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, 1993-10-01

Abstracts in Other Languages: 中文, 日本語, 한국어

Introduction

Until the mid 1980's, agricultural policy and research in Korea was focused mainly on obtaining the maximum yield, through varietal improvement and new cultural practices. Great progress has been made in agricultural production in Korea over the past two decades, although food self-sufficiency in 1990 was still only about 43% (RDA 1991), while the remaining food was imported at an annual cost of US$7,304 million. A further increase in the importation of agricultural goods seems to be inevitable in the years to come. Under these circumstances, the sustainability of Korean agriculture will depend on whether it is able to develop labor-saving technology based on low inputs.

Farm mechanization and effective weed management are recognized as two vitally important components of labor-saving technology. Herbicides are the most cost-effective weed control technology currently available. However, these are becoming unpopular among members of the public. It is anticipated that environmental policies on the use of pesticides, including herbicides, will be strengthened, and become more restrictive than before. However, it seems inevitable that farmers will still use a number of conventional herbicides, particularly those which meet health and environmental requirements, since no alternatives are available which give such an effective result for such a relatively low cost (Kim 1990). Improved weed management systems must be developed which maximize weed control at a minimum cost. The choice of system will depend on the farmer's preferences.

This paper discusses the current status of biological control, the use of allelopathy, and the integrated management of paddy weeds. The important weeds found in Korean paddy fields, and the herbicides used to control them, are also described to give the reader a better understanding of integrated weed management in Korea.

Important Weeds in Paddy Fields

It has been reported that 92 weed species belonging to 27 families are found in Korean paddy fields, of which 30 species are considered to be common weeds (Kim 1981). It is interesting to note the changes in weed flora which have taken place over the past three decades. Table 1 presents data summarized from Kim (1992) on the nationwide weed surveys conducted by the Rural Development Administration in 1971 and 1981, plus data from the 1960s from Kim (1969), and from 1990 collected by a private research organization.

In 1965, 17 species were listed as the most common weeds in paddy fields (Table 1) (Kim 1969). Of these, E. crus-galli was the most troublesome weed in paddy fields, followed by M. vaginalis, which was a serious weed in seedbeds as well as in paddy fields. Others were listed as minor weeds. Before 1970 herbicides were seldom used, but Kim in 1965 already anticipated that perennial weeds such as E. kuroguwai and P. distinctus would become more serious if herbicides were to be used for weed control.

In the first nationwide weed survey carried out in 1971, Rotala indica was recognized as the dominant species, followed by Eleocharis acicularis, M. vaginalis, Cyperus difformis, E. crus-galli, Lindernia procumbens, P. distinctus, Aneilema japonica, E. kuroguwai, Scirpus juncodes and Persicaria hydropiper, in that order (Table 1). This group of species constituted approximately 86% of all paddy weeds. Annual weeds were predominant in paddy fields, constituting 81% of this dominant group.

In a second nationwide weed survey carried out 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 represented by this group of weed species was about 87%, of which 54% were perennial weeds.

In the most recent weed survey carried out in 1990, S. trifolia had become the dominant weed species 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 be summarized as follows. Firstly, E. crus-galli gradually decreased from 1965 to 1981 but then increased again 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 paddy fields throughout the rice-growing season.

Secondly, R. indica, the dominant weed species in 1971, had markedly decreased by 1981 and 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 of annual herbicides such as the acid amide, carbamate and diphenylether types, which up until the mid 1980s had more than 80% of the market share in terms of quantity sold. M. vaginalis was poorly controlled by these herbicides and became the dominant weed because it characteristically produces a large number of seeds, and requires a longer time for emergence than E. crus-galli. Another interesting result is that perennial weeds such as S. pygmaea, S. trifolia, P. distinctus and C. serotinus also became much more common by 1981.

Little difference in weed flora was noted between 1981 and 1990, except that E. kuroguwai and E. crus-galli became important species. The perennials which dominated in 1981 still remained the most common species in 1990, except that S. trifolia replaced S. pygmaea. Similarity coefficients of changes in the level of dominance of different weed species over time were about 37-39 in the years 1971, 1981, but 62 between 1981 and 1991 (Kim 1991). These results confirm that there was a major 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, have been the major weeds in transplanted rice fields 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 they both require a relatively high cumulative temperature (117°C in the case of E. kuroguwai, and 297°C for S. trifolia).

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

Current Status of Biological Control

Biological weed control involves the use of living organisms to reduce the population of a target weed. Wapshere (1982) suggested three kinds of biocontrol method: the classical (inoculative) approach, the bioherbicide (inundative) approach, and herbivore management. Insects, mites, nematodes, plant pathogens, and aquatic and terrestrial herbivores, have all been used as biotic agents for the biological control of weeds (Watson 1992).

For the classical approach, the ideal target is a widespread introduced weed infesting large areas such as rangelands or aquatic habitats. In Korea, land size per farm household is very small, and farmers are also growing a diversified range of crops, particularly in the uplands. It is thus rather difficult to apply the classical approach to control a single weed species among a weed complex in cultivated crops.

The bioherbicide approach uses the living inoculum of a plant pathogen, in a manner similar to that of a chemical herbicide, to control or suppress the growth of a weed species. The first attempt made in Korea evaluated the potential of nine paddy weeds as hosts for bacterial leaf blight. Of these nine weed species, Alopecurus spp., Setaria viridis and Leersia japonica were shown to be most susceptible to the disease, showing a response similar to that of Milyang 23, a susceptible check rice variety (Table 3). However, most broadleaf weeds were not infected by the disease (Kim et al. 1981). It is possible that a number of weed species growing in or near paddy fields can be used as hosts of this pathogen, but no further work has yet been done.

In recent years, there has been some success in the use of a plant pathogen to control E. kuroguwai, a perennial sedge which is very difficult to control with the herbicides currently available. Epicoccosorus nematosporus has been identified as a very selective plant pathogen which can give good control of E. kuroguwai (Hong et al. 1991). None of the fifteen crops tested, including rice, were infected by E nematosporous, and out of the 15 weed species tested only sedges were infected. This indicates that there is a good possibility of using this pathogen as a bioherbicide to control sedges.

When the concentration of E. nematosporous was increased from 1.2 x 10³ to 0.8 x 10⁷ conidia/ml, the percentage of infected and dead plants also increased significantly, from 35.7 to 96.1%, and from 15.4 to 92.3%, respectively (Table 4). Three applications of E. nematosporus inoculum at a concentration of 6.3 x 10⁸ conidia/ml killed 94.9% of E. kuroguwai, compared with 68.0% after only one application (Table 5). The number of E. kuroguwai tubers at the time of the rice harvest was greatly reduced, from 76.3 tubers/pot for the untreated control to 9.6/pot after a single application and 1.0/pot after three applications of E. nematosporus (Table 5). Fig. 1 shows that applications of the inoculum take longer to control E. kuroguwai than bentazon, a synthetic herbicide known to be the most effective chemical treatment for this weed, but even so, about 90.0% of weeds had been controlled by the pathogen at 20 days after inoculation, an efficacy similar to that of bentazon. Further studies are in progress to determine the toxicity of the pathogen against weeds, methods of mass production, and formulation as a bioherbicide.

Another possibility of biocontrol by a pathogenic fungus, Alternaria sp., has been reported in the control of Scirpus planiculmis, a perennial sedge found on newly reclaimed land on the western coast of Korea. An interesting observation is that S. planiculmis appears to be the only sedge susceptible to this pathogen (Table 6). Under field conditions, Alternaria disease developed on S. planiculmis at 3 days after inoculation, while disease incidence had increased to 80% by mid-July, 12 days after inoculation at a concentration of 4 x 10⁶ cell/ml. Aafter one 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 control agents. No data are available on the use of herbivores to control submerged vegetation in aquatic systems in Korea. However, it is believed that geese and ducks were driven into rice and cotton fields to peck the weeds before herbicides came into intensive use. This is still a common practice in south China, where similarly chickens are kept in fruit gardens to eat young weeds and insects (Li 1992). A herbivorous fish native to China, the grass carp, has received a great deal of attention as a potential weed control agent because it is a strict vegetarian, feeding on submerged weeds such as duckweed and filamentous algae (Li 1992). Sheep, goats and cattle have some utility in supressing pasture weeds, but the use of grazing animals to control weeds has limited potential under Korean conditions.

Allelopathy

Molish (1937) defined the term `allelopathy' to refer to biochemical interactions between all types of plants, including microorganisms. Allelopathy was later defined as any negative or positive plant response mediated through chemical compounds naturally produced by another plant or microorganism (Putnam 1988). The potential of allelopathy for weed control has been reviewed by Einhellig and Leather (1988), Purvis (1990) and Watson (1992). They have reported that natural chemicals exerting an allelopathic effect may be either simple or complex secondary compounds. Many of the compounds implicated have yet to be identified, and often the effect is not due to a single compound (Einhellig and Leather 1988). Allelopathy can be categorized as either cultural control (habitat management), as when crop selection is involved, or biological control when natural chemicals are involved (Einhellig and Leather 1985, Purvis 1990).

There are a number of papers on the use of residues of crops such as barley, wheat and rye in order to utilize their allelopathic potential. Residues of fall-planted, spring-killed rye significantly reduced the total weed biomass, compared to the untreated control (Barnes et al. 1983). Barley used as a smother crop may be successful in eliminating weeds because it inhibits seed germination and the growth of selected plant species (Overland 1966). Wheat also seems to have allelopathic potential in repressing the germination of many weed seeds and also the growth of weed seedlings (Steinsiek 1982, Kwon and Kim 1985). Furthermore, Kwak and Kim (1984) and Kwon and Kim (1985) have reported that aqueous extracts from barley, wheat and rye residues inhibited the germination and growth of paddy and upland weeds. These extracts were particularly effective against P. distinctus, an important perennial broadleaf weed in paddy fields.

The fact that some crop residues exert an inhibitory effect on certain weeds may allow them to be used as an alternative control measure in the form of a mulch, or as part of a rotational sequence. The phenolic compounds in plant residues have been intensively studied to evaluate their allelopathic potential as protective agents to control weeds, insects and fungi (Court et al. 1982, Beart et al. 1985, Rice 1984, Kim et al. 1987). Aqueous and alcohol extracts from crop straw markedly inhibited the 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.

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 perfoliata, Datura stramonium and Cephalonoplos segetum) had allelopathic potential, a significant inhibitory effect on lettuce germination. Phenolic compounds such as ferulic, p-hydrobenzoic acids and cinnamic acids played a major role in the allelopathic activity in upland weed species such as Artemisia asiatica, Capsella bursa-pastris, Portulaca oleracea, and Trifolium repens (Chun et al. 1988). Phenolic substances also proved to have allelopathic potential in five upland weed species. The largest total amount of phenol was detected in Galinsoga ciliata, with 1.27 mg/g, followed by Polygonum perfoliata (1.8 mg/g), Stellaria aquatica (1.7 mg/g), Amaranthus lividus (1.28 mg/g), and Achyranthes japonica (1.27 mg/g). These plant species also contained a large quantity of ferulic, caffeic, 7-OH-coumarine and protocatechuic acids (Back and Kim 1988). Fatty acids and organic acids were also present, at levels of 4-9 mg/g and 24-22 mg/g, respectively (Back and Kim 1988). These are so far the most detailed studies on the allelopathic potential of weeds in Korea. Other papers show similar results in other weed species (e.g. Polygonum hydropiper, Polygonum aviculare) (Woo and Kim 1987) and Portulaca oleracea (Park and Kim 1988), and in wild plants, (e.g. Styrax japonica and Aralia continetalis) (Kim et al. 1990).

Unfortunately, the only work in Korea on allelopathic potential as a means of controlling paddy weeds is that on crop residues discussed above. Further study is needed to identify specific allelochemicals.

Chemical Control

Today, herbicides have become the main method of weed control for both paddy and upland crops in Korea, and there is no suitable alternative to these. Herbicide use in paddy fields was about 119%. (Some farmers apply herbicide more than once per rice culture, particularly to control E. kuroguwai, the most difficult weed in paddy fields). Only about 79% of upland fields were treated with herbicides. The lower use of herbicides in upland crops is probably because it is difficult for farmers to apply herbicides to diversified crops grown in small upland fields (Kim 1992). Only a negligible amount of 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, while acid amide type herbicides such as butachlor and alachlor shared more than 60% of the total herbicide market for more than 15 years, until 1989 (Kim 1990). One important feature was a dramatic increase in the use of mixture type herbicides, from 3.0% in 1981 to 68.0% in 1990 (Table 10). This reflects farmers' demand for a one-shot herbicide capable of controlling annual as well as perennial weeds with a single application. Twenty-seven mixture herbicides suitable for rice were available in Korea in 1991. Half of these were mixtures based on sulfonyl urea types, such as bensulfuron-methyl and pyrazosulfuron-ethyl, formulated with the aim of controlling both annual and perennial weeds. A single application of a mixture herbicide seems to be rational and economic, but a relatively early application within 10 DAT (days after transplanting) does not give satisfactory control of E. kuroguwai or S. trifolia, both of which require a long period for emergence in paddy fields. Thus, these two weeds often escape the residual effect of the herbicides, and have become the most difficult weeds to control in lowland rice fields.

Public awareness, and also investment into new research programs, to find alternatives to pesticides, could result in a range of new, environmentally sound pest management measures which involve a broader use of currently available alternatives such as cultural and biological methods.

Impact of Changes in Planting Methods

Rice has been a staple food for well over a thousand years in Korea. Cultivation methods have changed over time, from the primitive direct seeding to transplanting by hand, and finally transplanting by machine, as was the case for 88% of rice grown area in Korea in 1991. About 22% of this area transplanted by machine was planted in very young seedlings, only 8 to 10 days old. The use of very young seedlings is much cheaper than using ordinary seedlings 30 to 35 days old, because it reduces the cost of nursery culture by 54%. The transplanting of very young seedlings is thus expected to increase. However, the shallow depth of the irrigation water when extremely young seedlings are transplanted by machine favors weed germination and growth. This is a disadvantage, but on the other hand, the establishment of young seedlings is much faster than that of ordinary seedlings. Until recently, maximum yield has been the main target of rice production, based on good land preparation, good fertilizer and water management, and pest management. There is now a widespread feeling that maximum input technology should be replaced by low-input methods. Direct seeding is receiving a great deal of attention as an alternative planting method.

Any reduction of inputs used in cultural practices can be expected to promote weed growth, so that weeds are likely to become a more important problem in low-input technology. Furthermore, any implementation of direct seeding will give rise to important problems such as lodging and poor crop stands, as well as weeds. In direct seeded rice, weeds usually emerge ahead of the rice, absorbing the available nutrients earlier so that they flourish and show rapid growth. Any small advantage of one species during seedling establishment will have great impact on this species in later competition with others. It is evident that a change in planting methods from transplanting to direct seeding will result in a shift of weed dominance from a complex of perennial and annual weeds to annual grasses.

It is recommended that if dry-seeded rice is being cultivated, the field should be maintained in a relatively dry condition for 30 days after seeding, followed by irrigation as for transplanted rice culture. 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 (Kim 1991). Applications of systematic herbicides using pre-emergence types followed by post-emergence ones gives excellent weed control. A tank mixture of propanil with pre-emergence type annual herbicides can give satisfactory control of E. crus-galli. This can be used as a basis for developing appropriate IWM for dry-seeded rice.

Integrated Management of Paddy Weeds

The goal of weed management should not be to eradicate or exterminate, but rather to prevent weed damage from exceeding the economic injury threshold by taking advantage of a range of weed control methods. IWM is not only effective, but takes into consideration protection of the environment and food safety.

There is an increasing desire on the part of many farmers to adopt management practices such as IWM to enhance profitability, but the complexity of IWM technology has seriously hindered its widespread adoption. For instance, farmers have difficulty in understanding the economic threshold for a given weed, and weed response to different control methods under various cultural and climatic conditions. Therefore, many farmers have become accustomed to using herbicides as their primary defense against potential weed damage (Beyer 1991).

IWM techniques should follow a number of guiding principles.

• Crops should be matched to local growing 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 methods 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 in autumn, followed by the application of 3 mt/ha of barley straw before transplanting, reduced the population of P. distinctus by 40%, compared to the untreated control. This indicates that cultural practices such as autumn plowing and mulching with barley straw can play an important role in reducing the population of specific weeds, so that less herbicide is required to control them.

Fewer weeds were observed in double-cropped paddy fields where rice was followed by winter crops such as barley, rape, garlic or onion. However, the area of double cropping has greatly decreased in recent years, and most rice crops in Korea are grown in a system of monocropping. Crop rotation, whereby rice fields are converted to upland crops for a cropping season or a year, is now under study, and competitive crops are being sought which match international market prices. Cultivation of soybean on a yearly rotation in paddy fields markedly reduced the emergence of aquatic weeds and greatly reduced weed problems in the subsequent rice crop. This type of annual rotation of rice with soybean hindered the establishment of stable populations of either paddy weeds or upland weeds. Furthermore, direct seeding of rice can probably be implemented with minimum tillage after soybean culture. Thus crop rotation, minimum tillage, and use of straw mulch may all be important components of IWM. Application rates of herbicides can be greatly reduced if all other available methods are put to proper use. There is still a great deal of room for reducing the cost of herbicides if farmers choose the right herbicide and apply it at the proper time. Improvements in application methods and equipment, and the modification of formulation types into e.g. slow-release and liquid forms, could also reduce the cost and increase the efficiency of herbicides.

Cost saving and reduced environmental impact are the two most important aspects to be considered in developing labor-saving technology. Reduction of labor costs is badly needed for Korean agriculture to be able to survive after the agricultural commodity market is liberalized and Korea becomes open to imports. The production cost of 1 kg milled rice is about US$0.86 in Korea, but less than one third of this (about $0.24) in the USA. Furthermore, labor requirements for rice production per hectare are 36.5 times higher in Korea than in the USA (Kim 1992). IWM will be an important component of cost-saving technology.

In order to implement an IWM system, basic information should be put on a computerized data base, including a thorough knowledge of the biological characteristics of major weeds, local climatic information, weed population dynamics, and economic threshold concepts for a mixed or single-species weed population. Modelling of competition and population dynamics is necessary to develop an economic threshold level. This information can then be used as a basis to help make the development of IWM a reality.

Fig. 3 shows the components which could be included in a package of IWM under Korean conditions. In presents schematically some of the ways in which, cultural and biological methods can be integrated with appropriate herbicides to achieve superior control. It is almost certain that IWM will be the most profitable strategy, but its main limitation is its relative complexity compared with herbicide use. Questions arise of how we can simplify the complexity of IWM technology. This will be an important task for weed scientists in the years to come.

References

• Back, K.W. and K.U. Kim. 1988. Identification of phytotoxic compounds and allelopathic effects of various upland weeds. Korean Journal of Weed Science 8,3: 283-290.
• Barnes, J.P. and A.R. Putnam. 1983. Rye residues contribute to weed suppression in no-tillage cropping systems. Journal of Chemical Ecology 9: 1045-1057.
• Beart, J.E., T.H. Lilley and E. Haslami. 1985. Plant polyphenol_secondary metabolism and chemical defense: Some observations. Phytochemistry 24,1: 33-38.
• Beyer, E.M. Jr.. 1991. Crop Protection: Meeting The Challenge. Brighton Crop Protection Conference-Weeds 1: 3-22.
• Chun, J.C., K.W. Han, B.C. Jang and H.S. Shin. 1988. Determination of phenolic compounds responsible for allelopathy in upland weeds. Korean Journal of Weed Science 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 of flue-cured tobacco. Can. Jour. Plant Science 62,2: 489-496.
• Einhellig, F.A. and G.A. Leather. 1988. Potentials for exploiting allelopathy to enhance crop production. Journal of Chemical Ecology 14: 1829-1844.
• Hong, Y.K., J.C. Kim, K.L. Ryu, and S.C. Kim. 1991. Research Report of Youngnam Crop Experiment Station, Rural Development Administration, Milyang, Korea pp. 938-946.
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• Kim, K.U. and H.O. Choi. 1976. Perennial weed control in paddy rice fields. (1) Effect of various plowing depths in autumn on perennial weed control. Korean Journal of Crop Science 21,1: 20-23.
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• Kim, K.U., I.J. Lee, H.J. Jeong and D.S. Kim. 1987. Potential allelopathic substances identified from annual crop straws. Proceedings of 11th Asian-Pacific Weed Science 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 the possible reservoirs for rice bacterial leaf blight. Korean Journal of Crop Science 26,1: 40-44.
• Kim, K.U. 1990. Progress of weed control methods and their prospects in Korea. Proceedings of the 8th Sistership Meeting of Korea, Japan and Republic of China's Agrochemical Association, held in Seoul, Korea, 19th October 1990, pp. 1-30.
• Kim, K.U., K.W. Back and I.J. Lee. 1990. Development of naturally active compounds from noncultivated plants. Korean Journal of Weed Science 10,1: 22-29.
• Kim, K.U.. 1991. Weed population shift in rice culture and development of cost-effective weed control method. Symposium on 21st Crop Protection Strategies and Direction of Agrochemical Development, held at Rural Development Administration (RDA). RDA Symposium 16, pp. 30-42.
• Kim, K.U.. 1992. Weed management in Korea: Present status and prospect. Proceedings of the symposium on Weed Management in Asia and the Pacific Region. Research Bulletin of Institute of Agricultural Science and Technology, Kyungpook National University, Special Supplement 7: 59-71.
• Klassen, P.. 1990. New developments in pest control strategies. Farm Chemical International 4,5: 34-41.
• Korea Research Co.. 1990. Most important weeds in paddy fields in Korea (personal communication).
• Kwon, S.T. and K.U. Kim. 1985. Effect of phenolic compounds identified from crop residues (wheat, rye) on the germination and growth of various weeds. Korean Journal of Weed Science 5,2: 121-130.
• Li, Y.H.. 1992. Present status of weed management in China. Proceedings of the Symposium on Weed Management in Asia and the Pacific Region. Research Bulletin of Institute of Agricultural Science and Technology, Kyungpook National University, Special Supplement 7: 33-44.
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• Overland, L.. 1966. The role of allelopathic substances in the smother crop barley. American Journal of Botany 53: 423-432.
• Park, J.S. and K.U. Kim. 1988. Identification of physiologically active compounds from Purslane (Portulaca oleracea L.). Korean Journal of Weed Science 8,2: 169-175.
• Purvis, C.E. 1990. Allelopathy: a new direction in weed control. Plant Protection Quarterly 5: 55-59.
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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.

Index of Images

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  Table 2 Similarity Coefficient in Terms of Changes in Dominant Weed Species Overtime

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  Figure 2 Composition and DRY Weight of Weeds in DRY-Seeded Rice

  Source:Yeun<I>etal.</I>1991
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  Figure 3 Proposed Components of Iwm in Korea

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  Table 1 Changes in Dominant Weed Species in Paddy Fields in Korea, 1965-1990

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  Table 3 Effect on Various Plant Species at Different Growth Stages of the Pin Inoculation of Bacterial Leaf Blight (Group I)

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  Table 4 Effect of <I>Epicoccosorus Nematosporus</I> on <I>Eleocharis Kuroguwai</I><Sup>1)</Sup>

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  Figure 1 Comparison of Efficacy of a Plant Pathogen, <I>Epicoccosorus Nematosporus</I>, and Bentazon for the Control of <I>Eleocharis Kuroguwai</I>

  Source:Hong<I>etal.</I>1991
• 

  Table 5 Effect of Repeated Applications of <I>Epicoccosorus Nematosporus

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  Table 6 Host Range of Pathogenic Fungus, <I>Alternaria</I> SP., Collected from Gehwado in Western Korea

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  Table 7 Effect of <I>Alternaria</I> SP., Collected from Gehwado (Western Korea) on Disease Development<Sup>1)</Sup>

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  Table 8 Effect of Alcohol Extracts<Sup>1) </Sup>from Crop Straw on Germination of Test Plants (Shown As % Germination)

  Source:Kim<I>etal.</I>1987<LI>Source:Kim<I>etal.</I>1987<LI>Source:AgriculturalChemicalIndustrialAssociation
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  Table 9 Constitution of Phenolic Compounds Identified from Different Types of Crop Straw

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  Table 10 The Major Types of Chemical Herbicide Being Used in Paddy Fields in Korea, 1981-90

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