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visible effects. These methods are supplemented by cultural and ecological practices suitable to conditions in a particular area that indirectly suppress weed growth. The effects of indirect methods are not immediate and sometimes are considered not necessary if direct methods are adequate. Over time, however, indirect methods help reduce the need for direct control inputs. This impact on weed control systems should be considered in long-range weed management strategies. By combining methods, many rice growers knowingly or unknowingly practice a type of IWM. More widespread, deliberate adoption of IWM by Asian farmers is constrained by a lack of basic knowledge and understanding of IWM principles and how to integrate IWM into crop protection and crop production systems. Among the direct weed control methods used, the proportion that is cultural or chemical is dictated primarily by economic factors, from 100% hand or rotary weeding in areas with relatively inexpensive labor, such as in South and Southeast Asia, to 90% or higher use of herbicides where labor is expensive, as is practiced in the industrialized countries of East Asia. In highly industrialized countries where labor costs are very high, many farmers prefer one-time herbicide applications to save costs. Herbicides are used to control rice weeds in virtually all of Asia. The first group of herbicides (the phenoxys), used since 1950 for postemergence broadleaf and sedge control, are still popular. The second group (chloroacetamides, thiocarbamates), introduced in 1960 for preemergence grass control, continue to be widely used. Their specificity with regard to time of application (pre- or postemergence) and to type of weed controlled (annual grass or annual broadleaf/sedge) resulted in two significant developments, an increase in perennial weed populations and the need to apply the herbicides two to three times within a season to control all three types of weeds. Recent development of new herbicides or mixtures of old and new herbicides that provide wider application windows and efficacy against broader weed spectra are helping to control all types of weeds in a one-time application, saving on both herbicide and labor costs. These new compounds also have new chemistries, new modes of action, and much lower application rates than older compounds. How these new compounds and related practices will impact weed species shifts and the various components of rice weed control systems is unknown. Biological control agents that have been identified as having potential for possible widespread use need to be studied for their selectivity in all types of rice culture, in particular in direct-seeded rice. Deeper understanding is needed of both the biology of the control agent and that of the host weed, and of the relationship between the two. Although initial results indicate the potential of biocontrol methods as another direct control option, only a few weed species are involved, making biocontrol not an alternative, but a supplement to other methods. There is also a need to identify formulation and application techniques that are efficient and practical for small farms as well as commercial farms. Among indirect weed control methods, water management remains a critical factor, particularly in tropical Asia because of poor water control and highly unpredict- Integrated weed management in rice in Asia 159
able rainfall, with the resulting extreme moisture fluctuations, from drought to flood within a single cropping season. Soil moisture has been identified in most studies as the single most critical factor affecting weed control, even surpassing the effects of fertilizer. In rainfed areas, which make up a large part of the rice areas of Asia, rainfall dictates when to plant rice and affects not only rice and weed growth but also factors related to weed control, notably herbicide efficacy. Because herbicides require optimum moisture for optimum efficacy, too little moisture during a drought will result in poor weed control, and too much moisture during a flood will injure rice (De Datta 1980, Bernasor and De Datta 1983). This is significant in the growing trend toward adoption of wet-seeded rice, where crop stand and herbicide injury are major problems, in particular when grass herbicides are used (Moody 1992). Studying the relationship of moisture and herbicide activity may help provide answers on how to improve weed control during drought and how to avoid rice injury during floods. The use of weed-competitive rice cultivars has possibilities as an indirect control alternative, should there be an opportunity to breed for this trait. A major issue that needs to be resolved is how to combine contrasting traits, those that confer high yielding ability with those that confer competitiveness, or whether this is even possible. Some morphophysiological traits of weeds that make them more aggressive competitors than rice have been identified that could serve as bases for considering traits to be incorporated into a competitive, high-yielding cultivar. Related to the development of competitive cultivars is the development of allelopathic cultivars. Allelopathic cultivars are being identified and factors related to their potential use in Asia are being studied. Identification of allelochemicals, either for use in developing botanical herbicides or for transfer of allelopathic traits into commercially acceptable cultivars (if allelopathy is genetically determined), is another option being explored. Development of transgenic, herbicide-resistant rice cultivars is another indirect control option. Two important questions need to be considered: 1) will herbicideresistant cultivars encourage increased herbicide use, and 2) will herbicide-resistant cultivars result in the development of resistant weeds This new technology should be carefully studied in terms of both its feasibility and its short- and long-term impact on socioeconomic, environmental, and health-related issues in Asian rice production systems. Integrated weed management elements should take into consideration all other practices in rice and rice-based production systems, if adoption of new technology is to be successful. While basic and strategic research are important, research that builds on farmer knowledge and practices is equally important. It is the partnership of science and rice farming that will facilitate the development of relevant integrated pest management (IPM) practices that include weed management strategies. 160 De Datta and Baltazar
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DEDICATION To Keith Moody WEED SCIE
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The International Rice Research Ins
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Bioherbicides and weed management i
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ful insects caused adverse response
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The need to raise yields and mainta
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ACKNOWLEDGMENTS Any volume that dra
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Overview
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as more lucrative, full-time job op
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Table 3. Herbicide use in rice prod
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only 3.3:1. The benefit-cost ratio
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EFFECTS ON SOCIETY AND ECOSYSTEMS S
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mulate in soils and in water tables
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unds from the field into canals and
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tion. Even if herbicide mixtures ar
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plant pathogens for controlling gra
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spectrum herbicides glufosinate-amm
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De Datta SK. 1981. Principles and p
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Li KM, Li PZ. 1996. Effect of herbi
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Vongsaroj P. 1994. Weed control in
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i ce env ironments and how they res
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Soil fertility. Monochoria vaginali
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Such problems could be avoided by a
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Ho NK, Md Zuki I. 1988. Weed popula
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HERBICIDES IN UNITED STATES RICE PR
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LOSSES TO WEEDS IN U.S. RICE US. ri
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propanil, which remains the backbon
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4. U.S. rice yields and adoption of
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6. Use of herbicides to control bro
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8. Thiobencarb and molinate transpo
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oping programs that curtail negativ
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Dunshee CF, Jones JW. 1924. Results
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Impacts of herbicides
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Pesticide users The pesticide user
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Table 3. Frequency of pesticide app
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Poly- Multiple Eye Pulmonary neurop
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(weight/height) had the expected ne
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Table 5. Anticipated health costs o
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When no consideration is given to h
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MAFF. 1991. Pesticides approved und
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and analyzed. We used this data bas
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soluble herbicide is distributed in
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of the rice herbicides thiobencarb
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germination because they compete fo
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in N 2 fixation and nitrate reducti
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Table 3. Effects of pesticides on n
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transformations in soils are studie
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Floodwater invertebrates Applicatio
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tally prevented photosynthetic acti
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CITED REFERENCES Almazan LL, Robles
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Mandal BB, Bandyopadhyay P, Bandyop
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Smith RJ, Flinchum WT, Seaman DE. 1
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IMPACT OF HERBICIDE USE ON THE ENVI
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Factors that reduce toxic risk Phys
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Table 3. Persistence in soil of ric
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One infamous herbicide spill occurr
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investigated because of the large,
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Given the current state of knowledg
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Way JM, Newman JF, Moore NW, Knaggs
Page 125 and 126: powder)—one-third the total amoun
Page 127 and 128: tants, however—such as cyanide an
Page 129 and 130: In both 1987 and 1988, 42 t of copp
Page 131 and 132: compounds such as the germicide Aso
Page 133 and 134: otation of one wet season, one dry
Page 136 and 137: ECOLOGICAL FORCES INFLUENCING CROP-
Page 138 and 139: may also vary with the resource use
Page 140 and 141: crease competitive ability into cro
Page 142: Tilman D. 1988. Plant strategies an
Page 145 and 146: Table 1. Important rice weeds in te
Page 147 and 148: 2. Costs of production of rice and
Page 149 and 150: covered only about 7,500 ha in 1993
Page 151 and 152: Table 5. Herbicide-treated area in
Page 153 and 154: Table 6. Dominant weed species in K
Page 155 and 156: cides in the mix. Most mixtures, wh
Page 157 and 158: Kim KU, Shin DH. 1993. Development
Page 160 and 161: INTEGRATED WEED MANAGEMENT IN RICE
Page 162 and 163: after crop establishment) mostly co
Page 164 and 165: Table 2. Most common rice herbicide
Page 166 and 167: Table 4. Examples of rice herbicide
Page 168 and 169: portant where direct method options
Page 170 and 171: too wet. Optimum moisture is needed
Page 172 and 173: For direct-seeded rice, the availab
Page 176 and 177: CITED REFERENCES Ampong-Nyarko K, D
Page 178 and 179: Jikihara K, Kimura I. 1977. Benthio
Page 180: Watson AK. 1992. Current status of
Page 183 and 184: 1. Framework for planning and imple
Page 185 and 186: Baseline data The problems associat
Page 187: 3. Filling vacant areas in the fiel
Page 190 and 191: emained the dominant weed, their in
Page 192 and 193: 6. Cultural practices in the Muda a
Page 194 and 195: LESSONS LEARNED FROM THE PROGRAM St
Page 196: Ho NK, Md Zuki I, Asna BO. 1990. Th
Page 199 and 200: Using mycoherbicides involves apply
Page 201 and 202: Community education also is importa
Page 203 and 204: the crop of interest. This should i
Page 205 and 206: asically biological, technological,
Page 207 and 208: cause they are likely to be applied
Page 209 and 210: Similarly, the risks of bioherbicid
Page 211 and 212: Baki MM, Azmi M. 1992. Integrated m
Page 213 and 214: Matthews GA, Bateman RP. 1990. Appl
Page 215 and 216: Watson AK. 1985. Host specificity o
Page 217 and 218: vars as it was with very short-dura
Page 219 and 220: While it may not be possible to ide
Page 221 and 222: CITED REFERENCES Ahmad S, Majid A,
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USING BIOTECHNOLOGY IN GENETIC IMPR
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Molecular markers are being used ro
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use of herbicides as part of a rice
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A final strategy would be to use bi
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Rathore KS, Chowdhury VK, Hodges TK
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Putnam and Duke (1974) postulated t
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Visual ratings were made 7 wk after
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nels; and most of the genotypes tha
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Leaf Root Leaf Root Leaf Root Leaf
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One hypothesis is that rice plants
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Use of herbicides in Asian rice
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icides such as 2,4-D and butachlor
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lems, it has resulted in quantitati
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Reasons for herbicide selection Far
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Carey VP III, Talbert RE, Baltazar
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EXPERIENCE WITH RICE HERBICIDES IN
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2. Time spent weeding Japanese rice
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3. Area of Japanese ricefields rece
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4. Sequence of herbicide applicatio
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Table 6. Important weed species in
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CURRENT PROBLEMS IN WEED MANAGEMENT
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DEVELOPING A WEED MANAGEMENT STRATE
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(Timmer et al 1983, World Bank 1993
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gets of plant growth regulators and
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in their role of implementing agric
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clearly needed if farmers are to av
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Kuhrt WJ. 1992. The California grap
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Participants Dr. Heidi Albers Food
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Dr. Gurdev S. Khush International R
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