Relations of rice seeding rates to crop and weed growth in aerobic rice

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Field Crops Research 121 (2011) 105–115

Contents lists available at ScienceDirect

Field Crops Research

journa l homepage: www.e lsev ier .com/ locate / fc r

elations of rice seeding rates to crop and weed growth in aerobic rice

hagirath S. Chauhana,∗, Virender P. Singhb, Avnish Kumarb, David E. Johnsona

Crop and Environmental Sciences Division, International Rice Research Institute, Los Banos, PhilippinesDepartment of Agronomy, Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, India

r t i c l e i n f o

rticle history:eceived 18 September 2010eceived in revised form9 November 2010ccepted 29 November 2010

eywords:lant densityompetitioniomassieldhilippinesndiarrigationultural control

a b s t r a c t

Aerobic rice describes a management adaptation to reduced irrigation water supplies but, due to reducedintervals of flooding in this system, this requires revised weed management approaches to reduce costsand provide effective weed control. One approach is to make the crop more competitive and reduce theeffects of weeds on the crop by using higher rice seeding rates. A study was conducted in the Philippinesand India in 2008 and 2009 to assess the relations of seeding rates (15–125 kg ha−1) of hybrid and inbredvarieties to crop and weed growth in aerobic rice. Plant densities, tillers, and biomass of rice increasedlinearly with increased in seeding rates under both weedy and weed free environments. Weed biomassdecreased linearly with increasing seeding rates from 15 to 125 kg ha−1. Panicles and grain yields of rice incompetition with weeds increased in a quadratic relation with increased seeding rates at both locations;however, the response was flat in the weed free plots. A quadratic model predicted that seeding rates of48–80 kg ha−1 for the inbred varieties and 47–67 kg ha−1 for the hybrid varieties were needed to achievemaximum grain yield when grown in the absence of weeds, while rates of 95–125 kg seed ha−1 for theinbred varieties and 83–92 kg seed ha−1 for the hybrid varieties were needed to achieve maximum yieldsin competition with weeds. On the basis of these results, seeding rates greater than 80 kg ha−1 are advis-
able where there are risks of severe weed competition. Such high seeding rates may be prohibitive whenusing expensive seed, and maximum yields are not the only consideration for developing recommenda-tions for optimizing economic returns for farmers. Results of the present study do suggest however thatincreasing seeding rates of aerobic rice does suppress weed growth and reduce grain yield losses fromweed competition. This information could be incorporated in integrated crop management packages to
ctivel
manage weeds more effe
. Introduction

Rice (Oryza sativa L.) is a principal source of food for more thanalf of the world population, and more than 90% of rice worldwide

s grown and consumed in Asia. A change in rice establishmentethod from manual transplanting of seedlings to direct-seeding

as occurred in many Asian countries in the last two decades inesponse to rising production costs, especially for labor and waterPandey and Velasco, 2005), and this trend continues. Farmers in

any areas of the tropics have limited availability of irrigationater for rice and, in the future, most of the dry-season areas in

outh and Southeast Asia are anticipated to suffer “economic watercarcity” (Bouman and Tuong, 2003). This threatens the sustainabil-ty of production in these irrigated ecosystems since the rice mayuffer from drought and, even limited water shortages may make

∗ Corresponding author at: Crop and Environmental Sciences Division, Inter-ational Rice Research Institute, DAPO Box 7777, Metro Manila 4031, Laguna,hilippines.

E-mail address: [email protected] (B.S. Chauhan).

378-4290/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.fcr.2010.11.019

y.© 2010 Elsevier B.V. All rights reserved.

it unfeasible for farmers to flood rice fields. Aerobic rice describesa crop sown in dry soil and where irrigation is applied only to keepthe soil sufficiently moist for crop growth rather than saturated orflooded (Tuong and Bouman, 2003). As water shortages increase,the area under aerobic rice is expected to increase. Absence ofstanding water to suppress weeds and the simultaneous emer-gence of crop and weeds in these direct seeded systems means,however, that there is serious risk of substantial crop yield lossesdue to competition from weeds (Tuong et al., 2005; Chauhan andJohnson, 2010b). Weeds are a major constraint to direct-seeded ricesystems, and may cause yield losses up to 35% worldwide (Oerkeand Dehne, 2004). Despite the yield protection of manual weedingin dry seeded rice over four years in India, either after conventionalor zero-tillage, rice yield losses were approximately 15% of weed-free crops (Singh et al., in press). Weeds are therefore a substantialthreat to the productivity of dry seeded rice systems.

Herbicides are widely used to manage weeds in rice, thoughconcerns over the evolution of herbicide resistance in weeds, thedecline in the number of new herbicide molecules being released,weed species shifts, surface-water pollution, and an increase incosts may limit the herbicide options available to farmers in the

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106 B.S. Chauhan et al. / Field Crops Research 121 (2011) 105–115

July Aug Sep Oct Nov

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15

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35Maximum temperatureMinimum temperatureTotal rainfall

Jan Feb Mar Apr May

To

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m)

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100

200

300

400

nths

June July Aug Sep Oct15

20

25

30

35

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June July Aug Sep Oct0

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400

600

800

1000

2008 2009Philippines

India

infall

fmh(beTttbCwwTomtcg2sdt

etpZ3aiewosso

Mo

Fig. 1. Mean maximum and minimum temperatures (◦C), and total ra

uture (Buhler et al., 2002; Johnson and Mortimer, 2008). Weedanagement systems in wheat, for instance, that rely heavily on

erbicides (Chauhan et al., 2007) are considered as unsustainableLemerle et al., 2004). In the Philippines, the repeated use of her-icides in direct-seeded rice to control weeds has already led tovolution of resistance in Echinochloa crus-galli (Juliano et al., 2010).o provide more sustainable weed control measures and protecthe environment, reducing reliance on herbicides and applying cul-ural measures in integrated weed management approaches haseen advocated (Azmi et al., 2005; Chauhan and Johnson, 2010b,c;hauhan et al., 2010). Making the crop more competitive againsteeds has been proposed as a strategy to reduce the effects ofeeds on the crop (Mortensen et al., 1998; Gibson et al., 2002).

his may be achieved through crop nutrition (e.g., better timingf application), seed-bed conditions (e.g., leveled field and wateranagement), cultivar choice, or adjusting the crop plant popula-

ion density. Inbred and hybrid rice varieties may also differ in weedompetitiveness. Hybrid rice has the potential to produce 15–25%reater yield when compared with inbred cultivars (Walker et al.,008), and hybrid rice is increasingly being grown in Asia as farmerseek the means to improve productivity. Transplanting rather thanirect seeding is usually recommended for hybrid rice varieties dueo the relative high cost of hybrid seed.

Research with wheat (Olsen et al., 2005), barley (O’Donovant al., 2001), and rice (Ni et al., 2004; Phuong et al., 2005) has shownhat increased seeding rates improve the ability of crops to sup-ress weeds and can reduce yield loss under weedy conditions.hao et al. (2007) reported that increasing seeding rate from 100 to00 viable seeds m−2 raised yields and decreased weed biomass inerobic rice; however, further increases to 500 seeds m−2 resultedn no further improvement in yield or weed suppression. There isvidence therefore that seeding rate can affect the outcome of rice
eed competition but the information available on the interactions
f seeding rate and the effects of weed competition across differentites, and how it may be influenced by different rice genotypes, iscanted. Such information is required to develop agronomic rec-mmendations for aerobic rice in South and Southeast Asia. The

(mm) recorded at the experimental sites in the Philippines and India.

need for such information is heightened by recommendations forfarmers to reduce rice seeding rates in order to reduce costs (Guptaet al., 2006; Gopal et al., 2010).

A study was conducted in the Philippines and India to test thehypothesis that increase in rice seeding rates improves the cropcompetitiveness with weeds, decreases weed growth and rice yieldlosses, and that relations may differ according to rice genotype. Theobjectives of this study were to assess (1) the relations of seedingrates of rice to crop and weed growth and (2) if these relations differdepending on whether an inbred or a hybrid rice is grown.

2. Materials and methods

2.1. Experimental details

Experiments were conducted at the International Rice ResearchInstitute, Los Banos (14◦13′N, 121◦13′E), Philippines and GovindBallabh Pant University of Agriculture and Technology, Pantnagar(29◦03′N, 79◦31′E), India to study the relations of seeding rates ofhybrid and inbred varieties to crop and weed growth in aerobic rice.The field study in the Philippines was conducted during the wetseason of 2008 and dry season of 2009. The soil at the experimentalsite has a pH of 6.3; organic carbon of 1.4%; and has a sand, silt,and clay content of 35%, 36%, and 29%, respectively. In India, thestudy was conducted during the wet (Kharif) seasons of 2008 and2009. The soil at this site has a pH of 7.7; organic carbon of 1.0%and sand, silt, and clay content of 32, 39, and 29%, respectively.The experimental area at each location was dry cultivated using atwin axle tractor before sowing rice. The maximum and minimumtemperatures (◦C), and total rainfall recorded at the experimentalsites are presented in Fig. 1.

Treatments consisted of two rice varieties (one hybrid and one
inbred at each location) sown at six seeding rates, viz. 15, 25, 50,75, 100 and 125 kg ha−1 under weed-free and weedy conditions(Table 1). The rates covered the range of seeding rates currentlyrecommended for dry seeded rice. Locally grown varieties werechosen at each site. The experiments at each location and year were

B.S. Chauhan et al. / Field Crops Research 121 (2011) 105–115 107

Table 1The details of items in the Philippines and India.

Items Philippines India

VarietyInbred Apo Pant Sugandha 17Hybrid Mestizo 3 PRH 10

FertilizerN (kg ha−1) 100 100P2O5 (kg ha−1) 40 60K2O (kg ha−1) 40 40

Row spacing 25-cm 20-cmIrrigation method Sprinkler FlushDate of sowing

amswftissa(t4csbPtiap(hs4

2

ptir(scwcAwGi(

irposi

Philippines

Seed rate (kg ha-1)0 25 50 75 100 125

Ric

e p

lan

ts (

no

. m-2

)

0

50

100

150

200

250

300

350

2008 July 09 June 252009 January 16 June 14

Harvest area (m2) 6.0 4.8

rranged in a split–split plot design with weed management as theain plots, seeding rates as the subplots, and varieties as the sub-

ubplots. There were four replicates of each treatment. Rice seedsere sown by hand in dry soil in uniformly spaced (Table 1) shallow

urrows and covered with soil. Farmers commonly use seed drillshat sow rice at 20-cm row spacing in India and at 25-cm row spac-ng in the Philippines, and therefore, different row spacings wereelected at different sites. Irrigation was applied immediately afterowing and then as required by the crop. At both sites, phosphorusnd potassium were incorporated into the soil before crop sowingTable 1). In the Philippines, urea was applied at 100 kg N ha−1 inhree split doses: 30 kg at 2 weeks after sowing (WAS), 30 kg atWAS, and 40 kg at 8 WAS. These rates of application and timings

orrespond to those commonly recommended to farmers in direct-eeded rice in the Philippines. In India, 23 kg N ha−1 was applied asasal and 40 kg N ha−1 at 45 DAS, and 37 kg N ha−1 at 75 DAS. In thehilippines, the weed-free plots were treated with an application ofhe herbicide bispyribac-sodium (0.30 kg ha−1) 10 days after sow-ng, and these plots were then hand-weeded as needed to removell weeds throughout the crop season. In India, the weed-freelots were initially treated with an application of pendimethalin1.0 kg ha−1) within 3 days of sowing, and plots were subsequentlyand-weeded as required to remove weeds throughout the cropeason. Weedy plots at both locations were hand-weeded once atWAS, and weeds were allowed to grow before and after that.

.2. Measurements and data analysis

Rice plant stands at 2 WAS were counted from four randomlylaced row lengths of 1-m in each plot from the experiments inhe Philippines, but this data were not recorded in the experimentn India. At both locations, two quadrats of 0.25 m2 were placed atandom in each plot to determine rice tillers (no. m−2) and biomassg m−2) at 4 and 8 WAS. Weeds were sampled from the 0.25 m2

ampling area at 4 and 8 WAS (the same sampling areas as forrop biomass) in each weedy plot in both years. Weed densitiesere recorded and weed biomass (g m−2) determined. Weed and

rop biomass were measured after drying samples at 70 ◦C for 72 h.t harvest, rice panicle number and aboveground weed biomassere measured from a randomly placed 1 m2 area within each plot.rains per panicle were determined by randomly sampling 20 pan-

cles per plot. Rice grain yield was determined from the harvest areaTable 1) and grain weight adjusted to 14% moisture content.

As our primary objective was to assess the relations of seed-ng rates to crop and weed variables, the data were analyzed using
egression analysis (SigmaPlot 10.0). No attempt was made to com-are means of variables at different seeding rates. The structuref the independent variable (e.g., seeding rate) allowed regres-ion analysis to be used to illustrate the association between thendependent variable and the dependent variable (response). This
Fig. 2. The relation of rice plants to rice seed rate in 2008 (solid line) and 2009 (dashline) in the Philippines. The data were combined over varieties.

allowed the nature of responses to be described and predictionsof values intermediate to those actually studied, and provided aconcise way of comparing responses across the entire range oftreatments (Cousens, 1988). R2 values were used to determine‘goodness of fit’ for the selected equation. The relations of plantstand, tiller number, and biomass of rice and weed biomass to theinvestigated range of rice seeding rates were described using thelinear model:

y = a + bx (1)

where y is the estimated rice plant stand (m−2), tiller number (m−2),biomass (g m−2), or weed biomass (g m−2) as a function of seed-ing rate (x), a is the y intercept, and b describes the slope of theregression curve. A quadratic model was fitted to describe the rela-tionships of rice panicles and grain yield to rice seeding rates as

y = a + bx + cx2 (2)

where y is the estimated rice panicle number (m−2) or grain yield(kg ha−1) as a function of seeding rate (x), a is the y intercept,and b and c describe the slope of the regression curve. In weedyplots, the relationships between grain yield and weed biomass weredescribed using a linear model:

G = a + bw (3)

where G is the estimated grain yield (kg ha−1) as a function of weedbiomass (w, g m−2), a is the G intercept, and b describes the slopeof the regression curve.

3. Results

3.1. Relations between rice seeding rate and rice plant density

Rice plant densities were similar across the varieties and weedcontrol treatments in the Philippines and rice plants ranged from18 to 353 plants m−2 (Fig. 2). Plant densities increased linearly withinvestigated range of seeding rates in both years; however, densi-ties were lower in 2009 than in 2008. Estimates of the fitted modelindicate a seeding rate of 15 kg ha−1 produced 35 and 23 plants m−2

while 125 kg ha−1 resulted in 299 and 166 plants m−2 in 2008
and 2009, respectively; hence a 34–44% reduction in plant standbetween the years. This equates to only 51% of the sown seedsresulted in rice seedlings at 2 WAS in 2008, and 32% in 2009 inthe Philippines.


Hybrid

0

100

200

300

400

500 4 WAS; y = 230 - 0.39x; R2 = 0.178 WAS; y = 58 - 0.19x; R2 = 0.28AH; y = 293 - 1.92x; R2 = 0.62

Inbred

0

100

200

300

400


2009

Seed rate (kg ha-1)

0 25 50 75 100 125

Wee

d b

iom

ass

(g m

-2)

0

100

200

300

400

500


2008

0 25 50 75 100 1250

100

200

300

400

500


Philippines

Hybrid

0

40

80

120

160

200


Inbred

0

40

80

120

160

200

2404 WAS; y = 196 - 0.69x; R2 = 0.368 WAS; y = 88 - 0.42x; R2 = 0.43AH; y = 100 - 0.45x; R2 = 0.67

Seed rate (kg ha-1)

0 25 50 75 100 125

Wee

d b

iom

ass

(g m

-2)

0

40

80


0 25 50 75 100 1250

40

80

120

160


India

2008

2009

a

b

F sh-dotT iomasa es and

3

tArE

ig. 3. (a) The relation of weed biomass to rice seed rate at 4 (solid line) and 8 (dahe data are presented separately for varieties and years. (b) The relation of weed bnd at harvest (AH, dash line) in India. The data are presented separately for varieti

.2. Relations between rice seeding rate and weed biomass

The common weed species in the experiments conducted inhe Philippines were Ageratum conyzoides, Amaranthus viridis,. spinosus, Cleome rutidosperma, Commelina benghalensis, Cype-us iria, C. rotundus, Dactyloctenium aegyptium, Digitaria ciliaris,. colona, Eleusine indica, Euphorbia hirta, Ludwigia hyssopifolia,

line) weeks after sowing (WAS), and at harvest (AH, dash line) in the Philippines.s to rice seed rate at 4 (solid line) and 8 (dash-dot line) weeks after sowing (WAS),years.

Mimosa pudica, Portulaca oleracea, and Syndrella nodiflora. In India,the prevalent species common in both years were Brachiaria spp.,
Cyperus difformis, C. iria, E. colona, E. indica, and Fimbristylis mili-acea.
A linear model was ‘best fit’ to describe the relations betweenweed biomass and seeding rate at 4 and 8 WAS, and at crop harvest.The responses varied between locations and timings of observa-


Hybrid0

200

400

600

800

1000 WC; y = 175.5 + 3.94x; R2 = 0.66WF; y = 413.4 + 3.09x; R2 = 0.66

Inbred0

200

400

600

800 WC; y = 126.1 + 2.82x; R2 = 0.76WF; y = 200.4 + 2.92x; R2 = 0.78

Hybrid

Till

ers

(no

. m-2

)

0

200

400

600

800

1000WC; y = 200.7 + 3.57x; R2 = 0.62WF; y = 399.8 + 3.70x; R2 = 0.56

Inbred

0 25 50 75 100 1250

200

400

600

800 WC; y = 137.6 + 1.45x; R2 = 0.47WF; y = 238.7 + 1.95x; R2 = 0.49

Philippines

Hybrid

0

100

200

300

400

500

600

WC; y = 200.7 + 2.08x; R2 = 0.66WF; y = 384.2 + 1.47x; R2 = 0.79

Inbred

0

100

200

300

400

500

WC; y = 146.4 + 2.03x; R2 = 0.89WF; y = 229.4 + 1.76x; R2 = 0.79

Hybrid

0

100

200

300

400

500

600

700

WC; y = 226.6 + 2.18x; R2 = 0.90WF; y = 425.9 + 1.79x; R2 = 0.81

Inbred

Seed rate (kg ha-1)

0 25 50 75 100 1250

100

200

300

400

500

600

700

WC; y = 144.9 + 1.92x; R2 = 0.94WF; y = 231.0 + 2.64x; R2 = 0.71

2008

2009

India

F dy (WT

towtiwiPvs4f2h(

ig. 4. The relation of rice tillers to rice seed rate at 8 weeks after sowing under weehe data are presented separately for varieties and years.

ions (Fig. 3a and b). In the Philippines, the best model fit wasbserved with weed biomass observations taken at harvest (Fig. 3a),hereas in India, the model accounted best for the observed varia-

ion when observations were used at harvest in 2008 and at 8 WASn 2009 (Fig. 3b). There was a trend of decreasing weed biomass

ith increasing seeding rate at both locations. The fitted modelndicates increasing seeding rate from 25 to 100 kg ha−1 in thehilippines, across varieties, reduced weed biomass at crop har-est by 59% in 2008 and 47% in 2009 (Fig. 3a). In India, increasedeeding rate from 25 to 100 kg ha−1 reduced weed biomass at

WAS by 40% in 2008 to 58% in 2009 (Fig. 3b). Varietal dif-erences in weed suppression were not apparent, except in the008-experiment in India, where the fitted model indicates theybrid was superior to the inbred in weed suppression at 4 WASFig. 3b).

C; solid line) and weed free (WF; dash line) conditions in the Philippines and India.

3.3. Relations between rice seeding rate and rice tillers andbiomass

At both locations, tiller density at 4 (data not shown) and 8WAS (Fig. 4) increased linearly with increasing seeding rates (inthe range of seeding rates used in the trial), and this was consistentacross years, varieties, and weed control treatments. Tiller densi-ties, averaged over varieties in the Philippines, at 8 WAS increasedfrom 202 and 207 tillers m−2 at 15 kg ha−1 in 2008 and 2009 to 574and 483 tillers m−2 at 125 kg ha−1 in the weedy treatment, whereas
in the weed free treatment, tillers increased from 352 and 362 tillersm−2 at 15 kg ha−1 in 2008 and 2009 to 683 and 672 tillers m−2 at125 kg ha−1 (Fig. 4). Depending on variety and year, the weed freetreatment at a seeding rate of 15 kg ha−1 had 31–50% greater tillerdensities than in the weedy treatment, while at 125 kg ha−1 15–44%


Hybrid0

100

200

300

400

500 WC; y = 57.29 + 1.32x; R2 = 0.68WF; y = 171.53 + 1.63x; R2 = 0.57

Inbred

Cro

p b

iom

ass

(g m

-2)

0

100

200

300

400 WC; y = 45.45 + 1.13x; R2 = 0.75WF; y = 101.06 + 1.94x; R2 = 0.73

Hybrid0

50

100

150

200

250

300 WC; y = 42.19 + 0.78x; R2 = 0.61WF; y = 79.37 + 1.13x; R2 = 0.61

Inbred

0 25 50 75 100 1250

50

100

150

200

250

300 WC; y = 50.39 + 0.42x; R2 = 0.32WF; y = 68.56 + 0.84x; R2 = 0.45

Philippines

Hybrid

0

100

200

300

400

500

WC; y = 160.7 + 1.62x; R2 = 0.61WF; y = 272.6 + 1.14x; R2 = 0.56

Inbred

0

100

200

300

400

500

WC; y = 143.1 + 1.23x; R2 = 0.53WF; y = 173.5 + 1.19x; R2 = 0.61

Hybrid

0

100

200

300

400

500

WC; y = 156.9 + 1.64x; R2 = 0.85WF; y = 268.5 + 1.14x; R2 = 0.80

Inbred

Seed rate (kg ha-1)

0 25 50 75 100 1250

100

200

300

400

WC; y = 80.1 + 1.31x; R2 = 0.91WF; y = 166.1 + 1.17x; R2 = 0.80

2008

2009

India

F weedI

gt

btsw

3

ebiat

ig. 5. The relation of rice biomass to rice seed rate at 8 weeks after sowing underndia. The data are presented separately for varieties and years.

reater tiller densities were recorded in the weed free compared tohe weedy treatment. Similar trend was observed in India (Fig. 4).

Crop biomass increased linearly with increased seeding rates atoth locations (Fig. 5) in a similar pattern to that of tiller densi-ies. There was no indication however that, up to 8 WAS, increasedeeding rate reduced the differences in crop biomass between theeedy and weed free conditions.

.4. Relations between rice seeding rate and rice panicles

At both locations, rice panicles were best described by quadratic
quations with increased in investigated seeding rate (Fig. 6a and). In both weedy and weed-free treatments, panicle numbers
ncreased with seeding rates from 15 kg ha−1 to 100 or 125 kg ha−1,nd were dependent on varieties, years, and locations. The responseo increased seeding rate up to 75 kg ha−1, however, tended to be

y (WC; solid line) and weed free (WF; dash line) conditions in the Philippines and

greater in the weedy plots than in weed-free plots. At both loca-tions, differences in panicle numbers between weedy and weed freeplots decreased with increased seeding rates. In the Philippines in2008, for example, weed free treatment had 50–67% greater paniclenumbers than weedy treatment at 15 kg ha−1 while increased seed-ing rate of 100 kg ha−1 reduced this value by 16–25% depending onvariety (Fig. 6a). Similarly in 2009, at seeding rates of 15 kg ha−1

there were 84% greater panicle numbers in the weed free plotsthan in weedy treatment, across varieties, while at 125 kg ha−1

there were 35% greater panicle numbers in weed free plots than inweedy plots. A similar trend was observed in India (Fig. 6b) though
differences in panicle numbers between weed free and weedytreatments at 15 kg ha−1 were smaller than in the Philippines. Onaverage, the weedy treatment in the Philippines had 71% lowerpanicle numbers than weed free treatment at 15 kg ha−1, while inIndia this value was only 35%.


a

b

F weedv olid linf

3

e(lb

ig. 6. (a) The relation of rice panicles to rice seed rate under weedy (solid line) andarieties and years. (b) The relation of rice panicles to rice seed rate under weedy (sor varieties and years.

.5. Relations between rice seeding rate and rice grain yield

Grain yields at both locations were best described by quadraticquations with increased in investigated range of seeding ratesFig. 7a and b). In weed free conditions at both locations, there wasittle or no response to increasing seeding rates up to 75 kg ha−1 foroth varieties though yields decreased at greater seeding rates. In

free (dash line) conditions in the Philippines. The data are presented separately fore) and weed free (dash line) conditions in India. The data are presented separately

the Philippines, grain yields of the hybrid variety in weed free plotswas little affected as seeding rates increased from 15 to 75 kg ha−1,
and these ranged from 3340 to 3480 kg ha−1 in 2008 and 4220 to4250 kg ha−1 in 2009 (Fig. 7a), while in India, yield of the hybrid inweed free conditions ranged from 4190 to 4220 kg ha−1 in 2008 and3990 to 4330 kg ha−1 in 2009 over 15 to 75 kg seed ha−1 (Fig. 7b).The model predicts that maximum yield in the Philippines with


20080

1000

2000

3000

4000

5000

2009

Seed rate (kg ha-1)0 25 50 75 100 125

Gra

in y

ield

(kg

ha-1

)

0

1000

2000

3000

4000

5000

Philippines

20080

1000

2000

3000

4000

5000

2009

Seed rate (kg ha-1)

0 25 50 75 100 125

Gra

in y

ield

(kg

ha-1

)

0

1000

2000

3000

4000

5000

Indiaa b

F for hyd on ofl d) con

tiapwiav21fyMP82iiI1

eat7hete1wd

3

tPt(b

ig. 7. (a) The relation of rice grain yield to rice seed rate under weedy (solid lineash-dot line for inbred) conditions in 2008 and 2009 in Philippines. (b) The relati

ine for inbred) and weed free (long-dash line for hybrid and dash-dot line for inbre

he hybrid grown in weed-free conditions was achieved at seed-ng rates of 60 kg ha−1 in 2008, 47 kg ha−1 in 2009, and in Indiat 48 kg ha−1 in 2008 and 67 kg ha−1 in 2009. The correspondingredicted values in the Philippines for the inbred variety grown ineed-free conditions was achieved at seeding rates of 73 kg ha−1

n 2008 and 48 kg ha−1 in 2009 and, in India, 79 kg ha−1 in 2008nd 80 kg ha−1 in 2009. In the Philippines, yields of the inbredariety in the weedy plots in 2008 increased from 750 kg ha−1 to390 kg ha−1 (70% yield increase) with increased seeding rates from5 to 125 kg ha−1 (Fig. 7a). Similarly in India, increasing seeding raterom 15 to 125 kg ha−1 under weedy conditions increased grainield of inbred variety by 28% in 2008 and 51% in 2009 (Fig. 7b).aximum yield with the hybrid grown in weedy conditions in the

hilippines was predicted at seeding rates of 88 kg ha−1 in 2008 and9 kg ha−1 in 2009 and in India, 83 kg ha−1 in 2008 and 92 kg ha−1 in009. The corresponding predicted values in the Philippines for the

nbred variety grown in weedy conditions was achieved at seed-ng rates of 125 kg ha−1 in 2008 and 95 kg ha−1 in 2009, while inndia, this was achieved at the maximum seeding rate tested of25 kg ha−1 in 2008 and 2009 (Fig. 7a).

The fitted models predicted greater yields for the hybrid vari-ties than the inbred varieties at both locations, and that theverage yield response to increased seeding rate was greater inhe presence of weeds than in the weed free conditions (Fig. 7a andb). In the Philippines, for example, the hybrid variety at 15 kg ha−1

ad 15% greater yields (averaged over years) than the inbred vari-ty in the weed free conditions, while in the presence of weeds,he average yield advantage of the hybrid over the inbred vari-ty was 34% (Fig. 7a). Similarly in India, the hybrid variety at the5 kg ha−1 seeding rate had 16% greater yields than the inbred ineed free conditions and 26% more than the inbred in weedy con-itions (Fig. 7b).

.6. Relations between rice grain yield and weed biomass

Grain yields at both locations were negatively correlated with
he weed biomass at 4 WAS and at harvest (Fig. 8a and b). In thehilippines, weed biomass at harvest accounted for a greater part ofhe observed variation in grain yield than weed biomass at 4 WASFig. 8a). In India, the trend varied between years (Fig. 8b), as weediomass at harvest accounted for a greater part of the observed
brid and short-dash line for inbred) and weed free (long-dash line for hybrid andrice grain yield to rice seed rate under weedy (solid line for hybrid and short-dash

ditions in 2008 and 2009 in India.

variation in grain yield in 2008, whereas in 2009, weed biomass at4 WAS accounted for a greater part of the observed variation in grainyield. In the Philippines, weed biomass was greater at harvest thanat 4 WAS (Fig. 8a) while the converse was true in India (Fig. 8b). Thedifferent patterns of weed growth could be attributed to differentirrigation systems (Table 1) and weed species composition at thetwo locations.

4. Discussion

Uniform stands of adequate rice plant densities are criti-cal to achieve high yields in aerobic rice. In our study in thePhilippines, there was a linear increase in plant densities withseeding rates from 15 to 125 kg ha−1; however, the rice plantdensities achieved with a given seeding rate were inconsistentbetween years. Plant densities were 34–44% lower in 2009 thanin 2008. This illustrates large and variable differences betweenthe numbers of seed sown and seedlings established. The lossesat germination and emergence are likely to be exacerbated bysoil conditions, soil temperature, unfavorable moisture regimes,depth and uniformity of planting, seed quality, and damage bybirds and rats. These factors could have been responsible forthe different crop establishment between years but this raisesthe question “what is the level of variability that occurs acrossfarmers’ fields?”. To reduce risk in crop establishment, however,there is a need to better understand the causes of such variabil-ity between years or seasons, and the impact that managementpractices have on seedling establishment. Recommendations forseeding rate need to account for this variability, balance againstthe costs of seeds, and allow for an adequate rice density for opti-mum yields even in the seasons when establishment is relativelypoor.

Our study showed that seeding rates within the range of15–125 kg ha−1 for both hybrids and inbreds had relatively littleeffect on grain yield of rice grown in weed free conditions. Theseresults support the findings of a previous study with aerobic rice,
in which grain yields in the weed free condition were not influ-enced by the seeding rates within the range of 100–500 viable seedsm−2 (Zhao et al., 2007), which approximates to about 25–125 kgseed ha−1. In weedy conditions in our study, however, seedingrates of 95–125 kg ha−1 for the inbred variety and 83–92 kg ha−1


0 100 200 300 400 500 6000

500

1000

1500

2000

2500

3000

3500

4 WAS; y = 4810 - 13.3x; R2 = 0.54AH; y = 3123 - 6.2x; R2 = 0.76

0 100 200 300 400 500 6000

500

1000

1500

2000

2500

3000

3500

4 WAS; y = 4489 - 13.9x; R2 = 0.47AH; y = 2882 - 5.7x; R2 = 0.84

Weed biomass (g m-2)

0 100 200 300 400 500 600

Gra

in y

ield

(kg

ha-1

)

0

500

1000

1500

2000

2500

3000 4 WAS; y = 3021 - 14.5x; R2 = 0.49AH; y = 2988 - 5.1x; R2 = 0.75

0 100 200 300 400 500 6000

500

1000

1500

2000

2500

3000 4 WAS; y = 2640 - 12.8x; R2 = 0.40AH; y = 2479 - 3.9x; R2 = 0.73

derbnIdirbyH 2008

Philippines

2009

2008

0 50 100 150 200 2500

500

1000

1500

2000

2500

3000

4 WAS; y = 3027 - 6.1x; R2 = 0.53AH; y = 3494 - 16.0x; R2 = 0.65

20090 50 100 150 200 250

0

500

1000

1500

2000

2500

3000

4 WAS; y = 2345 - 2.9x; R2 = 0.20AH; y = 2561 - 9.3x; R2 = 0.46

Hybrid

Weed biomass (g m-2)

0 50 100 150 200 250

Gra

in y

ield

(kg

ha-1

)

0

500

1000

1500

2000

2500

3000

4 WAS; y = 3287 - 10.7x; R2 = 0.75AH; y = 3670 - 62.6x; R2 = 0.53

Inbred

0 50 100 150 200 2500

500

1000

1500

2000

2500

3000

4 WAS; y = 2734 - 10.1x; R2 = 0.69AH; y = 3442 - 113.2x; R2 = 0.52

India

a

b

F g (WAp ed bii

fcgcve

ig. 8. (a) The relation of rice grain yield to weed biomass at 4 weeks after sowinresented separately for varieties and years. (b) The relation of rice grain yield to we

n India. The data are presented separately for varieties and years.

or the hybrid variety were needed to achieve maximum yields in
ompetition with weeds. Increasing seeding rate suppressed weedrowth and reduced losses in grain yield from weed competitiononsistently across different locations, years, and varieties. Pre-ious studies with aerobic rice (Zhao et al., 2007), wheat (Olsent al., 2005), and barley (O’Donovan et al., 2001) support our find-
S; solid line) and at crop harvest (AH; dash line) in the Philippines. The data areomass at 4 weeks after sowing (WAS; solid line) and at crop harvest (AH; dash line)

ing that increased seeding rate reduces weed growth and yield
loss in weedy conditions. Zhao et al. (2007), for example, reportedthat increasing seeding rates from 100 to 300 viable seeds m−2
resulted in a significant decrease in weed biomass and yield lossin weedy conditions. Weed competition was severe in our studiesand, at a seeding rate of 50 kg ha−1, yield losses with the inbred

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ariety over the two years in India were about 50% of the weedree yields. In a previous study however, over four years, yieldosses due to weeds without weed control in direct seeded ricet the same seeding rate were approximately 85% of the weed-freeields (Singh et al., in press). While such high losses are consid-red unusual, these do illustrate the potential yield reductionsue to weed competition where control measures are not under-aken.

Global rice production must increase considerably by 2025 toeet the increasing demand (Peng and Yang, 2003). Much of this

ncreased production may come from hybrid rice varieties whichave the potential to produce 15–25% greater yield than inbredsWalker et al., 2008). In our study, hybrids grown in weed free con-ition at a seeding rate of 15 kg ha−1 had 15% higher yield thanhe inbred varieties in the Philippines, and the hybrids had a 16%dvantage in India.

Row spacing (25- vs. 20-cm) and mode of irrigation (flush vs.prinkler) were differed between sites, which may have impactedn crop and weed growth. Greater rice yield and lower weediomass has been reported for 20-cm rows compared to 30-cmows (Chauhan and Johnson, 2010a). Differences could also haveeen due to the contrasting rice varieties, weed flora, soil mois-ure conditions, and soil and climatic conditions. Rice in India wasrown in wet seasons, while in the Philippines the crop was grownn dry and wet seasons.

In India, seeding rates of 15–20 kg ha−1 are suggested for dryeeded rice (Gopal et al., 2010). The pressure to reducing seed-ng rate to reduce costs, however, should be balanced with theeed to reduce farmers’ risks due to increased losses due toeed competition and poor rice seedling establishment. Costs ofybrid seed make seeding rates as high as 100 kg ha−1 prohibitivelyxpensive, and thus the use of hybrids seems unlikely whereigh seeding rates are required. Even with inbred seeds, there iswareness as the importance of “seed quality” and higher qual-ty usually translates to higher costs. Farmers in the Philippinesrowing inbred varieties, however, are already use seeding ratesp to 100 kg ha−1 to achieve adequate and uniform crop establish-ent, and reduce competition due to weeds (Joel Janiya, personal

ommunication). Appropriate recommendations for a seeding rateherefore need to be based on a combination of agronomic andconomic data with respect to seed costs being balanced againsthe yield losses associated with either poor establishment, weedompetition or other pests. Based on our results alone, seed-ng rates of 55 kg ha−1 for hybrid seed and 70 kg ha−1 for inbredeed could provide “robust recommendations” for areas wherehere is effective weed control, while 90–125 kg ha−1 may be

ore appropriate where there are risks of severe weed compe-ition. High costs of hybrid seed, in particular, will encouragearmers to reduce the seeding rates and in these circumstanceshe needs to reduce costs can be balanced with risks associatedith either poor emergence or establishment and/weed competi-

ion.Increasing rice seeding rates may suppress weed growth and

educe losses to weeds, however, other problems that are detri-ental to yield such as lodging, rat damage, and insect and disease

nfection might be exacerbated by high seeding rates (Zhao et al.,007). The combination of optimum seeding rate, crop nutrition,nd rice varieties capable of rapid canopy closure, to reduce lightvailability for weeds, can assist in the suppression of weed growth,nd as components these can be incorporated in integrated cropnd weed management programs.

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Relations of rice seeding rates to crop and weed growth in aerobic rice

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