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Research Article Response of Rice Genotypes to Weed Competition in Dry Direct-Seeded Rice in India Gulshan Mahajan, 1 Mugalodi S. Ramesha, 2 and Bhagirath S. Chauhan 3 1 Punjab Agricultural University, Ludhiana 141004, India 2 International Rice Research Institute, India Office, Hyderabad 500030, India 3 Queensland Alliance for Agriculture and Food Innovation (QAAFI), University of Queensland, Toowoomba, QLD 4350, Australia Correspondence should be addressed to Bhagirath S. Chauhan; [email protected] Received 23 April 2014; Accepted 11 June 2014; Published 30 June 2014 Academic Editor: Antonio Ferrante Copyright © 2014 Gulshan Mahajan et al. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. e differential weed-competitive abilities of eight rice genotypes and the traits that may confer such attributes were investigated under partial weedy and weed-free conditions in naturally occurring weed flora in dry direct-seeded rice during the rainy seasons of 2011 and 2012 at Ludhiana, Punjab, India. e results showed genotypic differences in competitiveness against weeds. In weed-free plots, grain yield varied from 6.6 to 8.9 t ha −1 across different genotypes; it was lowest for PR-115 and highest for the hybrid H-97158. In partial weedy plots, grain yield and weed biomass at flowering varied from 3.6 to 6.7 t ha −1 and from 174 to 419 g m −2 , respectively. In partial weedy plots, grain yield was lowest for PR-115 and highest for PR-120. Average yield loss due to weed competition ranged from 21 to 46% in different rice genotypes. e study showed that early canopy closure, high leaf area index at early stage, and high root biomass and volume correlated positively with competitiveness. is study suggests that some traits (root biomass, leaf area index, and shoot biomass at the early stage) could play an important role in conferring weed competitiveness and these traits can be explored for dry-seeded rice. 1. Introduction Dry-seeded rice (DSR) is an emerging rice production system in the northwestern Indo-Gangetic Plains amidst a looming water crisis and labor scarcity [1]. Weeds are among the most important biological constraints to successful production of DSR and, therefore, the cultivation of DSR warrants intensive use of herbicides for weed control [2]. DSR production sys- tems are subject to greater weed pressure than conventional production systems, in which weeds are suppressed by flood- ing and transplanted rice seedlings have a “head start” over germinating weed seedlings. Many options exist for weed control in DSR, perhaps the most common being the use of herbicides. Reducing farmers’ dependence on herbicides is desirable to reduce herbicide costs, minimize environmental pollution, and delay the evolution of herbicide-resistant weeds [1, 3]. In DSR, effective weed control requires proper herbicide application timing and method [4], which are oſten not met, resulting in poor weed control. Alternative weed management technologies are therefore much needed [5]. In herbicide-dominant systems, overall weed control efficiency can be improved when herbicides were combined with crop species or genotypes of superior competitiveness [6, 7]. Variation among genotypes in their ability to compete with weeds has been documented for many crops, including rice [8, 9]. Although some studies exist on the differences in competitiveness, including attempts on deciphering rice traits related to weed competitiveness and yield [1012], only a limited number of cultivars have been evaluated so far. Crop competitiveness against weeds is composed of tolerance to weed infestation, which is the ability to maintain high yields under weedy conditions, and weed-suppressive ability, which is the capacity to suppress weed growth in terms of dry matter [13]. Screening weed-competitive genotypes could offer an opportunity for using them as a component of integrated weed management strategies in DSR. However, Hindawi Publishing Corporation e Scientific World Journal Volume 2014, Article ID 641589, 8 pages http://dx.doi.org/10.1155/2014/641589

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Research ArticleResponse of Rice Genotypes to Weed Competition inDry Direct-Seeded Rice in India

Gulshan Mahajan1 Mugalodi S Ramesha2 and Bhagirath S Chauhan3

1 Punjab Agricultural University Ludhiana 141004 India2 International Rice Research Institute India Office Hyderabad 500030 India3 Queensland Alliance for Agriculture and Food Innovation (QAAFI) University of Queensland Toowoomba QLD 4350 Australia

Correspondence should be addressed to Bhagirath S Chauhan bchauhanuqeduau

Received 23 April 2014 Accepted 11 June 2014 Published 30 June 2014

Academic Editor Antonio Ferrante

Copyright copy 2014 Gulshan Mahajan et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

The differential weed-competitive abilities of eight rice genotypes and the traits that may confer such attributes were investigatedunder partial weedy andweed-free conditions in naturally occurringweed flora in dry direct-seeded rice during the rainy seasons of2011 and 2012 at Ludhiana Punjab India The results showed genotypic differences in competitiveness against weeds In weed-freeplots grain yield varied from 66 to 89 t haminus1 across different genotypes it was lowest for PR-115 and highest for the hybridH-97158In partial weedy plots grain yield andweed biomass at flowering varied from 36 to 67 t haminus1 and from 174 to 419 gmminus2 respectivelyIn partial weedy plots grain yield was lowest for PR-115 and highest for PR-120 Average yield loss due to weed competition rangedfrom 21 to 46 in different rice genotypes The study showed that early canopy closure high leaf area index at early stage and highroot biomass and volume correlated positively with competitiveness This study suggests that some traits (root biomass leaf areaindex and shoot biomass at the early stage) could play an important role in conferring weed competitiveness and these traits canbe explored for dry-seeded rice

1 Introduction

Dry-seeded rice (DSR) is an emerging rice production systemin the northwestern Indo-Gangetic Plains amidst a loomingwater crisis and labor scarcity [1] Weeds are among the mostimportant biological constraints to successful production ofDSR and therefore the cultivation of DSRwarrants intensiveuse of herbicides for weed control [2] DSR production sys-tems are subject to greater weed pressure than conventionalproduction systems in which weeds are suppressed by flood-ing and transplanted rice seedlings have a ldquohead startrdquo overgerminating weed seedlings Many options exist for weedcontrol in DSR perhaps the most common being the use ofherbicides Reducing farmersrsquo dependence on herbicides isdesirable to reduce herbicide costs minimize environmentalpollution and delay the evolution of herbicide-resistantweeds [1 3] In DSR effective weed control requires properherbicide application timing andmethod [4] which are often

not met resulting in poor weed control Alternative weedmanagement technologies are therefore much needed [5]

In herbicide-dominant systems overall weed controlefficiency can be improved when herbicides were combinedwith crop species or genotypes of superior competitiveness[6 7] Variation among genotypes in their ability to competewith weeds has been documented for many crops includingrice [8 9] Although some studies exist on the differencesin competitiveness including attempts on deciphering ricetraits related to weed competitiveness and yield [10ndash12] onlya limited number of cultivars have been evaluated so far

Crop competitiveness against weeds is composed oftolerance to weed infestation which is the ability to maintainhigh yields under weedy conditions and weed-suppressiveability which is the capacity to suppress weed growth in termsof dry matter [13] Screening weed-competitive genotypescould offer an opportunity for using them as a componentof integrated weed management strategies in DSR However

Hindawi Publishing Corporatione Scientific World JournalVolume 2014 Article ID 641589 8 pageshttpdxdoiorg1011552014641589

2 The Scientific World Journal

to date only few genotypes of rice for instance Haefele et al[14] with superior weed competitiveness are known Hybridsdue to their early vigor may have the potential to com-plement the limited set of available competitive germplasmfor DSR The most important breeding objectives for theDSR genotypes were yield potential high environmentaladaptation early vigor and potentially favorable growthtraits for weed suppression Empirical evidence of superiorperformance of hybrids and new inbred lines adapted toDSR in particular the ability to better cope with weedsis still awaited Various authors suggested the evaluation ofhybrids in DSR to confirm possession of weed-competitivetraits and provide farmers with a wider choice of optionswhen cultivating DSR [15]

Morphological physiological and biochemical traits arethought to control plant competitiveness [16] and manystudies have been conducted to determine plant charactersconferring competitive ability in cereals Plant height playsa role in the competitive ability of rice [17] Crop heightappeared to have the greatest impact on competitive abilitywith the shortest cultivars experiencing the largest yieldreductions and allowing the greatest weed growth Howeverheight alone does not explain competitive ability becausesome shorter cultivars have been found to be good competi-tors in rice [18] Some workers found that leaf area index(LAI) to be negatively correlated with specific leaf area drymatter partitioning of leaves and mean tip elevation angle[19] They concluded that specific leaf area and tilleringability are major determinants of vegetative vigor Theyfurther emphasized that vegetative vigor and crop durationaffecting the ability of genotypes to recover from earlycompetition are the useful traits in the selection of weed-competitive rice Evidence that early season ground coveralso reduces subsequent weed biomass has been reportedby many researchers [20ndash22] A better understanding ofthe mechanisms by which a rice genotype becomes morecompetitive to weeds would not only serve to assist plantbreeders in developing competitive cultivars more quicklyand effectively but would also justify the use of plant breedingto increase crop-competitive ability [8] Yield gains of 7ndash9have been identified in ldquocompetitiverdquo aerobic cultivars whencompared with ldquononcompetitiverdquo cultivars [23]

The first objective of this study was to evaluate the weedcompetitiveness and yield potential of some popular hybridsand inbreds to identify superior genotypes that are potentiallysuitable for use in integrated weed management strategies inDSR Second objective was to determine rice traits measuredin weed-free and weedy conditions could be related to weeddry matter recorded under weedy conditions This is thefirst study presenting a comprehensive assessment of weed-competitive ability of the popular rice genotypeshybrids ofthis region for the DSR system

2 Materials and Methods

21 Site Description Field studies were conducted to assessthe effect of weed competition on the performance of ricegenotypes in direct-seeded conditions in the rainy seasons(JunendashOctober) of 2011 and 2012 at the research farm of the

Punjab Agricultural University Ludhiana (30∘561015840N 75∘521015840E)India The climate of the region is semiarid with an averageannual rainfall of 400ndash700mm (75ndash80 of which is receivedfrom July to September) aminimum temperature of 0ndash4∘C inJanuary and amaximum temperature of 41ndash45∘C in JuneThesoil type at the experimental site was sandy loam with 03organic matter and a pH of 72 The total N content in thesoil was 0042 The available P (18 kg haminus1) and K contents(290 kg haminus1) in soil were medium and high respectivelyField had the history of dry direct-seeded rice for 3 yearsThe weeds present in the field were Commelina benghalen-sis L Cyperus rotundus L Dactyloctenium aegyptium (L)WilldDigera arvensis ForskDigitaria sanguinalis (L) ScopEchinochloa colona (L) Link Eragrostis spp and Leptochloachinensis (L) Nees Weed composition was near to uniformin the entire field

22 Experimental Design The experiment in each yearwas established in a split-plot design with three replicatesThe main plots included two levels of weed infestation(weed-free and partial weedy) and subplots (62m times 14m)included eight rice genotypes Among genotypes there werefive inbred lines (two cultivars PR-115 and PR-120 threeadvanced breeding lines CR2707 IR83927 and IR88633)and three hybrids (H-97158 RH-257 and US-310) Thesegenotypes were selected on the basis of yield in our previousbreeding trials on DSR All the selected genotypes were ofindica rice

In the weed-free plots pendimethalin (750 g ai haminus1)as preemergence (PRE) was applied at 3 d after sowing(DAS) and bispyribac-sodium (25 g ai haminus1) as postemer-gence (POST) at 18 DAS Following bispyribac-sodium appli-cation plots were hand-weeded as needed to remove allweeds in the remainder of the season Herbicides wereapplied using a knapsack sprayer with a flat fan nozzle andwater as carrier at 375 L haminus1 In completely weedy plots yieldlosses in DSR are greater than 90 [24] In addition it is notcommon for farmers to leave their rice fields infested withweeds in irrigated areas The middle of the critical weed-freeperiod in DSR is around 28 DAS [24] Therefore to makepartial weedy plots the weedy plots were hand-weeded onceat 28 DAS and weeds were allowed to grow before and afterthe hand-weeding throughout the season [8]

23 Crop Management Fields were prepared by cultivatingtwice using a disc harrow followed by leveling with a woodenboard Seeds were sown by a single-row drill at a seedingrate of 30 kg haminus1 at 20-cm row spacing on June 21 2011 andJune 16 2012 The field was surface-irrigated immediatelyafter sowing Nitrogen (N) fertilizer at 130 kg haminus1 (as urea)was applied in four equal splits at 14 28 49 and 70 DASRecommended rates of chlorpyrifos (500 g ai haminus1) and prop-iconazole (625 g ai haminus1) were used to control insect pests anddiseases respectively

The LAI at tillering panicle initiation and floweringwas measured with a digital plant canopy imager (modelCI110CI-120 CID Inc CamasWA) but the data are shownonly for the tillering stage (28 DAS) due to similar trend

The Scientific World Journal 3

Under partial weedy condition digital plant canopy imageralso assesses the LAI of crop and weeds so LAI was onlyrecorded under weed free conditions Measurements weremade with the probe set parallel to the rows at two fixedlocations in each plot Two plants were randomly selectedfrom each plot at the early ripening stage for root samplingRoot samples were collected by removing soil to a depth of45 cm along with the plants with a 10-cm-diameter augerA uniform soil volume (3534 cm3) was excavated to collectroot samples from all the treatments Roots were carefullywashed and various parameters (root biomass and volume)were measured and calculated Root volume was measuredusing the water displacement method

Twoquadrats 025m2 size were placed at random in eachplot to determine weed biomass at 28 DAS and at floweringOnly at 28 DAS weeds were counted species-wise anddifferentiated into categories of sedges grass and broadleafweeds Weeds were cut at ground level washed with tapwater sun-dried and then oven-dried at 70∘C until constantweight and weighed For crop biomass plant samples werecollected in quadrats (040m times 025m) placed randomlyat two locations in each plot at tillering panicle initiationand flowering stages The plants were cut at ground leveloven-dried at 70∘C for 72 h and weighed Grain yield wasmeasured at 14moisture from a sampling area of 52m2 perplot At the same time five plants were selected randomlyfrom each plot to measure agronomic parameters which in-cluded plant height grains panicleminus1 and spikelet sterility() Competitiveness was measured as weed competitiveindex (CI) and calculated as [25 26]

CI =[119881infest119881mean]

[119882119894

119882mean] (1)

where 119881infest is yield of variety (119894) in terms of weed infested119881mean is the average yield of all varieties in the presence ofweed 119882

119894

is weed biomass varieties of 119894 119882mean is averageweed biomass is mixed with all varieties

Crop vigor recorded as visually rated crop biomass at2 week after sowing on a per-plot basis on a 1-to-9 scalewhere 9 was the greatest crop biomass and 1 was the least[8] The relative yield loss (YL) of the crop challenged byweed competition under field conditions was estimated usingequation YL () = 1minus(119884CW119884CM)times100 where119884CW and119884CMare crop yields in competition with weeds and in weed-freeconditions respectively

24 Statistical Analyses In a combined analysis of data theinteractions of years with the level of weed infestation andorgenotypes were nonsignificant therefore the data werepooled over the years (therefore a total of six replications)for further analyses (GenStat 80) Treatment means wereseparated using the least significant difference (LSD) testat the 5 level of significance The relationships betweengrain yield (t haminus1) and various crop traitsparameters wereassessed using linear regression analysis (SigmaPlot 100)

Table 1 Weed density (number mminus2) in response to differentgenotypes at 28 days after sowing

GenotypeWeed density

Grasses Broadleaved Sedgesnumber mminus2

PR-115 42 20 24H-97158 54 25 31CR2707 61 29 35US-310 36 17 20IR88633 26 12 15IR83927 39 18 22RH-257 32 15 18PR-120 30 14 17LSD (005) 7 3 4

3 Results and Discussion

31 Weed Composition Ten weed species were frequentlyobserved in the weedy plots during both years The mostdominant weed species (on the basis of density) encounteredin the weedy plots at 28 DAS in both years were Cyperusiria L (21) Cyperus rotundus (7) Echinochloa colona(10)Alternanthera sessilis (L) R Br exDC (4)Leptochloachinensis (12) Digitaria sanguinalis (21) Dactylocteniumaegyptium (6)Digera arvensis (5)Commelina benghalen-sis (5) and Eragrostis spp (9) Genotype CR-2707 hadthe highest sedges grasses and broad-leaf density among allthe tested genotypes (Table 1) Sedges grasses and broadleafweed density were similar for genotypes PR-120 IR88633and RH-257 however lower than CR-2707 and H-97158Genotypes PR-115 and IR-83927 had similar sedges grassesand broadleaf weed density but lower than that of CR-2707Weed species found in the trial do prefer upland rather thanflooded conditionsThe fieldwas exposed to alternate wettingand drying conditions and these conditions are likely to favorspecies diversification depending upon the competitive abil-ity of the genotypes In DSR weed composition is generallyfound to be more diverse than in transplanted rice [4]

32 Weed Biomass and Grain Yield At 28 DAS H-97158and CR2707 accrued with higher weed biomass than othergenotypes (Table 2) At this stage lowest weed biomass wasrecorded in PR-120 Weed biomass at flowering was alsolowest in PR-120 and it was highest in hybrid H-97158(Table 2) Genotypes IR88633 IR83927 PR-120 and RH-257accrued with similar but lower weed biomass than that ofH-97158 Genotypes US-310 CR2707 and H-97158 accruedwith similar weed biomass however they had higher weedbiomass thanPR-120 PR-115 hadweed biomass similar to thatof IR88633 IR83927 and RH-257

Weed competition caused significant reduction in grainyield of all the tested genotypes and the reduction waspositively correlated with weed biomass (119903 = 065 119875 = 040)Grain yield in weedy plots varied from 36 to 67 t haminus1 acrossdifferent genotypes and it was lowest and highest for PR-115and PR-120 respectively However in weed-free plots grain

4 The Scientific World Journal

Table 2 Weed biomass (gmminus2) in response to different genotypesat 28 d after sowing (DAS) and flowering

GenotypeWeed biomass

28 DAS Floweringgmminus2

PR-115 854 3330H-97158 1175 4185CR2707 1257 3865US-310 794 3812IR88633 567 2211IR83927 706 2808RH-257 613 2380PR-120 448 1743LSD (005) 261 1184

yield varied from 66 to 89 t haminus1 across different genotypesand it was lowest for PR-115 and highest forH-97158 (Table 3)Average yield loss due to weed competition ranged from21 to 46 among different genotypes The higher the weedbiomass the greater the percent yield reduction The highestrelative yield loss due to weed competition was noted in PR-115 and the relative yield loss of this cultivar was similar tothat of H-97158 CR2707 US-310 and RH-257 The lowestrelative yield loss due to weed competition was in PR-120which was similar to that of IR88633 and IR83927 Theseresults suggest that rice genotypes responded differently intheir competitiveness in suppressing weeds under severeinfestation PR-120 IR88633 and IR83927 proved to be betterweed competitors than the other genotypes

Correlation analyses were used to explore the relationshipbetween grain yield of different rice genotypes and plant char-acteristics in both weed-free and partial weedy conditionsIn weed-free conditions only grain yield had a significantpositive correlation with panicle density mminus2 (119903 = 077)(Table 4) In partial weedy conditions however grain yieldhad a significant positive correlation with panicle densitymminus2 grain panicleminus1 weed competitive index LAI rootweight and root volume (Table 5) These results indicate thatthese parameters could have played a vital role in improvinggrain yield under crop-weed competition In partial weedyconditions grain yield was negatively related with spikeletsterility relative yield loss and weed biomass

In weed-free plots the highest grain yield was found inH-97158 followed by RH-257 This was attributed to higherpanicle density in H-97158 Lowest grain yield was recordedin PR-115Weed competitive indexwas highest for PR-120 andother genotypes had lower weed competitive index than thiscultivar (Figure 1) PR-115 had the lowest weed competitiveindex which was similar to that of genotypes H-97158 CR2707 and US-310 On the other hand IR88633 IR83927and RH-257 had better weed competitive index than PR-115The high performance of these genotypes in terms of weedcompetitive index could be attributed to less weed biomassobserved due to their ability to suppress weeds Weeds canbe suppressed by allelopathic effects of rice genotypes inaddition to genotypic smothering effect on weeds however

Genotype

25

20

15

10

05

00

Com

petit

ive i

ndex

LSD

PR-115

H-97158

CR2707

-185

-16

-1-1

-1

US-310

IR88633

-1-79

-B-1

IR83927

-B-B

-278

-CRA

-1-1

-1

RH-257

PR-120

Figure 1 Weed competitive index of different rice genotypes

the genotypes used in our study have not been reported fortheir allelopathic effect

The interaction effects of weed infestation levels andgenotypes were significant for grain yield panicle numbermminus2 and grain number panicleminus1 (Table 3) Across weedinfestation levels 1000-grain weight was highest for IR88633followed by IR83927 (Table 6) Averaged over weed infesta-tion levels 1000-grain weight of all the other genotypes wassimilar to but lower than that of IR88633 In the partial weedyconditions 1000-grain weight had a positive correlation withweed competitive index whereas such correlation was notfound in weed-free conditions Plant height differed acrossthe genotypes (Table 6) IR88633 attained the highest plantheight followed by IR83927 and CR2707 Lowest plant heightwas found for PR-115 Genotypes H-97158 US-310 andRH-257 attained similar plant height Under partial weedyconditions plant height had a negative correlation withrelative yield loss

As expected grain yield of all genotypes in weedyconditions decreased significantly as compared with yield inweed-free conditions (Table 3) In partial weedy conditionshighest grain yield was found for PR-120 closely followed byIR83927 Grain yield of PR-120 in partial weedy conditionswas similar to that of PR-115 US-310 and IR88633 in weed-free conditions This response was because of the bettercompetitive ability of PR-120 that is it had the highest weedcompetitive index (Figure 1) and root biomass and volume(Table 7) Root biomass and volume were highest in PR-120and lowest in PR-115 however these root parameters of PR-115 H-97158 CR2707 and US-310 were the same SimilarlyIR88633 IR83927 and RH-257 had almost identical root bio-mass and volume High root biomass and volume offeredcrop-weed competition in favor of the crop providing greater

The Scientific World Journal 5

Table 3 Grain yield (t haminus1) panicles mminus2 and grains panicleminus1 in response to the interaction effect of weed infestation levels and genotypes

GenotypeGrain yield Panicles Filled grains

Weed-freea Partial weedy Weed-free Partial weedy Weed-free Partial weedyt haminus1 number mminus2 number panicleminus1

PR-115 659 358 290 230 114 96H-97158 889 515 351 236 133 103CR2707 743 480 305 238 136 96US-310 678 452 252 247 148 95IR88633 695 520 278 249 130 91IR83927 809 603 336 259 122 108RH-257 863 550 340 252 136 115PR-120 847 666 362 308 124 120LSD (005) 07 29 14aWeed-free plots were kept weed-free throughout the season Partial weedy plots plots were hand-weeded once at 28DAS and weeds were allowed to growbefore and after the hand-weeding throughout the season

Table 4 Correlation among different traits in weed-free conditions The critical value of 119903 was 040 at 5 level of significance

Traits GY Pan Gr TGW SS Ht CI RYL WP LAI VS RWGYPan 077Gr minus006 minus030TGW 013 minus003 minus016SS minus036 minus041 minus009 minus039Ht minus005 minus024 011 077 minus028CI 025 035 minus001 020 minus056 009RYL minus008 minus021 minus002 minus041 044 minus022 minus069WP minus002 minus019 028 minus033 032 minus013 minus078 065LAI 039 032 005 062 minus066 062 060 minus039 minus040VS 001 minus004 minus009 060 minus038 059 035 minus030 minus017 072RW 032 045 minus015 041 minus049 010 058 minus064 minus056 048 037RV 032 045 minus015 041 minus049 010 058 minus064 minus056 048 037 100GY grain yield LAI leaf area index at 28DAS Ht height RW root weight RV root volume Pan panicle mminus2 Gr filled grains panicleminus1 TGW 1000-grainweight SS spikelet sterility CI weed competitive index WP weed biomass and RYL relative yield loss

Table 5 Correlation among different traits in partial weedy conditions The critical value of 119903 was 040 at 5 level of significance

Traits GY Pan Gr TGW SS Ht CI RYL WP LAI VS RWGYPan 061Gr 053 062TGW 033 018 000SS minus047 minus034 minus039 minus038Ht 023 001 minus017 078 minus024CI 071 032 026 021 minus049 017RYL minus084 minus051 minus031 minus041 026 minus042 minus069WP minus056 minus029 minus027 minus034 032 minus020 minus078 065LAI 051 014 014 059 minus062 063 060 minus039 minus040VS 022 008 minus010 062 minus041 070 035 minus030 minus017 072RW 070 063 038 055 minus047 033 058 minus064 minus056 048 037RV 070 063 038 055 minus047 033 058 minus064 minus056 048 037GY grain yield LAI leaf area index at 28DAS Ht height RW root weight RV root volume Pan panicle mminus2 Gr filled grains panicleminus1 TGW 1000-grainweight SS spikelet sterility CI weed competitive index WP weed biomass and RYL relative yield loss

6 The Scientific World Journal

Table 6 Final plant height (cm plantminus1) 1000-grain weight (g)and spikelet sterility () in response to weed infestation levels andgenotypes

Treatment Plant height 1000-grainweight

Spikeletsterility

cm g Weed infestation level

Weed-freea 1153 260 195Partial weedy 1165 256 201LSD (005) NS NS 16

GenotypePR-115 935 247 262H-97158 1094 255 182CR2707 1246 254 157US-310 1092 243 258IR88633 1502 289 166IR83927 1269 270 171RH-257 1126 258 194PR-120 1001 255 146LSD (005) 71 05 39

Nonsignificant NSaWeed free plots plots were kept weed free throughout the season Partialweedy plots plotswere hand-weeded once at 28DAS andweedswere allowedto grow before and after the hand-weeding throughout the season

Table 7 Leaf area index (LAI) and vigor score at 28 days aftersowing and root biomass and volume at flowering of variousgenotypes under weed-free conditions

Genotype LAI Vigor score Root biomass Root volumegmminus2 mLmminus2

PR-115 186 43 1478 5615H-97158 273 47 2009 7635CR2707 326 90 1965 7466US-310 190 47 1757 6676IR88633 337 90 3428 13027IR83927 289 70 2914 11073RH-257 310 67 3034 11530PR-120 282 67 4295 16538LSD (005) 043 06 755 2870

vigor and early canopy closure as evidenced by the higher LAIof these genotypes

Vigor score was highest in cultivars IR88633 and CR2707(Table 7) Among the tested genotypes vigor score was lowestfor PR-115 followed by H-97158 and US-310 Early vigorhas been shown as an important characteristics related tocompetitive ability [16] and tends to be reflected in greatershoot and root biomass during early vegetative growth Inthe present study crop biomass of PR-120 remained similarin both partial weedy and weed-free conditions (Table 8)At panicle initiation PR-120 in the partial weedy conditionsaccruedwith similar crop biomass withH-97158 and IR88633in the weed-free conditions In the partial weedy conditions

at panicle initiation and flowering stages IR88633 IR83927RH-257 and PR-120 accrued with higher crop biomass thanthat of PR-115 H-97158 CR2707 and US-310 respectively(Table 8)

The present study revealed that higher biomass coupledwith higher plant height at early stages provided greaterstored assimilates for grain filling via translocation thatmay prevent the yield loss of these genotypes under par-tial weedy conditions In partial weedy conditions grainnumber panicleminus1 remained similar only in IR83927 andPR-120 as compared with that in weed-free conditions andthis response was mainly because of their better weed-competitive ability In partial weedy conditions the highestnumber of grains panicleminus1 was found in PR-120 Grainnumber panicleminus1 for PR-120 in partial weedy conditions wassimilar to that of PR-115 H-97158 IR88633 and IR83927 inthe weed-free conditions

High vigor score and LAI during early crop developmenthave been shown to bemajor plant traits contributing toweedcompetitiveness [27 28]The LAI is an important contributorto competitive ability as light is an important resource inplant growth Greater shading ability of the crop makes lesslight available to the competing weeds hence weed growthcan be reduced In the weed-free plots IR88633 attained thehighest LAI at 28 DAS whereas PR-115 attained the lowestLAI (Table 7) The LAI of PR-115 and US-310 was similarGenotypes H-97158 RH-257 PR-120 and IR83927 recordedsimilar LAI but this was higher than that of PR-115 The LAIof CR2707 and IR88633 was similar

The findings of our study indicate that fast-growing habitand the high LAI at the early stage of the crop along withhigh root biomass and volume are the important traits forweed competitiveness confirming studies by various authors[18 29] However some workers [30] demonstrated that anadvantage of early growth habit even for a few days couldshift the competitive balance between crops and weeds Inanother study some workers found a significant correlationbetween crop competitive ability and plant height [31] In thepresent study a negative correlation was found between plantheight and relative yield loss In contrast Fischer et al foundno correlation between the height of semidwarf cultivars andweed biomass [11] As in our study research by some scientists[32] suggested that rice cultivars capable of producing moregrain yield in competition with weeds have greater earlybiomass Similarly workers [8 33] proposed that early vigorshould be a key selection trait for weed competitiveness PR-120 owing to its high root biomass and volume which helpedin attaining high crop biomass and greater panicle numbermminus2 produced the highest yield in partial weedy conditionsOur results confirm the findings of researchers who reportedsignificant and negative correlation between weed biomassand panicle number mminus2 suggesting that panicle density wasdecreased by high weed growth [31]

Competitive ability is the ability of a genotype to suppressweed biomass or to maintain crop yields by tolerating weedcompetition In this study competitive ability was foundhighest for PR-120 The competitiveness of a cultivar mightbe due to the higher number of panicles mminus2 early growth

The Scientific World Journal 7

Table 8 Crop biomass (t haminus1) in response to the interaction effect of weed infestation levels and genotypes at panicle initiation and floweringstages

Weed infestation levelCrop biomass

PR-115 H-97158 CR2707 US-310 IR88633 IR83927 RH-257 PR-120t haminus1

Panicle initiation stageWeed-freea 43 48 320 380 490 440 450 460Partial weedy 330 325 315 340 422 414 440 480LSD (005) 03

Flowering stageWeed-free 698 748 698 721 802 712 732 718Partial weedy 382 425 410 412 490 512 530 592LSD (005) 05aWeed free plots plots were kept weed free throughout the season Partial weedy plots plots were hand-weeded once at 28DAS and weeds were allowed togrow before and after the hand-weeding throughout the season

and yield under both partial weedy andweed-free conditionsIn this study panicle number mminus2 also had a correlationwith grain yield under both partial weedy and weed-freeconditions Cultivars able to produce higher grain yields inpartial weedy conditions correlated with greater early ricegrowth and LAI [8 34 35] Likewise this study demonstratedthat high LAI along with high root biomass and volumewere the useful traits for weed-competitive genotypes CR-2707 and RH-257 had higher LAI and vigor and similarlyRH-257 had higher root biomass and volume but yield lossof these genotypes was found more than 35 This was dueto their (CR-2707 and RH-257) more tendencies for lodgingunder partial weedy conditions

In summary genotypes PR-115 and H-97158 were theworst competitors and PR-120 IR88633 and IR83927 weregood weed competitors Genotypes PR-120 IR88633 andIR83927 have potential for increasing the weed-competitiveability of genotypes in breeding programs and could beused to increase the competitiveness of highly productivegenotypes that are not competitive The study revealed thatearly canopy closure high LAI at the early stage and greaterroot biomass and volume correlated positively with compet-itiveness This study demonstrated that breeding to increasethe competiveness of highly productive rice plant typeswouldbe possible without compromising yieldThe competitivenessobserved in these studies for genotypes PR-120 IR88633 andIR83927 would be adequate in improving farmersrsquo incomeand reducing herbicide use and can be successfully used inintegrated weed management programs in DSR

4 Future Implications

This study implies that breeding for weed-suppressive ricegenotypes should be conducted under partial weedy con-ditions In a mixed weed population the weed-competitiveability of rice genotypes may be affected by weed composi-tion However the competitive ability of rice genotypes is acomplex trait and could not be explained by only one or twocharacteristics Interactions between traits for instance rootbiomass LAI and high shoot biomass at early stage play an

important role and these traits can be explored for identifyingweed-competitive genotypes in DSR Competitiveness of ricegenotypes against weeds will be an important key to thesuccessful adoption of weed-suppressive rice genotypes ina sustainable weed management program The developmentof such genotypes could play an important role in DSR byreducing herbicide load in agroeco system

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors are grateful to Bill Hardy International RiceResearch Institute Philippines for providing comments onthe paper The authors would also like to acknowledge thefunding from the Cereal Systems Initiatives for South Asia(CSISA)

References

[1] G Mahajan B S Chauhan and M S Gill ldquoDry-seeded riceculture in Punjab State of India lessons learned from farmersrdquoField Crops Research vol 144 pp 89ndash99 2013

[2] G Mahajan and B S Chauhan ldquoHerbicide options for weedControl in dry-seeded aromatic rice in IndiardquoWeed Technologyvol 27 pp 682ndash689 2013

[3] B S Chauhan ldquoWeed ecology andweedmanagement strategiesfor dry-seeded rice in AsiardquoWeed Technology vol 26 no 1 pp1ndash13 2012

[4] A N Rao D E Johnson B Sivaprasad J K Ladha and A MMortimer ldquoWeed management in direct-seeded ricerdquo Advancesin Agronomy vol 93 pp 153ndash255 2007

[5] D L Zhao G N Atlin L Bastiaans and J H J SpiertzldquoComparing rice germplasm groups for growth grain yield andweed-suppressive ability under aerobic soil conditionsrdquo WeedResearch vol 46 no 6 pp 444ndash452 2006

8 The Scientific World Journal

[6] D Lemerle B Verbeek R D Cousens and N E CoombesldquoThe potential for selecting wheat varieties strongly competitiveagainst weedsrdquoWeed Research vol 36 no 6 pp 505ndash513 1996

[7] G Mahajan and B S Chauhan ldquoThe role of cultivars in man-aging weeds in dry-seeded rice production systemsrdquo CropProtection vol 49 pp 52ndash57 2013

[8] D L Zhao G N Atlin L Bastiaans and J H J SpiertzldquoDeveloping selection protocols for weed competitiveness inaerobic ricerdquo Field Crops Research vol 97 no 2-3 pp 272ndash2852006

[9] K D Gibson A J Fischer T C Foin and J E Hill ldquoCrop traitsrelated to weed suppression in water-seeded rice (Oryza sativaL)rdquoWeed Science vol 51 no 1 pp 87ndash93 2003

[10] D E JohnsonMDingkuhnM P Jones andM CMahamaneldquoThe influence of rice plant type on the effect of weed competi-tion onOryza sativa andOryza glaberrimardquoWeed Research vol38 no 3 pp 207ndash216 1998

[11] A J Fischer H V Ramirez K D Gibson and B Da SilveiraPinheiro ldquoCompetitiveness of semidwarf upland rice cultivarsagainst palisadegrass (Brachiaria brizantha) and signalgrass (Bdecumbens)rdquo Agronomy Journal vol 93 no 5 pp 967ndash9732001

[12] P Anwar A S Juraimi A Man A Puteh A Selamat andM Begum ldquoWeed suppressive ability of rice (Oryza sativa L)germplasm under aerobic soil conditionsrdquoAustralian Journal ofCrop Science vol 4 no 9 pp 706ndash717 2010

[13] A Fischer H V Ramırez and J Lozano ldquoSuppression ofjunglerice [Echinochloa colona (L) Link] by irrigated ricecultivars in Latin AmericardquoAgronomy Journal vol 89 no 3 pp516ndash521 1997

[14] S M Haefele D E Johnson D MrsquoBodj M C S Wopereisand K M Miezan ldquoField screening of diverse rice genotypesforweed competitiveness in irrigated lowland ecosystemsrdquoFieldCrops Research vol 88 no 1 pp 39ndash56 2004

[15] B S Chauhan G Mahajan V Sardana J Timsina and M LJat ldquoProductivity and sustainability of the rice-wheat croppingsystem in the indo-gangetic plains of the Indian subcontinentproblems opportunities and strategiesrdquoAdvances inAgronomyvol 117 pp 315ndash369 2012

[16] D Lemerle B Verbeek and B Orchard ldquoRanking the abilityof wheat varieties to compete with Lolium rigidumrdquo WeedResearch vol 41 no 3 pp 197ndash209 2001

[17] D P Garrity M Movillon and K Moody ldquoDifferential weedsuppression ability in upland rice cultivarsrdquo Agronomy Journalvol 84 pp 586ndash591 1992

[18] K D Gibson J E Hill T C Foin B P Caton and A J FischerldquoWater-seeded rice cultivars differ in ability to interfere withwatergrassrdquo Agronomy Journal vol 93 no 2 pp 326ndash332 2001

[19] M Dingkuhn D E Johnson A Sow and A Y AudebertldquoRelationships between upland rice canopy characteristics andweed competitivenessrdquo Field Crops Research vol 61 no 1 pp79ndash95 1999

[20] M C Richards and G P Whytock ldquoVarietal competitivenesswith weedsrdquo Aspects of Applied Biology vol 34 pp 345ndash3541993

[21] D L ZhaoGNAtlin L Bastiaans and JH J Spiertz ldquoCultivarweed-competitiveness in aerobic rice heritability correlatedtraits and the potential for indirect selection in weed-freeenvironmentsrdquo Crop Science vol 46 no 1 pp 372ndash380 2006

[22] D G Huel and P Hucl ldquoGenotypic variation for competitiveability in spring wheatrdquo Plant Breeding vol 115 no 5 pp 325ndash329 1996

[23] H Q Wang B A M Bouman D L Zhao C Wang and PF Moya ldquoAerobic rice in northern China opportunities andchallengesrdquo in Proceedings of the International Workshop onWater-Wise Rice Production B A M Bouman H HengsdijkB Hardy P S Bindraban T P Tuong and J K Ladha Eds pp143ndash154 International Rice Research Institute April 2002

[24] B S Chauhan and D E Johnson ldquoRow spacing and weedcontrol timing affect yield of aerobic ricerdquo Field Crops Researchvol 121 no 2 pp 226ndash231 2011

[25] M A Baghestani E Zand and S Soufizadeh ldquoIranian winterwheats (Triticum aestivum L) interference with weeds I Grainyield and competitive indexrdquo Pakistan Journal of Weed ScienceResearch vol 12 pp 119ndash129 2006

[26] A Rezakhanlou M Aghabeigi and H Bagheri ldquoEvaluation ofsome of competitiveness indexes in competition between cottonvarieties and common cocklebur (Xanthium strumarium L)rdquoInternational Research Journal of Applied and Basic Sciences vol3 pp 1274ndash1278 2012

[27] R K Coleman G S Gill and G J Rebetzke ldquoIdentification ofquantitative trait loci for traits conferring weed competitivenessin wheat (Triticum aestivum L)rdquo Australian Journal of Agricul-tural Research vol 52 no 11-12 pp 1235ndash1246 2001

[28] J Rodenburg K Saito R G Kakaı A Toure M Mariko andP Kiepe ldquoWeed competitiveness of the lowland rice varietiesof NERICA in the southern Guinea Savannardquo Field CropsResearch vol 114 no 3 pp 411ndash418 2009

[29] B P Caton A E Cope and M Mortimer ldquoGrowth traits ofdiverse rice cultivars under severe competition implications forscreening for competitivenessrdquo Field Crops Research vol 83 no2 pp 157ndash172 2003

[30] M J Kropff ldquoEco-physiological models for crop-weed compe-titionrdquo inModeling Crop-Weed Interactions M J Kropff and HH van Laar Eds pp 25ndash32 CAB International WallingfordUK 1993

[31] F Ekeleme A Y Kamara S O Oikeh et al ldquoResponse ofupland rice cultivars to weed competition in the savannas ofWest Africardquo Crop Protection vol 28 no 1 pp 90ndash96 2009

[32] O S Namuco J E Cairns and D E Johnson ldquoInvestigatingearly vigour in upland rice (Oryza sativa L) part I Seedlinggrowth and grain yield in competition with weedsrdquo Field CropsResearch vol 113 no 3 pp 197ndash206 2009

[33] K Saito K Azoma and J Rodenburg ldquoPlant characteristicsassociated with weed competitiveness of rice under upland andlowland conditions in West Africardquo Field Crops Research vol116 no 3 pp 308ndash317 2010

[34] H Ni K Moody R P Robles E C Paller Jr and J S LalesldquoOryza sativa plant traits conferring competitive ability againstweedsrdquoWeed Science vol 48 no 2 pp 200ndash204 2000

[35] G Wu L T Wilson and A M McClung ldquoContribution of ricetillers to drymatter accumulation and yieldrdquoAgronomy Journalvol 90 no 3 pp 317ndash323 1998

Submit your manuscripts athttpwwwhindawicom

Nutrition and Metabolism

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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

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ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

GenomicsInternational Journal of

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Plant GenomicsInternational Journal of

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Biotechnology Research International

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Cell BiologyInternational Journal of

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2 The Scientific World Journal

to date only few genotypes of rice for instance Haefele et al[14] with superior weed competitiveness are known Hybridsdue to their early vigor may have the potential to com-plement the limited set of available competitive germplasmfor DSR The most important breeding objectives for theDSR genotypes were yield potential high environmentaladaptation early vigor and potentially favorable growthtraits for weed suppression Empirical evidence of superiorperformance of hybrids and new inbred lines adapted toDSR in particular the ability to better cope with weedsis still awaited Various authors suggested the evaluation ofhybrids in DSR to confirm possession of weed-competitivetraits and provide farmers with a wider choice of optionswhen cultivating DSR [15]

Morphological physiological and biochemical traits arethought to control plant competitiveness [16] and manystudies have been conducted to determine plant charactersconferring competitive ability in cereals Plant height playsa role in the competitive ability of rice [17] Crop heightappeared to have the greatest impact on competitive abilitywith the shortest cultivars experiencing the largest yieldreductions and allowing the greatest weed growth Howeverheight alone does not explain competitive ability becausesome shorter cultivars have been found to be good competi-tors in rice [18] Some workers found that leaf area index(LAI) to be negatively correlated with specific leaf area drymatter partitioning of leaves and mean tip elevation angle[19] They concluded that specific leaf area and tilleringability are major determinants of vegetative vigor Theyfurther emphasized that vegetative vigor and crop durationaffecting the ability of genotypes to recover from earlycompetition are the useful traits in the selection of weed-competitive rice Evidence that early season ground coveralso reduces subsequent weed biomass has been reportedby many researchers [20ndash22] A better understanding ofthe mechanisms by which a rice genotype becomes morecompetitive to weeds would not only serve to assist plantbreeders in developing competitive cultivars more quicklyand effectively but would also justify the use of plant breedingto increase crop-competitive ability [8] Yield gains of 7ndash9have been identified in ldquocompetitiverdquo aerobic cultivars whencompared with ldquononcompetitiverdquo cultivars [23]

The first objective of this study was to evaluate the weedcompetitiveness and yield potential of some popular hybridsand inbreds to identify superior genotypes that are potentiallysuitable for use in integrated weed management strategies inDSR Second objective was to determine rice traits measuredin weed-free and weedy conditions could be related to weeddry matter recorded under weedy conditions This is thefirst study presenting a comprehensive assessment of weed-competitive ability of the popular rice genotypeshybrids ofthis region for the DSR system

2 Materials and Methods

21 Site Description Field studies were conducted to assessthe effect of weed competition on the performance of ricegenotypes in direct-seeded conditions in the rainy seasons(JunendashOctober) of 2011 and 2012 at the research farm of the

Punjab Agricultural University Ludhiana (30∘561015840N 75∘521015840E)India The climate of the region is semiarid with an averageannual rainfall of 400ndash700mm (75ndash80 of which is receivedfrom July to September) aminimum temperature of 0ndash4∘C inJanuary and amaximum temperature of 41ndash45∘C in JuneThesoil type at the experimental site was sandy loam with 03organic matter and a pH of 72 The total N content in thesoil was 0042 The available P (18 kg haminus1) and K contents(290 kg haminus1) in soil were medium and high respectivelyField had the history of dry direct-seeded rice for 3 yearsThe weeds present in the field were Commelina benghalen-sis L Cyperus rotundus L Dactyloctenium aegyptium (L)WilldDigera arvensis ForskDigitaria sanguinalis (L) ScopEchinochloa colona (L) Link Eragrostis spp and Leptochloachinensis (L) Nees Weed composition was near to uniformin the entire field

22 Experimental Design The experiment in each yearwas established in a split-plot design with three replicatesThe main plots included two levels of weed infestation(weed-free and partial weedy) and subplots (62m times 14m)included eight rice genotypes Among genotypes there werefive inbred lines (two cultivars PR-115 and PR-120 threeadvanced breeding lines CR2707 IR83927 and IR88633)and three hybrids (H-97158 RH-257 and US-310) Thesegenotypes were selected on the basis of yield in our previousbreeding trials on DSR All the selected genotypes were ofindica rice

In the weed-free plots pendimethalin (750 g ai haminus1)as preemergence (PRE) was applied at 3 d after sowing(DAS) and bispyribac-sodium (25 g ai haminus1) as postemer-gence (POST) at 18 DAS Following bispyribac-sodium appli-cation plots were hand-weeded as needed to remove allweeds in the remainder of the season Herbicides wereapplied using a knapsack sprayer with a flat fan nozzle andwater as carrier at 375 L haminus1 In completely weedy plots yieldlosses in DSR are greater than 90 [24] In addition it is notcommon for farmers to leave their rice fields infested withweeds in irrigated areas The middle of the critical weed-freeperiod in DSR is around 28 DAS [24] Therefore to makepartial weedy plots the weedy plots were hand-weeded onceat 28 DAS and weeds were allowed to grow before and afterthe hand-weeding throughout the season [8]

23 Crop Management Fields were prepared by cultivatingtwice using a disc harrow followed by leveling with a woodenboard Seeds were sown by a single-row drill at a seedingrate of 30 kg haminus1 at 20-cm row spacing on June 21 2011 andJune 16 2012 The field was surface-irrigated immediatelyafter sowing Nitrogen (N) fertilizer at 130 kg haminus1 (as urea)was applied in four equal splits at 14 28 49 and 70 DASRecommended rates of chlorpyrifos (500 g ai haminus1) and prop-iconazole (625 g ai haminus1) were used to control insect pests anddiseases respectively

The LAI at tillering panicle initiation and floweringwas measured with a digital plant canopy imager (modelCI110CI-120 CID Inc CamasWA) but the data are shownonly for the tillering stage (28 DAS) due to similar trend

The Scientific World Journal 3

Under partial weedy condition digital plant canopy imageralso assesses the LAI of crop and weeds so LAI was onlyrecorded under weed free conditions Measurements weremade with the probe set parallel to the rows at two fixedlocations in each plot Two plants were randomly selectedfrom each plot at the early ripening stage for root samplingRoot samples were collected by removing soil to a depth of45 cm along with the plants with a 10-cm-diameter augerA uniform soil volume (3534 cm3) was excavated to collectroot samples from all the treatments Roots were carefullywashed and various parameters (root biomass and volume)were measured and calculated Root volume was measuredusing the water displacement method

Twoquadrats 025m2 size were placed at random in eachplot to determine weed biomass at 28 DAS and at floweringOnly at 28 DAS weeds were counted species-wise anddifferentiated into categories of sedges grass and broadleafweeds Weeds were cut at ground level washed with tapwater sun-dried and then oven-dried at 70∘C until constantweight and weighed For crop biomass plant samples werecollected in quadrats (040m times 025m) placed randomlyat two locations in each plot at tillering panicle initiationand flowering stages The plants were cut at ground leveloven-dried at 70∘C for 72 h and weighed Grain yield wasmeasured at 14moisture from a sampling area of 52m2 perplot At the same time five plants were selected randomlyfrom each plot to measure agronomic parameters which in-cluded plant height grains panicleminus1 and spikelet sterility() Competitiveness was measured as weed competitiveindex (CI) and calculated as [25 26]

CI =[119881infest119881mean]

[119882119894

119882mean] (1)

where 119881infest is yield of variety (119894) in terms of weed infested119881mean is the average yield of all varieties in the presence ofweed 119882

119894

is weed biomass varieties of 119894 119882mean is averageweed biomass is mixed with all varieties

Crop vigor recorded as visually rated crop biomass at2 week after sowing on a per-plot basis on a 1-to-9 scalewhere 9 was the greatest crop biomass and 1 was the least[8] The relative yield loss (YL) of the crop challenged byweed competition under field conditions was estimated usingequation YL () = 1minus(119884CW119884CM)times100 where119884CW and119884CMare crop yields in competition with weeds and in weed-freeconditions respectively

24 Statistical Analyses In a combined analysis of data theinteractions of years with the level of weed infestation andorgenotypes were nonsignificant therefore the data werepooled over the years (therefore a total of six replications)for further analyses (GenStat 80) Treatment means wereseparated using the least significant difference (LSD) testat the 5 level of significance The relationships betweengrain yield (t haminus1) and various crop traitsparameters wereassessed using linear regression analysis (SigmaPlot 100)

Table 1 Weed density (number mminus2) in response to differentgenotypes at 28 days after sowing

GenotypeWeed density

Grasses Broadleaved Sedgesnumber mminus2

PR-115 42 20 24H-97158 54 25 31CR2707 61 29 35US-310 36 17 20IR88633 26 12 15IR83927 39 18 22RH-257 32 15 18PR-120 30 14 17LSD (005) 7 3 4

3 Results and Discussion

31 Weed Composition Ten weed species were frequentlyobserved in the weedy plots during both years The mostdominant weed species (on the basis of density) encounteredin the weedy plots at 28 DAS in both years were Cyperusiria L (21) Cyperus rotundus (7) Echinochloa colona(10)Alternanthera sessilis (L) R Br exDC (4)Leptochloachinensis (12) Digitaria sanguinalis (21) Dactylocteniumaegyptium (6)Digera arvensis (5)Commelina benghalen-sis (5) and Eragrostis spp (9) Genotype CR-2707 hadthe highest sedges grasses and broad-leaf density among allthe tested genotypes (Table 1) Sedges grasses and broadleafweed density were similar for genotypes PR-120 IR88633and RH-257 however lower than CR-2707 and H-97158Genotypes PR-115 and IR-83927 had similar sedges grassesand broadleaf weed density but lower than that of CR-2707Weed species found in the trial do prefer upland rather thanflooded conditionsThe fieldwas exposed to alternate wettingand drying conditions and these conditions are likely to favorspecies diversification depending upon the competitive abil-ity of the genotypes In DSR weed composition is generallyfound to be more diverse than in transplanted rice [4]

32 Weed Biomass and Grain Yield At 28 DAS H-97158and CR2707 accrued with higher weed biomass than othergenotypes (Table 2) At this stage lowest weed biomass wasrecorded in PR-120 Weed biomass at flowering was alsolowest in PR-120 and it was highest in hybrid H-97158(Table 2) Genotypes IR88633 IR83927 PR-120 and RH-257accrued with similar but lower weed biomass than that ofH-97158 Genotypes US-310 CR2707 and H-97158 accruedwith similar weed biomass however they had higher weedbiomass thanPR-120 PR-115 hadweed biomass similar to thatof IR88633 IR83927 and RH-257

Weed competition caused significant reduction in grainyield of all the tested genotypes and the reduction waspositively correlated with weed biomass (119903 = 065 119875 = 040)Grain yield in weedy plots varied from 36 to 67 t haminus1 acrossdifferent genotypes and it was lowest and highest for PR-115and PR-120 respectively However in weed-free plots grain

4 The Scientific World Journal

Table 2 Weed biomass (gmminus2) in response to different genotypesat 28 d after sowing (DAS) and flowering

GenotypeWeed biomass

28 DAS Floweringgmminus2

PR-115 854 3330H-97158 1175 4185CR2707 1257 3865US-310 794 3812IR88633 567 2211IR83927 706 2808RH-257 613 2380PR-120 448 1743LSD (005) 261 1184

yield varied from 66 to 89 t haminus1 across different genotypesand it was lowest for PR-115 and highest forH-97158 (Table 3)Average yield loss due to weed competition ranged from21 to 46 among different genotypes The higher the weedbiomass the greater the percent yield reduction The highestrelative yield loss due to weed competition was noted in PR-115 and the relative yield loss of this cultivar was similar tothat of H-97158 CR2707 US-310 and RH-257 The lowestrelative yield loss due to weed competition was in PR-120which was similar to that of IR88633 and IR83927 Theseresults suggest that rice genotypes responded differently intheir competitiveness in suppressing weeds under severeinfestation PR-120 IR88633 and IR83927 proved to be betterweed competitors than the other genotypes

Correlation analyses were used to explore the relationshipbetween grain yield of different rice genotypes and plant char-acteristics in both weed-free and partial weedy conditionsIn weed-free conditions only grain yield had a significantpositive correlation with panicle density mminus2 (119903 = 077)(Table 4) In partial weedy conditions however grain yieldhad a significant positive correlation with panicle densitymminus2 grain panicleminus1 weed competitive index LAI rootweight and root volume (Table 5) These results indicate thatthese parameters could have played a vital role in improvinggrain yield under crop-weed competition In partial weedyconditions grain yield was negatively related with spikeletsterility relative yield loss and weed biomass

In weed-free plots the highest grain yield was found inH-97158 followed by RH-257 This was attributed to higherpanicle density in H-97158 Lowest grain yield was recordedin PR-115Weed competitive indexwas highest for PR-120 andother genotypes had lower weed competitive index than thiscultivar (Figure 1) PR-115 had the lowest weed competitiveindex which was similar to that of genotypes H-97158 CR2707 and US-310 On the other hand IR88633 IR83927and RH-257 had better weed competitive index than PR-115The high performance of these genotypes in terms of weedcompetitive index could be attributed to less weed biomassobserved due to their ability to suppress weeds Weeds canbe suppressed by allelopathic effects of rice genotypes inaddition to genotypic smothering effect on weeds however

Genotype

25

20

15

10

05

00

Com

petit

ive i

ndex

LSD

PR-115

H-97158

CR2707

-185

-16

-1-1

-1

US-310

IR88633

-1-79

-B-1

IR83927

-B-B

-278

-CRA

-1-1

-1

RH-257

PR-120

Figure 1 Weed competitive index of different rice genotypes

the genotypes used in our study have not been reported fortheir allelopathic effect

The interaction effects of weed infestation levels andgenotypes were significant for grain yield panicle numbermminus2 and grain number panicleminus1 (Table 3) Across weedinfestation levels 1000-grain weight was highest for IR88633followed by IR83927 (Table 6) Averaged over weed infesta-tion levels 1000-grain weight of all the other genotypes wassimilar to but lower than that of IR88633 In the partial weedyconditions 1000-grain weight had a positive correlation withweed competitive index whereas such correlation was notfound in weed-free conditions Plant height differed acrossthe genotypes (Table 6) IR88633 attained the highest plantheight followed by IR83927 and CR2707 Lowest plant heightwas found for PR-115 Genotypes H-97158 US-310 andRH-257 attained similar plant height Under partial weedyconditions plant height had a negative correlation withrelative yield loss

As expected grain yield of all genotypes in weedyconditions decreased significantly as compared with yield inweed-free conditions (Table 3) In partial weedy conditionshighest grain yield was found for PR-120 closely followed byIR83927 Grain yield of PR-120 in partial weedy conditionswas similar to that of PR-115 US-310 and IR88633 in weed-free conditions This response was because of the bettercompetitive ability of PR-120 that is it had the highest weedcompetitive index (Figure 1) and root biomass and volume(Table 7) Root biomass and volume were highest in PR-120and lowest in PR-115 however these root parameters of PR-115 H-97158 CR2707 and US-310 were the same SimilarlyIR88633 IR83927 and RH-257 had almost identical root bio-mass and volume High root biomass and volume offeredcrop-weed competition in favor of the crop providing greater

The Scientific World Journal 5

Table 3 Grain yield (t haminus1) panicles mminus2 and grains panicleminus1 in response to the interaction effect of weed infestation levels and genotypes

GenotypeGrain yield Panicles Filled grains

Weed-freea Partial weedy Weed-free Partial weedy Weed-free Partial weedyt haminus1 number mminus2 number panicleminus1

PR-115 659 358 290 230 114 96H-97158 889 515 351 236 133 103CR2707 743 480 305 238 136 96US-310 678 452 252 247 148 95IR88633 695 520 278 249 130 91IR83927 809 603 336 259 122 108RH-257 863 550 340 252 136 115PR-120 847 666 362 308 124 120LSD (005) 07 29 14aWeed-free plots were kept weed-free throughout the season Partial weedy plots plots were hand-weeded once at 28DAS and weeds were allowed to growbefore and after the hand-weeding throughout the season

Table 4 Correlation among different traits in weed-free conditions The critical value of 119903 was 040 at 5 level of significance

Traits GY Pan Gr TGW SS Ht CI RYL WP LAI VS RWGYPan 077Gr minus006 minus030TGW 013 minus003 minus016SS minus036 minus041 minus009 minus039Ht minus005 minus024 011 077 minus028CI 025 035 minus001 020 minus056 009RYL minus008 minus021 minus002 minus041 044 minus022 minus069WP minus002 minus019 028 minus033 032 minus013 minus078 065LAI 039 032 005 062 minus066 062 060 minus039 minus040VS 001 minus004 minus009 060 minus038 059 035 minus030 minus017 072RW 032 045 minus015 041 minus049 010 058 minus064 minus056 048 037RV 032 045 minus015 041 minus049 010 058 minus064 minus056 048 037 100GY grain yield LAI leaf area index at 28DAS Ht height RW root weight RV root volume Pan panicle mminus2 Gr filled grains panicleminus1 TGW 1000-grainweight SS spikelet sterility CI weed competitive index WP weed biomass and RYL relative yield loss

Table 5 Correlation among different traits in partial weedy conditions The critical value of 119903 was 040 at 5 level of significance

Traits GY Pan Gr TGW SS Ht CI RYL WP LAI VS RWGYPan 061Gr 053 062TGW 033 018 000SS minus047 minus034 minus039 minus038Ht 023 001 minus017 078 minus024CI 071 032 026 021 minus049 017RYL minus084 minus051 minus031 minus041 026 minus042 minus069WP minus056 minus029 minus027 minus034 032 minus020 minus078 065LAI 051 014 014 059 minus062 063 060 minus039 minus040VS 022 008 minus010 062 minus041 070 035 minus030 minus017 072RW 070 063 038 055 minus047 033 058 minus064 minus056 048 037RV 070 063 038 055 minus047 033 058 minus064 minus056 048 037GY grain yield LAI leaf area index at 28DAS Ht height RW root weight RV root volume Pan panicle mminus2 Gr filled grains panicleminus1 TGW 1000-grainweight SS spikelet sterility CI weed competitive index WP weed biomass and RYL relative yield loss

6 The Scientific World Journal

Table 6 Final plant height (cm plantminus1) 1000-grain weight (g)and spikelet sterility () in response to weed infestation levels andgenotypes

Treatment Plant height 1000-grainweight

Spikeletsterility

cm g Weed infestation level

Weed-freea 1153 260 195Partial weedy 1165 256 201LSD (005) NS NS 16

GenotypePR-115 935 247 262H-97158 1094 255 182CR2707 1246 254 157US-310 1092 243 258IR88633 1502 289 166IR83927 1269 270 171RH-257 1126 258 194PR-120 1001 255 146LSD (005) 71 05 39

Nonsignificant NSaWeed free plots plots were kept weed free throughout the season Partialweedy plots plotswere hand-weeded once at 28DAS andweedswere allowedto grow before and after the hand-weeding throughout the season

Table 7 Leaf area index (LAI) and vigor score at 28 days aftersowing and root biomass and volume at flowering of variousgenotypes under weed-free conditions

Genotype LAI Vigor score Root biomass Root volumegmminus2 mLmminus2

PR-115 186 43 1478 5615H-97158 273 47 2009 7635CR2707 326 90 1965 7466US-310 190 47 1757 6676IR88633 337 90 3428 13027IR83927 289 70 2914 11073RH-257 310 67 3034 11530PR-120 282 67 4295 16538LSD (005) 043 06 755 2870

vigor and early canopy closure as evidenced by the higher LAIof these genotypes

Vigor score was highest in cultivars IR88633 and CR2707(Table 7) Among the tested genotypes vigor score was lowestfor PR-115 followed by H-97158 and US-310 Early vigorhas been shown as an important characteristics related tocompetitive ability [16] and tends to be reflected in greatershoot and root biomass during early vegetative growth Inthe present study crop biomass of PR-120 remained similarin both partial weedy and weed-free conditions (Table 8)At panicle initiation PR-120 in the partial weedy conditionsaccruedwith similar crop biomass withH-97158 and IR88633in the weed-free conditions In the partial weedy conditions

at panicle initiation and flowering stages IR88633 IR83927RH-257 and PR-120 accrued with higher crop biomass thanthat of PR-115 H-97158 CR2707 and US-310 respectively(Table 8)

The present study revealed that higher biomass coupledwith higher plant height at early stages provided greaterstored assimilates for grain filling via translocation thatmay prevent the yield loss of these genotypes under par-tial weedy conditions In partial weedy conditions grainnumber panicleminus1 remained similar only in IR83927 andPR-120 as compared with that in weed-free conditions andthis response was mainly because of their better weed-competitive ability In partial weedy conditions the highestnumber of grains panicleminus1 was found in PR-120 Grainnumber panicleminus1 for PR-120 in partial weedy conditions wassimilar to that of PR-115 H-97158 IR88633 and IR83927 inthe weed-free conditions

High vigor score and LAI during early crop developmenthave been shown to bemajor plant traits contributing toweedcompetitiveness [27 28]The LAI is an important contributorto competitive ability as light is an important resource inplant growth Greater shading ability of the crop makes lesslight available to the competing weeds hence weed growthcan be reduced In the weed-free plots IR88633 attained thehighest LAI at 28 DAS whereas PR-115 attained the lowestLAI (Table 7) The LAI of PR-115 and US-310 was similarGenotypes H-97158 RH-257 PR-120 and IR83927 recordedsimilar LAI but this was higher than that of PR-115 The LAIof CR2707 and IR88633 was similar

The findings of our study indicate that fast-growing habitand the high LAI at the early stage of the crop along withhigh root biomass and volume are the important traits forweed competitiveness confirming studies by various authors[18 29] However some workers [30] demonstrated that anadvantage of early growth habit even for a few days couldshift the competitive balance between crops and weeds Inanother study some workers found a significant correlationbetween crop competitive ability and plant height [31] In thepresent study a negative correlation was found between plantheight and relative yield loss In contrast Fischer et al foundno correlation between the height of semidwarf cultivars andweed biomass [11] As in our study research by some scientists[32] suggested that rice cultivars capable of producing moregrain yield in competition with weeds have greater earlybiomass Similarly workers [8 33] proposed that early vigorshould be a key selection trait for weed competitiveness PR-120 owing to its high root biomass and volume which helpedin attaining high crop biomass and greater panicle numbermminus2 produced the highest yield in partial weedy conditionsOur results confirm the findings of researchers who reportedsignificant and negative correlation between weed biomassand panicle number mminus2 suggesting that panicle density wasdecreased by high weed growth [31]

Competitive ability is the ability of a genotype to suppressweed biomass or to maintain crop yields by tolerating weedcompetition In this study competitive ability was foundhighest for PR-120 The competitiveness of a cultivar mightbe due to the higher number of panicles mminus2 early growth

The Scientific World Journal 7

Table 8 Crop biomass (t haminus1) in response to the interaction effect of weed infestation levels and genotypes at panicle initiation and floweringstages

Weed infestation levelCrop biomass

PR-115 H-97158 CR2707 US-310 IR88633 IR83927 RH-257 PR-120t haminus1

Panicle initiation stageWeed-freea 43 48 320 380 490 440 450 460Partial weedy 330 325 315 340 422 414 440 480LSD (005) 03

Flowering stageWeed-free 698 748 698 721 802 712 732 718Partial weedy 382 425 410 412 490 512 530 592LSD (005) 05aWeed free plots plots were kept weed free throughout the season Partial weedy plots plots were hand-weeded once at 28DAS and weeds were allowed togrow before and after the hand-weeding throughout the season

and yield under both partial weedy andweed-free conditionsIn this study panicle number mminus2 also had a correlationwith grain yield under both partial weedy and weed-freeconditions Cultivars able to produce higher grain yields inpartial weedy conditions correlated with greater early ricegrowth and LAI [8 34 35] Likewise this study demonstratedthat high LAI along with high root biomass and volumewere the useful traits for weed-competitive genotypes CR-2707 and RH-257 had higher LAI and vigor and similarlyRH-257 had higher root biomass and volume but yield lossof these genotypes was found more than 35 This was dueto their (CR-2707 and RH-257) more tendencies for lodgingunder partial weedy conditions

In summary genotypes PR-115 and H-97158 were theworst competitors and PR-120 IR88633 and IR83927 weregood weed competitors Genotypes PR-120 IR88633 andIR83927 have potential for increasing the weed-competitiveability of genotypes in breeding programs and could beused to increase the competitiveness of highly productivegenotypes that are not competitive The study revealed thatearly canopy closure high LAI at the early stage and greaterroot biomass and volume correlated positively with compet-itiveness This study demonstrated that breeding to increasethe competiveness of highly productive rice plant typeswouldbe possible without compromising yieldThe competitivenessobserved in these studies for genotypes PR-120 IR88633 andIR83927 would be adequate in improving farmersrsquo incomeand reducing herbicide use and can be successfully used inintegrated weed management programs in DSR

4 Future Implications

This study implies that breeding for weed-suppressive ricegenotypes should be conducted under partial weedy con-ditions In a mixed weed population the weed-competitiveability of rice genotypes may be affected by weed composi-tion However the competitive ability of rice genotypes is acomplex trait and could not be explained by only one or twocharacteristics Interactions between traits for instance rootbiomass LAI and high shoot biomass at early stage play an

important role and these traits can be explored for identifyingweed-competitive genotypes in DSR Competitiveness of ricegenotypes against weeds will be an important key to thesuccessful adoption of weed-suppressive rice genotypes ina sustainable weed management program The developmentof such genotypes could play an important role in DSR byreducing herbicide load in agroeco system

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors are grateful to Bill Hardy International RiceResearch Institute Philippines for providing comments onthe paper The authors would also like to acknowledge thefunding from the Cereal Systems Initiatives for South Asia(CSISA)

References

[1] G Mahajan B S Chauhan and M S Gill ldquoDry-seeded riceculture in Punjab State of India lessons learned from farmersrdquoField Crops Research vol 144 pp 89ndash99 2013

[2] G Mahajan and B S Chauhan ldquoHerbicide options for weedControl in dry-seeded aromatic rice in IndiardquoWeed Technologyvol 27 pp 682ndash689 2013

[3] B S Chauhan ldquoWeed ecology andweedmanagement strategiesfor dry-seeded rice in AsiardquoWeed Technology vol 26 no 1 pp1ndash13 2012

[4] A N Rao D E Johnson B Sivaprasad J K Ladha and A MMortimer ldquoWeed management in direct-seeded ricerdquo Advancesin Agronomy vol 93 pp 153ndash255 2007

[5] D L Zhao G N Atlin L Bastiaans and J H J SpiertzldquoComparing rice germplasm groups for growth grain yield andweed-suppressive ability under aerobic soil conditionsrdquo WeedResearch vol 46 no 6 pp 444ndash452 2006

8 The Scientific World Journal

[6] D Lemerle B Verbeek R D Cousens and N E CoombesldquoThe potential for selecting wheat varieties strongly competitiveagainst weedsrdquoWeed Research vol 36 no 6 pp 505ndash513 1996

[7] G Mahajan and B S Chauhan ldquoThe role of cultivars in man-aging weeds in dry-seeded rice production systemsrdquo CropProtection vol 49 pp 52ndash57 2013

[8] D L Zhao G N Atlin L Bastiaans and J H J SpiertzldquoDeveloping selection protocols for weed competitiveness inaerobic ricerdquo Field Crops Research vol 97 no 2-3 pp 272ndash2852006

[9] K D Gibson A J Fischer T C Foin and J E Hill ldquoCrop traitsrelated to weed suppression in water-seeded rice (Oryza sativaL)rdquoWeed Science vol 51 no 1 pp 87ndash93 2003

[10] D E JohnsonMDingkuhnM P Jones andM CMahamaneldquoThe influence of rice plant type on the effect of weed competi-tion onOryza sativa andOryza glaberrimardquoWeed Research vol38 no 3 pp 207ndash216 1998

[11] A J Fischer H V Ramirez K D Gibson and B Da SilveiraPinheiro ldquoCompetitiveness of semidwarf upland rice cultivarsagainst palisadegrass (Brachiaria brizantha) and signalgrass (Bdecumbens)rdquo Agronomy Journal vol 93 no 5 pp 967ndash9732001

[12] P Anwar A S Juraimi A Man A Puteh A Selamat andM Begum ldquoWeed suppressive ability of rice (Oryza sativa L)germplasm under aerobic soil conditionsrdquoAustralian Journal ofCrop Science vol 4 no 9 pp 706ndash717 2010

[13] A Fischer H V Ramırez and J Lozano ldquoSuppression ofjunglerice [Echinochloa colona (L) Link] by irrigated ricecultivars in Latin AmericardquoAgronomy Journal vol 89 no 3 pp516ndash521 1997

[14] S M Haefele D E Johnson D MrsquoBodj M C S Wopereisand K M Miezan ldquoField screening of diverse rice genotypesforweed competitiveness in irrigated lowland ecosystemsrdquoFieldCrops Research vol 88 no 1 pp 39ndash56 2004

[15] B S Chauhan G Mahajan V Sardana J Timsina and M LJat ldquoProductivity and sustainability of the rice-wheat croppingsystem in the indo-gangetic plains of the Indian subcontinentproblems opportunities and strategiesrdquoAdvances inAgronomyvol 117 pp 315ndash369 2012

[16] D Lemerle B Verbeek and B Orchard ldquoRanking the abilityof wheat varieties to compete with Lolium rigidumrdquo WeedResearch vol 41 no 3 pp 197ndash209 2001

[17] D P Garrity M Movillon and K Moody ldquoDifferential weedsuppression ability in upland rice cultivarsrdquo Agronomy Journalvol 84 pp 586ndash591 1992

[18] K D Gibson J E Hill T C Foin B P Caton and A J FischerldquoWater-seeded rice cultivars differ in ability to interfere withwatergrassrdquo Agronomy Journal vol 93 no 2 pp 326ndash332 2001

[19] M Dingkuhn D E Johnson A Sow and A Y AudebertldquoRelationships between upland rice canopy characteristics andweed competitivenessrdquo Field Crops Research vol 61 no 1 pp79ndash95 1999

[20] M C Richards and G P Whytock ldquoVarietal competitivenesswith weedsrdquo Aspects of Applied Biology vol 34 pp 345ndash3541993

[21] D L ZhaoGNAtlin L Bastiaans and JH J Spiertz ldquoCultivarweed-competitiveness in aerobic rice heritability correlatedtraits and the potential for indirect selection in weed-freeenvironmentsrdquo Crop Science vol 46 no 1 pp 372ndash380 2006

[22] D G Huel and P Hucl ldquoGenotypic variation for competitiveability in spring wheatrdquo Plant Breeding vol 115 no 5 pp 325ndash329 1996

[23] H Q Wang B A M Bouman D L Zhao C Wang and PF Moya ldquoAerobic rice in northern China opportunities andchallengesrdquo in Proceedings of the International Workshop onWater-Wise Rice Production B A M Bouman H HengsdijkB Hardy P S Bindraban T P Tuong and J K Ladha Eds pp143ndash154 International Rice Research Institute April 2002

[24] B S Chauhan and D E Johnson ldquoRow spacing and weedcontrol timing affect yield of aerobic ricerdquo Field Crops Researchvol 121 no 2 pp 226ndash231 2011

[25] M A Baghestani E Zand and S Soufizadeh ldquoIranian winterwheats (Triticum aestivum L) interference with weeds I Grainyield and competitive indexrdquo Pakistan Journal of Weed ScienceResearch vol 12 pp 119ndash129 2006

[26] A Rezakhanlou M Aghabeigi and H Bagheri ldquoEvaluation ofsome of competitiveness indexes in competition between cottonvarieties and common cocklebur (Xanthium strumarium L)rdquoInternational Research Journal of Applied and Basic Sciences vol3 pp 1274ndash1278 2012

[27] R K Coleman G S Gill and G J Rebetzke ldquoIdentification ofquantitative trait loci for traits conferring weed competitivenessin wheat (Triticum aestivum L)rdquo Australian Journal of Agricul-tural Research vol 52 no 11-12 pp 1235ndash1246 2001

[28] J Rodenburg K Saito R G Kakaı A Toure M Mariko andP Kiepe ldquoWeed competitiveness of the lowland rice varietiesof NERICA in the southern Guinea Savannardquo Field CropsResearch vol 114 no 3 pp 411ndash418 2009

[29] B P Caton A E Cope and M Mortimer ldquoGrowth traits ofdiverse rice cultivars under severe competition implications forscreening for competitivenessrdquo Field Crops Research vol 83 no2 pp 157ndash172 2003

[30] M J Kropff ldquoEco-physiological models for crop-weed compe-titionrdquo inModeling Crop-Weed Interactions M J Kropff and HH van Laar Eds pp 25ndash32 CAB International WallingfordUK 1993

[31] F Ekeleme A Y Kamara S O Oikeh et al ldquoResponse ofupland rice cultivars to weed competition in the savannas ofWest Africardquo Crop Protection vol 28 no 1 pp 90ndash96 2009

[32] O S Namuco J E Cairns and D E Johnson ldquoInvestigatingearly vigour in upland rice (Oryza sativa L) part I Seedlinggrowth and grain yield in competition with weedsrdquo Field CropsResearch vol 113 no 3 pp 197ndash206 2009

[33] K Saito K Azoma and J Rodenburg ldquoPlant characteristicsassociated with weed competitiveness of rice under upland andlowland conditions in West Africardquo Field Crops Research vol116 no 3 pp 308ndash317 2010

[34] H Ni K Moody R P Robles E C Paller Jr and J S LalesldquoOryza sativa plant traits conferring competitive ability againstweedsrdquoWeed Science vol 48 no 2 pp 200ndash204 2000

[35] G Wu L T Wilson and A M McClung ldquoContribution of ricetillers to drymatter accumulation and yieldrdquoAgronomy Journalvol 90 no 3 pp 317ndash323 1998

Submit your manuscripts athttpwwwhindawicom

Nutrition and Metabolism

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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Agronomy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

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The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

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PsycheHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

GenomicsInternational Journal of

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Plant GenomicsInternational Journal of

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Biotechnology Research International

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Cell BiologyInternational Journal of

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The Scientific World Journal 3

Under partial weedy condition digital plant canopy imageralso assesses the LAI of crop and weeds so LAI was onlyrecorded under weed free conditions Measurements weremade with the probe set parallel to the rows at two fixedlocations in each plot Two plants were randomly selectedfrom each plot at the early ripening stage for root samplingRoot samples were collected by removing soil to a depth of45 cm along with the plants with a 10-cm-diameter augerA uniform soil volume (3534 cm3) was excavated to collectroot samples from all the treatments Roots were carefullywashed and various parameters (root biomass and volume)were measured and calculated Root volume was measuredusing the water displacement method

Twoquadrats 025m2 size were placed at random in eachplot to determine weed biomass at 28 DAS and at floweringOnly at 28 DAS weeds were counted species-wise anddifferentiated into categories of sedges grass and broadleafweeds Weeds were cut at ground level washed with tapwater sun-dried and then oven-dried at 70∘C until constantweight and weighed For crop biomass plant samples werecollected in quadrats (040m times 025m) placed randomlyat two locations in each plot at tillering panicle initiationand flowering stages The plants were cut at ground leveloven-dried at 70∘C for 72 h and weighed Grain yield wasmeasured at 14moisture from a sampling area of 52m2 perplot At the same time five plants were selected randomlyfrom each plot to measure agronomic parameters which in-cluded plant height grains panicleminus1 and spikelet sterility() Competitiveness was measured as weed competitiveindex (CI) and calculated as [25 26]

CI =[119881infest119881mean]

[119882119894

119882mean] (1)

where 119881infest is yield of variety (119894) in terms of weed infested119881mean is the average yield of all varieties in the presence ofweed 119882

119894

is weed biomass varieties of 119894 119882mean is averageweed biomass is mixed with all varieties

Crop vigor recorded as visually rated crop biomass at2 week after sowing on a per-plot basis on a 1-to-9 scalewhere 9 was the greatest crop biomass and 1 was the least[8] The relative yield loss (YL) of the crop challenged byweed competition under field conditions was estimated usingequation YL () = 1minus(119884CW119884CM)times100 where119884CW and119884CMare crop yields in competition with weeds and in weed-freeconditions respectively

24 Statistical Analyses In a combined analysis of data theinteractions of years with the level of weed infestation andorgenotypes were nonsignificant therefore the data werepooled over the years (therefore a total of six replications)for further analyses (GenStat 80) Treatment means wereseparated using the least significant difference (LSD) testat the 5 level of significance The relationships betweengrain yield (t haminus1) and various crop traitsparameters wereassessed using linear regression analysis (SigmaPlot 100)

Table 1 Weed density (number mminus2) in response to differentgenotypes at 28 days after sowing

GenotypeWeed density

Grasses Broadleaved Sedgesnumber mminus2

PR-115 42 20 24H-97158 54 25 31CR2707 61 29 35US-310 36 17 20IR88633 26 12 15IR83927 39 18 22RH-257 32 15 18PR-120 30 14 17LSD (005) 7 3 4

3 Results and Discussion

31 Weed Composition Ten weed species were frequentlyobserved in the weedy plots during both years The mostdominant weed species (on the basis of density) encounteredin the weedy plots at 28 DAS in both years were Cyperusiria L (21) Cyperus rotundus (7) Echinochloa colona(10)Alternanthera sessilis (L) R Br exDC (4)Leptochloachinensis (12) Digitaria sanguinalis (21) Dactylocteniumaegyptium (6)Digera arvensis (5)Commelina benghalen-sis (5) and Eragrostis spp (9) Genotype CR-2707 hadthe highest sedges grasses and broad-leaf density among allthe tested genotypes (Table 1) Sedges grasses and broadleafweed density were similar for genotypes PR-120 IR88633and RH-257 however lower than CR-2707 and H-97158Genotypes PR-115 and IR-83927 had similar sedges grassesand broadleaf weed density but lower than that of CR-2707Weed species found in the trial do prefer upland rather thanflooded conditionsThe fieldwas exposed to alternate wettingand drying conditions and these conditions are likely to favorspecies diversification depending upon the competitive abil-ity of the genotypes In DSR weed composition is generallyfound to be more diverse than in transplanted rice [4]

32 Weed Biomass and Grain Yield At 28 DAS H-97158and CR2707 accrued with higher weed biomass than othergenotypes (Table 2) At this stage lowest weed biomass wasrecorded in PR-120 Weed biomass at flowering was alsolowest in PR-120 and it was highest in hybrid H-97158(Table 2) Genotypes IR88633 IR83927 PR-120 and RH-257accrued with similar but lower weed biomass than that ofH-97158 Genotypes US-310 CR2707 and H-97158 accruedwith similar weed biomass however they had higher weedbiomass thanPR-120 PR-115 hadweed biomass similar to thatof IR88633 IR83927 and RH-257

Weed competition caused significant reduction in grainyield of all the tested genotypes and the reduction waspositively correlated with weed biomass (119903 = 065 119875 = 040)Grain yield in weedy plots varied from 36 to 67 t haminus1 acrossdifferent genotypes and it was lowest and highest for PR-115and PR-120 respectively However in weed-free plots grain

4 The Scientific World Journal

Table 2 Weed biomass (gmminus2) in response to different genotypesat 28 d after sowing (DAS) and flowering

GenotypeWeed biomass

28 DAS Floweringgmminus2

PR-115 854 3330H-97158 1175 4185CR2707 1257 3865US-310 794 3812IR88633 567 2211IR83927 706 2808RH-257 613 2380PR-120 448 1743LSD (005) 261 1184

yield varied from 66 to 89 t haminus1 across different genotypesand it was lowest for PR-115 and highest forH-97158 (Table 3)Average yield loss due to weed competition ranged from21 to 46 among different genotypes The higher the weedbiomass the greater the percent yield reduction The highestrelative yield loss due to weed competition was noted in PR-115 and the relative yield loss of this cultivar was similar tothat of H-97158 CR2707 US-310 and RH-257 The lowestrelative yield loss due to weed competition was in PR-120which was similar to that of IR88633 and IR83927 Theseresults suggest that rice genotypes responded differently intheir competitiveness in suppressing weeds under severeinfestation PR-120 IR88633 and IR83927 proved to be betterweed competitors than the other genotypes

Correlation analyses were used to explore the relationshipbetween grain yield of different rice genotypes and plant char-acteristics in both weed-free and partial weedy conditionsIn weed-free conditions only grain yield had a significantpositive correlation with panicle density mminus2 (119903 = 077)(Table 4) In partial weedy conditions however grain yieldhad a significant positive correlation with panicle densitymminus2 grain panicleminus1 weed competitive index LAI rootweight and root volume (Table 5) These results indicate thatthese parameters could have played a vital role in improvinggrain yield under crop-weed competition In partial weedyconditions grain yield was negatively related with spikeletsterility relative yield loss and weed biomass

In weed-free plots the highest grain yield was found inH-97158 followed by RH-257 This was attributed to higherpanicle density in H-97158 Lowest grain yield was recordedin PR-115Weed competitive indexwas highest for PR-120 andother genotypes had lower weed competitive index than thiscultivar (Figure 1) PR-115 had the lowest weed competitiveindex which was similar to that of genotypes H-97158 CR2707 and US-310 On the other hand IR88633 IR83927and RH-257 had better weed competitive index than PR-115The high performance of these genotypes in terms of weedcompetitive index could be attributed to less weed biomassobserved due to their ability to suppress weeds Weeds canbe suppressed by allelopathic effects of rice genotypes inaddition to genotypic smothering effect on weeds however

Genotype

25

20

15

10

05

00

Com

petit

ive i

ndex

LSD

PR-115

H-97158

CR2707

-185

-16

-1-1

-1

US-310

IR88633

-1-79

-B-1

IR83927

-B-B

-278

-CRA

-1-1

-1

RH-257

PR-120

Figure 1 Weed competitive index of different rice genotypes

the genotypes used in our study have not been reported fortheir allelopathic effect

The interaction effects of weed infestation levels andgenotypes were significant for grain yield panicle numbermminus2 and grain number panicleminus1 (Table 3) Across weedinfestation levels 1000-grain weight was highest for IR88633followed by IR83927 (Table 6) Averaged over weed infesta-tion levels 1000-grain weight of all the other genotypes wassimilar to but lower than that of IR88633 In the partial weedyconditions 1000-grain weight had a positive correlation withweed competitive index whereas such correlation was notfound in weed-free conditions Plant height differed acrossthe genotypes (Table 6) IR88633 attained the highest plantheight followed by IR83927 and CR2707 Lowest plant heightwas found for PR-115 Genotypes H-97158 US-310 andRH-257 attained similar plant height Under partial weedyconditions plant height had a negative correlation withrelative yield loss

As expected grain yield of all genotypes in weedyconditions decreased significantly as compared with yield inweed-free conditions (Table 3) In partial weedy conditionshighest grain yield was found for PR-120 closely followed byIR83927 Grain yield of PR-120 in partial weedy conditionswas similar to that of PR-115 US-310 and IR88633 in weed-free conditions This response was because of the bettercompetitive ability of PR-120 that is it had the highest weedcompetitive index (Figure 1) and root biomass and volume(Table 7) Root biomass and volume were highest in PR-120and lowest in PR-115 however these root parameters of PR-115 H-97158 CR2707 and US-310 were the same SimilarlyIR88633 IR83927 and RH-257 had almost identical root bio-mass and volume High root biomass and volume offeredcrop-weed competition in favor of the crop providing greater

The Scientific World Journal 5

Table 3 Grain yield (t haminus1) panicles mminus2 and grains panicleminus1 in response to the interaction effect of weed infestation levels and genotypes

GenotypeGrain yield Panicles Filled grains

Weed-freea Partial weedy Weed-free Partial weedy Weed-free Partial weedyt haminus1 number mminus2 number panicleminus1

PR-115 659 358 290 230 114 96H-97158 889 515 351 236 133 103CR2707 743 480 305 238 136 96US-310 678 452 252 247 148 95IR88633 695 520 278 249 130 91IR83927 809 603 336 259 122 108RH-257 863 550 340 252 136 115PR-120 847 666 362 308 124 120LSD (005) 07 29 14aWeed-free plots were kept weed-free throughout the season Partial weedy plots plots were hand-weeded once at 28DAS and weeds were allowed to growbefore and after the hand-weeding throughout the season

Table 4 Correlation among different traits in weed-free conditions The critical value of 119903 was 040 at 5 level of significance

Traits GY Pan Gr TGW SS Ht CI RYL WP LAI VS RWGYPan 077Gr minus006 minus030TGW 013 minus003 minus016SS minus036 minus041 minus009 minus039Ht minus005 minus024 011 077 minus028CI 025 035 minus001 020 minus056 009RYL minus008 minus021 minus002 minus041 044 minus022 minus069WP minus002 minus019 028 minus033 032 minus013 minus078 065LAI 039 032 005 062 minus066 062 060 minus039 minus040VS 001 minus004 minus009 060 minus038 059 035 minus030 minus017 072RW 032 045 minus015 041 minus049 010 058 minus064 minus056 048 037RV 032 045 minus015 041 minus049 010 058 minus064 minus056 048 037 100GY grain yield LAI leaf area index at 28DAS Ht height RW root weight RV root volume Pan panicle mminus2 Gr filled grains panicleminus1 TGW 1000-grainweight SS spikelet sterility CI weed competitive index WP weed biomass and RYL relative yield loss

Table 5 Correlation among different traits in partial weedy conditions The critical value of 119903 was 040 at 5 level of significance

Traits GY Pan Gr TGW SS Ht CI RYL WP LAI VS RWGYPan 061Gr 053 062TGW 033 018 000SS minus047 minus034 minus039 minus038Ht 023 001 minus017 078 minus024CI 071 032 026 021 minus049 017RYL minus084 minus051 minus031 minus041 026 minus042 minus069WP minus056 minus029 minus027 minus034 032 minus020 minus078 065LAI 051 014 014 059 minus062 063 060 minus039 minus040VS 022 008 minus010 062 minus041 070 035 minus030 minus017 072RW 070 063 038 055 minus047 033 058 minus064 minus056 048 037RV 070 063 038 055 minus047 033 058 minus064 minus056 048 037GY grain yield LAI leaf area index at 28DAS Ht height RW root weight RV root volume Pan panicle mminus2 Gr filled grains panicleminus1 TGW 1000-grainweight SS spikelet sterility CI weed competitive index WP weed biomass and RYL relative yield loss

6 The Scientific World Journal

Table 6 Final plant height (cm plantminus1) 1000-grain weight (g)and spikelet sterility () in response to weed infestation levels andgenotypes

Treatment Plant height 1000-grainweight

Spikeletsterility

cm g Weed infestation level

Weed-freea 1153 260 195Partial weedy 1165 256 201LSD (005) NS NS 16

GenotypePR-115 935 247 262H-97158 1094 255 182CR2707 1246 254 157US-310 1092 243 258IR88633 1502 289 166IR83927 1269 270 171RH-257 1126 258 194PR-120 1001 255 146LSD (005) 71 05 39

Nonsignificant NSaWeed free plots plots were kept weed free throughout the season Partialweedy plots plotswere hand-weeded once at 28DAS andweedswere allowedto grow before and after the hand-weeding throughout the season

Table 7 Leaf area index (LAI) and vigor score at 28 days aftersowing and root biomass and volume at flowering of variousgenotypes under weed-free conditions

Genotype LAI Vigor score Root biomass Root volumegmminus2 mLmminus2

PR-115 186 43 1478 5615H-97158 273 47 2009 7635CR2707 326 90 1965 7466US-310 190 47 1757 6676IR88633 337 90 3428 13027IR83927 289 70 2914 11073RH-257 310 67 3034 11530PR-120 282 67 4295 16538LSD (005) 043 06 755 2870

vigor and early canopy closure as evidenced by the higher LAIof these genotypes

Vigor score was highest in cultivars IR88633 and CR2707(Table 7) Among the tested genotypes vigor score was lowestfor PR-115 followed by H-97158 and US-310 Early vigorhas been shown as an important characteristics related tocompetitive ability [16] and tends to be reflected in greatershoot and root biomass during early vegetative growth Inthe present study crop biomass of PR-120 remained similarin both partial weedy and weed-free conditions (Table 8)At panicle initiation PR-120 in the partial weedy conditionsaccruedwith similar crop biomass withH-97158 and IR88633in the weed-free conditions In the partial weedy conditions

at panicle initiation and flowering stages IR88633 IR83927RH-257 and PR-120 accrued with higher crop biomass thanthat of PR-115 H-97158 CR2707 and US-310 respectively(Table 8)

The present study revealed that higher biomass coupledwith higher plant height at early stages provided greaterstored assimilates for grain filling via translocation thatmay prevent the yield loss of these genotypes under par-tial weedy conditions In partial weedy conditions grainnumber panicleminus1 remained similar only in IR83927 andPR-120 as compared with that in weed-free conditions andthis response was mainly because of their better weed-competitive ability In partial weedy conditions the highestnumber of grains panicleminus1 was found in PR-120 Grainnumber panicleminus1 for PR-120 in partial weedy conditions wassimilar to that of PR-115 H-97158 IR88633 and IR83927 inthe weed-free conditions

High vigor score and LAI during early crop developmenthave been shown to bemajor plant traits contributing toweedcompetitiveness [27 28]The LAI is an important contributorto competitive ability as light is an important resource inplant growth Greater shading ability of the crop makes lesslight available to the competing weeds hence weed growthcan be reduced In the weed-free plots IR88633 attained thehighest LAI at 28 DAS whereas PR-115 attained the lowestLAI (Table 7) The LAI of PR-115 and US-310 was similarGenotypes H-97158 RH-257 PR-120 and IR83927 recordedsimilar LAI but this was higher than that of PR-115 The LAIof CR2707 and IR88633 was similar

The findings of our study indicate that fast-growing habitand the high LAI at the early stage of the crop along withhigh root biomass and volume are the important traits forweed competitiveness confirming studies by various authors[18 29] However some workers [30] demonstrated that anadvantage of early growth habit even for a few days couldshift the competitive balance between crops and weeds Inanother study some workers found a significant correlationbetween crop competitive ability and plant height [31] In thepresent study a negative correlation was found between plantheight and relative yield loss In contrast Fischer et al foundno correlation between the height of semidwarf cultivars andweed biomass [11] As in our study research by some scientists[32] suggested that rice cultivars capable of producing moregrain yield in competition with weeds have greater earlybiomass Similarly workers [8 33] proposed that early vigorshould be a key selection trait for weed competitiveness PR-120 owing to its high root biomass and volume which helpedin attaining high crop biomass and greater panicle numbermminus2 produced the highest yield in partial weedy conditionsOur results confirm the findings of researchers who reportedsignificant and negative correlation between weed biomassand panicle number mminus2 suggesting that panicle density wasdecreased by high weed growth [31]

Competitive ability is the ability of a genotype to suppressweed biomass or to maintain crop yields by tolerating weedcompetition In this study competitive ability was foundhighest for PR-120 The competitiveness of a cultivar mightbe due to the higher number of panicles mminus2 early growth

The Scientific World Journal 7

Table 8 Crop biomass (t haminus1) in response to the interaction effect of weed infestation levels and genotypes at panicle initiation and floweringstages

Weed infestation levelCrop biomass

PR-115 H-97158 CR2707 US-310 IR88633 IR83927 RH-257 PR-120t haminus1

Panicle initiation stageWeed-freea 43 48 320 380 490 440 450 460Partial weedy 330 325 315 340 422 414 440 480LSD (005) 03

Flowering stageWeed-free 698 748 698 721 802 712 732 718Partial weedy 382 425 410 412 490 512 530 592LSD (005) 05aWeed free plots plots were kept weed free throughout the season Partial weedy plots plots were hand-weeded once at 28DAS and weeds were allowed togrow before and after the hand-weeding throughout the season

and yield under both partial weedy andweed-free conditionsIn this study panicle number mminus2 also had a correlationwith grain yield under both partial weedy and weed-freeconditions Cultivars able to produce higher grain yields inpartial weedy conditions correlated with greater early ricegrowth and LAI [8 34 35] Likewise this study demonstratedthat high LAI along with high root biomass and volumewere the useful traits for weed-competitive genotypes CR-2707 and RH-257 had higher LAI and vigor and similarlyRH-257 had higher root biomass and volume but yield lossof these genotypes was found more than 35 This was dueto their (CR-2707 and RH-257) more tendencies for lodgingunder partial weedy conditions

In summary genotypes PR-115 and H-97158 were theworst competitors and PR-120 IR88633 and IR83927 weregood weed competitors Genotypes PR-120 IR88633 andIR83927 have potential for increasing the weed-competitiveability of genotypes in breeding programs and could beused to increase the competitiveness of highly productivegenotypes that are not competitive The study revealed thatearly canopy closure high LAI at the early stage and greaterroot biomass and volume correlated positively with compet-itiveness This study demonstrated that breeding to increasethe competiveness of highly productive rice plant typeswouldbe possible without compromising yieldThe competitivenessobserved in these studies for genotypes PR-120 IR88633 andIR83927 would be adequate in improving farmersrsquo incomeand reducing herbicide use and can be successfully used inintegrated weed management programs in DSR

4 Future Implications

This study implies that breeding for weed-suppressive ricegenotypes should be conducted under partial weedy con-ditions In a mixed weed population the weed-competitiveability of rice genotypes may be affected by weed composi-tion However the competitive ability of rice genotypes is acomplex trait and could not be explained by only one or twocharacteristics Interactions between traits for instance rootbiomass LAI and high shoot biomass at early stage play an

important role and these traits can be explored for identifyingweed-competitive genotypes in DSR Competitiveness of ricegenotypes against weeds will be an important key to thesuccessful adoption of weed-suppressive rice genotypes ina sustainable weed management program The developmentof such genotypes could play an important role in DSR byreducing herbicide load in agroeco system

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors are grateful to Bill Hardy International RiceResearch Institute Philippines for providing comments onthe paper The authors would also like to acknowledge thefunding from the Cereal Systems Initiatives for South Asia(CSISA)

References

[1] G Mahajan B S Chauhan and M S Gill ldquoDry-seeded riceculture in Punjab State of India lessons learned from farmersrdquoField Crops Research vol 144 pp 89ndash99 2013

[2] G Mahajan and B S Chauhan ldquoHerbicide options for weedControl in dry-seeded aromatic rice in IndiardquoWeed Technologyvol 27 pp 682ndash689 2013

[3] B S Chauhan ldquoWeed ecology andweedmanagement strategiesfor dry-seeded rice in AsiardquoWeed Technology vol 26 no 1 pp1ndash13 2012

[4] A N Rao D E Johnson B Sivaprasad J K Ladha and A MMortimer ldquoWeed management in direct-seeded ricerdquo Advancesin Agronomy vol 93 pp 153ndash255 2007

[5] D L Zhao G N Atlin L Bastiaans and J H J SpiertzldquoComparing rice germplasm groups for growth grain yield andweed-suppressive ability under aerobic soil conditionsrdquo WeedResearch vol 46 no 6 pp 444ndash452 2006

8 The Scientific World Journal

[6] D Lemerle B Verbeek R D Cousens and N E CoombesldquoThe potential for selecting wheat varieties strongly competitiveagainst weedsrdquoWeed Research vol 36 no 6 pp 505ndash513 1996

[7] G Mahajan and B S Chauhan ldquoThe role of cultivars in man-aging weeds in dry-seeded rice production systemsrdquo CropProtection vol 49 pp 52ndash57 2013

[8] D L Zhao G N Atlin L Bastiaans and J H J SpiertzldquoDeveloping selection protocols for weed competitiveness inaerobic ricerdquo Field Crops Research vol 97 no 2-3 pp 272ndash2852006

[9] K D Gibson A J Fischer T C Foin and J E Hill ldquoCrop traitsrelated to weed suppression in water-seeded rice (Oryza sativaL)rdquoWeed Science vol 51 no 1 pp 87ndash93 2003

[10] D E JohnsonMDingkuhnM P Jones andM CMahamaneldquoThe influence of rice plant type on the effect of weed competi-tion onOryza sativa andOryza glaberrimardquoWeed Research vol38 no 3 pp 207ndash216 1998

[11] A J Fischer H V Ramirez K D Gibson and B Da SilveiraPinheiro ldquoCompetitiveness of semidwarf upland rice cultivarsagainst palisadegrass (Brachiaria brizantha) and signalgrass (Bdecumbens)rdquo Agronomy Journal vol 93 no 5 pp 967ndash9732001

[12] P Anwar A S Juraimi A Man A Puteh A Selamat andM Begum ldquoWeed suppressive ability of rice (Oryza sativa L)germplasm under aerobic soil conditionsrdquoAustralian Journal ofCrop Science vol 4 no 9 pp 706ndash717 2010

[13] A Fischer H V Ramırez and J Lozano ldquoSuppression ofjunglerice [Echinochloa colona (L) Link] by irrigated ricecultivars in Latin AmericardquoAgronomy Journal vol 89 no 3 pp516ndash521 1997

[14] S M Haefele D E Johnson D MrsquoBodj M C S Wopereisand K M Miezan ldquoField screening of diverse rice genotypesforweed competitiveness in irrigated lowland ecosystemsrdquoFieldCrops Research vol 88 no 1 pp 39ndash56 2004

[15] B S Chauhan G Mahajan V Sardana J Timsina and M LJat ldquoProductivity and sustainability of the rice-wheat croppingsystem in the indo-gangetic plains of the Indian subcontinentproblems opportunities and strategiesrdquoAdvances inAgronomyvol 117 pp 315ndash369 2012

[16] D Lemerle B Verbeek and B Orchard ldquoRanking the abilityof wheat varieties to compete with Lolium rigidumrdquo WeedResearch vol 41 no 3 pp 197ndash209 2001

[17] D P Garrity M Movillon and K Moody ldquoDifferential weedsuppression ability in upland rice cultivarsrdquo Agronomy Journalvol 84 pp 586ndash591 1992

[18] K D Gibson J E Hill T C Foin B P Caton and A J FischerldquoWater-seeded rice cultivars differ in ability to interfere withwatergrassrdquo Agronomy Journal vol 93 no 2 pp 326ndash332 2001

[19] M Dingkuhn D E Johnson A Sow and A Y AudebertldquoRelationships between upland rice canopy characteristics andweed competitivenessrdquo Field Crops Research vol 61 no 1 pp79ndash95 1999

[20] M C Richards and G P Whytock ldquoVarietal competitivenesswith weedsrdquo Aspects of Applied Biology vol 34 pp 345ndash3541993

[21] D L ZhaoGNAtlin L Bastiaans and JH J Spiertz ldquoCultivarweed-competitiveness in aerobic rice heritability correlatedtraits and the potential for indirect selection in weed-freeenvironmentsrdquo Crop Science vol 46 no 1 pp 372ndash380 2006

[22] D G Huel and P Hucl ldquoGenotypic variation for competitiveability in spring wheatrdquo Plant Breeding vol 115 no 5 pp 325ndash329 1996

[23] H Q Wang B A M Bouman D L Zhao C Wang and PF Moya ldquoAerobic rice in northern China opportunities andchallengesrdquo in Proceedings of the International Workshop onWater-Wise Rice Production B A M Bouman H HengsdijkB Hardy P S Bindraban T P Tuong and J K Ladha Eds pp143ndash154 International Rice Research Institute April 2002

[24] B S Chauhan and D E Johnson ldquoRow spacing and weedcontrol timing affect yield of aerobic ricerdquo Field Crops Researchvol 121 no 2 pp 226ndash231 2011

[25] M A Baghestani E Zand and S Soufizadeh ldquoIranian winterwheats (Triticum aestivum L) interference with weeds I Grainyield and competitive indexrdquo Pakistan Journal of Weed ScienceResearch vol 12 pp 119ndash129 2006

[26] A Rezakhanlou M Aghabeigi and H Bagheri ldquoEvaluation ofsome of competitiveness indexes in competition between cottonvarieties and common cocklebur (Xanthium strumarium L)rdquoInternational Research Journal of Applied and Basic Sciences vol3 pp 1274ndash1278 2012

[27] R K Coleman G S Gill and G J Rebetzke ldquoIdentification ofquantitative trait loci for traits conferring weed competitivenessin wheat (Triticum aestivum L)rdquo Australian Journal of Agricul-tural Research vol 52 no 11-12 pp 1235ndash1246 2001

[28] J Rodenburg K Saito R G Kakaı A Toure M Mariko andP Kiepe ldquoWeed competitiveness of the lowland rice varietiesof NERICA in the southern Guinea Savannardquo Field CropsResearch vol 114 no 3 pp 411ndash418 2009

[29] B P Caton A E Cope and M Mortimer ldquoGrowth traits ofdiverse rice cultivars under severe competition implications forscreening for competitivenessrdquo Field Crops Research vol 83 no2 pp 157ndash172 2003

[30] M J Kropff ldquoEco-physiological models for crop-weed compe-titionrdquo inModeling Crop-Weed Interactions M J Kropff and HH van Laar Eds pp 25ndash32 CAB International WallingfordUK 1993

[31] F Ekeleme A Y Kamara S O Oikeh et al ldquoResponse ofupland rice cultivars to weed competition in the savannas ofWest Africardquo Crop Protection vol 28 no 1 pp 90ndash96 2009

[32] O S Namuco J E Cairns and D E Johnson ldquoInvestigatingearly vigour in upland rice (Oryza sativa L) part I Seedlinggrowth and grain yield in competition with weedsrdquo Field CropsResearch vol 113 no 3 pp 197ndash206 2009

[33] K Saito K Azoma and J Rodenburg ldquoPlant characteristicsassociated with weed competitiveness of rice under upland andlowland conditions in West Africardquo Field Crops Research vol116 no 3 pp 308ndash317 2010

[34] H Ni K Moody R P Robles E C Paller Jr and J S LalesldquoOryza sativa plant traits conferring competitive ability againstweedsrdquoWeed Science vol 48 no 2 pp 200ndash204 2000

[35] G Wu L T Wilson and A M McClung ldquoContribution of ricetillers to drymatter accumulation and yieldrdquoAgronomy Journalvol 90 no 3 pp 317ndash323 1998

Submit your manuscripts athttpwwwhindawicom

Nutrition and Metabolism

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Food ScienceInternational Journal of

Agronomy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Microbiology

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

AgricultureAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

PsycheHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BiodiversityInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

GenomicsInternational Journal of

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Plant GenomicsInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Biotechnology Research International

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Forestry ResearchInternational Journal of

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Journal of BotanyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EcologyInternational Journal of

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Veterinary Medicine International

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Cell BiologyInternational Journal of

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Evolutionary BiologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

4 The Scientific World Journal

Table 2 Weed biomass (gmminus2) in response to different genotypesat 28 d after sowing (DAS) and flowering

GenotypeWeed biomass

28 DAS Floweringgmminus2

PR-115 854 3330H-97158 1175 4185CR2707 1257 3865US-310 794 3812IR88633 567 2211IR83927 706 2808RH-257 613 2380PR-120 448 1743LSD (005) 261 1184

yield varied from 66 to 89 t haminus1 across different genotypesand it was lowest for PR-115 and highest forH-97158 (Table 3)Average yield loss due to weed competition ranged from21 to 46 among different genotypes The higher the weedbiomass the greater the percent yield reduction The highestrelative yield loss due to weed competition was noted in PR-115 and the relative yield loss of this cultivar was similar tothat of H-97158 CR2707 US-310 and RH-257 The lowestrelative yield loss due to weed competition was in PR-120which was similar to that of IR88633 and IR83927 Theseresults suggest that rice genotypes responded differently intheir competitiveness in suppressing weeds under severeinfestation PR-120 IR88633 and IR83927 proved to be betterweed competitors than the other genotypes

Correlation analyses were used to explore the relationshipbetween grain yield of different rice genotypes and plant char-acteristics in both weed-free and partial weedy conditionsIn weed-free conditions only grain yield had a significantpositive correlation with panicle density mminus2 (119903 = 077)(Table 4) In partial weedy conditions however grain yieldhad a significant positive correlation with panicle densitymminus2 grain panicleminus1 weed competitive index LAI rootweight and root volume (Table 5) These results indicate thatthese parameters could have played a vital role in improvinggrain yield under crop-weed competition In partial weedyconditions grain yield was negatively related with spikeletsterility relative yield loss and weed biomass

In weed-free plots the highest grain yield was found inH-97158 followed by RH-257 This was attributed to higherpanicle density in H-97158 Lowest grain yield was recordedin PR-115Weed competitive indexwas highest for PR-120 andother genotypes had lower weed competitive index than thiscultivar (Figure 1) PR-115 had the lowest weed competitiveindex which was similar to that of genotypes H-97158 CR2707 and US-310 On the other hand IR88633 IR83927and RH-257 had better weed competitive index than PR-115The high performance of these genotypes in terms of weedcompetitive index could be attributed to less weed biomassobserved due to their ability to suppress weeds Weeds canbe suppressed by allelopathic effects of rice genotypes inaddition to genotypic smothering effect on weeds however

Genotype

25

20

15

10

05

00

Com

petit

ive i

ndex

LSD

PR-115

H-97158

CR2707

-185

-16

-1-1

-1

US-310

IR88633

-1-79

-B-1

IR83927

-B-B

-278

-CRA

-1-1

-1

RH-257

PR-120

Figure 1 Weed competitive index of different rice genotypes

the genotypes used in our study have not been reported fortheir allelopathic effect

The interaction effects of weed infestation levels andgenotypes were significant for grain yield panicle numbermminus2 and grain number panicleminus1 (Table 3) Across weedinfestation levels 1000-grain weight was highest for IR88633followed by IR83927 (Table 6) Averaged over weed infesta-tion levels 1000-grain weight of all the other genotypes wassimilar to but lower than that of IR88633 In the partial weedyconditions 1000-grain weight had a positive correlation withweed competitive index whereas such correlation was notfound in weed-free conditions Plant height differed acrossthe genotypes (Table 6) IR88633 attained the highest plantheight followed by IR83927 and CR2707 Lowest plant heightwas found for PR-115 Genotypes H-97158 US-310 andRH-257 attained similar plant height Under partial weedyconditions plant height had a negative correlation withrelative yield loss

As expected grain yield of all genotypes in weedyconditions decreased significantly as compared with yield inweed-free conditions (Table 3) In partial weedy conditionshighest grain yield was found for PR-120 closely followed byIR83927 Grain yield of PR-120 in partial weedy conditionswas similar to that of PR-115 US-310 and IR88633 in weed-free conditions This response was because of the bettercompetitive ability of PR-120 that is it had the highest weedcompetitive index (Figure 1) and root biomass and volume(Table 7) Root biomass and volume were highest in PR-120and lowest in PR-115 however these root parameters of PR-115 H-97158 CR2707 and US-310 were the same SimilarlyIR88633 IR83927 and RH-257 had almost identical root bio-mass and volume High root biomass and volume offeredcrop-weed competition in favor of the crop providing greater

The Scientific World Journal 5

Table 3 Grain yield (t haminus1) panicles mminus2 and grains panicleminus1 in response to the interaction effect of weed infestation levels and genotypes

GenotypeGrain yield Panicles Filled grains

Weed-freea Partial weedy Weed-free Partial weedy Weed-free Partial weedyt haminus1 number mminus2 number panicleminus1

PR-115 659 358 290 230 114 96H-97158 889 515 351 236 133 103CR2707 743 480 305 238 136 96US-310 678 452 252 247 148 95IR88633 695 520 278 249 130 91IR83927 809 603 336 259 122 108RH-257 863 550 340 252 136 115PR-120 847 666 362 308 124 120LSD (005) 07 29 14aWeed-free plots were kept weed-free throughout the season Partial weedy plots plots were hand-weeded once at 28DAS and weeds were allowed to growbefore and after the hand-weeding throughout the season

Table 4 Correlation among different traits in weed-free conditions The critical value of 119903 was 040 at 5 level of significance

Traits GY Pan Gr TGW SS Ht CI RYL WP LAI VS RWGYPan 077Gr minus006 minus030TGW 013 minus003 minus016SS minus036 minus041 minus009 minus039Ht minus005 minus024 011 077 minus028CI 025 035 minus001 020 minus056 009RYL minus008 minus021 minus002 minus041 044 minus022 minus069WP minus002 minus019 028 minus033 032 minus013 minus078 065LAI 039 032 005 062 minus066 062 060 minus039 minus040VS 001 minus004 minus009 060 minus038 059 035 minus030 minus017 072RW 032 045 minus015 041 minus049 010 058 minus064 minus056 048 037RV 032 045 minus015 041 minus049 010 058 minus064 minus056 048 037 100GY grain yield LAI leaf area index at 28DAS Ht height RW root weight RV root volume Pan panicle mminus2 Gr filled grains panicleminus1 TGW 1000-grainweight SS spikelet sterility CI weed competitive index WP weed biomass and RYL relative yield loss

Table 5 Correlation among different traits in partial weedy conditions The critical value of 119903 was 040 at 5 level of significance

Traits GY Pan Gr TGW SS Ht CI RYL WP LAI VS RWGYPan 061Gr 053 062TGW 033 018 000SS minus047 minus034 minus039 minus038Ht 023 001 minus017 078 minus024CI 071 032 026 021 minus049 017RYL minus084 minus051 minus031 minus041 026 minus042 minus069WP minus056 minus029 minus027 minus034 032 minus020 minus078 065LAI 051 014 014 059 minus062 063 060 minus039 minus040VS 022 008 minus010 062 minus041 070 035 minus030 minus017 072RW 070 063 038 055 minus047 033 058 minus064 minus056 048 037RV 070 063 038 055 minus047 033 058 minus064 minus056 048 037GY grain yield LAI leaf area index at 28DAS Ht height RW root weight RV root volume Pan panicle mminus2 Gr filled grains panicleminus1 TGW 1000-grainweight SS spikelet sterility CI weed competitive index WP weed biomass and RYL relative yield loss

6 The Scientific World Journal

Table 6 Final plant height (cm plantminus1) 1000-grain weight (g)and spikelet sterility () in response to weed infestation levels andgenotypes

Treatment Plant height 1000-grainweight

Spikeletsterility

cm g Weed infestation level

Weed-freea 1153 260 195Partial weedy 1165 256 201LSD (005) NS NS 16

GenotypePR-115 935 247 262H-97158 1094 255 182CR2707 1246 254 157US-310 1092 243 258IR88633 1502 289 166IR83927 1269 270 171RH-257 1126 258 194PR-120 1001 255 146LSD (005) 71 05 39

Nonsignificant NSaWeed free plots plots were kept weed free throughout the season Partialweedy plots plotswere hand-weeded once at 28DAS andweedswere allowedto grow before and after the hand-weeding throughout the season

Table 7 Leaf area index (LAI) and vigor score at 28 days aftersowing and root biomass and volume at flowering of variousgenotypes under weed-free conditions

Genotype LAI Vigor score Root biomass Root volumegmminus2 mLmminus2

PR-115 186 43 1478 5615H-97158 273 47 2009 7635CR2707 326 90 1965 7466US-310 190 47 1757 6676IR88633 337 90 3428 13027IR83927 289 70 2914 11073RH-257 310 67 3034 11530PR-120 282 67 4295 16538LSD (005) 043 06 755 2870

vigor and early canopy closure as evidenced by the higher LAIof these genotypes

Vigor score was highest in cultivars IR88633 and CR2707(Table 7) Among the tested genotypes vigor score was lowestfor PR-115 followed by H-97158 and US-310 Early vigorhas been shown as an important characteristics related tocompetitive ability [16] and tends to be reflected in greatershoot and root biomass during early vegetative growth Inthe present study crop biomass of PR-120 remained similarin both partial weedy and weed-free conditions (Table 8)At panicle initiation PR-120 in the partial weedy conditionsaccruedwith similar crop biomass withH-97158 and IR88633in the weed-free conditions In the partial weedy conditions

at panicle initiation and flowering stages IR88633 IR83927RH-257 and PR-120 accrued with higher crop biomass thanthat of PR-115 H-97158 CR2707 and US-310 respectively(Table 8)

The present study revealed that higher biomass coupledwith higher plant height at early stages provided greaterstored assimilates for grain filling via translocation thatmay prevent the yield loss of these genotypes under par-tial weedy conditions In partial weedy conditions grainnumber panicleminus1 remained similar only in IR83927 andPR-120 as compared with that in weed-free conditions andthis response was mainly because of their better weed-competitive ability In partial weedy conditions the highestnumber of grains panicleminus1 was found in PR-120 Grainnumber panicleminus1 for PR-120 in partial weedy conditions wassimilar to that of PR-115 H-97158 IR88633 and IR83927 inthe weed-free conditions

High vigor score and LAI during early crop developmenthave been shown to bemajor plant traits contributing toweedcompetitiveness [27 28]The LAI is an important contributorto competitive ability as light is an important resource inplant growth Greater shading ability of the crop makes lesslight available to the competing weeds hence weed growthcan be reduced In the weed-free plots IR88633 attained thehighest LAI at 28 DAS whereas PR-115 attained the lowestLAI (Table 7) The LAI of PR-115 and US-310 was similarGenotypes H-97158 RH-257 PR-120 and IR83927 recordedsimilar LAI but this was higher than that of PR-115 The LAIof CR2707 and IR88633 was similar

The findings of our study indicate that fast-growing habitand the high LAI at the early stage of the crop along withhigh root biomass and volume are the important traits forweed competitiveness confirming studies by various authors[18 29] However some workers [30] demonstrated that anadvantage of early growth habit even for a few days couldshift the competitive balance between crops and weeds Inanother study some workers found a significant correlationbetween crop competitive ability and plant height [31] In thepresent study a negative correlation was found between plantheight and relative yield loss In contrast Fischer et al foundno correlation between the height of semidwarf cultivars andweed biomass [11] As in our study research by some scientists[32] suggested that rice cultivars capable of producing moregrain yield in competition with weeds have greater earlybiomass Similarly workers [8 33] proposed that early vigorshould be a key selection trait for weed competitiveness PR-120 owing to its high root biomass and volume which helpedin attaining high crop biomass and greater panicle numbermminus2 produced the highest yield in partial weedy conditionsOur results confirm the findings of researchers who reportedsignificant and negative correlation between weed biomassand panicle number mminus2 suggesting that panicle density wasdecreased by high weed growth [31]

Competitive ability is the ability of a genotype to suppressweed biomass or to maintain crop yields by tolerating weedcompetition In this study competitive ability was foundhighest for PR-120 The competitiveness of a cultivar mightbe due to the higher number of panicles mminus2 early growth

The Scientific World Journal 7

Table 8 Crop biomass (t haminus1) in response to the interaction effect of weed infestation levels and genotypes at panicle initiation and floweringstages

Weed infestation levelCrop biomass

PR-115 H-97158 CR2707 US-310 IR88633 IR83927 RH-257 PR-120t haminus1

Panicle initiation stageWeed-freea 43 48 320 380 490 440 450 460Partial weedy 330 325 315 340 422 414 440 480LSD (005) 03

Flowering stageWeed-free 698 748 698 721 802 712 732 718Partial weedy 382 425 410 412 490 512 530 592LSD (005) 05aWeed free plots plots were kept weed free throughout the season Partial weedy plots plots were hand-weeded once at 28DAS and weeds were allowed togrow before and after the hand-weeding throughout the season

and yield under both partial weedy andweed-free conditionsIn this study panicle number mminus2 also had a correlationwith grain yield under both partial weedy and weed-freeconditions Cultivars able to produce higher grain yields inpartial weedy conditions correlated with greater early ricegrowth and LAI [8 34 35] Likewise this study demonstratedthat high LAI along with high root biomass and volumewere the useful traits for weed-competitive genotypes CR-2707 and RH-257 had higher LAI and vigor and similarlyRH-257 had higher root biomass and volume but yield lossof these genotypes was found more than 35 This was dueto their (CR-2707 and RH-257) more tendencies for lodgingunder partial weedy conditions

In summary genotypes PR-115 and H-97158 were theworst competitors and PR-120 IR88633 and IR83927 weregood weed competitors Genotypes PR-120 IR88633 andIR83927 have potential for increasing the weed-competitiveability of genotypes in breeding programs and could beused to increase the competitiveness of highly productivegenotypes that are not competitive The study revealed thatearly canopy closure high LAI at the early stage and greaterroot biomass and volume correlated positively with compet-itiveness This study demonstrated that breeding to increasethe competiveness of highly productive rice plant typeswouldbe possible without compromising yieldThe competitivenessobserved in these studies for genotypes PR-120 IR88633 andIR83927 would be adequate in improving farmersrsquo incomeand reducing herbicide use and can be successfully used inintegrated weed management programs in DSR

4 Future Implications

This study implies that breeding for weed-suppressive ricegenotypes should be conducted under partial weedy con-ditions In a mixed weed population the weed-competitiveability of rice genotypes may be affected by weed composi-tion However the competitive ability of rice genotypes is acomplex trait and could not be explained by only one or twocharacteristics Interactions between traits for instance rootbiomass LAI and high shoot biomass at early stage play an

important role and these traits can be explored for identifyingweed-competitive genotypes in DSR Competitiveness of ricegenotypes against weeds will be an important key to thesuccessful adoption of weed-suppressive rice genotypes ina sustainable weed management program The developmentof such genotypes could play an important role in DSR byreducing herbicide load in agroeco system

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors are grateful to Bill Hardy International RiceResearch Institute Philippines for providing comments onthe paper The authors would also like to acknowledge thefunding from the Cereal Systems Initiatives for South Asia(CSISA)

References

[1] G Mahajan B S Chauhan and M S Gill ldquoDry-seeded riceculture in Punjab State of India lessons learned from farmersrdquoField Crops Research vol 144 pp 89ndash99 2013

[2] G Mahajan and B S Chauhan ldquoHerbicide options for weedControl in dry-seeded aromatic rice in IndiardquoWeed Technologyvol 27 pp 682ndash689 2013

[3] B S Chauhan ldquoWeed ecology andweedmanagement strategiesfor dry-seeded rice in AsiardquoWeed Technology vol 26 no 1 pp1ndash13 2012

[4] A N Rao D E Johnson B Sivaprasad J K Ladha and A MMortimer ldquoWeed management in direct-seeded ricerdquo Advancesin Agronomy vol 93 pp 153ndash255 2007

[5] D L Zhao G N Atlin L Bastiaans and J H J SpiertzldquoComparing rice germplasm groups for growth grain yield andweed-suppressive ability under aerobic soil conditionsrdquo WeedResearch vol 46 no 6 pp 444ndash452 2006

8 The Scientific World Journal

[6] D Lemerle B Verbeek R D Cousens and N E CoombesldquoThe potential for selecting wheat varieties strongly competitiveagainst weedsrdquoWeed Research vol 36 no 6 pp 505ndash513 1996

[7] G Mahajan and B S Chauhan ldquoThe role of cultivars in man-aging weeds in dry-seeded rice production systemsrdquo CropProtection vol 49 pp 52ndash57 2013

[8] D L Zhao G N Atlin L Bastiaans and J H J SpiertzldquoDeveloping selection protocols for weed competitiveness inaerobic ricerdquo Field Crops Research vol 97 no 2-3 pp 272ndash2852006

[9] K D Gibson A J Fischer T C Foin and J E Hill ldquoCrop traitsrelated to weed suppression in water-seeded rice (Oryza sativaL)rdquoWeed Science vol 51 no 1 pp 87ndash93 2003

[10] D E JohnsonMDingkuhnM P Jones andM CMahamaneldquoThe influence of rice plant type on the effect of weed competi-tion onOryza sativa andOryza glaberrimardquoWeed Research vol38 no 3 pp 207ndash216 1998

[11] A J Fischer H V Ramirez K D Gibson and B Da SilveiraPinheiro ldquoCompetitiveness of semidwarf upland rice cultivarsagainst palisadegrass (Brachiaria brizantha) and signalgrass (Bdecumbens)rdquo Agronomy Journal vol 93 no 5 pp 967ndash9732001

[12] P Anwar A S Juraimi A Man A Puteh A Selamat andM Begum ldquoWeed suppressive ability of rice (Oryza sativa L)germplasm under aerobic soil conditionsrdquoAustralian Journal ofCrop Science vol 4 no 9 pp 706ndash717 2010

[13] A Fischer H V Ramırez and J Lozano ldquoSuppression ofjunglerice [Echinochloa colona (L) Link] by irrigated ricecultivars in Latin AmericardquoAgronomy Journal vol 89 no 3 pp516ndash521 1997

[14] S M Haefele D E Johnson D MrsquoBodj M C S Wopereisand K M Miezan ldquoField screening of diverse rice genotypesforweed competitiveness in irrigated lowland ecosystemsrdquoFieldCrops Research vol 88 no 1 pp 39ndash56 2004

[15] B S Chauhan G Mahajan V Sardana J Timsina and M LJat ldquoProductivity and sustainability of the rice-wheat croppingsystem in the indo-gangetic plains of the Indian subcontinentproblems opportunities and strategiesrdquoAdvances inAgronomyvol 117 pp 315ndash369 2012

[16] D Lemerle B Verbeek and B Orchard ldquoRanking the abilityof wheat varieties to compete with Lolium rigidumrdquo WeedResearch vol 41 no 3 pp 197ndash209 2001

[17] D P Garrity M Movillon and K Moody ldquoDifferential weedsuppression ability in upland rice cultivarsrdquo Agronomy Journalvol 84 pp 586ndash591 1992

[18] K D Gibson J E Hill T C Foin B P Caton and A J FischerldquoWater-seeded rice cultivars differ in ability to interfere withwatergrassrdquo Agronomy Journal vol 93 no 2 pp 326ndash332 2001

[19] M Dingkuhn D E Johnson A Sow and A Y AudebertldquoRelationships between upland rice canopy characteristics andweed competitivenessrdquo Field Crops Research vol 61 no 1 pp79ndash95 1999

[20] M C Richards and G P Whytock ldquoVarietal competitivenesswith weedsrdquo Aspects of Applied Biology vol 34 pp 345ndash3541993

[21] D L ZhaoGNAtlin L Bastiaans and JH J Spiertz ldquoCultivarweed-competitiveness in aerobic rice heritability correlatedtraits and the potential for indirect selection in weed-freeenvironmentsrdquo Crop Science vol 46 no 1 pp 372ndash380 2006

[22] D G Huel and P Hucl ldquoGenotypic variation for competitiveability in spring wheatrdquo Plant Breeding vol 115 no 5 pp 325ndash329 1996

[23] H Q Wang B A M Bouman D L Zhao C Wang and PF Moya ldquoAerobic rice in northern China opportunities andchallengesrdquo in Proceedings of the International Workshop onWater-Wise Rice Production B A M Bouman H HengsdijkB Hardy P S Bindraban T P Tuong and J K Ladha Eds pp143ndash154 International Rice Research Institute April 2002

[24] B S Chauhan and D E Johnson ldquoRow spacing and weedcontrol timing affect yield of aerobic ricerdquo Field Crops Researchvol 121 no 2 pp 226ndash231 2011

[25] M A Baghestani E Zand and S Soufizadeh ldquoIranian winterwheats (Triticum aestivum L) interference with weeds I Grainyield and competitive indexrdquo Pakistan Journal of Weed ScienceResearch vol 12 pp 119ndash129 2006

[26] A Rezakhanlou M Aghabeigi and H Bagheri ldquoEvaluation ofsome of competitiveness indexes in competition between cottonvarieties and common cocklebur (Xanthium strumarium L)rdquoInternational Research Journal of Applied and Basic Sciences vol3 pp 1274ndash1278 2012

[27] R K Coleman G S Gill and G J Rebetzke ldquoIdentification ofquantitative trait loci for traits conferring weed competitivenessin wheat (Triticum aestivum L)rdquo Australian Journal of Agricul-tural Research vol 52 no 11-12 pp 1235ndash1246 2001

[28] J Rodenburg K Saito R G Kakaı A Toure M Mariko andP Kiepe ldquoWeed competitiveness of the lowland rice varietiesof NERICA in the southern Guinea Savannardquo Field CropsResearch vol 114 no 3 pp 411ndash418 2009

[29] B P Caton A E Cope and M Mortimer ldquoGrowth traits ofdiverse rice cultivars under severe competition implications forscreening for competitivenessrdquo Field Crops Research vol 83 no2 pp 157ndash172 2003

[30] M J Kropff ldquoEco-physiological models for crop-weed compe-titionrdquo inModeling Crop-Weed Interactions M J Kropff and HH van Laar Eds pp 25ndash32 CAB International WallingfordUK 1993

[31] F Ekeleme A Y Kamara S O Oikeh et al ldquoResponse ofupland rice cultivars to weed competition in the savannas ofWest Africardquo Crop Protection vol 28 no 1 pp 90ndash96 2009

[32] O S Namuco J E Cairns and D E Johnson ldquoInvestigatingearly vigour in upland rice (Oryza sativa L) part I Seedlinggrowth and grain yield in competition with weedsrdquo Field CropsResearch vol 113 no 3 pp 197ndash206 2009

[33] K Saito K Azoma and J Rodenburg ldquoPlant characteristicsassociated with weed competitiveness of rice under upland andlowland conditions in West Africardquo Field Crops Research vol116 no 3 pp 308ndash317 2010

[34] H Ni K Moody R P Robles E C Paller Jr and J S LalesldquoOryza sativa plant traits conferring competitive ability againstweedsrdquoWeed Science vol 48 no 2 pp 200ndash204 2000

[35] G Wu L T Wilson and A M McClung ldquoContribution of ricetillers to drymatter accumulation and yieldrdquoAgronomy Journalvol 90 no 3 pp 317ndash323 1998

Submit your manuscripts athttpwwwhindawicom

Nutrition and Metabolism

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Food ScienceInternational Journal of

Agronomy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Microbiology

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

AgricultureAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

PsycheHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BiodiversityInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

GenomicsInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Plant GenomicsInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Biotechnology Research International

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Forestry ResearchInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of BotanyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Veterinary Medicine International

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Cell BiologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Evolutionary BiologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World Journal 5

Table 3 Grain yield (t haminus1) panicles mminus2 and grains panicleminus1 in response to the interaction effect of weed infestation levels and genotypes

GenotypeGrain yield Panicles Filled grains

Weed-freea Partial weedy Weed-free Partial weedy Weed-free Partial weedyt haminus1 number mminus2 number panicleminus1

PR-115 659 358 290 230 114 96H-97158 889 515 351 236 133 103CR2707 743 480 305 238 136 96US-310 678 452 252 247 148 95IR88633 695 520 278 249 130 91IR83927 809 603 336 259 122 108RH-257 863 550 340 252 136 115PR-120 847 666 362 308 124 120LSD (005) 07 29 14aWeed-free plots were kept weed-free throughout the season Partial weedy plots plots were hand-weeded once at 28DAS and weeds were allowed to growbefore and after the hand-weeding throughout the season

Table 4 Correlation among different traits in weed-free conditions The critical value of 119903 was 040 at 5 level of significance

Traits GY Pan Gr TGW SS Ht CI RYL WP LAI VS RWGYPan 077Gr minus006 minus030TGW 013 minus003 minus016SS minus036 minus041 minus009 minus039Ht minus005 minus024 011 077 minus028CI 025 035 minus001 020 minus056 009RYL minus008 minus021 minus002 minus041 044 minus022 minus069WP minus002 minus019 028 minus033 032 minus013 minus078 065LAI 039 032 005 062 minus066 062 060 minus039 minus040VS 001 minus004 minus009 060 minus038 059 035 minus030 minus017 072RW 032 045 minus015 041 minus049 010 058 minus064 minus056 048 037RV 032 045 minus015 041 minus049 010 058 minus064 minus056 048 037 100GY grain yield LAI leaf area index at 28DAS Ht height RW root weight RV root volume Pan panicle mminus2 Gr filled grains panicleminus1 TGW 1000-grainweight SS spikelet sterility CI weed competitive index WP weed biomass and RYL relative yield loss

Table 5 Correlation among different traits in partial weedy conditions The critical value of 119903 was 040 at 5 level of significance

Traits GY Pan Gr TGW SS Ht CI RYL WP LAI VS RWGYPan 061Gr 053 062TGW 033 018 000SS minus047 minus034 minus039 minus038Ht 023 001 minus017 078 minus024CI 071 032 026 021 minus049 017RYL minus084 minus051 minus031 minus041 026 minus042 minus069WP minus056 minus029 minus027 minus034 032 minus020 minus078 065LAI 051 014 014 059 minus062 063 060 minus039 minus040VS 022 008 minus010 062 minus041 070 035 minus030 minus017 072RW 070 063 038 055 minus047 033 058 minus064 minus056 048 037RV 070 063 038 055 minus047 033 058 minus064 minus056 048 037GY grain yield LAI leaf area index at 28DAS Ht height RW root weight RV root volume Pan panicle mminus2 Gr filled grains panicleminus1 TGW 1000-grainweight SS spikelet sterility CI weed competitive index WP weed biomass and RYL relative yield loss

6 The Scientific World Journal

Table 6 Final plant height (cm plantminus1) 1000-grain weight (g)and spikelet sterility () in response to weed infestation levels andgenotypes

Treatment Plant height 1000-grainweight

Spikeletsterility

cm g Weed infestation level

Weed-freea 1153 260 195Partial weedy 1165 256 201LSD (005) NS NS 16

GenotypePR-115 935 247 262H-97158 1094 255 182CR2707 1246 254 157US-310 1092 243 258IR88633 1502 289 166IR83927 1269 270 171RH-257 1126 258 194PR-120 1001 255 146LSD (005) 71 05 39

Nonsignificant NSaWeed free plots plots were kept weed free throughout the season Partialweedy plots plotswere hand-weeded once at 28DAS andweedswere allowedto grow before and after the hand-weeding throughout the season

Table 7 Leaf area index (LAI) and vigor score at 28 days aftersowing and root biomass and volume at flowering of variousgenotypes under weed-free conditions

Genotype LAI Vigor score Root biomass Root volumegmminus2 mLmminus2

PR-115 186 43 1478 5615H-97158 273 47 2009 7635CR2707 326 90 1965 7466US-310 190 47 1757 6676IR88633 337 90 3428 13027IR83927 289 70 2914 11073RH-257 310 67 3034 11530PR-120 282 67 4295 16538LSD (005) 043 06 755 2870

vigor and early canopy closure as evidenced by the higher LAIof these genotypes

Vigor score was highest in cultivars IR88633 and CR2707(Table 7) Among the tested genotypes vigor score was lowestfor PR-115 followed by H-97158 and US-310 Early vigorhas been shown as an important characteristics related tocompetitive ability [16] and tends to be reflected in greatershoot and root biomass during early vegetative growth Inthe present study crop biomass of PR-120 remained similarin both partial weedy and weed-free conditions (Table 8)At panicle initiation PR-120 in the partial weedy conditionsaccruedwith similar crop biomass withH-97158 and IR88633in the weed-free conditions In the partial weedy conditions

at panicle initiation and flowering stages IR88633 IR83927RH-257 and PR-120 accrued with higher crop biomass thanthat of PR-115 H-97158 CR2707 and US-310 respectively(Table 8)

The present study revealed that higher biomass coupledwith higher plant height at early stages provided greaterstored assimilates for grain filling via translocation thatmay prevent the yield loss of these genotypes under par-tial weedy conditions In partial weedy conditions grainnumber panicleminus1 remained similar only in IR83927 andPR-120 as compared with that in weed-free conditions andthis response was mainly because of their better weed-competitive ability In partial weedy conditions the highestnumber of grains panicleminus1 was found in PR-120 Grainnumber panicleminus1 for PR-120 in partial weedy conditions wassimilar to that of PR-115 H-97158 IR88633 and IR83927 inthe weed-free conditions

High vigor score and LAI during early crop developmenthave been shown to bemajor plant traits contributing toweedcompetitiveness [27 28]The LAI is an important contributorto competitive ability as light is an important resource inplant growth Greater shading ability of the crop makes lesslight available to the competing weeds hence weed growthcan be reduced In the weed-free plots IR88633 attained thehighest LAI at 28 DAS whereas PR-115 attained the lowestLAI (Table 7) The LAI of PR-115 and US-310 was similarGenotypes H-97158 RH-257 PR-120 and IR83927 recordedsimilar LAI but this was higher than that of PR-115 The LAIof CR2707 and IR88633 was similar

The findings of our study indicate that fast-growing habitand the high LAI at the early stage of the crop along withhigh root biomass and volume are the important traits forweed competitiveness confirming studies by various authors[18 29] However some workers [30] demonstrated that anadvantage of early growth habit even for a few days couldshift the competitive balance between crops and weeds Inanother study some workers found a significant correlationbetween crop competitive ability and plant height [31] In thepresent study a negative correlation was found between plantheight and relative yield loss In contrast Fischer et al foundno correlation between the height of semidwarf cultivars andweed biomass [11] As in our study research by some scientists[32] suggested that rice cultivars capable of producing moregrain yield in competition with weeds have greater earlybiomass Similarly workers [8 33] proposed that early vigorshould be a key selection trait for weed competitiveness PR-120 owing to its high root biomass and volume which helpedin attaining high crop biomass and greater panicle numbermminus2 produced the highest yield in partial weedy conditionsOur results confirm the findings of researchers who reportedsignificant and negative correlation between weed biomassand panicle number mminus2 suggesting that panicle density wasdecreased by high weed growth [31]

Competitive ability is the ability of a genotype to suppressweed biomass or to maintain crop yields by tolerating weedcompetition In this study competitive ability was foundhighest for PR-120 The competitiveness of a cultivar mightbe due to the higher number of panicles mminus2 early growth

The Scientific World Journal 7

Table 8 Crop biomass (t haminus1) in response to the interaction effect of weed infestation levels and genotypes at panicle initiation and floweringstages

Weed infestation levelCrop biomass

PR-115 H-97158 CR2707 US-310 IR88633 IR83927 RH-257 PR-120t haminus1

Panicle initiation stageWeed-freea 43 48 320 380 490 440 450 460Partial weedy 330 325 315 340 422 414 440 480LSD (005) 03

Flowering stageWeed-free 698 748 698 721 802 712 732 718Partial weedy 382 425 410 412 490 512 530 592LSD (005) 05aWeed free plots plots were kept weed free throughout the season Partial weedy plots plots were hand-weeded once at 28DAS and weeds were allowed togrow before and after the hand-weeding throughout the season

and yield under both partial weedy andweed-free conditionsIn this study panicle number mminus2 also had a correlationwith grain yield under both partial weedy and weed-freeconditions Cultivars able to produce higher grain yields inpartial weedy conditions correlated with greater early ricegrowth and LAI [8 34 35] Likewise this study demonstratedthat high LAI along with high root biomass and volumewere the useful traits for weed-competitive genotypes CR-2707 and RH-257 had higher LAI and vigor and similarlyRH-257 had higher root biomass and volume but yield lossof these genotypes was found more than 35 This was dueto their (CR-2707 and RH-257) more tendencies for lodgingunder partial weedy conditions

In summary genotypes PR-115 and H-97158 were theworst competitors and PR-120 IR88633 and IR83927 weregood weed competitors Genotypes PR-120 IR88633 andIR83927 have potential for increasing the weed-competitiveability of genotypes in breeding programs and could beused to increase the competitiveness of highly productivegenotypes that are not competitive The study revealed thatearly canopy closure high LAI at the early stage and greaterroot biomass and volume correlated positively with compet-itiveness This study demonstrated that breeding to increasethe competiveness of highly productive rice plant typeswouldbe possible without compromising yieldThe competitivenessobserved in these studies for genotypes PR-120 IR88633 andIR83927 would be adequate in improving farmersrsquo incomeand reducing herbicide use and can be successfully used inintegrated weed management programs in DSR

4 Future Implications

This study implies that breeding for weed-suppressive ricegenotypes should be conducted under partial weedy con-ditions In a mixed weed population the weed-competitiveability of rice genotypes may be affected by weed composi-tion However the competitive ability of rice genotypes is acomplex trait and could not be explained by only one or twocharacteristics Interactions between traits for instance rootbiomass LAI and high shoot biomass at early stage play an

important role and these traits can be explored for identifyingweed-competitive genotypes in DSR Competitiveness of ricegenotypes against weeds will be an important key to thesuccessful adoption of weed-suppressive rice genotypes ina sustainable weed management program The developmentof such genotypes could play an important role in DSR byreducing herbicide load in agroeco system

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors are grateful to Bill Hardy International RiceResearch Institute Philippines for providing comments onthe paper The authors would also like to acknowledge thefunding from the Cereal Systems Initiatives for South Asia(CSISA)

References

[1] G Mahajan B S Chauhan and M S Gill ldquoDry-seeded riceculture in Punjab State of India lessons learned from farmersrdquoField Crops Research vol 144 pp 89ndash99 2013

[2] G Mahajan and B S Chauhan ldquoHerbicide options for weedControl in dry-seeded aromatic rice in IndiardquoWeed Technologyvol 27 pp 682ndash689 2013

[3] B S Chauhan ldquoWeed ecology andweedmanagement strategiesfor dry-seeded rice in AsiardquoWeed Technology vol 26 no 1 pp1ndash13 2012

[4] A N Rao D E Johnson B Sivaprasad J K Ladha and A MMortimer ldquoWeed management in direct-seeded ricerdquo Advancesin Agronomy vol 93 pp 153ndash255 2007

[5] D L Zhao G N Atlin L Bastiaans and J H J SpiertzldquoComparing rice germplasm groups for growth grain yield andweed-suppressive ability under aerobic soil conditionsrdquo WeedResearch vol 46 no 6 pp 444ndash452 2006

8 The Scientific World Journal

[6] D Lemerle B Verbeek R D Cousens and N E CoombesldquoThe potential for selecting wheat varieties strongly competitiveagainst weedsrdquoWeed Research vol 36 no 6 pp 505ndash513 1996

[7] G Mahajan and B S Chauhan ldquoThe role of cultivars in man-aging weeds in dry-seeded rice production systemsrdquo CropProtection vol 49 pp 52ndash57 2013

[8] D L Zhao G N Atlin L Bastiaans and J H J SpiertzldquoDeveloping selection protocols for weed competitiveness inaerobic ricerdquo Field Crops Research vol 97 no 2-3 pp 272ndash2852006

[9] K D Gibson A J Fischer T C Foin and J E Hill ldquoCrop traitsrelated to weed suppression in water-seeded rice (Oryza sativaL)rdquoWeed Science vol 51 no 1 pp 87ndash93 2003

[10] D E JohnsonMDingkuhnM P Jones andM CMahamaneldquoThe influence of rice plant type on the effect of weed competi-tion onOryza sativa andOryza glaberrimardquoWeed Research vol38 no 3 pp 207ndash216 1998

[11] A J Fischer H V Ramirez K D Gibson and B Da SilveiraPinheiro ldquoCompetitiveness of semidwarf upland rice cultivarsagainst palisadegrass (Brachiaria brizantha) and signalgrass (Bdecumbens)rdquo Agronomy Journal vol 93 no 5 pp 967ndash9732001

[12] P Anwar A S Juraimi A Man A Puteh A Selamat andM Begum ldquoWeed suppressive ability of rice (Oryza sativa L)germplasm under aerobic soil conditionsrdquoAustralian Journal ofCrop Science vol 4 no 9 pp 706ndash717 2010

[13] A Fischer H V Ramırez and J Lozano ldquoSuppression ofjunglerice [Echinochloa colona (L) Link] by irrigated ricecultivars in Latin AmericardquoAgronomy Journal vol 89 no 3 pp516ndash521 1997

[14] S M Haefele D E Johnson D MrsquoBodj M C S Wopereisand K M Miezan ldquoField screening of diverse rice genotypesforweed competitiveness in irrigated lowland ecosystemsrdquoFieldCrops Research vol 88 no 1 pp 39ndash56 2004

[15] B S Chauhan G Mahajan V Sardana J Timsina and M LJat ldquoProductivity and sustainability of the rice-wheat croppingsystem in the indo-gangetic plains of the Indian subcontinentproblems opportunities and strategiesrdquoAdvances inAgronomyvol 117 pp 315ndash369 2012

[16] D Lemerle B Verbeek and B Orchard ldquoRanking the abilityof wheat varieties to compete with Lolium rigidumrdquo WeedResearch vol 41 no 3 pp 197ndash209 2001

[17] D P Garrity M Movillon and K Moody ldquoDifferential weedsuppression ability in upland rice cultivarsrdquo Agronomy Journalvol 84 pp 586ndash591 1992

[18] K D Gibson J E Hill T C Foin B P Caton and A J FischerldquoWater-seeded rice cultivars differ in ability to interfere withwatergrassrdquo Agronomy Journal vol 93 no 2 pp 326ndash332 2001

[19] M Dingkuhn D E Johnson A Sow and A Y AudebertldquoRelationships between upland rice canopy characteristics andweed competitivenessrdquo Field Crops Research vol 61 no 1 pp79ndash95 1999

[20] M C Richards and G P Whytock ldquoVarietal competitivenesswith weedsrdquo Aspects of Applied Biology vol 34 pp 345ndash3541993

[21] D L ZhaoGNAtlin L Bastiaans and JH J Spiertz ldquoCultivarweed-competitiveness in aerobic rice heritability correlatedtraits and the potential for indirect selection in weed-freeenvironmentsrdquo Crop Science vol 46 no 1 pp 372ndash380 2006

[22] D G Huel and P Hucl ldquoGenotypic variation for competitiveability in spring wheatrdquo Plant Breeding vol 115 no 5 pp 325ndash329 1996

[23] H Q Wang B A M Bouman D L Zhao C Wang and PF Moya ldquoAerobic rice in northern China opportunities andchallengesrdquo in Proceedings of the International Workshop onWater-Wise Rice Production B A M Bouman H HengsdijkB Hardy P S Bindraban T P Tuong and J K Ladha Eds pp143ndash154 International Rice Research Institute April 2002

[24] B S Chauhan and D E Johnson ldquoRow spacing and weedcontrol timing affect yield of aerobic ricerdquo Field Crops Researchvol 121 no 2 pp 226ndash231 2011

[25] M A Baghestani E Zand and S Soufizadeh ldquoIranian winterwheats (Triticum aestivum L) interference with weeds I Grainyield and competitive indexrdquo Pakistan Journal of Weed ScienceResearch vol 12 pp 119ndash129 2006

[26] A Rezakhanlou M Aghabeigi and H Bagheri ldquoEvaluation ofsome of competitiveness indexes in competition between cottonvarieties and common cocklebur (Xanthium strumarium L)rdquoInternational Research Journal of Applied and Basic Sciences vol3 pp 1274ndash1278 2012

[27] R K Coleman G S Gill and G J Rebetzke ldquoIdentification ofquantitative trait loci for traits conferring weed competitivenessin wheat (Triticum aestivum L)rdquo Australian Journal of Agricul-tural Research vol 52 no 11-12 pp 1235ndash1246 2001

[28] J Rodenburg K Saito R G Kakaı A Toure M Mariko andP Kiepe ldquoWeed competitiveness of the lowland rice varietiesof NERICA in the southern Guinea Savannardquo Field CropsResearch vol 114 no 3 pp 411ndash418 2009

[29] B P Caton A E Cope and M Mortimer ldquoGrowth traits ofdiverse rice cultivars under severe competition implications forscreening for competitivenessrdquo Field Crops Research vol 83 no2 pp 157ndash172 2003

[30] M J Kropff ldquoEco-physiological models for crop-weed compe-titionrdquo inModeling Crop-Weed Interactions M J Kropff and HH van Laar Eds pp 25ndash32 CAB International WallingfordUK 1993

[31] F Ekeleme A Y Kamara S O Oikeh et al ldquoResponse ofupland rice cultivars to weed competition in the savannas ofWest Africardquo Crop Protection vol 28 no 1 pp 90ndash96 2009

[32] O S Namuco J E Cairns and D E Johnson ldquoInvestigatingearly vigour in upland rice (Oryza sativa L) part I Seedlinggrowth and grain yield in competition with weedsrdquo Field CropsResearch vol 113 no 3 pp 197ndash206 2009

[33] K Saito K Azoma and J Rodenburg ldquoPlant characteristicsassociated with weed competitiveness of rice under upland andlowland conditions in West Africardquo Field Crops Research vol116 no 3 pp 308ndash317 2010

[34] H Ni K Moody R P Robles E C Paller Jr and J S LalesldquoOryza sativa plant traits conferring competitive ability againstweedsrdquoWeed Science vol 48 no 2 pp 200ndash204 2000

[35] G Wu L T Wilson and A M McClung ldquoContribution of ricetillers to drymatter accumulation and yieldrdquoAgronomy Journalvol 90 no 3 pp 317ndash323 1998

Submit your manuscripts athttpwwwhindawicom

Nutrition and Metabolism

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Food ScienceInternational Journal of

Agronomy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Microbiology

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

AgricultureAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

PsycheHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BiodiversityInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

GenomicsInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Plant GenomicsInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Biotechnology Research International

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Forestry ResearchInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of BotanyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Veterinary Medicine International

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Cell BiologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Evolutionary BiologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

6 The Scientific World Journal

Table 6 Final plant height (cm plantminus1) 1000-grain weight (g)and spikelet sterility () in response to weed infestation levels andgenotypes

Treatment Plant height 1000-grainweight

Spikeletsterility

cm g Weed infestation level

Weed-freea 1153 260 195Partial weedy 1165 256 201LSD (005) NS NS 16

GenotypePR-115 935 247 262H-97158 1094 255 182CR2707 1246 254 157US-310 1092 243 258IR88633 1502 289 166IR83927 1269 270 171RH-257 1126 258 194PR-120 1001 255 146LSD (005) 71 05 39

Nonsignificant NSaWeed free plots plots were kept weed free throughout the season Partialweedy plots plotswere hand-weeded once at 28DAS andweedswere allowedto grow before and after the hand-weeding throughout the season

Table 7 Leaf area index (LAI) and vigor score at 28 days aftersowing and root biomass and volume at flowering of variousgenotypes under weed-free conditions

Genotype LAI Vigor score Root biomass Root volumegmminus2 mLmminus2

PR-115 186 43 1478 5615H-97158 273 47 2009 7635CR2707 326 90 1965 7466US-310 190 47 1757 6676IR88633 337 90 3428 13027IR83927 289 70 2914 11073RH-257 310 67 3034 11530PR-120 282 67 4295 16538LSD (005) 043 06 755 2870

vigor and early canopy closure as evidenced by the higher LAIof these genotypes

Vigor score was highest in cultivars IR88633 and CR2707(Table 7) Among the tested genotypes vigor score was lowestfor PR-115 followed by H-97158 and US-310 Early vigorhas been shown as an important characteristics related tocompetitive ability [16] and tends to be reflected in greatershoot and root biomass during early vegetative growth Inthe present study crop biomass of PR-120 remained similarin both partial weedy and weed-free conditions (Table 8)At panicle initiation PR-120 in the partial weedy conditionsaccruedwith similar crop biomass withH-97158 and IR88633in the weed-free conditions In the partial weedy conditions

at panicle initiation and flowering stages IR88633 IR83927RH-257 and PR-120 accrued with higher crop biomass thanthat of PR-115 H-97158 CR2707 and US-310 respectively(Table 8)

The present study revealed that higher biomass coupledwith higher plant height at early stages provided greaterstored assimilates for grain filling via translocation thatmay prevent the yield loss of these genotypes under par-tial weedy conditions In partial weedy conditions grainnumber panicleminus1 remained similar only in IR83927 andPR-120 as compared with that in weed-free conditions andthis response was mainly because of their better weed-competitive ability In partial weedy conditions the highestnumber of grains panicleminus1 was found in PR-120 Grainnumber panicleminus1 for PR-120 in partial weedy conditions wassimilar to that of PR-115 H-97158 IR88633 and IR83927 inthe weed-free conditions

High vigor score and LAI during early crop developmenthave been shown to bemajor plant traits contributing toweedcompetitiveness [27 28]The LAI is an important contributorto competitive ability as light is an important resource inplant growth Greater shading ability of the crop makes lesslight available to the competing weeds hence weed growthcan be reduced In the weed-free plots IR88633 attained thehighest LAI at 28 DAS whereas PR-115 attained the lowestLAI (Table 7) The LAI of PR-115 and US-310 was similarGenotypes H-97158 RH-257 PR-120 and IR83927 recordedsimilar LAI but this was higher than that of PR-115 The LAIof CR2707 and IR88633 was similar

The findings of our study indicate that fast-growing habitand the high LAI at the early stage of the crop along withhigh root biomass and volume are the important traits forweed competitiveness confirming studies by various authors[18 29] However some workers [30] demonstrated that anadvantage of early growth habit even for a few days couldshift the competitive balance between crops and weeds Inanother study some workers found a significant correlationbetween crop competitive ability and plant height [31] In thepresent study a negative correlation was found between plantheight and relative yield loss In contrast Fischer et al foundno correlation between the height of semidwarf cultivars andweed biomass [11] As in our study research by some scientists[32] suggested that rice cultivars capable of producing moregrain yield in competition with weeds have greater earlybiomass Similarly workers [8 33] proposed that early vigorshould be a key selection trait for weed competitiveness PR-120 owing to its high root biomass and volume which helpedin attaining high crop biomass and greater panicle numbermminus2 produced the highest yield in partial weedy conditionsOur results confirm the findings of researchers who reportedsignificant and negative correlation between weed biomassand panicle number mminus2 suggesting that panicle density wasdecreased by high weed growth [31]

Competitive ability is the ability of a genotype to suppressweed biomass or to maintain crop yields by tolerating weedcompetition In this study competitive ability was foundhighest for PR-120 The competitiveness of a cultivar mightbe due to the higher number of panicles mminus2 early growth

The Scientific World Journal 7

Table 8 Crop biomass (t haminus1) in response to the interaction effect of weed infestation levels and genotypes at panicle initiation and floweringstages

Weed infestation levelCrop biomass

PR-115 H-97158 CR2707 US-310 IR88633 IR83927 RH-257 PR-120t haminus1

Panicle initiation stageWeed-freea 43 48 320 380 490 440 450 460Partial weedy 330 325 315 340 422 414 440 480LSD (005) 03

Flowering stageWeed-free 698 748 698 721 802 712 732 718Partial weedy 382 425 410 412 490 512 530 592LSD (005) 05aWeed free plots plots were kept weed free throughout the season Partial weedy plots plots were hand-weeded once at 28DAS and weeds were allowed togrow before and after the hand-weeding throughout the season

and yield under both partial weedy andweed-free conditionsIn this study panicle number mminus2 also had a correlationwith grain yield under both partial weedy and weed-freeconditions Cultivars able to produce higher grain yields inpartial weedy conditions correlated with greater early ricegrowth and LAI [8 34 35] Likewise this study demonstratedthat high LAI along with high root biomass and volumewere the useful traits for weed-competitive genotypes CR-2707 and RH-257 had higher LAI and vigor and similarlyRH-257 had higher root biomass and volume but yield lossof these genotypes was found more than 35 This was dueto their (CR-2707 and RH-257) more tendencies for lodgingunder partial weedy conditions

In summary genotypes PR-115 and H-97158 were theworst competitors and PR-120 IR88633 and IR83927 weregood weed competitors Genotypes PR-120 IR88633 andIR83927 have potential for increasing the weed-competitiveability of genotypes in breeding programs and could beused to increase the competitiveness of highly productivegenotypes that are not competitive The study revealed thatearly canopy closure high LAI at the early stage and greaterroot biomass and volume correlated positively with compet-itiveness This study demonstrated that breeding to increasethe competiveness of highly productive rice plant typeswouldbe possible without compromising yieldThe competitivenessobserved in these studies for genotypes PR-120 IR88633 andIR83927 would be adequate in improving farmersrsquo incomeand reducing herbicide use and can be successfully used inintegrated weed management programs in DSR

4 Future Implications

This study implies that breeding for weed-suppressive ricegenotypes should be conducted under partial weedy con-ditions In a mixed weed population the weed-competitiveability of rice genotypes may be affected by weed composi-tion However the competitive ability of rice genotypes is acomplex trait and could not be explained by only one or twocharacteristics Interactions between traits for instance rootbiomass LAI and high shoot biomass at early stage play an

important role and these traits can be explored for identifyingweed-competitive genotypes in DSR Competitiveness of ricegenotypes against weeds will be an important key to thesuccessful adoption of weed-suppressive rice genotypes ina sustainable weed management program The developmentof such genotypes could play an important role in DSR byreducing herbicide load in agroeco system

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors are grateful to Bill Hardy International RiceResearch Institute Philippines for providing comments onthe paper The authors would also like to acknowledge thefunding from the Cereal Systems Initiatives for South Asia(CSISA)

References

[1] G Mahajan B S Chauhan and M S Gill ldquoDry-seeded riceculture in Punjab State of India lessons learned from farmersrdquoField Crops Research vol 144 pp 89ndash99 2013

[2] G Mahajan and B S Chauhan ldquoHerbicide options for weedControl in dry-seeded aromatic rice in IndiardquoWeed Technologyvol 27 pp 682ndash689 2013

[3] B S Chauhan ldquoWeed ecology andweedmanagement strategiesfor dry-seeded rice in AsiardquoWeed Technology vol 26 no 1 pp1ndash13 2012

[4] A N Rao D E Johnson B Sivaprasad J K Ladha and A MMortimer ldquoWeed management in direct-seeded ricerdquo Advancesin Agronomy vol 93 pp 153ndash255 2007

[5] D L Zhao G N Atlin L Bastiaans and J H J SpiertzldquoComparing rice germplasm groups for growth grain yield andweed-suppressive ability under aerobic soil conditionsrdquo WeedResearch vol 46 no 6 pp 444ndash452 2006

8 The Scientific World Journal

[6] D Lemerle B Verbeek R D Cousens and N E CoombesldquoThe potential for selecting wheat varieties strongly competitiveagainst weedsrdquoWeed Research vol 36 no 6 pp 505ndash513 1996

[7] G Mahajan and B S Chauhan ldquoThe role of cultivars in man-aging weeds in dry-seeded rice production systemsrdquo CropProtection vol 49 pp 52ndash57 2013

[8] D L Zhao G N Atlin L Bastiaans and J H J SpiertzldquoDeveloping selection protocols for weed competitiveness inaerobic ricerdquo Field Crops Research vol 97 no 2-3 pp 272ndash2852006

[9] K D Gibson A J Fischer T C Foin and J E Hill ldquoCrop traitsrelated to weed suppression in water-seeded rice (Oryza sativaL)rdquoWeed Science vol 51 no 1 pp 87ndash93 2003

[10] D E JohnsonMDingkuhnM P Jones andM CMahamaneldquoThe influence of rice plant type on the effect of weed competi-tion onOryza sativa andOryza glaberrimardquoWeed Research vol38 no 3 pp 207ndash216 1998

[11] A J Fischer H V Ramirez K D Gibson and B Da SilveiraPinheiro ldquoCompetitiveness of semidwarf upland rice cultivarsagainst palisadegrass (Brachiaria brizantha) and signalgrass (Bdecumbens)rdquo Agronomy Journal vol 93 no 5 pp 967ndash9732001

[12] P Anwar A S Juraimi A Man A Puteh A Selamat andM Begum ldquoWeed suppressive ability of rice (Oryza sativa L)germplasm under aerobic soil conditionsrdquoAustralian Journal ofCrop Science vol 4 no 9 pp 706ndash717 2010

[13] A Fischer H V Ramırez and J Lozano ldquoSuppression ofjunglerice [Echinochloa colona (L) Link] by irrigated ricecultivars in Latin AmericardquoAgronomy Journal vol 89 no 3 pp516ndash521 1997

[14] S M Haefele D E Johnson D MrsquoBodj M C S Wopereisand K M Miezan ldquoField screening of diverse rice genotypesforweed competitiveness in irrigated lowland ecosystemsrdquoFieldCrops Research vol 88 no 1 pp 39ndash56 2004

[15] B S Chauhan G Mahajan V Sardana J Timsina and M LJat ldquoProductivity and sustainability of the rice-wheat croppingsystem in the indo-gangetic plains of the Indian subcontinentproblems opportunities and strategiesrdquoAdvances inAgronomyvol 117 pp 315ndash369 2012

[16] D Lemerle B Verbeek and B Orchard ldquoRanking the abilityof wheat varieties to compete with Lolium rigidumrdquo WeedResearch vol 41 no 3 pp 197ndash209 2001

[17] D P Garrity M Movillon and K Moody ldquoDifferential weedsuppression ability in upland rice cultivarsrdquo Agronomy Journalvol 84 pp 586ndash591 1992

[18] K D Gibson J E Hill T C Foin B P Caton and A J FischerldquoWater-seeded rice cultivars differ in ability to interfere withwatergrassrdquo Agronomy Journal vol 93 no 2 pp 326ndash332 2001

[19] M Dingkuhn D E Johnson A Sow and A Y AudebertldquoRelationships between upland rice canopy characteristics andweed competitivenessrdquo Field Crops Research vol 61 no 1 pp79ndash95 1999

[20] M C Richards and G P Whytock ldquoVarietal competitivenesswith weedsrdquo Aspects of Applied Biology vol 34 pp 345ndash3541993

[21] D L ZhaoGNAtlin L Bastiaans and JH J Spiertz ldquoCultivarweed-competitiveness in aerobic rice heritability correlatedtraits and the potential for indirect selection in weed-freeenvironmentsrdquo Crop Science vol 46 no 1 pp 372ndash380 2006

[22] D G Huel and P Hucl ldquoGenotypic variation for competitiveability in spring wheatrdquo Plant Breeding vol 115 no 5 pp 325ndash329 1996

[23] H Q Wang B A M Bouman D L Zhao C Wang and PF Moya ldquoAerobic rice in northern China opportunities andchallengesrdquo in Proceedings of the International Workshop onWater-Wise Rice Production B A M Bouman H HengsdijkB Hardy P S Bindraban T P Tuong and J K Ladha Eds pp143ndash154 International Rice Research Institute April 2002

[24] B S Chauhan and D E Johnson ldquoRow spacing and weedcontrol timing affect yield of aerobic ricerdquo Field Crops Researchvol 121 no 2 pp 226ndash231 2011

[25] M A Baghestani E Zand and S Soufizadeh ldquoIranian winterwheats (Triticum aestivum L) interference with weeds I Grainyield and competitive indexrdquo Pakistan Journal of Weed ScienceResearch vol 12 pp 119ndash129 2006

[26] A Rezakhanlou M Aghabeigi and H Bagheri ldquoEvaluation ofsome of competitiveness indexes in competition between cottonvarieties and common cocklebur (Xanthium strumarium L)rdquoInternational Research Journal of Applied and Basic Sciences vol3 pp 1274ndash1278 2012

[27] R K Coleman G S Gill and G J Rebetzke ldquoIdentification ofquantitative trait loci for traits conferring weed competitivenessin wheat (Triticum aestivum L)rdquo Australian Journal of Agricul-tural Research vol 52 no 11-12 pp 1235ndash1246 2001

[28] J Rodenburg K Saito R G Kakaı A Toure M Mariko andP Kiepe ldquoWeed competitiveness of the lowland rice varietiesof NERICA in the southern Guinea Savannardquo Field CropsResearch vol 114 no 3 pp 411ndash418 2009

[29] B P Caton A E Cope and M Mortimer ldquoGrowth traits ofdiverse rice cultivars under severe competition implications forscreening for competitivenessrdquo Field Crops Research vol 83 no2 pp 157ndash172 2003

[30] M J Kropff ldquoEco-physiological models for crop-weed compe-titionrdquo inModeling Crop-Weed Interactions M J Kropff and HH van Laar Eds pp 25ndash32 CAB International WallingfordUK 1993

[31] F Ekeleme A Y Kamara S O Oikeh et al ldquoResponse ofupland rice cultivars to weed competition in the savannas ofWest Africardquo Crop Protection vol 28 no 1 pp 90ndash96 2009

[32] O S Namuco J E Cairns and D E Johnson ldquoInvestigatingearly vigour in upland rice (Oryza sativa L) part I Seedlinggrowth and grain yield in competition with weedsrdquo Field CropsResearch vol 113 no 3 pp 197ndash206 2009

[33] K Saito K Azoma and J Rodenburg ldquoPlant characteristicsassociated with weed competitiveness of rice under upland andlowland conditions in West Africardquo Field Crops Research vol116 no 3 pp 308ndash317 2010

[34] H Ni K Moody R P Robles E C Paller Jr and J S LalesldquoOryza sativa plant traits conferring competitive ability againstweedsrdquoWeed Science vol 48 no 2 pp 200ndash204 2000

[35] G Wu L T Wilson and A M McClung ldquoContribution of ricetillers to drymatter accumulation and yieldrdquoAgronomy Journalvol 90 no 3 pp 317ndash323 1998

Submit your manuscripts athttpwwwhindawicom

Nutrition and Metabolism

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Food ScienceInternational Journal of

Agronomy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Microbiology

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

AgricultureAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

PsycheHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BiodiversityInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

GenomicsInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Plant GenomicsInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Biotechnology Research International

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Forestry ResearchInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of BotanyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Veterinary Medicine International

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Cell BiologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Evolutionary BiologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World Journal 7

Table 8 Crop biomass (t haminus1) in response to the interaction effect of weed infestation levels and genotypes at panicle initiation and floweringstages

Weed infestation levelCrop biomass

PR-115 H-97158 CR2707 US-310 IR88633 IR83927 RH-257 PR-120t haminus1

Panicle initiation stageWeed-freea 43 48 320 380 490 440 450 460Partial weedy 330 325 315 340 422 414 440 480LSD (005) 03

Flowering stageWeed-free 698 748 698 721 802 712 732 718Partial weedy 382 425 410 412 490 512 530 592LSD (005) 05aWeed free plots plots were kept weed free throughout the season Partial weedy plots plots were hand-weeded once at 28DAS and weeds were allowed togrow before and after the hand-weeding throughout the season

and yield under both partial weedy andweed-free conditionsIn this study panicle number mminus2 also had a correlationwith grain yield under both partial weedy and weed-freeconditions Cultivars able to produce higher grain yields inpartial weedy conditions correlated with greater early ricegrowth and LAI [8 34 35] Likewise this study demonstratedthat high LAI along with high root biomass and volumewere the useful traits for weed-competitive genotypes CR-2707 and RH-257 had higher LAI and vigor and similarlyRH-257 had higher root biomass and volume but yield lossof these genotypes was found more than 35 This was dueto their (CR-2707 and RH-257) more tendencies for lodgingunder partial weedy conditions

In summary genotypes PR-115 and H-97158 were theworst competitors and PR-120 IR88633 and IR83927 weregood weed competitors Genotypes PR-120 IR88633 andIR83927 have potential for increasing the weed-competitiveability of genotypes in breeding programs and could beused to increase the competitiveness of highly productivegenotypes that are not competitive The study revealed thatearly canopy closure high LAI at the early stage and greaterroot biomass and volume correlated positively with compet-itiveness This study demonstrated that breeding to increasethe competiveness of highly productive rice plant typeswouldbe possible without compromising yieldThe competitivenessobserved in these studies for genotypes PR-120 IR88633 andIR83927 would be adequate in improving farmersrsquo incomeand reducing herbicide use and can be successfully used inintegrated weed management programs in DSR

4 Future Implications

This study implies that breeding for weed-suppressive ricegenotypes should be conducted under partial weedy con-ditions In a mixed weed population the weed-competitiveability of rice genotypes may be affected by weed composi-tion However the competitive ability of rice genotypes is acomplex trait and could not be explained by only one or twocharacteristics Interactions between traits for instance rootbiomass LAI and high shoot biomass at early stage play an

important role and these traits can be explored for identifyingweed-competitive genotypes in DSR Competitiveness of ricegenotypes against weeds will be an important key to thesuccessful adoption of weed-suppressive rice genotypes ina sustainable weed management program The developmentof such genotypes could play an important role in DSR byreducing herbicide load in agroeco system

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors are grateful to Bill Hardy International RiceResearch Institute Philippines for providing comments onthe paper The authors would also like to acknowledge thefunding from the Cereal Systems Initiatives for South Asia(CSISA)

References

[1] G Mahajan B S Chauhan and M S Gill ldquoDry-seeded riceculture in Punjab State of India lessons learned from farmersrdquoField Crops Research vol 144 pp 89ndash99 2013

[2] G Mahajan and B S Chauhan ldquoHerbicide options for weedControl in dry-seeded aromatic rice in IndiardquoWeed Technologyvol 27 pp 682ndash689 2013

[3] B S Chauhan ldquoWeed ecology andweedmanagement strategiesfor dry-seeded rice in AsiardquoWeed Technology vol 26 no 1 pp1ndash13 2012

[4] A N Rao D E Johnson B Sivaprasad J K Ladha and A MMortimer ldquoWeed management in direct-seeded ricerdquo Advancesin Agronomy vol 93 pp 153ndash255 2007

[5] D L Zhao G N Atlin L Bastiaans and J H J SpiertzldquoComparing rice germplasm groups for growth grain yield andweed-suppressive ability under aerobic soil conditionsrdquo WeedResearch vol 46 no 6 pp 444ndash452 2006

8 The Scientific World Journal

[6] D Lemerle B Verbeek R D Cousens and N E CoombesldquoThe potential for selecting wheat varieties strongly competitiveagainst weedsrdquoWeed Research vol 36 no 6 pp 505ndash513 1996

[7] G Mahajan and B S Chauhan ldquoThe role of cultivars in man-aging weeds in dry-seeded rice production systemsrdquo CropProtection vol 49 pp 52ndash57 2013

[8] D L Zhao G N Atlin L Bastiaans and J H J SpiertzldquoDeveloping selection protocols for weed competitiveness inaerobic ricerdquo Field Crops Research vol 97 no 2-3 pp 272ndash2852006

[9] K D Gibson A J Fischer T C Foin and J E Hill ldquoCrop traitsrelated to weed suppression in water-seeded rice (Oryza sativaL)rdquoWeed Science vol 51 no 1 pp 87ndash93 2003

[10] D E JohnsonMDingkuhnM P Jones andM CMahamaneldquoThe influence of rice plant type on the effect of weed competi-tion onOryza sativa andOryza glaberrimardquoWeed Research vol38 no 3 pp 207ndash216 1998

[11] A J Fischer H V Ramirez K D Gibson and B Da SilveiraPinheiro ldquoCompetitiveness of semidwarf upland rice cultivarsagainst palisadegrass (Brachiaria brizantha) and signalgrass (Bdecumbens)rdquo Agronomy Journal vol 93 no 5 pp 967ndash9732001

[12] P Anwar A S Juraimi A Man A Puteh A Selamat andM Begum ldquoWeed suppressive ability of rice (Oryza sativa L)germplasm under aerobic soil conditionsrdquoAustralian Journal ofCrop Science vol 4 no 9 pp 706ndash717 2010

[13] A Fischer H V Ramırez and J Lozano ldquoSuppression ofjunglerice [Echinochloa colona (L) Link] by irrigated ricecultivars in Latin AmericardquoAgronomy Journal vol 89 no 3 pp516ndash521 1997

[14] S M Haefele D E Johnson D MrsquoBodj M C S Wopereisand K M Miezan ldquoField screening of diverse rice genotypesforweed competitiveness in irrigated lowland ecosystemsrdquoFieldCrops Research vol 88 no 1 pp 39ndash56 2004

[15] B S Chauhan G Mahajan V Sardana J Timsina and M LJat ldquoProductivity and sustainability of the rice-wheat croppingsystem in the indo-gangetic plains of the Indian subcontinentproblems opportunities and strategiesrdquoAdvances inAgronomyvol 117 pp 315ndash369 2012

[16] D Lemerle B Verbeek and B Orchard ldquoRanking the abilityof wheat varieties to compete with Lolium rigidumrdquo WeedResearch vol 41 no 3 pp 197ndash209 2001

[17] D P Garrity M Movillon and K Moody ldquoDifferential weedsuppression ability in upland rice cultivarsrdquo Agronomy Journalvol 84 pp 586ndash591 1992

[18] K D Gibson J E Hill T C Foin B P Caton and A J FischerldquoWater-seeded rice cultivars differ in ability to interfere withwatergrassrdquo Agronomy Journal vol 93 no 2 pp 326ndash332 2001

[19] M Dingkuhn D E Johnson A Sow and A Y AudebertldquoRelationships between upland rice canopy characteristics andweed competitivenessrdquo Field Crops Research vol 61 no 1 pp79ndash95 1999

[20] M C Richards and G P Whytock ldquoVarietal competitivenesswith weedsrdquo Aspects of Applied Biology vol 34 pp 345ndash3541993

[21] D L ZhaoGNAtlin L Bastiaans and JH J Spiertz ldquoCultivarweed-competitiveness in aerobic rice heritability correlatedtraits and the potential for indirect selection in weed-freeenvironmentsrdquo Crop Science vol 46 no 1 pp 372ndash380 2006

[22] D G Huel and P Hucl ldquoGenotypic variation for competitiveability in spring wheatrdquo Plant Breeding vol 115 no 5 pp 325ndash329 1996

[23] H Q Wang B A M Bouman D L Zhao C Wang and PF Moya ldquoAerobic rice in northern China opportunities andchallengesrdquo in Proceedings of the International Workshop onWater-Wise Rice Production B A M Bouman H HengsdijkB Hardy P S Bindraban T P Tuong and J K Ladha Eds pp143ndash154 International Rice Research Institute April 2002

[24] B S Chauhan and D E Johnson ldquoRow spacing and weedcontrol timing affect yield of aerobic ricerdquo Field Crops Researchvol 121 no 2 pp 226ndash231 2011

[25] M A Baghestani E Zand and S Soufizadeh ldquoIranian winterwheats (Triticum aestivum L) interference with weeds I Grainyield and competitive indexrdquo Pakistan Journal of Weed ScienceResearch vol 12 pp 119ndash129 2006

[26] A Rezakhanlou M Aghabeigi and H Bagheri ldquoEvaluation ofsome of competitiveness indexes in competition between cottonvarieties and common cocklebur (Xanthium strumarium L)rdquoInternational Research Journal of Applied and Basic Sciences vol3 pp 1274ndash1278 2012

[27] R K Coleman G S Gill and G J Rebetzke ldquoIdentification ofquantitative trait loci for traits conferring weed competitivenessin wheat (Triticum aestivum L)rdquo Australian Journal of Agricul-tural Research vol 52 no 11-12 pp 1235ndash1246 2001

[28] J Rodenburg K Saito R G Kakaı A Toure M Mariko andP Kiepe ldquoWeed competitiveness of the lowland rice varietiesof NERICA in the southern Guinea Savannardquo Field CropsResearch vol 114 no 3 pp 411ndash418 2009

[29] B P Caton A E Cope and M Mortimer ldquoGrowth traits ofdiverse rice cultivars under severe competition implications forscreening for competitivenessrdquo Field Crops Research vol 83 no2 pp 157ndash172 2003

[30] M J Kropff ldquoEco-physiological models for crop-weed compe-titionrdquo inModeling Crop-Weed Interactions M J Kropff and HH van Laar Eds pp 25ndash32 CAB International WallingfordUK 1993

[31] F Ekeleme A Y Kamara S O Oikeh et al ldquoResponse ofupland rice cultivars to weed competition in the savannas ofWest Africardquo Crop Protection vol 28 no 1 pp 90ndash96 2009

[32] O S Namuco J E Cairns and D E Johnson ldquoInvestigatingearly vigour in upland rice (Oryza sativa L) part I Seedlinggrowth and grain yield in competition with weedsrdquo Field CropsResearch vol 113 no 3 pp 197ndash206 2009

[33] K Saito K Azoma and J Rodenburg ldquoPlant characteristicsassociated with weed competitiveness of rice under upland andlowland conditions in West Africardquo Field Crops Research vol116 no 3 pp 308ndash317 2010

[34] H Ni K Moody R P Robles E C Paller Jr and J S LalesldquoOryza sativa plant traits conferring competitive ability againstweedsrdquoWeed Science vol 48 no 2 pp 200ndash204 2000

[35] G Wu L T Wilson and A M McClung ldquoContribution of ricetillers to drymatter accumulation and yieldrdquoAgronomy Journalvol 90 no 3 pp 317ndash323 1998

Submit your manuscripts athttpwwwhindawicom

Nutrition and Metabolism

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Food ScienceInternational Journal of

Agronomy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Microbiology

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

AgricultureAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

PsycheHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BiodiversityInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

GenomicsInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Plant GenomicsInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Biotechnology Research International

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Forestry ResearchInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of BotanyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Veterinary Medicine International

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Cell BiologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Evolutionary BiologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

8 The Scientific World Journal

[6] D Lemerle B Verbeek R D Cousens and N E CoombesldquoThe potential for selecting wheat varieties strongly competitiveagainst weedsrdquoWeed Research vol 36 no 6 pp 505ndash513 1996

[7] G Mahajan and B S Chauhan ldquoThe role of cultivars in man-aging weeds in dry-seeded rice production systemsrdquo CropProtection vol 49 pp 52ndash57 2013

[8] D L Zhao G N Atlin L Bastiaans and J H J SpiertzldquoDeveloping selection protocols for weed competitiveness inaerobic ricerdquo Field Crops Research vol 97 no 2-3 pp 272ndash2852006

[9] K D Gibson A J Fischer T C Foin and J E Hill ldquoCrop traitsrelated to weed suppression in water-seeded rice (Oryza sativaL)rdquoWeed Science vol 51 no 1 pp 87ndash93 2003

[10] D E JohnsonMDingkuhnM P Jones andM CMahamaneldquoThe influence of rice plant type on the effect of weed competi-tion onOryza sativa andOryza glaberrimardquoWeed Research vol38 no 3 pp 207ndash216 1998

[11] A J Fischer H V Ramirez K D Gibson and B Da SilveiraPinheiro ldquoCompetitiveness of semidwarf upland rice cultivarsagainst palisadegrass (Brachiaria brizantha) and signalgrass (Bdecumbens)rdquo Agronomy Journal vol 93 no 5 pp 967ndash9732001

[12] P Anwar A S Juraimi A Man A Puteh A Selamat andM Begum ldquoWeed suppressive ability of rice (Oryza sativa L)germplasm under aerobic soil conditionsrdquoAustralian Journal ofCrop Science vol 4 no 9 pp 706ndash717 2010

[13] A Fischer H V Ramırez and J Lozano ldquoSuppression ofjunglerice [Echinochloa colona (L) Link] by irrigated ricecultivars in Latin AmericardquoAgronomy Journal vol 89 no 3 pp516ndash521 1997

[14] S M Haefele D E Johnson D MrsquoBodj M C S Wopereisand K M Miezan ldquoField screening of diverse rice genotypesforweed competitiveness in irrigated lowland ecosystemsrdquoFieldCrops Research vol 88 no 1 pp 39ndash56 2004

[15] B S Chauhan G Mahajan V Sardana J Timsina and M LJat ldquoProductivity and sustainability of the rice-wheat croppingsystem in the indo-gangetic plains of the Indian subcontinentproblems opportunities and strategiesrdquoAdvances inAgronomyvol 117 pp 315ndash369 2012

[16] D Lemerle B Verbeek and B Orchard ldquoRanking the abilityof wheat varieties to compete with Lolium rigidumrdquo WeedResearch vol 41 no 3 pp 197ndash209 2001

[17] D P Garrity M Movillon and K Moody ldquoDifferential weedsuppression ability in upland rice cultivarsrdquo Agronomy Journalvol 84 pp 586ndash591 1992

[18] K D Gibson J E Hill T C Foin B P Caton and A J FischerldquoWater-seeded rice cultivars differ in ability to interfere withwatergrassrdquo Agronomy Journal vol 93 no 2 pp 326ndash332 2001

[19] M Dingkuhn D E Johnson A Sow and A Y AudebertldquoRelationships between upland rice canopy characteristics andweed competitivenessrdquo Field Crops Research vol 61 no 1 pp79ndash95 1999

[20] M C Richards and G P Whytock ldquoVarietal competitivenesswith weedsrdquo Aspects of Applied Biology vol 34 pp 345ndash3541993

[21] D L ZhaoGNAtlin L Bastiaans and JH J Spiertz ldquoCultivarweed-competitiveness in aerobic rice heritability correlatedtraits and the potential for indirect selection in weed-freeenvironmentsrdquo Crop Science vol 46 no 1 pp 372ndash380 2006

[22] D G Huel and P Hucl ldquoGenotypic variation for competitiveability in spring wheatrdquo Plant Breeding vol 115 no 5 pp 325ndash329 1996

[23] H Q Wang B A M Bouman D L Zhao C Wang and PF Moya ldquoAerobic rice in northern China opportunities andchallengesrdquo in Proceedings of the International Workshop onWater-Wise Rice Production B A M Bouman H HengsdijkB Hardy P S Bindraban T P Tuong and J K Ladha Eds pp143ndash154 International Rice Research Institute April 2002

[24] B S Chauhan and D E Johnson ldquoRow spacing and weedcontrol timing affect yield of aerobic ricerdquo Field Crops Researchvol 121 no 2 pp 226ndash231 2011

[25] M A Baghestani E Zand and S Soufizadeh ldquoIranian winterwheats (Triticum aestivum L) interference with weeds I Grainyield and competitive indexrdquo Pakistan Journal of Weed ScienceResearch vol 12 pp 119ndash129 2006

[26] A Rezakhanlou M Aghabeigi and H Bagheri ldquoEvaluation ofsome of competitiveness indexes in competition between cottonvarieties and common cocklebur (Xanthium strumarium L)rdquoInternational Research Journal of Applied and Basic Sciences vol3 pp 1274ndash1278 2012

[27] R K Coleman G S Gill and G J Rebetzke ldquoIdentification ofquantitative trait loci for traits conferring weed competitivenessin wheat (Triticum aestivum L)rdquo Australian Journal of Agricul-tural Research vol 52 no 11-12 pp 1235ndash1246 2001

[28] J Rodenburg K Saito R G Kakaı A Toure M Mariko andP Kiepe ldquoWeed competitiveness of the lowland rice varietiesof NERICA in the southern Guinea Savannardquo Field CropsResearch vol 114 no 3 pp 411ndash418 2009

[29] B P Caton A E Cope and M Mortimer ldquoGrowth traits ofdiverse rice cultivars under severe competition implications forscreening for competitivenessrdquo Field Crops Research vol 83 no2 pp 157ndash172 2003

[30] M J Kropff ldquoEco-physiological models for crop-weed compe-titionrdquo inModeling Crop-Weed Interactions M J Kropff and HH van Laar Eds pp 25ndash32 CAB International WallingfordUK 1993

[31] F Ekeleme A Y Kamara S O Oikeh et al ldquoResponse ofupland rice cultivars to weed competition in the savannas ofWest Africardquo Crop Protection vol 28 no 1 pp 90ndash96 2009

[32] O S Namuco J E Cairns and D E Johnson ldquoInvestigatingearly vigour in upland rice (Oryza sativa L) part I Seedlinggrowth and grain yield in competition with weedsrdquo Field CropsResearch vol 113 no 3 pp 197ndash206 2009

[33] K Saito K Azoma and J Rodenburg ldquoPlant characteristicsassociated with weed competitiveness of rice under upland andlowland conditions in West Africardquo Field Crops Research vol116 no 3 pp 308ndash317 2010

[34] H Ni K Moody R P Robles E C Paller Jr and J S LalesldquoOryza sativa plant traits conferring competitive ability againstweedsrdquoWeed Science vol 48 no 2 pp 200ndash204 2000

[35] G Wu L T Wilson and A M McClung ldquoContribution of ricetillers to drymatter accumulation and yieldrdquoAgronomy Journalvol 90 no 3 pp 317ndash323 1998

Submit your manuscripts athttpwwwhindawicom

Nutrition and Metabolism

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Food ScienceInternational Journal of

Agronomy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Microbiology

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

AgricultureAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

PsycheHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BiodiversityInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

GenomicsInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Plant GenomicsInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Biotechnology Research International

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Forestry ResearchInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of BotanyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Veterinary Medicine International

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Cell BiologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Evolutionary BiologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Submit your manuscripts athttpwwwhindawicom

Nutrition and Metabolism

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Food ScienceInternational Journal of

Agronomy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Microbiology

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

AgricultureAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

PsycheHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BiodiversityInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

GenomicsInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Plant GenomicsInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Biotechnology Research International

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Forestry ResearchInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of BotanyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Veterinary Medicine International

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Cell BiologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Evolutionary BiologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014