Ii. Integrated Management of Paddy Weeds in Korea, with an Emphasis on Allelopathy

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

Kil Ung Kim Department of Agronomy, College of Agriculture Kyungpook National University, Taegu 702-701, Korea

ABSTRACT

Annual weeds such as Echinochloa crus-galli and Monochoria vaginalis, and perennials such as Sagittaria trifolia, Sagittaria pygmaea, Eleocharis kuroguwai, Cyperus serotinus and Potamogeton distinctus, have been the most important weed species in paddy fields in Korea during the past two decades. Perennial weeds have increased from about 19% in 1971 to about 54% in 1981 and 60% in 1990. E. kuroguwai and S. trifolia are now the weeds most resistant to current herbicides, because their emergence takes place over a prolonged period. Biocontrol of E. kuroguwai has become possible by means of a plant pathogen, Epicoccosorus nematosporus, and of Scirpus planculmis by a pathogen Alternaria sp.. Residues of crops such as barley, wheat and rye had the allelopathic potential to reduce a population of P. distinctus in paddy fields. Phenolic compounds found in crop residues such as ferulic acid have been determined to have allelopathic potential, and also p-coumaric, sinapic, protocathechuic and caffeic acids. The integration of efficient herbicides with cultural control (using crop rotation, appropriate 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 in Korea’s rice production.

INTRODUCTION currently available. However, these are becoming unpopular among members of the public. It is Until the mid 1980’s, agricultural policy anticipated that environmental policies on the use of and research in Korea was focused mainly on obtain- pesticides, including herbicides, will be strength- ing the maximum yield, through varietal improve- ened, and become more restrictive than before. ment and new cultural practices. Great progress has However, it seems inevitable that farmers will still been made in agricultural production in Korea over use a number of conventional herbicides, particularly the past two decades, although food self-sufficiency those which meet health and environmental require- in 1990 was still only about 43% (RDA 1991), while ments, since no alternatives are available which give the remaining food was imported at an annual cost of such an effective result for such a relatively low cost US$7,304 million. A further increase in the impor- (Kim 1990). Improved weed management systems tation of agricultural goods seems to be inevitable in must be developed which maximize weed control at the years to come. Under these circumstances, the a minimum cost. The choice of system will depend sustainability of Korean agriculture will depend on on the farmer’s preferences. whether it is able to develop labor-saving technology This paper discusses the current status of based on low inputs. biological control, the use of allelopathy, and the Farm mechanization and effective weed integrated management of paddy weeds. The impor- management are recognized as two vitally important tant weeds found in Korean paddy fields, and the components of labor-saving technology. Herbicides herbicides used to control them, are also described to are the most cost-effective weed control technology give the reader a better understanding of integrated

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

8 weed management in Korea. such as E. kuroguwai and P. distinctus would become more serious if herbicides were to be used for IMPORTANT WEEDS IN PADDY FIELDS weed control. In the first nationwide weed survey carried It has been reported that 92 weed species out in 1971, Rotala indica was recognized as the belonging to 27 families are found in Korean paddy dominant species, followed by Eleocharis acicularis, fields, of which 30 species are considered to be M. vaginalis, Cyperus difformis, E. crus-galli, common weeds (Kim 1981). It is interesting to note Lindernia procumbens, P. distinctus, Aneilema the changes in weed flora which have taken place japonica, E. kuroguwai, Scirpus juncodes and over the past three decades. Table 1 presents data Persicaria hydropiper, in that order (Table 1). This summarized from Kim (1992) on the nationwide group of species constituted approximately 86% of weed surveys conducted by the Rural Development all paddy weeds. Annual weeds were predominant in Administration in 1971 and 1981, plus data from the paddy fields, constituting 81% of this dominant 1960s from Kim (1969), and from 1990 collected by group. a private research organization. In a second nationwide weed survey carried In 1965, 17 species were listed as the most out in 1981, the most important weed was M. common weeds in paddy fields (Table 1) (Kim 1969). vaginalis, followed by S. pygmaea, S. trifolia, P. Of these, E. crus-galli was the most troublesome distinctus, C. serotinus, R. indica, A. japonica, L. weed in paddy fields, followed by M. vaginalis, procumbens, E. kuroguwai and Ludwigia prostrata, which was a serious weed in seedbeds as well as in in that order (Table 1). The dominance represented paddy fields. Others were listed as minor weeds. by this group of weed species was about 87%, of Before 1970 herbicides were seldom used, but Kim which 54% were perennial weeds. in 1965 already anticipated that 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

9 in 1990, S. trifolia had become the dominant weed they both require a relatively high cumulative tem- species in paddy fields, followed by S. pygmaea, E. perature (117°C in the case of E. kuroguwai, and kuroguwai, E. crus-galli, P. distinctus, C. serotinus, 297°C for S. trifolia). and M. vaginalis (Table 1). Of these, about 60% The ten most important weed species indi- were perennial weeds. cated here can be expected to continue as the major Thus, the changes in weed flora can be weeds in paddy fields for at least the next 10 or 20 summarized as follows. Firstly, E. crus-galli gradu- years, as long as the current control methods which ally decreased from 1965 to 1981 but then increased rely on the heavy use of one-shot herbicides continue again from 1981 to 1990. This indicates that E. crus- to be used. galli, which has a very similar habitat to that of rice, has remained the dominant weed species in paddy CURRENT STATUS OF fields throughout the rice-growing season. BIOLOGICAL CONTROL Secondly, R. indica, the dominant weed species in 1971, had markedly decreased by 1981 Biological weed control involves the use of and had become relatively unimportant by 1990. living organisms to reduce the population of a target This fall in the occurance of E. crus-galli and R. weed. Wapshere (1982) suggested three kinds of indica by 1981 is related to the intensive use of biocontrol method: the classical (inoculative) ap- annual herbicides such as the acid amide, carbamate proach, the bioherbicide (inundative) approach, and and diphenylether types, which up until the mid herbivore management. Insects, mites, nematodes, 1980s had more than 80% of the market share in plant pathogens, and aquatic and terrestrial herbi- terms of quantity sold. M. vaginalis was poorly vores, have all been used as biotic agents for the controlled by these herbicides and became the domi- biological control of weeds (Watson 1992). nant weed because it characteristically produces a For the classical approach, the ideal target large number of seeds, and requires a longer time for is a widespread introduced weed infesting large emergence than E. crus-galli. Another interesting areas such as rangelands or aquatic habitats. In result is that perennial weeds such as S. pygmaea, S. Korea, land size per farm household is very small, trifolia, P. distinctus and C. serotinus also became and farmers are also growing a diversified range of much more common by 1981. crops, particularly in the uplands. It is thus rather Little difference in weed flora was noted difficult to apply the classical approach to control a between 1981 and 1990, except that E. kuroguwai single weed species among a weed complex in culti- and E. crus-galli became important species. The vated crops. perennials which dominated in 1981 still remained The bioherbicide approach uses the living the most common species in 1990, except that S. inoculum of a plant pathogen, in a manner similar to trifolia replaced S. pygmaea. Similarity coefficients that of a chemical herbicide, to control or suppress of changes in the level of dominance of different the growth of a weed species. The first attempt made weed species over time were about 37-39 in the years in Korea evaluated the potential of nine paddy weeds 1971, 1981, but 62 between 1981 and 1991 (Kim as hosts for bacterial leaf blight. Of these nine weed 1991). These results confirm that there was a major species, Alopecurus spp., Setaria viridis and Leersia change in weed flora between 1971 and 1981, but little change between 1981 and 1991 (Table 2). In conclusion, it would seem that about ten species, including both annual and perennial weeds, Table 2. Similarity coefficient in terms of have been the major weeds in transplanted rice fields changes in dominant weed species overtime during the past three decades. However, the relative importance of the various species has fluctuated according to the control measures used, and particularly the type of herbicide applied. The heavy applications of herbicide used for weed control since 1980 have stabilized weed populations and made perennial weeds dominant in paddy fields. E. kuroguwai and S. trifolia are regarded as the two most difficult weeds to control. Their emergence takes place over a prolonged period, partly because source: Kim 1991

10 japonica were shown to be most susceptible to the 94.9% of E. kuroguwai, compared with 68.0% after disease, showing a response similar to that of Milyang only one application (Table 5). The number of E. 23, a susceptible check rice variety (Table 3). How- kuroguwai tubers at the time of the rice harvest was ever, most broadleaf weeds were not infected by the greatly reduced, from 76.3 tubers/pot for the un- disease (Kim et al. 1981). It is possible that a number treated control to 9.6/pot after a single application of weed species growing in or near paddy fields can and 1.0/pot after three applications of E. be used as hosts of this pathogen, but no further work nematosporus (Table 5). Fig. 1 shows that applica- has yet been done. tions of the inoculum take longer to control E. In recent years, there has been some suc- kuroguwai than bentazon, a synthetic herbicide known cess in the use of a plant pathogen to control E. to be the most effective chemical treatment for this kuroguwai, a perennial sedge which is very difficult weed, but even so, about 90.0% of weeds had been to control with the herbicides currently available. controlled by the pathogen at 20 days after inocula- Epicoccosorus nematosporus has been identified as tion, an efficacy similar to that of bentazon. Further a very selective plant pathogen which can give good studies are in progress to determine the toxicity of control of E. kuroguwai (Hong et al. 1991). None of the pathogen against weeds, methods of mass pro- the fifteen crops tested, including rice, were infected duction, and formulation as a bioherbicide. by E nematosporous, and out of the 15 weed species Another possibility of biocontrol by a patho- tested only sedges were infected. This indicates that genic fungus, Alternaria sp., has been reported in the there is a good possibility of using this pathogen as control of Scirpus planiculmis, a perennial sedge a bioherbicide to control sedges. found on newly reclaimed land on the western coast When the concentration of E. of Korea. An interesting observation is that S. nematosporous was increased from 1.2 x 103 to 0.8 planiculmis appears to be the only sedge susceptible x 107 conidia/ml, the percentage of infected and dead to this pathogen (Table 6). Under field conditions, plants also increased significantly, from 35.7 to Alternaria disease developed on S. planiculmis at 3 96.1%, and from 15.4 to 92.3%, respectively (Table days after inoculation, while disease incidence had 4). Three applications of E. nematosporus inoculum increased to 80% by mid-July, 12 days after inocula- at a concentration of 6.3 x 108 conidia/ml killed 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 of bacterial leaf blight (Group I)

Source: Kim et al. 1981

11 application at a concentration of 3.0 x 106 mycelle potential under Korean conditions. (cell/ml), 85% of the sedge had been killed (Table 7). The third approach uses herbivores as con- ALLELOPATHY trol agents. No data are available on the use of herbivores to control submerged vegetation in aquatic Molish (1937) defined the term ‘allelopa- systems in Korea. However, it is believed that geese thy’ to refer to biochemical interactions between all and ducks were driven into rice and cotton fields to types of plants, including microorganisms. Allelopa- peck the weeds before herbicides came into intensive thy was later defined as any negative or positive plant use. This is still a common practice in south China, response mediated through chemical compounds where similarly chickens are kept in fruit gardens to naturally produced by another plant or microorgan- eat young weeds and insects (Li 1992). A herbivo- ism (Putnam 1988). The potential of allelopathy for rous fish native to China, the grass carp, has received weed control has been reviewed by Einhellig and a great deal of attention as a potential weed control Leather (1988), Purvis (1990) and Watson (1992). agent because it is a strict vegetarian, feeding on They have reported that natural chemicals exerting submerged weeds such as duckweed and filamen- an allelopathic effect may be either simple or com- tous algae (Li 1992). Sheep, goats and cattle have plex secondary compounds. Many of the com- some utility in supressing pasture weeds, but the use pounds implicated have yet to be identified, and of grazing animals to control weeds has limited often the effect is not due to a single compound

Table 4. Effect of Epicoccosorus nematosporus on Eleocharis kuroguwai1)

Source: Hong et al. 1991

Table 5. Effect of repeated applications of Epicoccosorus nematosporus on control of Eleocharis kuroguwai

Source: Hong et al. 1991

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

Source: Hong et al. 1991

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

Source: Park et al. 1992

13 Table 7. Effect of Alternaria sp., collected 1984, Kim et al. 1987). Aqueous and alcohol from Gehwado (western Korea) on extracts from crop straw markedly inhibited the disease development1) growth of lettuce seedlings. The inhibition effect of barley extract was greatest, followed by wheat and rye, in that order (Kim et al. 1987) (Table 8). Twelve simple free phenolic acids were identified in straw from barley, wheat and rye (Table 9). Ferulic acid was the most abundant (about 20% or more), followed by p-coumaric, sinapic, protocatecheuic and caffeic acid, and polyphenols such as scopoletin and rutin. Fatty acids such as linoleic acid, and organic acids, were also determined (Kim et al. 1987). It was suggested that phenolic substances, together with fatty acids and organic acids, may be potential allelopathic substances since they exert a strong inhibitory effect on the germination and growth of lettuce. Source: Park et al. 1992 In Korea, there are several reports available on the allelopathic potential of various types of upland weed. Kim et al. (1987) reported that out of 49 upland weed species tested, five species (Lotus corniculatus, Bidens frondosa, Polygonum (Einhellig and Leather 1988). Allelopathy can be perfoliata, Datura stramonium and Cephalonoplos categorized as either cultural control (habitat man- segetum) had allelopathic potential, a significant agement), as when crop selection is involved, or inhibitory effect on lettuce germination. Phenolic biological control when natural chemicals are in- compounds such as ferulic, p-hydrobenzoic acids volved (Einhellig and Leather 1985, Purvis 1990). and cinnamic acids played a major role in the allelo- There are a number of papers on the use of pathic activity in upland weed species such as Arte- residues of crops such as barley, wheat and rye in misia asiatica, Capsella bursa-pastris, Portulaca order to utilize their allelopathic potential. Residues oleracea, and Trifolium repens (Chun et al. 1988). of fall-planted, spring-killed rye significantly re- Phenolic substances also proved to have allelopathic duced the total weed biomass, compared to the potential in five upland weed species. The largest untreated control (Barnes et al. 1983). Barley used total amount of phenol was detected in Galinsoga as a smother crop may be successful in eliminating ciliata, with 1.27 mg/g, followed by Polygonum weeds because it inhibits seed germination and the perfoliata (1.8 mg/g), Stellaria aquatica (1.7 mg/g), growth of selected plant species (Overland 1966). Amaranthus lividus (1.28 mg/g), and Achyranthes Wheat also seems to have allelopathic potential in japonica (1.27 mg/g). These plant species also repressing the germination of many weed seeds and contained a large quantity of ferulic, caffeic, 7-OH- also the growth of weed seedlings (Steinsiek 1982, coumarine and protocatechuic acids (Back and Kim Kwon and Kim 1985). Furthermore, Kwak and Kim 1988). Fatty acids and organic acids were also (1984) and Kwon and Kim (1985) have reported that present, at levels of 4-9 mg/g and 24-22 mg/g, aqueous extracts from barley, wheat and rye residues respectively (Back and Kim 1988). These are so far inhibited the germination and growth of paddy and the most detailed studies on the allelopathic potential upland weeds. These extracts were particularly of weeds in Korea. Other papers show similar results effective against P. distinctus, an important peren- in other weed species (e.g. Polygonum hydropiper, nial broadleaf weed in paddy fields. Polygonum aviculare) (Woo and Kim 1987) and The fact that some crop residues exert an Portulaca oleracea (Park and Kim 1988), and in inhibitory effect on certain weeds may allow them to wild plants, (e.g. Styrax japonica and Aralia be used as an alternative control measure in the form continetalis) (Kim et al. 1990). of a mulch, or as part of a rotational sequence. The Unfortunately, the only work in Korea on phenolic compounds in plant residues have been allelopathic potential as a means of controlling paddy intensively studied to evaluate their allelopathic po- weeds is that on crop residues discussed above. tential as protective agents to control weeds, insects Further study is needed to identify specific and fungi (Court et al. 1982, Beart et al. 1985, Rice allelochemicals.

14 CHEMICAL CONTROL machine, as was the case for 88% of rice grown area in Korea in 1991. About 22% of this area trans- Today, herbicides have become the main planted by machine was planted in very young seed- method of weed control for both paddy and upland lings, only 8 to 10 days old. The use of very young crops in Korea, and there is no suitable alternative to seedlings is much cheaper than using ordinary seed- these. Herbicide use in paddy fields was about lings 30 to 35 days old, because it reduces the cost 119%. (Some farmers apply herbicide more than of nursery culture by 54%. The transplanting of very once per rice culture, particularly to control E. young seedlings is thus expected to increase. How- kuroguwai, the most difficult weed in paddy fields). ever, the shallow depth of the irrigation water when Only about 79% of upland fields were treated with extremely young seedlings are transplanted by ma- herbicides. The lower use of herbicides in upland chine favors weed germination and growth. This is crops is probably because it is difficult for farmers to a disadvantage, but on the other hand, the establish- apply herbicides to diversified crops grown in small ment of young seedlings is much faster than that of upland fields (Kim 1992). Only a negligible amount ordinary seedlings. Until recently, maximum yield of herbicide was used in the mid 1960’s, mainly 2,4- has been the main target of rice production, based on D, with PCP coming into use in the late 1960s. good land preparation, good fertilizer and water Nitrofen became popular in the early 1970s, while management, and pest management. There is now a acid amide type herbicides such as butachlor and widespread feeling that maximum input technology alachlor shared more than 60% of the total herbicide should be replaced by low-input methods. Direct market for more than 15 years, until 1989 (Kim seeding is receiving a great deal of attention as an 1990). One important feature was a dramatic in- alternative planting method. crease in the use of mixture type herbicides, from Any reduction of inputs used in cultural 3.0% in 1981 to 68.0% in 1990 (Table 10). This practices can be expected to promote weed growth, reflects farmers’ demand for a one-shot herbicide so that weeds are likely to become a more important capable of controlling annual as well as perennial problem in low-input technology. Furthermore, any weeds with a single application. Twenty-seven implementation of direct seeding will give rise to mixture herbicides suitable for rice were available in important problems such as lodging and poor crop Korea in 1991. Half of these were mixtures based on stands, as well as weeds. In direct seeded rice, weeds sulfonyl urea types, such as bensulfuron-methyl and usually emerge ahead of the rice, absorbing the pyrazosulfuron-ethyl, formulated with the aim of available nutrients earlier so that they flourish and controlling both annual and perennial weeds. A show rapid growth. Any small advantage of one single application of a mixture herbicide seems to be species during seedling establishment will have great rational and economic, but a relatively early applica- impact on this species in later competition with tion within 10 DAT (days after transplanting) does others. It is evident that a change in planting not give satisfactory control of E. kuroguwai or S. methods from transplanting to direct seeding will trifolia, both of which require a long period for result in a shift of weed dominance from a complex emergence in paddy fields. Thus, these two weeds of perennial and annual weeds to annual grasses. often escape the residual effect of the herbicides, and It is recommended that if dry-seeded rice is have become the most difficult weeds to control in being cultivated, the field should be maintained in a lowland rice fields. relatively dry condition for 30 days after seeding, Public awareness, and also investment into followed by irrigation as for transplanted rice cul- new research programs, to find alternatives to pesti- ture. In fields planted with dry-seeded rice, E. crus- cides, could result in a range of new, environmentally galli is the dominant weed species, followed by C. sound pest management measures which involve a amuricus and Rorippa islandica (Fig. 2) (Yeun et al. broader use of currently available alternatives such 1991). After the repeated culture of dry-seeded rice, as cultural and biological methods. C4-type grasses become the dominant weeds (Kim 1991). Applications of systematic herbicides using IMPACT OF CHANGES IN pre-emergence types followed by post-emergence PLANTING METHODS ones gives excellent weed control. A tank mixture of propanil with pre-emergence type annual herbicides Rice has been a staple food for well over a can give satisfactory control of E. crus-galli. This thousand years in Korea. Cultivation methods have can be used as a basis for developing appropriate changed over time, from the primitive direct seeding IWM for dry-seeded rice. to transplanting by hand, and finally transplanting by 15 from crop straw on germination of test plants (shown as % germination) 1) 1987 et al. Source: Kim Table 8. Effect of alcohol extracts

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

Source: Kim et al. 1987

Table 10. The major types of chemical herbicide being used in paddy fields in Korea, 198190

Source: Agricultural Chemical Industrial Association

17 INTEGRATED MANAGEMENT OF study, and competitive crops are being sought which PADDY WEEDS match international market prices. Cultivation of soybean on a yearly rotation in paddy fields markedly The goal of weed management should not reduced the emergence of aquatic weeds and greatly be to eradicate or exterminate, but rather to prevent reduced weed problems in the subsequent rice crop. weed damage from exceeding the economic injury This type of annual rotation of rice with soybean threshold by taking advantage of a range of weed hindered the establishment of stable populations of control methods. IWM is not only effective, but either paddy weeds or upland weeds. Furthermore, takes into consideration protection of the environ- direct seeding of rice can probably be implemented ment and food safety. with minimum tillage after soybean culture. Thus There is an increasing desire on the part of crop rotation, minimum tillage, and use of straw many farmers to adopt management practices such mulch may all be important components of IWM. as IWM to enhance profitability, but the complexity Application rates of herbicides can be greatly re- of IWM technology has seriously hindered its wide- duced if all other available methods are put to proper spread adoption. For instance, farmers have diffi- use. There is still a great deal of room for reducing culty in understanding the economic threshold for a the cost of herbicides if farmers choose the right given weed, and weed response to different control herbicide and apply it at the proper time. Improve- methods under various cultural and climatic condi- ments in application methods and equipment, and the tions. Therefore, many farmers have become accus- modification of formulation types into e.g. slow- tomed to using herbicides as their primary defense release and liquid forms, could also reduce the cost against potential weed damage (Beyer 1991). and increase the efficiency of herbicides. IWM techniques should follow a number of Cost saving and reduced environmental guiding principles. impact are the two most important aspects to be • Crops should be matched to local grow- considered in developing labor-saving technology. ing conditions; Reduction of labor costs is badly needed for Korean • Crop rotation should be practiced; agriculture to be able to survive after the agricultural • The soil should be protected throughout commodity market is liberalized and Korea becomes the year by cover crops; open to imports. The production cost of 1 kg milled • Cultivars should be selected which are rice is about US$0.86 in Korea, but less than one competitive against weeds; third of this (about $0.24) in the USA. Furthermore, • There should be accurate application of labor requirements for rice production per hectare fertilizers to meet crop requirements; are 36.5 times higher in Korea than in the USA (Kim • Where possible, biological control meth- 1992). IWM will be an important component of ods should be used (Klassen 1990). cost-saving technology. In practice, it is rather difficult for farmers In order to implement an IWM system, to adopt these principles in their crop production. basic information should be put on a computerized A good example of IWM in Korea has been data base, including a thorough knowledge of the suggested by Kim et al. (1976). A single plowing in biological characteristics of major weeds, local cli- autumn, followed by the application of 3 mt/ha of matic information, weed population dynamics, and barley straw before transplanting, reduced the popu- economic threshold concepts for a mixed or single- lation of P. distinctus by 40%, compared to the species weed population. Modelling of competition untreated control. This indicates that cultural prac- and population dynamics is necessary to develop an tices such as autumn plowing and mulching with economic threshold level. This information can then barley straw can play an important role in reducing be used as a basis to help make the development of the population of specific weeds, so that less herbi- IWM a reality. cide is required to control them. Fig. 3 shows the components which could Fewer weeds were observed in double- be included in a package of IWM under Korean cropped paddy fields where rice was followed by conditions. In presents schematically some of the winter crops such as barley, rape, garlic or onion. ways in which, cultural and biological methods can However, the area of double cropping has greatly be integrated with appropriate herbicides to achieve decreased in recent years, and most rice crops in superior control. It is almost certain that IWM will Korea are grown in a system of monocropping. Crop be the most profitable strategy, but its main limita- rotation, whereby rice fields are converted to upland tion is its relative complexity compared with herbi- crops for a cropping season or a year, is now under cide use. Questions arise of how we can simplify the

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

Source: Yeun et al. 1991

19 Fig. 3. Proposed components of IWM in Korea

complexity of IWM technology. This will be an the nonvolatile fatty and organic acids of important task for weed scientists in the years to flue-cured tobacco. Can. Jour. Plant Sci- come. ence 62,2: 489-496. Einhellig, F.A. and G.A. Leather. 1988. Po- tentials for exploiting allelopathy to en- REFERENCES hance crop production. Journal of Chemical Ecology 14: 1829-1844. Back, K.W. and K.U. Kim. 1988. Identifica- Hong, Y.K., J.C. Kim, K.L. Ryu, and S.C. tion of phytotoxic compounds and allelo- Kim. 1991. Research Report of pathic effects of various upland weeds. Youngnam Crop Experiment Station, Rural Korean Journal of Weed Science 8,3: Development Administration, Milyang, Ko- 283-290. rea pp. 938-946. Barnes, J.P. and A.R. Putnam. 1983. Rye Kwak, S.S. and K.U. Kim. 1984. Effect of residues contribute to weed suppression in major phenolic acids identified from barley no-tillage cropping systems. Journal of residues on the germination of paddy Chemical Ecology 9: 1045-1057. weeds. 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21 DISCUSSION

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