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Crop Protection 49 (2013) 52e57

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Crop Protection

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Review

The role of cultivars in managing weeds in dry-seeded rice productionsystems

Gulshan Mahajan a, Bhagirath Singh Chauhan b,*

a Punjab Agricultural University, Ludhiana, Punjab, Indiab International Rice Research Institute, Los Baños, Philippines

a r t i c l e i n f o

Article history:Received 1 October 2012Received in revised form7 March 2013Accepted 20 March 2013

Keywords:Weed-competitive cultivarHerbicide-resistant riceSeedling vigourAnaerobic conditions

* Corresponding author. Tel.: þ63 (0)9282749987.E-mail address: b.chauhan@irri.org (B.S. Chauhan)

0261-2194/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.cropro.2013.03.008

a b s t r a c t

Dry-seeded rice (DSR) is an emerging production system in Asia. However, weeds are a major biologicalconstraint in the success of DSR production. Although newly available herbicides may provide satis-factory weed control in DSR, an excessive use of herbicides may increase the risk of herbicide resistanceand shifts towards problematic weed species. Cultural management practices with the integrated use ofcultivars could be exploited to reduce selection pressure and delay herbicide resistance in weeds. Thelack of suitable weed-competitive cultivars, however, has been a major constraint in this direction andthere is a need to exploit the role of rice cultivars for weed management in DSR. The traits that are likelyto be most helpful for weed management in direct seeding include seed germination in anaerobicconditions and tolerance of early submergence for uniform crop establishment, high and early seedlingvigour with rapid leaf area development during the early vegetative stage for weed suppression, cultivarshaving an allelopathic effect, and herbicide-resistant rice cultivars.

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1. Introduction

Most of the rice in Asia is being grown by transplanting 25e30-day-old seedlings into puddled soil as puddled-transplantedsystems increase nutrient availability and weed suppression(Chauhan, 2012). But, now, as a result of the looming water crisisand shortage of labour during transplanting, farmers in Asia areconsidering the option of direct seeding (Pandey and Velasco,2005). In Asia, two types of direct seeding are mainly practiced:dry and wet seeding. Dry seeding consists of sowing dry seeds indry soils, while wet seeding involves sowing pre-germinated seedson the surface of wet-puddled soils. Dry seeding is probably theoldest method of rice establishment. Historical accounts of ricecultivation in Asia indicate that, during the early period ofdomestication, rice used to be dry-seeded (broadcast) in a mixturewith other crops (Grigg, 1974). In Asia, dry-seeded rice (DSR) isextensively practiced in the northwest Indo-Gangetic Plainsbecause DSR in this region provides the highest opportunity toattain optimal plant density and highwater and labour productivity(Chauhan et al., 2012b).

To date, few cultivars have been developed for DSR systems andfarmers are using cultivars that were developed for puddled-

.

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transplanted rice. As a result, with the existing cultivars, theremay be a yield penalty in DSR because of the trade-off betweenwater saving and grain yield. Breeding programs for DSR produc-tion systems in Asia therefore aim at a separate target environmentcompared with the traditional upland rice environments whereyields are substantially low due to the non-availability of water atcritical growth stages (Prasad, 2011). Since most upland rice israinfed, the emphasis in breeding programs has been on traits thatprotect the crop from drought. In traditional upland environments,the targeted cultivars are tall and they have low tillering ability andoften produce low but stable yields under low-fertility conditions.Such cultivars tend to have low harvest indices and they tend tolodge under high-fertility conditions (Atlin et al., 2006). In contrastto this, in DSR production environments, cultivars are targeted thatare semi-tall, have low tillering ability but high biomass at earlystages, have early canopy closure, and provide crop-weed compe-tition in favour of the crop (Rodenburg and Johnson, 2009). Thesecultivars must have tolerance for lodging under high-fertilityconditions.

Weeds are a major constraint in DSR cultivation and the successof DSR warrants the intensive use of herbicides. Herbicides havebeen proven effective in many cases, but intensive herbicide usecan cause environmental contamination and the development ofherbicide resistance in weeds (Heap, 2012). The increased use ofherbicides, risk of herbicide resistance, rising costs of production,and concerns about environmental pollution are creating an

G. Mahajan, B.S. Chauhan / Crop Protection 49 (2013) 52e57 53

interest among researchers in exploring non-chemical (cultural)methods of weed control (Chauhan, 2012).

Herbicide resistance in weeds is a major problem in herbicide-dominant systems. Although herbicide resistance is not prevalentin Asia, the abundance of propanil-resistant Echinochloa colona (L.)Link. populations in Central and South America and E. crus-galli (L.)Beauv. populations in the United States warrants a limited andjudicious use of herbicides for weed control. In Asia, weed speciessuch as Eleusine indicaGaertn. have also developed resistance to theinhibitors of acetyl CoA carboxylase (ACCase), while Monochoriavaginalis (Burm. f.) Kunth has evolved resistance to aceto-lactasesynthase (ALS) inhibitors (Heap, 2012). In the future, other weedspecies may also develop resistance, making weed control evenmore difficult.

Herbicide resistance has stimulated evaluations of culturalweed control tactics in integrated weed management programs(Chauhan and Johnson, 2010b). Reducing farmers’ dependence onherbicides is also desirable to reduce herbicide costs and selectionpressure and to delay the development of herbicide resistance inweeds. In herbicide-dominant systems, herbicide performance canbe improved when combined with crop species or cultivars of su-perior competitiveness (Lemerle et al., 1996). Variation amongcultivars in their ability to compete with weeds has been docu-mented for many crops, including rice (Gibson and Fischer, 2004;Zhao, 2006).

The role of competitive cultivars can be exploited further withagronomic manipulations such as altered plant spatial pattern andsowing time, and narrow crop rows, which might be helpful inproviding supplemental weed control when herbicide inputsdecrease (Mahajan and Chauhan, 2011). Therefore, the develop-ment of weed-competitive cultivars is useful for the managementof weeds in DSR and this could also prove to be a cost-effectivecomponent of an overall integrated weed management programin DSR.

2. The role of cultivars in managing weeds

2.1. Weed-competitive cultivars

Though the development of competitive rice cultivars wouldprovide a safe and environmentally benign tool for weed man-agement with less load of herbicides in the agroecosystem(Dingkuhn et al., 1999), it has not been addressed seriously by plantbreeders. Recently, researchers have paid more attention to the useof competitive cultivars for weed management in DSR as the use ofweed-competitive cultivars is a cost-effective method with mini-mum environmental contamination (Caton et al., 2003; Chauhan,2012; Mahajan and Chauhan, 2011). A competitive cultivar mighthelp in curtailing the dose of herbicides in DSR by suppressingweed emergence and growth. In wheat, for example, cultivar PBW343 provided greater competition to weeds than PDW 233 byhaving more tillers and thus helped in curtailing the dose of her-bicides in the wheat crop (Mahajan et al., 2004).

Using competitive cultivars to suppress weeds might substan-tially reduce selection pressure, herbicide use, and labour costs andpermit weeds to be controlled with a single application of herbicide(either pre- or post-emergence) in DSR. Now, for the managementof weeds in DSR, farmers in Asia use both pre- and post-emergenceherbicides. In addition, farmers may need to use one handweeding.Competitive cultivars may therefore be an important component ofintegrated weed management strategies (Mahajan et al., 2013).

Cultivar-weed competitiveness has two components: weedtolerance and weed-suppressive ability (Jannink et al., 2000; Zhao,2006). Weed tolerance is the ability of a crop to maintain high yielddespite weed competition, whereas weed-suppressive ability is the

ability to suppress the growth of weeds through competition. Bothcomponents are important since yield stability and the preventionof weed seed production and subsequent seed bank buildup aredesirable in crops growing in associationwithweeds (Jordan,1993).Cultivar differences in weed-suppressive ability are determinedby assessing variation in weed biomass in plots under weedcompetition. Jannink and colleagues suggested breeding for weed-suppressive ability over weed tolerance because suppressingweedsreduces weed seed production and benefits weed managementin the future, while tolerating weeds benefits only the currentgrowing season andmay result in an increased weed seed bank andweed pressure in subsequent seasons from unsuppressed weeds(Jannink et al., 2000). Cultivar differences in weed competitivenesshave been reported in many crops, including rice. It has beenobserved that early-maturing rice cultivars and rice hybrids have asmothering effect onweeds due to improved vigour and a tendencyof early canopy cover. However, little is known about the relativeimportance of shoot and root competition of hybrids and inbreds infield conditions in DSR systems and this may be the subject offuture research (Chauhan and Johnson, 2010a).

The importance of seedling vigour for optimum plant estab-lishment and increasing weed competitiveness has been evaluatedby many workers (Haefele et al., 2004; Zhang et al., 2004). Kanbaret al. (2006) reported that cultivars having high seedling vigoursuppressed weeds to a greater extent, in large rainfed and uplandtracts of tropics, where dry seeding is practiced. Seedling vigour isthe ability of a plant to emerge rapidly from soil or water(Heydecker, 1960). Seedling vigour could play a critical role in DSRas it helps in better plant establishment and offers successfulcompetition with weeds in favour of the crop. Vigour in rice alsoaffects many agronomic characters and indirectly influences grainyield by offering competition to weeds. Shoot length has beenfound to have a positive correlation with fresh and dry weights ofseedlings, and vigour index (Shashidhar,1990), therefore playing animportant role in the suppression of weeds.

Seedling shoot length, root length, mesocotyl length, primaryleaf length, and root-to-shoot ratio at a given growth stage havebeen used for characterizing weed-competitive rice cultivars(Shibuya and Oka, 1994; Sivasubramanian and Ramakrishnan,1978). When rice was grown together with E. colona or Ludwigiahyssopifolia (G. Don.) Exell., shoot competition reduced the growthand yield of rice more than root competition (Chauhan andJohnson, 2010a). These findings suggested that shoot competitionfor light may be the primary mechanism determining competitiveoutcomes between DSR and E. colona or L. hyssopifolia. Rice grainyield was highly correlated with above- and belowground biomass.The results also suggest the importance of measuring the wholeplant when seeking to understand differences in the competitiveability of DSR. Most importantly, there is a need to develop ricecultivars suitable for DSR as cultivars bred for transplanted rice arenow being used in DSR systems (Chauhan, 2012). In transplantedrice, plant breeders gave less emphasis to weed-competitive traitsfor evolving cultivars because of the trade-off relationship betweengrain yield and weed-competitive traits. Also, it is not a desirabletrait in transplanted conditions because early head advantage andpuddling followed by stagnation of water provide effective weedcontrol in transplanted rice. In contrast, this type of environment isnot available to DSR and therefore DSR crops face early crop-weedcompetition and suffer heavy yield loss if weeds are not controlled.

Rice plant characteristics reported to be associated with weedcompetitiveness are height together with early canopy cover, hightiller density, droopy leaves, high biomass accumulation at the earlycrop stage, high leaf area index and high specific leaf area duringvegetative growth, rapid canopy ground cover, and early vigour. It isargued that tall plants are associated with weed competitiveness

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but they often have low yield potential and tend to lodge. Semi-dwarf cultivars can also be as competitive as tall plant-type culti-vars and therefore intermediate height (between tall traditionaland modern semi-dwarf) may be more desirable for direct seeding(Fischer et al., 2001; Fukai et al., 2002). In transplanted rice,transplanting delays tillering; however, tillering does not seem tobe a constraint in direct seeding and therefore tillering ability is nota primary trait for selection (Song et al., 2009). Excessive tillering atan early stage could result in reduced leaf biomass and photosyn-thesis at a later stage and eventually become one of the majorreasons for low yields (Song et al., 2009). Oryza glaberrima, culti-vated African rice with low yield potential, possessing the trait ofdroopy leaves with high specific leaf area, is very effective in weedsuppression. If this trait is restricted to early growth and combinedwith the trait of erect leaves with low specific leaf area from Oryzasativa, such cultivars could be useful for direct seeding (Jones et al.,1997). Cultivars with superior weed competitiveness have beenconfirmed in other regions, such as Apo and UPLRi-7 (with traits ofrapid seedling and early biomass) in the Philippines (Zhao, 2006),Oryzica sabana 6with traits of high leaf area index and tiller densityfor high photon flux density in Latin America (Fischer et al., 2001),and M-202 in North America with traits of high leaf area and rootbiomass (Gibson et al., 2003), and these could be tested throughoutAsia in the future, where DSR is being promoted.

The use of weed-competitive cultivars is unlikely to be feasibleas a stand-alone technology but rather it may be a valuablecomponent of integrated measures (Chauhan, 2012). Suitable cul-tivars should, in addition to weed competitiveness, possess othertraits such as resistance to or tolerance of other biotic and abioticstresses (Dingkuhn et al., 1999). Furthermore, a suitable cultivarneeds to be well adapted to the environment and have the specificcharacteristics desired by farmers and consumers.

2.2. Cultivars having allelopathic traits

A large number of reviews have been published on crop alle-lopathy, which refers to the process of the release of chemicalcompounds by living and intact roots of crop plants that affect thegrowth of other plant species (Belz, 2007; Farooq et al., 2011).Allelopathy is one of the potential mechanisms to suppress weedsand is a possible component in integrated weed managementpractices. Weed suppressiveness and allelopathy may, however, beconfounded and they may coexist in the same cultivar. It wassuggested, but not demonstrated, that theweed-suppressive abilityof IG10 (O. glaberrima) may be, in part, due to allelopathy (Fofanaand Rauber, 2000). It was found that 412 rice accessions devel-oped an allelopathic zone around rice plants for Heterantheralimosa (Sw.) Willd. and 145 rice accessions for Ammannia species.Progress has been significant in isolating rice allelochemicals(Rimando et al., 2001) and locating genes controlling allelopathiceffects of rice (Jensen et al., 2001). Using a relay seeding technique,cultivar IAC 165 was shown to possess stronger allelopathic activitythan CO 39 (Jensen et al., 2001). These workers also identifiedquantitative trait loci (QTLs) associated with the rice allelochem-icals against E. crus-galli. This is an important step towards breedingallelopathic rice cultivars. It was found that 35% of the totalphenotypic variation of allelopathic activity of populations wasexplained by four main-effect QTLs situated on three chromo-somes. However, some questions are yet to be answered: Do alle-lopathic rice cultivars have autotoxic effects and does theallelopathic potential of rice cultivars adversely influence the cya-nobacteria population of the paddy field? (Jensen et al., 2001).Cyanobacteria are known to fix nitrogen in paddy fields. Anyadverse effect of allelopathic rice cultivars on nitrogen-fixing po-tential of cyanobacteria may not be desirable. Many authors

suggested that success in breeding new rice cultivars having goodweed-suppressing ability would benefit farmers in rice-cultivatingcountries and play an important role in sustainable agriculturalproduction (Jamil et al., 2011; Khanh et al., 2007). In a breedingprogram, both traditionally bred and hybrid rice with allelopathycould be exploited. Courtois and Olofsdotter (1998) revealed that, ifa high number of QTLs with low effect are involved, a traditionalbreeding method can be a reasonable alternative, in which twoparents with contrasting behaviour are crossed and recombinantinbred lines (RILs) are derived through the single-seed descentmethod. Hybrid rice with stronger weed suppression ability couldbe bred, but the quality factors associated with rice allelopathyshould be carefully considered in a breeding program as animportant standard for the new cultivars (Lin et al., 2000). Indeed,the significance of allelopathy for weed management in rice willremain conjectural until it is clearly demonstrated that differencesobserved in bioassays also occur in the field.

2.3. Cultivars capable of emerging under anaerobic conditions

Germplasm having anaerobic germination could be exploited inbreeding programs for direct seeding to facilitate early ponding ofwater in the field for weed control. Cultivars having anaerobicconditions are required for DSR to reduce the load of herbicides.Already some QTLs for this trait have been identified (Septiningsihet al., 2009) at the International Rice Research Institute (IRRI). Withthe availability of such cultivars, the risk of uncertainties of rainfalland possibilities of flooding after seeding may be eliminated. Therisk of possible flooding during the early stage hinders large-scaleadoption of DSR because of the high sensitivity of germinatingrice seeds to flooding and the likely failure of crop establishment.Poor establishment favours crop-weed competition towardsweeds.

Good land levelling is a prerequisite for DSR because it facilitatesuniform and better establishment, permits more precise watercontrol and good drainage that provide good weed control, andreduces the water requirement (Chauhan, 2012; Jat et al., 2009).Good land levelling is possible only with laser levellers. Fieldslevelled by plankers (wooden or metal boards) have frequent dikesand ditches, which cause large variability across the fields. Becauseof a lack of uniform water distribution associated with unevennessof land, the problem of excess or no water causing large yieldvariability within a field is common and these factors hinder uni-form establishment in DSR fields. In South Asia, the arrival ofmonsoon is tricky and sometimes heavy pre-monsoon rains afterseeding may cause rotting of seed that may result in poor cropestablishment. Poor crop establishment may favour competitiontowards weeds and therefore these problems could be solved byhaving cultivars capable of germinating and emerging underanaerobic conditions.

The sowing of seeds under the surface of flooded soil is knownas anaerobic seeding (Balasubramanian and Hill, 2002). With theuse of cultivars having anaerobic germination, the cost of herbi-cides may decline (Yamauchi, 1996), because early flooding couldprovide better weed control. If rice can germinate, establish, andgrow under anaerobic conditions, the constraints for both directwet and dry sowing would be reduced and cultivation of DSRcould be possible with less water and energy. Cultivars with Sub1-traits tolerate complete submergence at the seedling stage(Iftekharuddaula et al., 2011; Septiningsih et al., 2009; Xu et al.,2006); however, they are susceptible to anaerobic germination.The introgression of the SUB1 gene did not improve germinationunder flooded conditions. However, Nipponbare (a japonicacultivar) exhibited greater seedling vigour under submergencedue to rapid shoot elongation and it was found better than

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internationally recognized submergence-tolerant cultivar FR13A(Vu et al., 2010).

Cultivars having tolerance of anaerobic conditions duringgermination are increasingly required because of the shift in manyareas from transplanting to direct seeding. With the availability ofsuch cultivars, DSR can easily be submerged immediately after cropsowing and could provide economical and environmentallyfriendly weed control. Donors for anaerobic germination have beenidentified and are being used to develop improved breeding lines(Angaji et al., 2010; Ismail et al., 2009). Anaerobic germinationappears to be under the control of multiple QTLs with a relativelystrong effect, and at least some donors appear to have different,nonallelic QTLs (Mackill et al., 2012). This suggests that the QTLsfrom different donor cultivars can be combined into the samecultivar to increase the tolerance. Prototype breeding lines withSUB1 and anaerobic germination tolerance have demonstrated thatthe two traits can be combined in the same line and this is becausethe SUB1 gene does not appear to be expressed until at least 4 daysafter germination (Mackill et al., 2012).

2.4. Cultivars to manage weedy rice and volunteer rice seedlings

Weedy rice (Oryza sativa L.) is an emerging problem in DSRproduction systems and no selective herbicide controls theseweeds. Weedy rice, being physiologically and morphologicallysimilar to cultivated rice plants, is not easily distinguished beforeheading. Weedy rice matures earlier than cultivated rice andshatters its grains in the field; therefore, its problem is aggravatedin some places (Azmi et al., 2005). This problem could be solved byevolving purple-coloured or pinkish-stemmed high-yielding culti-vars. Farmers in the state of Himachal Pradesh, India, for example,adopted the practice of growing a purple-leaf rice cultivar (R-575)in weedy rice-infested fields (Kaushik et al., 2011). During hoeingand weeding operations, farmers removed all green-leaf plantsfrom their fields and thus eradicated weedy rice from their fields.

The problem of volunteer rice plants in DSR is emerging due tothe shattering of grains at the time of the previous harvest. Theproblem is more severe where cultivars have a tendency to lodge.These volunteer plants also act as weeds. The management ofvolunteer rice seedlings is also an issue for further research in DSR-based cropping systems. The problem of volunteer rice in DSR maybe solved by evolving cultivars that have some tolerance for shat-tering and the anti-dormancy gene (Gressel and Valverde, 2009).Differences for shattering have been observed in many cultivarsand these should be exploited in breeding programs for directseeding. Evolving cultivars having the anti-dormancy gene forcontrol of volunteer plants could be risky in riceerice croppingsystems (in southern India and mostly Southeast Asian countries);however, this strategy could be exploited in breeding programs fordirect seeding in parts of Bangladesh, India, and Pakistan wherewheat follows after rice.

2.5. Herbicide-resistant rice

The use of herbicides in DSR systems may require the devel-opment of herbicide-resistant cultivars. The adoption of herbicide-resistant rice (HRR) has the potential to solve the problem of weeds,especially weedy and wild rice, in DSR systems. To date, three HRRsystems have been developed: imidazolinone-, glufosinate-, andglyphosate-resistant cultivars (Gealy et al., 2003). Glufosinate- andglyphosate-resistant rice cultivars were developed through trans-genic technologies. Imidazolinone-resistant rice was developedthrough chemically induced seed mutagenesis and conventionalbreeding and it conveys resistance to the imidazolinone group ofherbicides.

These herbicides share some important and unique character-istics such as broad-spectrum control of grasses and broadleafweeds, long-term weed control, flexibility in crop rotation, abiodegradable nature, and effectiveness at low doses, thereforereducing the total amount of herbicide released in the environ-ment. These properties make these herbicides safer and environ-mentally compatible. Developing HRR is therefore a new way ofconferring selectivity and enhancing crop safety and production(Chauhan et al., 2012a; James, 2011). The use of nontransgenic HRRcultivars developed by seed mutagenesis could be a viable weedmanagement strategy in DSR systems. Clearfield� (imidazolinone-resistant) rice was originally developed to help rice farmers in theU.S. overcome the problem of weedy rice (called ‘red rice’ in theAmericas). In Asia, it was first released in Malaysia in 2010 and itmay be introduced in other Asian countries in the coming years.Developing a nontransgenic herbicide-resistant cultivar would be aclassical, safe, and yet novel and effective means of weed controlthrough the application of highly effective non-toxic and rapidlybiodegradable new-generation herbicides.

For a sustained production system, the key area of herbicideresistance or weed management involves using an integratedapproach that encompasses stringent application of herbicides,suitable cultural practices, a stale seedbed technique, herbicide andcrop rotations for effective management of weed populations, thecontrol of herbicide-tolerant crop volunteers, and also the man-agement of outcrossing of transgenic to non-transgenic crops andweeds. Explicit guidelines must be adhered to when using HRR.

If compatible species are sympatrically distributed and theirflowering time is similar, then gene flow is ubiquitous even in self-pollinated crops. Herbicide-resistant crops appear very promisingin India, particularly in soybean and maize, in which there is littlerisk of transgression of herbicide-resistance genes into wild types.The greatest risk in the use of HRR is the potential for transfer of agene conferring herbicide resistance to weedy rice. Weedy rice isnot a problem in many parts of Bangladesh, India, and Pakistan andtherefore the risk of gene flow from HRR to weedy rice is supposedto be low; however, this risk cannot be ruled out in DSR systems inthe future. Weedy rice, for example, is becoming an acute problemin areas where transplanting is being replaced by DSR such asVietnam, Malaysia, Sri Lanka, Thailand, and Korea (Mortimer et al.,2000). Such gene flow can impact crop invasiveness, the fitness ofwild species, the development of ‘super weeds’, and the loss ofnative biodiversity. The evolution of herbicide-resistant weeds dueto increased reliance on a single herbicide for weed control is apotential concern. About 310 weed biotypes have evolved resis-tance to 19 different target-site chemistries (Heap, 2012). Theevolution of herbicide resistance in weeds through selection pres-sure imparts greater risk than the development of resistancethrough gene flow in related species in which the frequency ofinterspecific hybridization and subsequent introgression is low(Warwick et al., 2004). Gene flow from imidazolinone-resistant ricecultivars to weedy rice could, however, hasten resistance devel-opment in weedy rice because this herbicide group exerts strongselection pressure. But, problems can be diminished with the use ofintegrated weed management strategies.

The use of HRR requires meticulous stewardship to preventoutcrossing of HRR toweedy rice (Tan et al., 2005). Consequently, inorder to follow stewardship recommendations, farmers wouldneed to understand the risks and the consequences of gene flowand they should be trained. Effective stewardship may also requirewell-functioning and cost effectivemarkets for farmers to be able topurchase any additional or alternative chemical products to eradi-cate weeds escaping the first chemical treatment. In addition tothis, the existing herbicide-resistant technologies require thefarmers to purchase new (certified) seeds for each new cropping

G. Mahajan, B.S. Chauhan / Crop Protection 49 (2013) 52e5756

season (Tan et al., 2005). The advantage of such a system is that thecertified seed, produced on quality-controlled production farms,will be free of weed seeds, including wild relatives of rice, and assuch will minimize the spread of weedy and wild rice biotypes.Establishment of effective public-private partnerships might go along way towards the spread of this technology. The use of ricecultivar mixtures in a field or different rice cultivars in neigh-bouring fields might cause gene flow fromHRR to conventional ricecultivars. Gene flow to conventional rice cultivars may cause asubsequent indirect flow of transgenes to wild rice in adjacentfields (Lu and Snow, 2005). Prevention of this indirect gene flowwould require community-based regulation and information sys-tems and good communication between rice producers.

It is concluded that, despite the potential benefits of HRR as aweed management tool for DSR systems, it is necessary to fullyassess the potential adverse consequences of the widespreadadoption of HRR throughout Asia. Evaluation is needed for thehybridization rates between HRR and its wild and weedy relativespresent in Asia to determine the compatibility differences andheterosis potential of hybrids between wild relatives and HRRcultivars.

3. Conclusions

DSR systems are expected to increase in the future due to con-cerns over labour and water shortages. However, weeds are animportant biotic constraint to the adoption of these productionsystems. To reduce reliance on herbicides, there is a need todevelop cultural weed management tools using cultivars thatfavour rice over weeds. The availability and use of such cultivarswould provide effective, season-long, and sustainable weed controlin DSR systems. There is a good diversity in rice genomes and, withthe availability of new modern tools in genetic engineering, it maybe possible to develop rice cultivars that may all have weed-competitive traits and can combat weeds in DSR systems.

Acknowledgments

The authors are grateful to Bill Hardy, International RiceResearch Institute, Philippines, for providing comments on themanuscript.

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