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Crop Protection 48 (2013) 51e56

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Crop Protection

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Review

Strategies to manage weedy rice in Asia

Bhagirath Singh Chauhan*

International Rice Research Institute, Los Baños, Philippines

a r t i c l e i n f o

Article history:Received 6 November 2012Received in revised form14 February 2013Accepted 16 February 2013

Keywords:Land preparationSeeding rateCompetitive cultivarEstablishment method

* Tel.: þ63 (0)9282749987.E-mail address: b.chauhan@irri.org.

0261-2194/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.cropro.2013.02.015

a b s t r a c t

Weedy rice, an emerging problem in Asia, increases production costs and reduces farmers’ incomethrough yield reduction and through lowered rice value at harvest. Rice farmers in many Asian countriesare shifting from transplanting to direct seeding; however, due to physical and physiological similaritiesof weedy rice to cultivated rice and the absence of standing water at the time of crop emergence,adoption of direct-seeded rice systems makes weedy rice infestation one of the most serious problems.Selective herbicides to control weedy rice in conventional rice cultivars are not available and thereforemanaging weedy rice is a challenging and increasing problem for farmers in Asia. In the absence ofselective herbicides, various cultural weed management strategies may help reduce the problem ofweedy rice. These strategies may include the use of clean seeds and machinery, use of stale seedbedpractice, thorough land preparation, rotation of different rice establishment methods, use of high seedingrate and row-seeded crop, use of purple-coloured cultivars, use of flooding, and adoption of croprotation.

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1. Introduction

Rice is a principal source of food for more than half of the worldpopulation. It is grown on approximately 153 million ha (Mha)globally, of which 90% of the area is in Asia. Rice is traditionallygrown in Asia by manual transplanting of seedlings into puddledsoil (soil is puddled after intensive cultivation inwet conditions). Inthe recent years, there have been concerns of labour and watershortages in many areas in Asia. Rural labour is migrating to thecities and labour costs have thus increased rapidly. It is also hard tofind labour at the time of rice transplanting (Mahajan et al., 2011,2013). By 2025, a significant amount of rice area may suffer from“physical” and “economic” water scarcity (Tuong and Bouman,2003). Therefore, farmers in Asia are slowly moving towardsdirect seeding of rice in response to increased costs or decreasedavailability of water and/or labour (Chauhan, 2012a; Mahajanet al., 2012).

Although there are several advantages of direct-seeded rice,weeds are the major constraints to direct-seeded rice productionbecause of the absence of the suppressive effect of standing wateron weed growth at crop emergence and the absence of a size dif-ferential between the crop and the weeds (Chauhan and Johnson,2010a). In general, two major weed complexes have been

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recognized to be of particular concern in rice: Echinochloa speciesand weedy rice (Labrada, 1997).

Weedy rice is defined as a weed in and around arable area (Suh,2008). In general term, weedy rice is unwanted plants of the genusOryza that infest and compete with rice and other crops and pro-duces grains with a distinctly red or rough pericarp (Delouche et al.,2007; Suh, 2008). In specific term, weedy rice is unwanted plants ofOryza sativa growing together with cultivated rice. Some sugges-tions were proposed about the origins of weedy rice (Kane andBaack, 2007): weedy rice may have evolved from wild rice; weedyrice may have originated from escaped domesticated rice seeds,which then evolved weedy traits; or weedy rice hybrids may haveevolved through interbreeding between cultivated and wild rice. Inthe United States, weedy rice (called red rice) has been regarded aserious problem for many years (Goss and Brown, 1939). In Asia,however, weedy rice is an emerging problem. Its infestation wasfirst reported in Malaysia in 1988, in the Philippines in 1990, and inVietnam in 1994 (Mortimer et al., 2000; Second, 1991; Wahab andSuhaimi, 1991). Weedy rice increases production costs and re-duces farmers’ income quantitatively through yield reduction andqualitatively through lower rice value at harvest (Mortimer et al.,2000). In Asia, rice yield losses due to weedy rice infestation werereported to be from 16% to 74% (Azmi et al., 1994; Chin, 2001). InMalaysia, infestations by about 35weedy rice paniclesm�2 caused ayield loss of about 1 t ha�1 (Azmi et al., 2005a). In the United States,1e3 plants m�2 of weedy rice were the threshold infestation toprevent yield losses of rice (Smith, 1988).

B.S. Chauhan / Crop Protection 48 (2013) 51e5652

An important characteristic of weedy rice is early shattering ofthe grain; however, shattering varies among different biotypes oraccessions (Azmi and Karim, 2008). Weedy rice has variable seeddormancy and it can remain dormant in the soil for several years. Inthe United States, weedy rice was found to have a greater responseto higher nitrogen rates compared with cultivated rice (Burgoset al., 2006). Recently, in Asia too, some weedy rice accessionshave been found to have greater nitrogen-use efficiency for shootbiomass than cultivated rice (Chauhan and Johnson, 2011b). Inanother study, compared with cultivated rice, weedy rice respon-ded more strongly to rising CO2 level with greater competitiveability, suggesting that weedy rice may become amore problematicweed in the future (Ziska et al., 2010).

Recently, weedy rice has been increasingly reported as one ofthe major weed problems in many Asian countries, includingMalaysia, Thailand, India, Republic of Korea, Philippines, Vietnam,and Sri Lanka (Delouche et al., 2007). The increase in weedy riceinfestation in Asia is closely associated with the increase in areaunder direct seeding. The growing trend towards the adoption ofdirect-seeded rice in Asia is extending the areas infested withweedy rice and increasing the severity of the infestations (Deloucheet al., 2007). In Malaysia, the widespread occurrence of weedy ricehas been favoured by the practice of implementing a direct seedingrice culture, the use of easy shattering cultivars, and the use ofcombine harvesters (Suh, 2008). Factors that determine the popu-lation growth rate of weedy rice populations are (Mortimer et al.,2000): weedy rice seed remain dormant in the soil over longtime, weedy rice seed spread through crop seed contamination, andweedy rice seed from plants in the previous rice crop. In Vietnam,farmers and specialists emphasized the importance of contami-nated rice seeds as a source of weedy rice infestations (Deloucheet al., 2007). In a survey in Vietnam, 37% of the farmers opinedthat weedy rice evolve from cultivated rice, 32% believed that itemerge from the soil seed bank, and 14% of the farmers thoughtthat weedy rice is introduced as contaminants in rice seed.

As mentioned earlier, farmers in many Asian countries areshifting from transplanting to direct seeding. However, due tophysical and physiological similarities of weedy rice to cultivatedrice and the absence of standing water at the time of crop emer-gence, adoption of direct-seeded rice systems makes weedy riceinfestation one of the most serious problems that farmersencounter (Azmi and Karim, 2008; Chauhan, 2012a,b; Chauhan andJohnson, 2010b). In some countries where direct seeding is alreadya common practice, farmers are reverting to mechanized trans-planting to manage weedy rice (Chauhan, 2012a).

Selective herbicides to control weedy rice in conventional ricecultivars are not available and therefore managing weedy rice is achallenging and increasing problem for farmers in Asia. In theabsence of selective herbicides, various cultural weedmanagementstrategies may help reduce the problem of weedy rice. This articlereviews potential strategies to manage weedy rice in Asia.

2. Weed management strategies

2.1. Preventive measures

The first and the most important step in reducing weedy riceinfestation is the use of clean rice seeds. The spread of weedy ricefrom an infested area to a clean area has occurred inmany countriesthrough the distribution of contaminated rice seeds to farmers(Azmi and Karim, 2008). Contamination with a small fraction ofweedy rice seeds may result in a heavy weedy rice infestationwithin a year in riceerice and riceericeerice cropping systems.Therefore, to obtain clean seeds, it is important that weedy riceplants in the field are not allowed to produce seeds. In a survey in

Vietnam, 40% of the collected rice seed samples were contaminatedwith weedy rice seed and these occurred in all the provinces (18)surveyed (Mai et al., 2000). In Sri Lanka, farmers who opted touproot weedy rice plants at heading sometimes throw the seed-bearing plants into canals and unknowingly helped in spreadingthe weedy rice seeds to adjacent fields (Abeysekara et al., 2010).

Farmers should inspect their fields regularly and must rogueweedy rice plants whenever these appear. For effective field in-spections, farmers should be familiar with the characteristics of therice variety planted and be able to distinguish them from the off-type and weedy rice accessions (Delouche et al., 2007). Therefore,there is a need to increase awareness of weedy rice among farmers.The machines used for land preparation, sowing, harvesting, andthreshing should be cleaned before moving from one field toanother. In addition, field margins and irrigation canals should befree from weeds, including weedy and wild rice.

2.2. Land preparation

Straw and stubble burning is considered an effective strategy ineradicating weedy rice seeds present on the soil surface (Azmi andKarim, 2008). However, this practice has been banned in manycountries because of smoke pollution. Similar to that of other weedspecies, the seed bank of weedy rice can be reduced using the staleseedbed practice (Chauhan, 2012a). After a light shower or irriga-tion, weedy rice and other weed seedlings are allowed to germinateand then killed with tillage or non-selective herbicides. The use ofstale seedbed practice has been reported to reduce the number ofweedy rice plants in the crop (Delouche et al., 2007). However, theefficacy of this practice will depend on the degree of dormancy inthe weedy rice seeds, as dormant seeds will not germinate.Although stale seedbed practice is useful in reducing weedy riceseed bank, the practical possibility of this practice needs to beevaluated by farmers themselves when the period between theharvesting of the preceding crop and sowing of the next crop isshort and when crop intensification is the main aim of the farming(Chauhan, 2012a). In some cases, there may be a yield penalty ifcrop planting is delayed by the preplanting operations (Chauhan,2012a; Liebman et al., 2001).

Thorough land preparation is done for seedbed preparation andthis has been used to control weeds ever since the origin of agri-culture. Repeated cultivation is effective in reducing the weedy riceseed bank and seedlings in rice fields. Weedy rice seedlings thatemerge after the first cultivation are killed by the subsequentcultivation operations. In different direct seeding systems, tillage isperformed in dry or wet soil conditions. In aerobic and dry-seededrice systems, soil is cultivated in dry conditions, whereas in wet-seeded rice, soil is cultivated in wet conditions (the process iscalled puddling). These tillage operations could be used to buryweedy rice seeds below the maximum depth of their emergence. Inthe United States, weedy rice ecotypes emerged from buried depthsof 7.5 cm (Gealy et al., 2000). In a recent study in the Philippines,three of four weedy rice accessions could not emerge from a burialdepth of 8 cm (Chauhan, 2012b). These results suggest that emer-gence of weedy rice could be suppressed by deep tillage that buriesseeds below 8 cm. However, subsequent tillage operations in thenext few seasons should be shallow to avoid bringing back theburied seeds on the soil surface. In earlier studies, weedy rice inVietnam showed a dormancy of 20e65 d (Thanh et al., 1999) andweedy rice in Italy showed a dormancy of 170 d (Vidotto andFerrero, 2000). In Asia, dormancy in weedy rice accessions hasbeen reported to be between 0 and 300 days (Suh, 2008), sug-gesting variable degrees of dormancy in different weedy rice ac-cessions. There is a need to study the weedy rice seed longevity fordifferent accessions occurring in different rice-based cropping

B.S. Chauhan / Crop Protection 48 (2013) 51e56 53

systems (riceerice, riceericeerice, riceewheat [Triticum aestivumL.], riceecorn [Zea mays L.], riceefallow, etc.) in Asia.

In addition, proper land levelling after tillage operations hasbeen found to result in better crop establishment, water savings,energy savings, greater nutrient-use efficiency, and improved weedcontrol (Chauhan, 2012a). Improved weed control in levelled fieldsis achieved mainly because of better crop establishment and watermanagement.

2.3. Rice establishment methods

As mentioned earlier, rice in many of the Asian countries isgrown using transplanted rice seedlings. However, due to labourand water shortages, farmers are moving towards direct seeding. Incountries such as Sri Lanka, Vietnam, andMalaysia, wet-seeded ricehas already been practiced for a long time. Now, farmers in thesecountries face the problem of weedy rice. In direct-seeded systems,it is hard to differentiate between weedy rice and cultivated rice atthe early stages because the rice seeds are broadcast andweedy riceand cultivated rice emerge at the same time. Reverting to trans-planting in weedy rice-infested areas may help reduce the weedyrice problem. Farmers cannot hire labour for transplanting in thesecountries because of unavailability of labourers and/or high labourwages. However, there is a scope for mechanized transplantingusing transplanters, which are now being commercialized in manyAsian countries.

In transplanted rice, there is the advantage of seedling size.Transplanted seedlings will be more competitive against the newlyemerged weed and weedy rice seedlings and it will be easy todistinguish cultivated rice seedlings from weedy rice seedlings. Inaddition, there is standing water in the field at the time of trans-planting and standing water/flooding is known to suppress weedyrice emergence (Azmi and Karim, 2008; Chauhan, 2012a,b; Chin,2001). In Malaysia and Vietnam, farmers have succeeded inreducing weedy rice in their field by introducing transplanted ricein one or two cropping seasons after a serious infestation of weedyrice in direct-seeded systems (Watanabe et al., 2000). A study wasconducted inMalaysia to determine the effectiveness of mechanicaltransplanting on weedy rice control (Azmi et al., 2005b). The studyreported a marked reduction in weedy rice infestation after twoconsecutive seasons of mechanical transplanting; weedy ricepanicle density was reduced from 58 to 20 panicles m�2. In Korea,more than 90% reduction in weedy rice plants was observed inmachine-transplanted rice over a 3-year study (Kim et al., 2000).Broadcasting of seedlings is also practiced in Vietnam and Sri Lankato manage weedy rice (Abeysekara et al., 2010; Luat, 2000); thispractice is called “parachute planting” in Sri Lanka. In this method,seedlings are grown in a nursery and two to three seedlings arebroadcast or thrown onto puddled soil. Water depth at the time ofbroadcasting is from 5 cm to 10 cm, considered sufficient to sup-press weedy rice (Azmi and Karim, 2008). A study in Malaysiafound that broadcasting seedlings was effective in reducing weedyrice infestation and that it was comparable with conventionalwater seeding and manual transplanting methods, resulting inbetter rice yields (Azmi and Karim, 2008).

In some parts of Vietnam, for example, in the Mekong Delta,farmers broadcast pre-germinated rice seeds in 10 to 50-cm-deepwater (Luat, 2000). This practice is called water-seeding, in whichweedy rice seeds flooded and buried in the soil are not able toemerge. This practice is not popular or practiced in many of theAsian countries because local varieties capable of germinating un-der anaerobic conditions are not available. Recently, water-seedingwas also tested in some parts of Sri Lanka to reduce weedy riceinfestation (Anurudhika Abeyesekara, personal communication).

2.4. Seeding rate and row seeding

Use of high seeding rates is known to suppress weeds in severalcrops, including rice (Chauhan et al., 2011; Zhao, 2006). Manyfarmers in many areas in Asia use high seeding rates to suppressweeds and to compensate for poor seed quality and crop emergenceand for losses due to birds, rodents, and snails (Chauhan, 2012a). In astudy inMalaysia, thegrainyieldof rice increasedwithan increase inseeding rate from 20 to 80 kg ha�1 in a direct-seeded field infestedwithweedy rice (Azmi et al., 2000). Recently, Azmi andKarim (2008)reported that seeding ratesgreater than150kgha�1wereadopted insomeweedy rice-infested areas inMalaysia to reduce theproblemofweedy rice. The authors reported that high seeding rates did notincrease rice yield, but farmers used them to suppress weed speciesand to ensure against uncertainty in crop establishment. In agreenhouse study, the shoot biomass offiveweedy rice accessions incompetitionwith60kgof IR72 seedsha�1was13e30% less than thatof weedy rice plants grown alone (Chauhan and Johnson, 2010b).The study suggested that increasing rice crop interference couldreduce weedy rice growth significantly. In a previous study in theUnited States, the aboveground biomass and grain yield of weedyrice decreased with increased in rice seeding rates from 50 to150 kg ha�1 (Estorninos et al., 2005). At the same time, the yield ofrice increased with the increased in crop seeding rates in theweedyrice infested area. There may be increased costs associated with theuse of high seeding rates. In someAsian countries, however, farmersare already using high seeding rates to suppress weeds and tocompensate for poor seed quality and crop establishment (Chauhan,2012a). High seeding rates are commonly used by farmers in LatinAmerica to suppress rice weeds (Fischer and Ramirez, 1993). Highseeding rates could improve the success of reduced herbicide dosesand reduce the perceived risks associated with the environmentaleffects of herbicides (Estorninos et al., 2005).

In many countries, especially those in Southeast Asia, direct-seeded rice is sown by broadcasting dry or pre-germinated seeds.In broadcast culture, however, it is difficult to control weedy rice asit is difficult to distinguish from cultivated rice until they reach theflowering stage. Therefore, row crops will have an advantage overbroadcast crops as weedy rice emerging between the rows can bedistinguished and pulled out (Chauhan, 2012a). Aerobic or dry-seeded rice crops can be sown using a seed-drill fitted with atwo- or four-wheel tractor. The area under these seeding systems isnow increasing in India, Bangladesh, and Pakistan. Wet-seeded ricecan be sown in rows by using a drum seeder. The wet-seeded ricesystem is common in Sri Lanka and Southeast Asia. In row crops,manual and mechanical weeding is possible. The rice crop grown inrows will also allow hand rouging of weedy rice to prevent seed set.Drum seeders have been introduced in Asia for a long time; how-ever, adoption is very low because farmers do not want to pull aseeder in puddled soil (personal observations). There is, therefore, aneed to invest in research and development on different seedersthat can be pulled by small two-wheel tractors in puddled soil. Asmany farmers in Asia are small landholders and as most of themcannot afford to buy seeders, there is a need to develop andstrengthen the “custom-hiring” system.

2.5. Weed-competitive and purple-coloured cultivars

The use of weed-competitive cultivars is the first requirement indeveloping weed management strategies in any crop or system.Crop cultivars having early vigour and quick canopy closure aremore competitive against weeds. Tall and traditional cultivars arefound to be more competitive, but they have lower yield potentialthan short-statured modern cultivars (Chauhan, 2012a). The trade-off between yield potential and competitiveness can be reduced by

B.S. Chauhan / Crop Protection 48 (2013) 51e5654

selecting traits other than plant height. Information on the effect ofweed-competitive cultivars on weedy rice is very limited in theliterature, especially in Asia. A study in Malaysia showed that IR64,which matured earlier than weedy rice, provided the highest grainyield under serious infestation by weedy rice (Azmi and Abdullah,1998). In addition, while weedy rice was still at the flowering stage,the early harvesting of IR64 also helped in reducing the weedy riceseed bank.

Cultivars with purple-coloured leavesmay help to reduceweedyrice seed banks. The use of such cultivars will be very helpful inbroadcast-rice culture, as weedy rice can be differentiated fromthese cultivars and pulled out. In eastern India, such cultivars (e.g.,P502) were grown to reduce the weedy rice problem (Tewari,2008). Similarly, farmers in the state of Himachal Pradesh, India,adopted the practice of growing a purple-leaf rice cultivar (R575) inweedy rice-infested fields (Kaushik et al., 2011). During hoeing andweeding operations, farmers removed all green-leaf plants fromtheir fields and thus eradicated weedy rice from their fields.Including cultivars with purple leaves in a season in double or triplerice cropping systems can substantially minimize the weedy riceseed bank. Farmers usually do not like to grow these cultivars intheir field because of the low yield, but this is a better option thanabandoning the field due to high infestation of weedy rice. Highyielding purple-stemmed cultivars with green foliage can also beused to get rid of weedy rice plants. There is a strong-felt need bythe farmers in eastern India for growing purple-base rice cultivarsin weedy rice infested areas (Tewari, 2008). Furthermore, effortscan be made in breeding to increase the yield potential of purple-coloured cultivars.

2.6. Water management

Water management plays an important role in weed manage-ment in any form of rice establishment system. Flooding isconsidered an important component of cultural weedmanagementin rice. However, it is optimal timing, duration, and depth offlooding that affect the growth of weeds, including weedy rice inrice fields (Chauhan, 2012a,b; Chauhan and Johnson, 2010a).Introduction of flooding at an early rather than at a later stage re-sults in less weed infestation (Chauhan and Johnson, 2010a, 2011a).In Vietnam, a flooding depth of 5 cm or more was able to suppressgermination and emergence ofweedy rice (Chin, 2001). InMalaysia,5e10-cm flooding depth was suggested to inhibit weedy riceemergence (Azmi and Karim, 2008). In a study in Italy, winterflooding between rice crops resulted in greater reduction in thenumber of weedy rice seeds on the soil surface as compared withfields left dry between rice crops (Fogliatto et al., 2010).

In a recent study in the Philippines, flooding depths from 0 to8 cm had no or very little effect on seedling emergence of fourweedy rice accessions when seeds were sown on the soil surface(Chauhan, 2012b). Flooding, on the other hand, decreased seedlingemergence in all weedy rice accessions when seeds were sown1 cm deep into the soil. In the same study, a flooding depth of 2 cmreduced seedling biomass of all weedy rice accessions by greaterthan 85%.

In a water-seeding system, there is flooding at the time of cropestablishment. In transplanted rice too, flooding can be introduceda few days after transplanting. In direct-seeded systems, however,flooding is carried out after the crop has emerged and, by that time,weedy rice seedlingsmay have also emerged andmay be difficult tomanage by flooding (Chauhan, 2012a,b). Rice cultivars capable ofemerging from the soil and floodwater are not common in Asia. Theavailability of rice cultivars capable of emerging from anaerobic soilconditions will prove useful in suppressing weedy rice emergenceand growth during crop emergence in direct-seeded systems

(Chauhan, 2012a,b). The use of such rice cultivars with shallowflooding will also help reduce herbicide use. Work on these culti-vars is in progress at the International Rice Research Institute,Philippines.

2.7. Herbicide

As weedy rice and cultivated rice belong to the same species,that is, similar in terms of anatomical and physiological traits, se-lective herbicides are not available to control weedy rice in con-ventional rice cultivars. However, herbicides can be applied beforeplanting the rice crop. In the stale seedbed technique, for example,non-selective herbicides can be applied after the emergence ofweedy rice seedlings. Pre-emergence herbicides can be appliedbefore rice sowing (pre-plant application); however, the methodand/or timing of herbicide application aremodified. InMalaysia, forexample, pretilachlor applied just before or after tillage under-standing water reduced the weedy rice seed bank (Azmi et al.,2004). In this case, standing water remained in the field for 2e4 d before it was drained; pre-germinated rice seeds were thenbroadcast. Recently, similar results were found in Sri Lanka whenpretilachlor was applied at least 2 d before crop sowing (Chauhanet al., unpublished data). In Thailand too, the application of pre-tilachlor by dripping the concentrate or slightly diluted productdirectly into the water during the last levelling offered an effectivecontrol of weedy rice (Allard et al., 2005). Some other herbicides,for example, oxadiazon and metolachlor may also provide effectivecontrol of weedy rice, but to avoid any phytotoxic effect on rice,these herbicides should be applied at least 15 d before rice planting(Eleftherohorinos and Dhima, 2002). There is a need to evaluate thepossibility of such control measures in intensive rice-based crop-ping systems, where the time interval between the two crops isshort.

Herbicide-resistant rice cultivars could be an option tomanageweedy rice inAsia. There are three kinds of herbicide resistantrice: imidazolinone-, glyphosate-, and glufosinate-resistant rice.Imidazolinone-resistant rice is non-transgenic as it was originated bynatural mutation. Imidazolinone herbicides control weeds by inhibit-ing the acetolactase synthase enzyme. In Asia, two imidazolinone-resistant rice (Clearfield) cultivars were released in Malaysia in 2010to reduce weedy rice infestation. In other Asian countries, herbicide-resistant rice has not been commercialized yet. Glyphosate- andglufosinate-resistant rice cultivars are transgenic innature.Glyphosateinhibits the enzyme 5-enolpyruvylshikimate phosphate (EPSP) syn-thase and glufosinate acts by inhibiting the essential ammoniaassimilation enzyme, glutamine synthetase (Suh, 2008).

The use of herbicide-resistant rice may benefit rice farmers byimproving weedy rice control and reducing weed control costs.Imidazolinone herbicides, for example, control a broad range ofweeds and they have a residual effect. Similarly, glyphosate andglufosinate can kill a broad range of weed flora, which wouldpermit less herbicide use in terms of amount and number of ap-plications (Olofsdotter et al., 2000). Both of these non-selectiveherbicides are applied as post-emergence and therefore, theirdoses can be adjusted according to the weed infestation and thesprayingwindowcan bewider (Olofsdotter et al., 2000). All of theseherbicides share some important characteristics such as broad-spectrum control of weeds, long-term weed control, flexibility incrop rotation, biodegradable in nature, and effectiveness at lowdoses, therefore reducing the total amount of herbicide released inthe environment. Such properties make these herbicides safer andenvironmentally friendly. Also, weeds resistant to currently usedherbicides could be controlled with these broad-spectrum herbi-cides (Olofsdotter et al., 2000). Although the transgenic cultivarsprovide an effective means of weed control, they have been dubbed

B.S. Chauhan / Crop Protection 48 (2013) 51e56 55

genetically modified. Because of bio-safety and regulatory con-cerns, herbicide-resistant transgenic rice cultivars have not yetbeen released for commercial cultivation.

Herbicide-resistant rice could be an effective tool to manageweedy rice in Asia; however, the use of herbicide-resistant ricecultivars may help in developing resistance in weedy rice throughgene flow, making weedy rice control even more difficult. Geneflow is the incorporation of a gene into the gene pool of one pop-ulation from other populations (Futuyma, 1998). The floral traits ofweedy rice aremore appropriate for outcross than that of cultivatedrice and thus, the gene of herbicide-resistant rice can be easilytransferred to weedy rice (Suh, 2008). There is a need to learn fromthe experience of other countries, where herbicide-resistant ricehas been cultivated for a long time. A study in the United Statesreported that gene flow between herbicide-resistant (Clearfield)rice and weedy rice was a problem and approximately170 resistant plants ha�1 of weedy rice resulted when outcrossingrates were very low (Shivrain et al., 2007). In another study, themaximum field outcrossing rate betweenweedy rice biotypes and arice cultivar (CL161) ranged from 0.03 to 0.25% (Shivrain et al.,2008).

Some strategies have been proposed to minimize gene flow(Suh, 2008): physical isolation of the herbicide-resistant crop,rotation of crops, and alter gene flow by interfering with pollinationand fertilization using maternal inheritance and male sterility. In arecent study, Gressel and Valverde (2009) proposed to use glyph-osate and glufosinate resistance separately as genes of choice, andglyphosate, glufosinate, and bentazon susceptibilities as mitigatinggenes, with a six-season rotation where each stage kills volunteerplants of transgenic crop from the previous season. New multiple-herbicide-resistant crops with resistance to other herbicides will bean important component of future weed management systems(Green, 2012).

Herbicide-resistant rice is a good tool to manage weedy rice andother weeds in the short term; however, once such a tool is intro-duced to farmers in Asia, it will be extremely hard for farmers to goback to their usual practice of rice cultivation (Chauhan, 2012a).There is a strong need to develop guidelines for the use ofherbicide-resistant rice cultivars. The herbicide-resistant riceshould not be grown for two growing seasons in a row in the samefield and this recommendation is probably the most important formaintaining the sustainability of this technology (Labrada, 2007).Any weedy rice plants that escaped should be pulled out immedi-ately before they shed seeds. If weedy rice has developed resis-tance, there is a need to develop a specific action plan such asrouging weedy rice plants or spot spraying with non-selectiveherbicides (Gealy et al., 2003). The authors also suggested that ifrouging cannot eliminate the resistant weedy rice population, theuse of herbicides with differentmodes of action in the next growingseasons can be helpful in minimizing selection of herbicide-resistant weedy rice plants. Crop rotation is an important man-agement practice that allows the use of chemicals that can controlweedy rice in non-rice crops. However, this option should befurther evaluated in Asian regions where farmers depend only onrice. In Asia, limited research has been conducted on weedy ricebiology and therefore, a detailed understanding of the impact ofimplementing herbicide-resistant rice is required along withknowledge on weedy rice biology (Olofsdotter et al., 2000).

2.8. Crop rotation

It is well known that continuous cultivation of a single crop orcrops having similar management practices allows certain weedspecies to become dominant in the cropping system. Growing adifferent crop in rotation allows different cultural practices, and

these practices help in disrupting the growth cycle of weeds. Croprotation is an effective control measure for weedy rice. One ricecrop, preferably in the dry season, in double or triple rice croppingsystems can be rotated with an upland crop such as corn andsoybean (Glycine max L.). The available moisture will encouragegermination of weedy rice and the emerged seedlings of weedy ricecan then be killed by using a combination of different herbicides(Azmi and Karim, 2008; Chin, 2001). This option is not possible in arice crop because of the non-availability of selective herbicides tocontrol weedy rice. Furthermore, glyphosate-resistant (RoundupReady) corn is already being used in some countries, for example,the Philippines. The use of glyphosate in these crops provideseffective control of weedy rice as well as other problematic weeds,such as purple nutsedge (Cyperus rotundus L.).

In Vietnam, introducing mungbean (Vigna radiata L.) in the dryseason resulted in a huge reduction inweedy rice plants (Watanabeet al., 2000). In the ongoing experiments in Vietnam, theweedy riceseed bank was greatly reduced in a triple rice cropping systemwhen dry-season rice was replaced by corn, sesame (Sesamumindicum L.), or mungbean (Chauhan et al.; unpublished data). Inthese crops, weedy rice and volunteer rice are easy to recognize andthey can be pulled out or killed by using herbicides. In Italy, in-clusion of soybean in rotation for a year led to a 97% reduction inweedy rice seed bank on the top 10-cm soil layer (Ferrero andVidotto, 1997). Suggestions are always given regarding the adop-tion of crop rotation on farmers’ fields; however, there are limita-tions, for example, availability of a suitable crop, drainage of the ricefields in some area, and the demand for rice for household con-sumption (Delouche et al., 2007). In addition, adoption of rotationalcrops will depend on their market prices.

3. Conclusions

In the absence of selective herbicides to control weedy rice inconventional rice cultivars, various strategies, such as the use ofclean seeds and machinery, use of stale seedbed practice, thoroughland preparation, rotation of different rice establishment methods,use of high seeding rates and row-seeded crops, use of purple-coloured cultivars, flooding, and crop rotation need to be inte-grated to achieve effective control of weedy rice. Herbicide-resistant rice could be an effective tool to manage weedy rice inAsia; however, the use of herbicide-resistant rice cultivars may helpin developing resistance in weedy rice through gene flow, makingweedy rice control even more difficult. There is a strong need todevelop guidelines for the use of herbicide-resistant rice cultivarsin Asia.

Acknowledgements

Some of the views in this article should be understood as thepersonal opinions of the author only; they do not reflect those ofthe International Rice Research Institute. The author is grateful toDr. Bill Hardy, International Rice Research Institute, Philippines, forediting the manuscript.

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