin,” a root exudate from spotted labor). Some specific tillage guidelines and //PRINCIPLES OF WEED MANAGEMENT PAGE 9
  10. 10. techniques for weed management include the cessive pruning of crop roots. Earliest cultiva-following: tion should avoid throwing soil toward the crop row, as this places new weed seed into the crop• Preplant tillage. Where weeds such as row where it may germinate before the cropquackgrass or johnsongrass exist, spring-tooth canopy can shade it out. Use row shields asharrows and similar tools can be effective in appropriate. As the crop canopy develops, soilcatching and pulling the rhizomes to the soil should be thrown into the crop row to coversurface, where they desiccate and die. Discing, emerging contrast, tends to cut and distribute rhizomesand may make the stand even more dense. • Interrow cultivation is best done as soon as possible after precipitation, once the soil is• Blind tillage. Blind cultivation is a pre-emer- dry enough to work. This avoids compaction,gent and early post-emergent tillage operation breaks surface crusting, and catches weeds asfor weed control. It usually employs either fin- they are germinating—the most vulnerableger weeders, tine harrows, or rotary hoes. These stage.implements are run across the entire field, in-cluding directly over the rows. The large-seeded Generally speaking, tillage systems tend to dis-corn, soybeans, or sunflowers survive with mini- courage most biennial and perennial weed spe-mal damage, while small-seeded weeds are eas- cies, leaving annual weeds as the primary prob-ily uprooted and killed. For corn, the first pass lem. Exceptions to this are several weeds withshould be made between three and five days especially resilient underground rhizome struc-after planting and a second at the spike stage. tures such as johnsongrass, field bindweed, andBlind cultivation may continue until the crop is quackgrass. Plowing of fields to bring up theabout five inches tall. For soybeans, the first rhizomes and roots has been used to controlpass should be done when germinating crop bindweed and quackgrass.seedlings are still about ½ inch below the soilsurface, but not when the “hook” is actually Another interesting application of timing toemerging. The second pass should be done three weed control is night tillage. Researchers haveto five days after soybean emergence, and twice found that germination of some weed species islater at four-day intervals. Sunflowers can be apparently triggered by exposure to light. Till-blind-tilled up to the six-leaf stage, giving them age done in darkness exposes far fewer seeds toan excellent head start on weeds. Grain sor- light and reduces weed pressure. So far, small-ghum may be rotary hoed prior to the spike seeded broadleaf weeds (lambsquarter, rag-stage, and again about one week after spike weed, pigweed, smartweed, mustard, and blackstage. Because the seed is small, timing for nightshade) appear to be most readily affectedblind-till in sorghum is very exacting. Post-emer- (24).gent blind tillage should be done in the hottestpart of the day, when crop plants are limber, to Flame weedingavoid excessive damage. Rotary hoes, not har- Preplant, pre-emergent, and post-emergentrows, should be used if the soil is crusted or too flame weeding has been successful in a numbertrashy. Seeding rates should be increased 5 to of crops. The preplant application has com-10% to compensate for losses in blind cultiva- monly been referred to as the “stale seedbedtion (22, 23). technique.” After seedbed tillage is completed, weed seeds, mostly in the upper two inches of• Row crop cultivation. Cultivation is best kept the soil, are allowed to sprout. Assuming ad-as shallow as possible to bring as few weed seeds equate moisture and a minimum soil tempera-as possible to the soil surface. Where perennial ture of 50º F (to a depth of 2 inches), this shouldrhizome weeds are a problem, the shovels far- occur within two weeks. A fine to slightly com-thest from the crop row may be set deeper on pacted seedbed will germinate a much largerthe first cultivation to bring rhizomes to the sur- number of weeds. The weeds are then “seared”face. Tines are more effective than duck feet with a flamer, or burned down with a broad-sweeps for this purpose. Later cultivations spectrum herbicide, preferably when theshould have all shovels set shallow to avoid ex- populationInsteIn, Instead, the ridge-till planterPAGE 10 //PRINCIPLES OF WEED MANAGEMENT
  11. 11. population is between the first and fifth true-leaf stages, a time when they are most suscep-tible. The crop should then be seeded as soonas possible, and with minimal soil disturbanceto avoid bringing new seed to the surface. Forthe same reason, subsequent cultivations shouldbe shallow (less than 2 inches deep) (25).Pre-emergent flaming may be done after seed-ing, and in some crops post-emergent flamingmay be done as well. Flaming is often used asa band treatment for the crop row, and usuallycombined with interrow cultivation. Earlyflaming may be done in corn when it is 1.5 to 2inches high. The growing tip is beneath thesoil surface at this stage, and the crop readilyrecovers from the leaf damage. Subsequentpost-emergent flamings may be done when cornreaches 6–10 inches in height, and later at lay-by. No flaming should be done when corn is atapproximately 4 inches high, as it is most vul-nerable then. The burners are offset to reduceturbulence and to avoid concentrating toomuch heat on the corn. Water shields are avail-able on some flame weeder models. Uniformseedbed preparation and uniform tractor speedare important elements in flaming. Hot anddry weather appears to increase the efficacy offlaming (26).Searing the plant is much more successful thancharring. Excessive burning of the weeds of-ten stimulates the roots and encourages re-growth, in addition to using more fuel. Flam-ing has generally proved most successful onyoung broadleaf weeds. It is reportedly less integrated approaches have been developed on-successful on grasses, as the seedlings develop farm, by farmers themselves. A useful booka protective sheath around the growing tip that spotlights farmers and other researcherswhen they are about 1 inch tall (27). Some con- and the integrated weed management strategiescerns with the use of fire include possible crop they are using is Controlling Weeds With Fewerdamage, potential dangers in fuel handling, and Chemicals, available from the Rodale Institutethe cost of fuel. For more information on flame (see Additional Resources). The next twoweeding, see the ATTRA publication Flame examples are taken from this book.Weeding for Agronomic Crops. Dick and Sharon Thompson of Boone, Iowa,Integrated Weed Management built a herbicide-free weed-management sys-An integrated approach means assembling a tem around ridge-till technology for corn andweed management plan that incorporates a soybeans. Fields are overseeded or drilled innumber of tools consistent with farm goals. the fall with combinations of hairy vetch, oats,Included are sanitation procedures, crop and grain rye as a winter cover crop. The vetchrotations, specialized tillage schemes, cover provides nitrogen, while the grasses providecrops, and herbicides. The best examples of weed suppression and erosion protection. The cover crop is not tilled in before planting. //PRINCIPLES OF WEED MANAGEMENT PAGE 11
  12. 12. skims off enough of the ridge top to create a • Sustainable Soil Managementclean seeding strip. Subsequent passes with the Assessing soil health; organic matter andridge-till cultivator eliminate any cover crop in humus management; organic amendments;the interrow area and help to re-shape the soil organisms; aggregation; fertilizers;ridges. The Thompsons estimate savings of $45 addtional $48 per acre using their methods. “Walkingthe Journey” is a 20-minute video chronicling • Sustainable Corn and Soybean Productionthe Thompson farm, available for $39. See Ad- Weed, seed, and pest management; strip cropditional Resources for ordering information. ping; farm experiences.In Windsor, North Dakota, Fred Kirschen- • Making the Transition to Sustainablemann has developed a diverse rotation includ- Farminging cool-weather crops like oats, rye, barley, and Planning, key ideas for transitions, andspring wheat, and warm-season crops like sun- practices.flower, buckwheat, and millet. He employs se-lective timing to manage his principal weed • Pursuing Conservation Tillage for Organicproblem, pigeon grass. By planting cool- Crop Productionweather grains early, he can get a competitive A look at the potential for applying conservationjump on the weed, which requires somewhat tillage to organic cropping systems.warmer soil to germinate. The warm-seasoncrops do best long after pigeon grass has germi- The following Current Topics are also available:nated, however. He uses shallow pre-plant till-age to control weeds in these crops. • Conservation TillageKirschenmann also composts manure before • Alternative Control of Johnsongrassspreading it. One of the many advantages of • Alternative Control of Field Bindweedcomposting is the reduction of viable weedseeds, which are killed by heat during the Referencescuring process. 1) Barker, Joel A. 1993. Paradigms—The Business of Discovering the Future. HarperDon and Deloris Easdale of Hurdland, Missouri, Business. New York. 240 p.reduced their annual herbicide costs from$10,000 to less than $1,000 in three years on 2) Savory, Allan. 1988. Holistic Resourcetheir 300-plus acres of grain crops (28). They Management. Island Press. Washington,use hairy vetch, winter rye, or Austrian winter DC. 564 p.peas in combination with their ridge-till system.They flail chop hairy vetch or winter peas ahead 3) Wilson, Robert G. 1988. Biology of weedof the ridge-till planter and plant directly into seeds in the soil. p. 25–40. In: Miguel Altierithe remaining cover crop residue. This practice and Matt Liebman (eds.). Weed Manage-eliminates using a burndown herbicide. The ment in Agroecosystems: Ecological Ap-legumes replace much of the nitrogen needed proaches. CRC Press, Inc. Boca Raton, FL.for the corn or milo crop. Some liquid starter 354 p.and liquid nitrogen is placed below the seed atplanting. They more than recover the seed costs 4) Stevens, O.A. 1954. Weed Seed Facts. Northof their cover crops in savings on fertilizer and Dakota Agriculture College Extensionherbicide. Circular. A-218. 4 p.Other ATTRA Publications of Interest 5) Lehnert, Dick. 1996. Breaking the weed-• Cover Crops and Green Manures seed bank. Soybean Digest. Mid-March. Uses, benefits, and limitations of cover crops p. 52. and green manures; vegetation manage- ment; and sources of information.PAGE 12 //PRINCIPLES OF WEED MANAGEMENT
  13. 13. 6) Clements, David R., Diane L. Benoit, Stephen 15) University of Connecticut IPM Program. D. Murphy, and Clarence J. Swanton. 1996. No date. Contribution of cover crop Tillage effects on weed seed return and mulches to weed management. seedbank composition. Weed Science. Vol- <>. ume 44. p. 314–322. 16) Crutchfield, Donald A., Gail A. Wicks, and7) Schlesselman, John T., Gary L. Ritenour, and Orvin C. Burnside. 1985. Effect of winter Mahlon Hile. 1989. Cultural and physical wheat straw mulch level on weed control. control methods. p. 45–62. In: Principles of Weed Science. Volume 34. p. 110–114. Weed Control in California. Second edition. California Weed Conference. Thompson 17) Lyon, Drew J., and David D. Baltensperger. Publications. Fresno, CA. 1995. Cropping systems control winter an- nual grass weeds in winter wheat. Journal8) Daar, Sheila. 1986. Update: Suppressing of Production Agriculture. Vol. 8, No. 4. p. weeds with allelopathic mulches. The IPM 535–539. Practitioner. April. p. 1–4. 18) Helsel, Z.R. and T. Reinbott. Circa 1989.9) Putnam, Alan R., Joseph DeFrank, and Jane Intercropping: planting soybeans into stand- P. Barnes. 1983. Exploitation of allelopa- ing green wheat. University of Missouri thy for weed control in annual and peren- Agronomy Department. University of Mis- nial cropping systems. Journal of Chemical souri, Columbia, MO. 3 p. Ecology. Volume 9, Number 8. p. 1001– 1010. 19) Quarles, William. 1999. Non-toxic weed control in the lawn and garden. Common10) Worsham, A.D. 1991. Allelopathic cover Sense Pest Control Quarterly. Summer. p. crops to reduce herbicide input. Proceed- 4–14. ings of the Southern Weed Science Society. 44th Annual. Volume 44. p. 58–69. 20) Anon. 2002. Vinegar wipes out thistles organically. Stockman Grass Farmer. July.11) Schilling, D.G., A.D. Worsham, and D.A. p. 1. Danehower. 1986. Influence of mulch, till- age, and diphenamid on weed control, yield, 21) Malone, Marty. 2003. Uses for Knapweed? and quality in no-till, flue-cured tobacco. The Ag Perspective. Park County Exten- Weed Science. Volume 34. p. 738–744. sion Service. Available online at: Yenish, J.P., and A.D. Worsham. 1993. News/Weed_news/weed_news.html. Replacing herbicides with herbage: poten- tial use for cover crops in no-tillage. p. 37- 22) Anon. 1991. Non-chemical weed control 42. In: P.K. Bollich, (ed.) Proceedings of for row crops. Sustainable Farming News. the Southern Conservation Tillage Confer- September. p. 1–8. ence for Sustainable Agriculture. Monroe, LA. June 15–17. 23) Jordan, C. Wayne. 1981. Sunflower Pro- duction In Mississippi. Cooperative Exten-13) Anon. 1993. Sweet potato plants vs. weeds. sion Service. Mississippi State University. HortIdeas. January. p. 8. Mississippi State, MS. 2 p.14) Boydston, Rick, and Ann Hang. 1995. 24) Becker, Hank. 1996. Nightmare in tilling Rapeseed green manure crop suppresses fields—a horror for weeds. Farmers’ Digest. weeds in potato. Weed Technology. Vol. 9. March. p. 20–24. p. 669–675. 25) Pieri, Paul B. No date. Flame Weeding. Maurolou Farm. Little Compton, RI. 6 p. //PRINCIPLES OF WEED MANAGEMENT PAGE 13
  14. 14. 26) Cramer, Craig. 1990. Turbocharge your cul- Cramer, Craig, and the New Farm Staff (eds.). tivator. New Farm. March–April. 1991. Controlling Weeds with Fewer Chemi- p. 27–30, 35. cals. Rodale Institute, Kutztown, PA. 138 p. Available for $14.95 from Rodale Institute (see27) Drlik, Tanya. 1994. Non-toxic weed con- address above). trol. The IPM Practitioner. October. p. 20. Bowman, Greg (ed.). 1997. Steel in the Field.28) Easdale, Deloris. 1996. Controlling weeds Sustainable Agriculture Network Handbook and maintaining soil fertility with cover # 2. 128 p. crops. National Conservation Tillage Digest. This book is a farmer’s guide to weed manage- February. Vol. 3, No. 2. p. 28–30. ment tools using cultivation equipment. Avail- able for $18.00 + $3.95 shipping and handlingAdditional Resources from the Rodale Institute listed above or:Shirley, C., and New Farm staff. 1993. What Sustainable Agriculture PublicationsReally Happens When You Cut Chemicals. 210 Hills BuildingFebruary. Vol. 3, No. 2. p. 28–30. 156 p. University of Vermont This book contains a series of farmers’ ex- Burlington, VT 05405-0082 periences with adopting new strategies for 802-656-0484 higher profits and lower input costs, while E-mail: enhancing the environment. Available for $14.95 from: Walking the Journey: Sustainable Agriculture Rodale Institute that Works. 1992. 611 Siegfriedale Road A 20–minute video of Dick and Sharon Kutztown, PA 19530 Thompson’s ridge-till farming in Iowa. 800-832-6285 Available for $25 from: 610-683-6009 Extension Communications E-mail: Attention: Lisa Scarborough 3614 ASB, Room 1712 Iowa State University Ames, IA 50011 515-294-4972PAGE 14 //PRINCIPLES OF WEED MANAGEMENT
  15. 15. Feedback1. Does this publication provide the information you were look- ing for? How could it be improved?2. Do you know a farmer who is implementing techniques dis- cussed in this publication? Can you provide their address and phone number?3. Do you know of any related research that would add to the information presented here?4. Do you know a good related website not listed in this publi- cation?5. Please add any other information or comments that you would wish to share.//PRINCIPLES OF WEED MANAGEMENT PAGE 15