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II. INTEGRATED MANAGEMENT OF PADDY WEEDS INKOREA, WITH AN EMPHASIS ON ALLELOPATHY

Kil Ung KimDepartment of Agronomy, College of Agriculture

Kyungpook National University, Taegu 702-701, Korea

ABSTRACT

Annual weeds such as Echinochloa crus-galli and Monochoria vaginalis, and perennialssuch as Sagittaria trifolia, Sagittaria pygmaea, Eleocharis kuroguwai, Cyperus serotinus andPotamogeton distinctus, have been the most important weed species in paddy fields in Korea dur-ing the past two decades. Perennial weeds have increased from about 19% in 1971 to about54% in 1981 and 60% in 1990. E. kuroguwai and S. trifolia are now the weeds most resistantto current herbicides, because their emergence takes place over a prolonged period. Biocontrolof E. kuroguwai has become possible by means of a plant pathogen, Epicoccosorusnematosporus, and of Scirpus planculmis by a pathogen Alternaria sp.. Residues of crops suchas barley, wheat and rye had the allelopathic potential to reduce a population of P. distinctusin paddy fields. Phenolic compounds found in crop residues such as ferulic acid have been de-termined to have allelopathic potential, and also p-coumaric, sinapic, protocathechuic and caffeicacids. The integration of efficient herbicides with cultural control (using crop rotation, appropri-ate cultivation methods, crop residues etc.) and biocontrol (use of plant pathogens such as E.nematosporus and Alternaria sp.) can help make integrated weed management (IWM) a reality inKorea’s rice production.

INTRODUCTION

Until the mid 1980’s, agricultural policyand research in Korea was focused mainly on obtain-ing the maximum yield, through varietal improve-ment and new cultural practices. Great progress hasbeen made in agricultural production in Korea overthe past two decades, although food self-sufficiencyin 1990 was still only about 43% (RDA 1991), whilethe remaining food was imported at an annual cost ofUS$7,304 million. A further increase in the impor-tation of agricultural goods seems to be inevitable inthe years to come. Under these circumstances, thesustainability of Korean agriculture will depend onwhether it is able to develop labor-saving technologybased on low inputs.

Farm mechanization and effective weedmanagement are recognized as two vitally importantcomponents of labor-saving technology. Herbicidesare the most cost-effective weed control technology

currently available. However, these are becomingunpopular among members of the public. It isanticipated that environmental policies on the use ofpesticides, including herbicides, will be strength-ened, and become more restrictive than before.However, it seems inevitable that farmers will stilluse a number of conventional herbicides, particularlythose which meet health and environmental require-ments, since no alternatives are available which givesuch an effective result for such a relatively low cost(Kim 1990). Improved weed management systemsmust be developed which maximize weed control ata minimum cost. The choice of system will dependon the farmer’s preferences.

This paper discusses the current status ofbiological control, the use of allelopathy, and theintegrated management of paddy weeds. The impor-tant weeds found in Korean paddy fields, and theherbicides used to control them, are also described togive the reader a better understanding of integrated

Keywords: Integrated weed management (IWM), Eleocharis kuroguwai, Sagittaria triforia,Epicoccorous nematosporus, Alternaria sp., phenolic compounds, allelopathy

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weed management in Korea.

IMPORTANT WEEDS IN PADDY FIELDS

It has been reported that 92 weed speciesbelonging to 27 families are found in Korean paddyfields, of which 30 species are considered to becommon weeds (Kim 1981). It is interesting to notethe changes in weed flora which have taken placeover the past three decades. Table 1 presents datasummarized from Kim (1992) on the nationwideweed surveys conducted by the Rural DevelopmentAdministration in 1971 and 1981, plus data from the1960s from Kim (1969), and from 1990 collected bya private research organization.

In 1965, 17 species were listed as the mostcommon weeds in paddy fields (Table 1) (Kim 1969).Of these, E. crus-galli was the most troublesomeweed in paddy fields, followed by M. vaginalis,which was a serious weed in seedbeds as well as inpaddy fields. Others were listed as minor weeds.Before 1970 herbicides were seldom used, but Kimin 1965 already anticipated that perennial weeds

such as E. kuroguwai and P. distinctus would be-come more serious if herbicides were to be used forweed control.

In the first nationwide weed survey carriedout in 1971, Rotala indica was recognized as thedominant species, followed by Eleocharis acicularis,M. vaginalis, Cyperus difformis, E. crus-galli,Lindernia procumbens, P. distinctus, Aneilemajaponica, E. kuroguwai, Scirpus juncodes andPersicaria hydropiper, in that order (Table 1). Thisgroup of species constituted approximately 86% ofall paddy weeds. Annual weeds were predominant inpaddy fields, constituting 81% of this dominantgroup.

In a second nationwide weed survey carriedout in 1981, the most important weed was M.vaginalis, followed by S. pygmaea, S. trifolia, P.distinctus, C. serotinus, R. indica, A. japonica, L.procumbens, E. kuroguwai and Ludwigia prostrata,in that order (Table 1). The dominance representedby this group of weed species was about 87%, ofwhich 54% were perennial weeds.

In the most recent weed survey carried out

Table 1. Changes in dominant weed species in paddy fields in Korea, 1965­1990

Source: Kim 1992

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in 1990, S. trifolia had become the dominant weedspecies in paddy fields, followed by S. pygmaea, E.kuroguwai, E. crus-galli, P. distinctus, C. serotinus,and M. vaginalis (Table 1). Of these, about 60%were perennial weeds.

Thus, the changes in weed flora can besummarized as follows. Firstly, E. crus-galli gradu-ally decreased from 1965 to 1981 but then increasedagain from 1981 to 1990. This indicates that E. crus-galli, which has a very similar habitat to that of rice,has remained the dominant weed species in paddyfields throughout the rice-growing season.

Secondly, R. indica, the dominant weedspecies in 1971, had markedly decreased by 1981and had become relatively unimportant by 1990.This fall in the occurance of E. crus-galli and R.indica by 1981 is related to the intensive use ofannual herbicides such as the acid amide, carbamateand diphenylether types, which up until the mid1980s had more than 80% of the market share interms of quantity sold. M. vaginalis was poorlycontrolled by these herbicides and became the domi-nant weed because it characteristically produces alarge number of seeds, and requires a longer time foremergence than E. crus-galli. Another interestingresult is that perennial weeds such as S. pygmaea, S.trifolia, P. distinctus and C. serotinus also becamemuch more common by 1981.

Little difference in weed flora was notedbetween 1981 and 1990, except that E. kuroguwaiand E. crus-galli became important species. Theperennials which dominated in 1981 still remainedthe most common species in 1990, except that S.trifolia replaced S. pygmaea. Similarity coefficientsof changes in the level of dominance of differentweed species over time were about 37-39 in the years1971, 1981, but 62 between 1981 and 1991 (Kim1991). These results confirm that there was a majorchange in weed flora between 1971 and 1981, butlittle change between 1981 and 1991 (Table 2).

In conclusion, it would seem that about tenspecies, including both annual and perennial weeds,have been the major weeds in transplanted rice fieldsduring the past three decades. However, the relativeimportance of the various species has fluctuatedaccording to the control measures used, and particu-larly the type of herbicide applied. The heavyapplications of herbicide used for weed control since1980 have stabilized weed populations and madeperennial weeds dominant in paddy fields. E.kuroguwai and S. trifolia are regarded as the twomost difficult weeds to control. Their emergencetakes place over a prolonged period, partly because

they both require a relatively high cumulative tem-perature (117°C in the case of E. kuroguwai, and297°C for S. trifolia).

The ten most important weed species indi-cated here can be expected to continue as the majorweeds in paddy fields for at least the next 10 or 20years, as long as the current control methods whichrely on the heavy use of one-shot herbicides continueto be used.

CURRENT STATUS OFBIOLOGICAL CONTROL

Biological weed control involves the use ofliving organisms to reduce the population of a targetweed. Wapshere (1982) suggested three kinds ofbiocontrol method: the classical (inoculative) ap-proach, the bioherbicide (inundative) approach, andherbivore management. Insects, mites, nematodes,plant pathogens, and aquatic and terrestrial herbi-vores, have all been used as biotic agents for thebiological control of weeds (Watson 1992).

For the classical approach, the ideal targetis a widespread introduced weed infesting largeareas such as rangelands or aquatic habitats. InKorea, land size per farm household is very small,and farmers are also growing a diversified range ofcrops, particularly in the uplands. It is thus ratherdifficult to apply the classical approach to control asingle weed species among a weed complex in culti-vated crops.

The bioherbicide approach uses the livinginoculum of a plant pathogen, in a manner similar tothat of a chemical herbicide, to control or suppressthe growth of a weed species. The first attempt madein Korea evaluated the potential of nine paddy weedsas hosts for bacterial leaf blight. Of these nine weedspecies, Alopecurus spp., Setaria viridis and Leersia

Table 2. Similarity coefficient in terms ofchanges in dominant weed speciesovertime

source: Kim 1991

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japonica were shown to be most susceptible to thedisease, showing a response similar to that of Milyang23, a susceptible check rice variety (Table 3). How-ever, most broadleaf weeds were not infected by thedisease (Kim et al. 1981). It is possible that a numberof weed species growing in or near paddy fields canbe used as hosts of this pathogen, but no further workhas yet been done.

In recent years, there has been some suc-cess in the use of a plant pathogen to control E.kuroguwai, a perennial sedge which is very difficultto control with the herbicides currently available.Epicoccosorus nematosporus has been identified asa very selective plant pathogen which can give goodcontrol of E. kuroguwai (Hong et al. 1991). None ofthe fifteen crops tested, including rice, were infectedby E nematosporous, and out of the 15 weed speciestested only sedges were infected. This indicates thatthere is a good possibility of using this pathogen asa bioherbicide to control sedges.

When the concentration of E.nematosporous was increased from 1.2 x 103 to 0.8x 107 conidia/ml, the percentage of infected and deadplants also increased significantly, from 35.7 to96.1%, and from 15.4 to 92.3%, respectively (Table4). Three applications of E. nematosporus inoculumat a concentration of 6.3 x 108 conidia/ml killed

94.9% of E. kuroguwai, compared with 68.0% afteronly one application (Table 5). The number of E.kuroguwai tubers at the time of the rice harvest wasgreatly reduced, from 76.3 tubers/pot for the un-treated control to 9.6/pot after a single applicationand 1.0/pot after three applications of E.nematosporus (Table 5). Fig. 1 shows that applica-tions of the inoculum take longer to control E.kuroguwai than bentazon, a synthetic herbicide knownto be the most effective chemical treatment for thisweed, but even so, about 90.0% of weeds had beencontrolled by the pathogen at 20 days after inocula-tion, an efficacy similar to that of bentazon. Furtherstudies are in progress to determine the toxicity ofthe pathogen against weeds, methods of mass pro-duction, and formulation as a bioherbicide.

Another possibility of biocontrol by a patho-genic fungus, Alternaria sp., has been reported in thecontrol of Scirpus planiculmis, a perennial sedgefound on newly reclaimed land on the western coastof Korea. An interesting observation is that S.planiculmis appears to be the only sedge susceptibleto this pathogen (Table 6). Under field conditions,Alternaria disease developed on S. planiculmis at 3days after inoculation, while disease incidence hadincreased to 80% by mid-July, 12 days after inocula-tion at a concentration of 4 x 106 cell/ml. Aafter one

Table 3. Effect on various plant species at different growth stages of the pin inoculation ofbacterial leaf blight (Group I)

Source: Kim et al. 1981

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application at a concentration of 3.0 x 106 mycelle(cell/ml), 85% of the sedge had been killed (Table 7).

The third approach uses herbivores as con-trol agents. No data are available on the use ofherbivores to control submerged vegetation in aquaticsystems in Korea. However, it is believed that geeseand ducks were driven into rice and cotton fields topeck the weeds before herbicides came into intensiveuse. This is still a common practice in south China,where similarly chickens are kept in fruit gardens toeat young weeds and insects (Li 1992). A herbivo-rous fish native to China, the grass carp, has receiveda great deal of attention as a potential weed controlagent because it is a strict vegetarian, feeding onsubmerged weeds such as duckweed and filamen-tous algae (Li 1992). Sheep, goats and cattle havesome utility in supressing pasture weeds, but the useof grazing animals to control weeds has limited

potential under Korean conditions.

ALLELOPATHY

Molish (1937) defined the term ‘allelopa-thy’ to refer to biochemical interactions between alltypes of plants, including microorganisms. Allelopa-thy was later defined as any negative or positive plantresponse mediated through chemical compoundsnaturally produced by another plant or microorgan-ism (Putnam 1988). The potential of allelopathy forweed control has been reviewed by Einhellig andLeather (1988), Purvis (1990) and Watson (1992).They have reported that natural chemicals exertingan allelopathic effect may be either simple or com-plex secondary compounds. Many of the com-pounds implicated have yet to be identified, andoften the effect is not due to a single compound

Table 5. Effect of repeated applications of Epicoccosorus nematosporuson control of Eleocharis kuroguwai

Table 4. Effect of Epicoccosorus nematosporus on Eleocharis kuroguwai1)

Source: Hong et al. 1991

Source: Hong et al. 1991

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Table 6. Host range of pathogenic fungus, Alternaria sp., collected from Gehwado in westernKorea

Fig. 1. Comparison of efficacy of a plant pathogen, Epicoccosorus nematosporus, andbentazon for the control of Eleocharis kuroguwai

Source: Hong et al. 1991

Source: Park et al. 1992

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(Einhellig and Leather 1988). Allelopathy can becategorized as either cultural control (habitat man-agement), as when crop selection is involved, orbiological control when natural chemicals are in-volved (Einhellig and Leather 1985, Purvis 1990).

There are a number of papers on the use ofresidues of crops such as barley, wheat and rye inorder to utilize their allelopathic potential. Residuesof fall-planted, spring-killed rye significantly re-duced the total weed biomass, compared to theuntreated control (Barnes et al. 1983). Barley usedas a smother crop may be successful in eliminatingweeds because it inhibits seed germination and thegrowth of selected plant species (Overland 1966).Wheat also seems to have allelopathic potential inrepressing the germination of many weed seeds andalso the growth of weed seedlings (Steinsiek 1982,Kwon and Kim 1985). Furthermore, Kwak and Kim(1984) and Kwon and Kim (1985) have reported thataqueous extracts from barley, wheat and rye residuesinhibited the germination and growth of paddy andupland weeds. These extracts were particularlyeffective against P. distinctus, an important peren-nial broadleaf weed in paddy fields.

The fact that some crop residues exert aninhibitory effect on certain weeds may allow them tobe used as an alternative control measure in the formof a mulch, or as part of a rotational sequence. Thephenolic compounds in plant residues have beenintensively studied to evaluate their allelopathic po-tential as protective agents to control weeds, insectsand fungi (Court et al. 1982, Beart et al. 1985, Rice

1984, Kim et al. 1987). Aqueous and alcoholextracts from crop straw markedly inhibited thegrowth of lettuce seedlings. The inhibition effect ofbarley extract was greatest, followed by wheat andrye, in that order (Kim et al. 1987) (Table 8). Twelvesimple free phenolic acids were identified in strawfrom barley, wheat and rye (Table 9). Ferulic acidwas the most abundant (about 20% or more), fol-lowed by p-coumaric, sinapic, protocatecheuic andcaffeic acid, and polyphenols such as scopoletin andrutin. Fatty acids such as linoleic acid, and organicacids, were also determined (Kim et al. 1987). It wassuggested that phenolic substances, together withfatty acids and organic acids, may be potential allelo-pathic substances since they exert a strong inhibitoryeffect on the germination and growth of lettuce.

In Korea, there are several reports availableon the allelopathic potential of various types ofupland weed. Kim et al. (1987) reported that out of49 upland weed species tested, five species (Lotuscorniculatus, Bidens frondosa, Polygonumperfoliata, Datura stramonium and Cephalonoplossegetum) had allelopathic potential, a significantinhibitory effect on lettuce germination. Phenoliccompounds such as ferulic, p-hydrobenzoic acidsand cinnamic acids played a major role in the allelo-pathic activity in upland weed species such as Arte-misia asiatica, Capsella bursa-pastris, Portulacaoleracea, and Trifolium repens (Chun et al. 1988).Phenolic substances also proved to have allelopathicpotential in five upland weed species. The largesttotal amount of phenol was detected in Galinsogaciliata, with 1.27 mg/g, followed by Polygonumperfoliata (1.8 mg/g), Stellaria aquatica (1.7 mg/g),Amaranthus lividus (1.28 mg/g), and Achyranthesjaponica (1.27 mg/g). These plant species alsocontained a large quantity of ferulic, caffeic, 7-OH-coumarine and protocatechuic acids (Back and Kim1988). Fatty acids and organic acids were alsopresent, at levels of 4-9 mg/g and 24-22 mg/g,respectively (Back and Kim 1988). These are so farthe most detailed studies on the allelopathic potentialof weeds in Korea. Other papers show similar resultsin other weed species (e.g. Polygonum hydropiper,Polygonum aviculare) (Woo and Kim 1987) andPortulaca oleracea (Park and Kim 1988), and inwild plants, (e.g. Styrax japonica and Araliacontinetalis) (Kim et al. 1990).

Unfortunately, the only work in Korea onallelopathic potential as a means of controlling paddyweeds is that on crop residues discussed above.Further study is needed to identify specificallelochemicals.

Table 7. Effect of Alternaria sp., collectedfrom Gehwado (western Korea) ondisease development1)

Source: Park et al. 1992

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CHEMICAL CONTROL

Today, herbicides have become the mainmethod of weed control for both paddy and uplandcrops in Korea, and there is no suitable alternative tothese. Herbicide use in paddy fields was about119%. (Some farmers apply herbicide more thanonce per rice culture, particularly to control E.kuroguwai, the most difficult weed in paddy fields).Only about 79% of upland fields were treated withherbicides. The lower use of herbicides in uplandcrops is probably because it is difficult for farmers toapply herbicides to diversified crops grown in smallupland fields (Kim 1992). Only a negligible amountof herbicide was used in the mid 1960’s, mainly 2,4-D, with PCP coming into use in the late 1960s.Nitrofen became popular in the early 1970s, whileacid amide type herbicides such as butachlor andalachlor shared more than 60% of the total herbicidemarket for more than 15 years, until 1989 (Kim1990). One important feature was a dramatic in-crease in the use of mixture type herbicides, from3.0% in 1981 to 68.0% in 1990 (Table 10). Thisreflects farmers’ demand for a one-shot herbicidecapable of controlling annual as well as perennialweeds with a single application. Twenty-sevenmixture herbicides suitable for rice were available inKorea in 1991. Half of these were mixtures based onsulfonyl urea types, such as bensulfuron-methyl andpyrazosulfuron-ethyl, formulated with the aim ofcontrolling both annual and perennial weeds. Asingle application of a mixture herbicide seems to berational and economic, but a relatively early applica-tion within 10 DAT (days after transplanting) doesnot give satisfactory control of E. kuroguwai or S.trifolia, both of which require a long period foremergence in paddy fields. Thus, these two weedsoften escape the residual effect of the herbicides, andhave become the most difficult weeds to control inlowland rice fields.

Public awareness, and also investment intonew research programs, to find alternatives to pesti-cides, could result in a range of new, environmentallysound pest management measures which involve abroader use of currently available alternatives suchas cultural and biological methods.

IMPACT OF CHANGES INPLANTING METHODS

Rice has been a staple food for well over athousand years in Korea. Cultivation methods havechanged over time, from the primitive direct seedingto transplanting by hand, and finally transplanting by

machine, as was the case for 88% of rice grown areain Korea in 1991. About 22% of this area trans-planted by machine was planted in very young seed-lings, only 8 to 10 days old. The use of very youngseedlings is much cheaper than using ordinary seed-lings 30 to 35 days old, because it reduces the costof nursery culture by 54%. The transplanting of veryyoung seedlings is thus expected to increase. How-ever, the shallow depth of the irrigation water whenextremely young seedlings are transplanted by ma-chine favors weed germination and growth. This isa disadvantage, but on the other hand, the establish-ment of young seedlings is much faster than that ofordinary seedlings. Until recently, maximum yieldhas been the main target of rice production, based ongood land preparation, good fertilizer and watermanagement, and pest management. There is now awidespread feeling that maximum input technologyshould be replaced by low-input methods. Directseeding is receiving a great deal of attention as analternative planting method.

Any reduction of inputs used in culturalpractices can be expected to promote weed growth,so that weeds are likely to become a more importantproblem in low-input technology. Furthermore, anyimplementation of direct seeding will give rise toimportant problems such as lodging and poor cropstands, as well as weeds. In direct seeded rice, weedsusually emerge ahead of the rice, absorbing theavailable nutrients earlier so that they flourish andshow rapid growth. Any small advantage of onespecies during seedling establishment will have greatimpact on this species in later competition withothers. It is evident that a change in plantingmethods from transplanting to direct seeding willresult in a shift of weed dominance from a complexof perennial and annual weeds to annual grasses.

It is recommended that if dry-seeded rice isbeing cultivated, the field should be maintained in arelatively dry condition for 30 days after seeding,followed by irrigation as for transplanted rice cul-ture. In fields planted with dry-seeded rice, E. crus-galli is the dominant weed species, followed by C.amuricus and Rorippa islandica (Fig. 2) (Yeun et al.1991). After the repeated culture of dry-seeded rice,C4-type grasses become the dominant weeds (Kim1991). Applications of systematic herbicides usingpre-emergence types followed by post-emergenceones gives excellent weed control. A tank mixture ofpropanil with pre-emergence type annual herbicidescan give satisfactory control of E. crus-galli. Thiscan be used as a basis for developing appropriateIWM for dry-seeded rice.

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Table 8. Effect of alcohol extracts1) from crop straw on germination of test plants (shown as % germination)

Source: Kim et al. 1987

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Table 9. Constitution of phenolic compounds identified from different types of crop straw

Table 10. The major types of chemical herbicide being used in paddy fields in Korea, 1981­90

Source: Kim et al. 1987

Source: Agricultural Chemical Industrial Association

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INTEGRATED MANAGEMENT OFPADDY WEEDS

The goal of weed management should notbe to eradicate or exterminate, but rather to preventweed damage from exceeding the economic injurythreshold by taking advantage of a range of weedcontrol methods. IWM is not only effective, buttakes into consideration protection of the environ-ment and food safety.

There is an increasing desire on the part ofmany farmers to adopt management practices suchas IWM to enhance profitability, but the complexityof IWM technology has seriously hindered its wide-spread adoption. For instance, farmers have diffi-culty in understanding the economic threshold for agiven weed, and weed response to different controlmethods under various cultural and climatic condi-tions. Therefore, many farmers have become accus-tomed to using herbicides as their primary defenseagainst potential weed damage (Beyer 1991).

IWM techniques should follow a number ofguiding principles.

• Crops should be matched to local grow-ing conditions;

• Crop rotation should be practiced;• The soil should be protected throughout

the year by cover crops;• Cultivars should be selected which are

competitive against weeds;• There should be accurate application of

fertilizers to meet crop requirements;• Where possible, biological control meth-

ods should be used (Klassen 1990).In practice, it is rather difficult for farmers

to adopt these principles in their crop production.A good example of IWM in Korea has been

suggested by Kim et al. (1976). A single plowing inautumn, followed by the application of 3 mt/ha ofbarley straw before transplanting, reduced the popu-lation of P. distinctus by 40%, compared to theuntreated control. This indicates that cultural prac-tices such as autumn plowing and mulching withbarley straw can play an important role in reducingthe population of specific weeds, so that less herbi-cide is required to control them.

Fewer weeds were observed in double-cropped paddy fields where rice was followed bywinter crops such as barley, rape, garlic or onion.However, the area of double cropping has greatlydecreased in recent years, and most rice crops inKorea are grown in a system of monocropping. Croprotation, whereby rice fields are converted to uplandcrops for a cropping season or a year, is now under

study, and competitive crops are being sought whichmatch international market prices. Cultivation ofsoybean on a yearly rotation in paddy fields markedlyreduced the emergence of aquatic weeds and greatlyreduced weed problems in the subsequent rice crop.This type of annual rotation of rice with soybeanhindered the establishment of stable populations ofeither paddy weeds or upland weeds. Furthermore,direct seeding of rice can probably be implementedwith minimum tillage after soybean culture. Thuscrop rotation, minimum tillage, and use of strawmulch may all be important components of IWM.Application rates of herbicides can be greatly re-duced if all other available methods are put to properuse. There is still a great deal of room for reducingthe cost of herbicides if farmers choose the rightherbicide and apply it at the proper time. Improve-ments in application methods and equipment, and themodification of formulation types into e.g. slow-release and liquid forms, could also reduce the costand increase the efficiency of herbicides.

Cost saving and reduced environmentalimpact are the two most important aspects to beconsidered in developing labor-saving technology.Reduction of labor costs is badly needed for Koreanagriculture to be able to survive after the agriculturalcommodity market is liberalized and Korea becomesopen to imports. The production cost of 1 kg milledrice is about US$0.86 in Korea, but less than onethird of this (about $0.24) in the USA. Furthermore,labor requirements for rice production per hectareare 36.5 times higher in Korea than in the USA (Kim1992). IWM will be an important component ofcost-saving technology.

In order to implement an IWM system,basic information should be put on a computerizeddata base, including a thorough knowledge of thebiological characteristics of major weeds, local cli-matic information, weed population dynamics, andeconomic threshold concepts for a mixed or single-species weed population. Modelling of competitionand population dynamics is necessary to develop aneconomic threshold level. This information can thenbe used as a basis to help make the development ofIWM a reality.

Fig. 3 shows the components which couldbe included in a package of IWM under Koreanconditions. In presents schematically some of theways in which, cultural and biological methods canbe integrated with appropriate herbicides to achievesuperior control. It is almost certain that IWM willbe the most profitable strategy, but its main limita-tion is its relative complexity compared with herbi-cide use. Questions arise of how we can simplify the

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Fig. 2. Composition and dry weight of weeds in dry­seeded rice

Source: Yeun et al. 1991

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complexity of IWM technology. This will be animportant task for weed scientists in the years tocome.

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Back, K.W. and K.U. Kim. 1988. Identifica-tion of phytotoxic compounds and allelo-pathic effects of various upland weeds.Korean Journal of Weed Science 8,3:283-290.

Barnes, J.P. and A.R. Putnam. 1983. Ryeresidues contribute to weed suppression inno-tillage cropping systems. Journal ofChemical Ecology 9: 1045-1057.

Beart, J.E., T.H. Lilley and E. Haslami.1985. Plant polyphenol–secondary me-tabolism and chemical defense: Some ob-servations. Phytochemistry 24,1: 33-38.

Beyer, E.M. Jr.. 1991. Crop Protection:Meeting The Challenge. Brighton CropProtection Conference-Weeds 1: 3-22.

Chun, J.C., K.W. Han, B.C. Jang and H.S.Shin. 1988. Determination of phenoliccompounds responsible for allelopathy inupland weeds. Korean Journal of WeedScience 8,3: 258-264.

Court, W.A., J.M. Elliot and J.G. Hendel.1982. Influence of applied nitrogen on

the nonvolatile fatty and organic acids offlue-cured tobacco. Can. Jour. Plant Sci-ence 62,2: 489-496.

Einhellig, F.A. and G.A. Leather. 1988. Po-tentials for exploiting allelopathy to en-hance crop production. Journal ofChemical Ecology 14: 1829-1844.

Hong, Y.K., J.C. Kim, K.L. Ryu, and S.C.Kim. 1991. Research Report ofYoungnam Crop Experiment Station, RuralDevelopment Administration, Milyang, Ko-rea pp. 938-946.

Kwak, S.S. and K.U. Kim. 1984. Effect ofmajor phenolic acids identified from barleyresidues on the germination of paddyweeds. Korean Journal of Weed Science4,1: 39-51.

Kim, D.S. 1969. An introduction to weedcontrol in Korea. Proceeding of 2ndAsian-Pacific Weed Control Interchange,pp. 34-42.

Kim, K.U. and H.O. Choi. 1976. Perennialweed control in paddy rice fields. (1)Effect of various plowing depths in au-tumn on perennial weed control. KoreanJournal of Crop Science 21,1: 20-23.

Kim, K.U. 1981. Weed control in Korea.In: Weeds and Weed Control in Asia.Book Series No. 20, Food and FertilizerTechnology Center, Taipei, Taiwan ROC,

Fig. 3. Proposed components of IWM in Korea

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pp. 37-50.Kim, K.U., I.J. Lee, H.J. Jeong and D.S.

Kim. 1987. Potential allelopathic sub-stances identified from annual crop straws.Proceedings of 11th Asian-Pacific WeedScience Society Conference, pp. 303-310.

Kim, K.U., S.Y. Jeh, J.K. J.K. Sohn and S.K.Lee. 1981. Paddy weeds serving as thepossible reservoirs for rice bacterial leafblight. Korean Journal of Crop Science26,1: 40-44.

Kim, K.U. 1990. Progress of weed controlmethods and their prospects in Korea.Proceedings of the 8th Sistership Meetingof Korea, Japan and Republic of China’sAgrochemical Association, held in Seoul,Korea, 19th October 1990, pp. 1-30.

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22

DISCUSSION

Dr. Okada referred to the practice in Korea of transplanting very young seedlings, and asked whetherthis gave rise to special problems in herbicide use. Dr. Kim replied that initially there had been some worriesabout the possibility of herbicide toxicity with very young seedlings (at the 2-2.5 leaf stage). Farmers hadaccordingly been very cautious in applying herbicides, but by 1992 it was felt that there were no special herbicideproblems when these very young seedlings were used. In nurseries for ordinary seedlings, the seed bed mustbe prepared in early April when the weather in Korea is still very cold. In raising young seedlings, the seedbedis prepared in May when the temperature is much higher. Because of this, less protection is needed andcultivation is easier. The same herbicides used for ordinary transplanted seedlings are also suitable for use onvery young ones.