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Responses of upland NERICA® ricevarieties to nitrogen and plant densitySylvester Oikeh a , Amadou Touré a , Baba Sidibé b , Abibou Nianga , Mande Semon a , Yoshimi Sokei a & Mariame Mariko aa Africa Rice Center, (WARDA) , Cotonou, Beninb IER-CRRA , Sikasso, MaliPublished online: 15 May 2009.
To cite this article: Sylvester Oikeh , Amadou Touré , Baba Sidibé , Abibou Niang , MandeSemon , Yoshimi Sokei & Mariame Mariko (2009) Responses of upland NERICA® rice varietiesto nitrogen and plant density, Archives of Agronomy and Soil Science, 55:3, 301-314, DOI:10.1080/03650340802360484
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Responses of upland NERICA1
rice varieties to nitrogen and plant density
Sylvester Oikeha*, Amadou Tourea, Baba Sidibeb, Abibou Nianga, Mande Semona,Yoshimi Sokeia and Mariame Marikoa
aAfrica Rice Center, (WARDA), Cotonou, Benin; bIER-CRRA, Sikasso, Mali
(Received 10 June 2008; final version received 22 July 2008)
Improved varieties, nitrogen fertilizer, and plant spacing have been identified forincreasing upland rice productivity. However, these factors have not been adequatelyinvestigated on interspecific rice, New Rice for Africa (NERICA1). Different levels ofnitrogen (0, 30, 60 and 120 kg ha71) and plant spacing (dibbling: 30 6 30 cm,20 6 20 cm, and drilling: 25 6 5 cm) on the growth and yield of three interspecific ricevarieties and a check variety were evaluated on Terre de barre soils. Rainfall in bothyears was unevenly distribution which caused drought in both years. Across both years,rice yield was significantly depressed with 60N and 120N by 53–81%, compared withother N levels. NERICA4 with 30N gave the highest panicles density and harvest index,and the best yield (1.2 Mg ha71). Wide spacing of 20 6 20 cm or 30 6 30 cm with fourplants stand71 was optimum for the NERICA. Drilling rice at 25 6 5 cm with oneplant stand71 depressed yield. Results showed that in smallholder upland ecosystemsprone to unpredictable drought, wide spacing and low N can be recommended forproduction of NERICA.
Keywords: degraded soil; drought; nitrogen; plant density; upland NERICA1
Introduction
Varieties of the New Rice for Africa (NERICA1) are interspecific, low-management riceplant types targeted for resource-limited, smallholder production systems (Dingkuhn et al.1998). These varieties were developed from crosses between high yielding Oryza sativa(Asian rice) and low yielding, resilient O. glaberrima (African rice). A recent FAO reportindicated that NERICA is enhancing upland rice production in many countries in Africa(FAO 2007). Studies on participatory varietal selection (PVS) carried out in southwesternNigeria on a wide range of upland varieties (O. sativa, O. glaberrima and NERICA)showed that farmers preferred varieties of NERICA because of their good tillering abilityand high tolerance to major biotic and abiotic stresses (Okeleye et al. 2006).
Upland rice production systems, where NERICA varieties are grown occupy almosthalf of the rice area and contribute to 29% of total rice production in West Africa. Theseare fragile and prone to soil degradation, nutrient depletion and limited productivity duepartly to limited use of nutrients and land-use intensification (Buresh et al. 1997; Beckerand Johnson 1999). Becker and Johnson (2001) reported intensification-induced yieldlosses of about 25% due mainly to increased weed infestation in the forest agro-ecosystemand declining soil organic C and N supplying potentials of soils in the derived
*Corresponding author. Email: [email protected]
Archives of Agronomy and Soil Science
Vol. 55, No. 3, June 2009, 301–314
ISSN 0365-0340 print/ISSN 1476-3567 online
� 2009 Taylor & Francis
DOI: 10.1080/03650340802360484
http://www.informaworld.com
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savanna. Because most smallholder farmers have limited resources for purchased inputssuch as inorganic nitrogen (N) fertilizers, deficiencies of N in upland rice arecommon. Therefore, split applications of 90–120 kg N ha71 have been recommendedfor upland Oryza sativa varieties to overcome these deficiencies and optimize yields in theNigerian forest agro-ecosystems (Aduayi et al. 2002). Recent studies showed that withearly seeding and reliance on high mineral N common in the topsoil (N flush) at thebeginning of the cropping season (Weber et al. 1995; Oikeh 1996), split application of60 kg N ha71, with the first split of one-third urea-N applied at 21 days after seeding(DAS) and the rest N at about panicle initiation stage (45–50 DAS), was sufficient for theproduction of some upland NERICA varieties (Oikeh et al. 2008).
The use of appropriate plant density is an important agronomic practice to optimizeyield. Because most upland rice varieties are low-tillering plant types with limited panicles,farmers tend to cultivate them using different plant spacing and densities based on thecropping systems. Wide plant spacing of 30 6 30 cm with seeds placed in a hill or pocket,and a density of 44 plants m72 is often used by farmers to enhance the production ofviable tillers and panicles. Other farmers, who have fairly good access to fertilizers andseeds, use 25 6 5 cm drilling at a high seed rate of 80–90 kg seeds ha71 to obtain a finalpopulation of 80 plants m72. Such high rice densities are sometimes used by farmers tosuppress weeds. This technology was adopted from south-east Asia (M. Sie, personalcommunication). For high yielding rice varieties, the number of panicles per unit area is animportant yield component (Kenneth and Halms 1996), and it is influenced by the tillersdeveloped during the vegetative growth stage (De Datta 1981). However, the productionof many tillers and panicles does not always translate into greater grain yield particularlywhen the crop suffers from moisture stress during grain set and grain-filling stages (Oikehet al. 2008). Therefore, plant spacing that will enhance the production of effective panicleswith filled grains will optimize yield.
Rice yield response to plant density is influenced by the rate of fertilizerapplication. Nguu and De Datta (1979) reported that at a high level of applied N(120 kg ha71), grain yield of O. sativa increased as plant density was increased to a certainlevel beyond which the yield decreased with increased plant density. High density couldalso promote lodging, thus limiting grain yield. Considering that the varieties of NERICAhave greater potential to produce more tillers than most traditional upland varieties(Okeleye et al. 2006), the question is: what, therefore, is the best plant density and Nfertilizer rate needed to optimize yields from NERICA? The objective of this study was toidentify the best combination of N fertilizer and plant density to enhance productivity ofsome released upland varieties of NERICA in the coastal savanna agro-ecosystem of WestAfrica.
Materials and methods
Location and site characterization
A field trial was conducted during the 2005 and 2006 wet seasons at the experimental farmof the National Agricultural Research Institute of Benin Republic at Niaouli (068440 N,028080 W, 15 m elevation) in the coastal savanna, on a flat, degraded sandy clay soilderived from coastal sedimentary formations (Terre de barre) (Gaiser et al. 2000). Therainfall pattern is bimodal from April to mid-July for the long wet season, and fromSeptember to October for the short wet season. Meteorological data were collected duringthe study period. The soil at the experimental site was classified as an acidic Acrisol(Ultisol). According to the analytical procedures of the International Institute of Tropical
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Agriculture (1989), the average chemical analysis of topsoil 0–20 cm showed soil pH inwater ¼ 5.0, total organic carbon content of 16.5 g kg71, total N 0.4 g kg71, available P(Bray II) 10.0 mg kg71, cation exchange capacity 3.6 molc kg
71, sand 750 g kg71, andclay 160 g kg71.
The dominant weeds in both years were a two-year fallow of Chromolaena odorata andCyperus spp. These two weed species accounted for more than 80% of the weed populationfound in the site before the experiment.
Experimental design and treatments
The study used three early maturing (90–100 days) upland varieties (NERICA1, 2, and 4)already adopted by many farmers in West Africa and parts of Eastern and CentralAfrica. One commonly-grown, medium-maturing (110–120 days) O. sativa cv. WAB 56-104, one of the parents of NERICA, was included as a check. In both years, the treatmentswere four levels of N, zero (0N), 30 (30N), 60 (60N), and 120 (120N) kg N ha71, threelevels of plant spacing: 30 6 30 cm (D1), 20 6 20 cm (D2); and 25 6 5 cm with seeddrilling (D3), and the four varieties. These treatments were arranged as split-split plots in arandomized complete block design, with three replications. The N levels were in the mainplots (16 6 13.5 m), plant spacing (plant density: 44 [D1], 100 [D2], and 80 [D3] plantm72) was in the subplots (5 6 13.5 m), while the varieties were in the sub-sub-plots(5 6 3 m).
Five to seven seeds were dibble-seeded on a flat surface previously ploughed andharrowed for D1 and D2 spacing arrangements at a seed rate of 50–60 kg seeds ha71. At14–18 days after seeding (DAS), seedlings were thinned to four per stand for D1 andD2. But D3 was drilled at a seed rate of 80–90 kg seeds ha71 and maintained at oneseedling per stand. In 2005, seeding was done on 16 June. But because of the drought thataffected the experiment in 2005, seeding in 2006 was done on 26 April to avoid thereoccurrence of late season drought.
Basal application of 26 kg P ha71 as triple superphosphate (46% P2O5) and 25 kg Kha71 as muriate of potash (KCl, 60% K2O) were given to all plots before seeding thevarieties. Nitrogen treatment in the form of urea (46% N) was programmed to beapplied in two splits, one-third at 21 DAS and the rest at about panicle initiationstage (45–50 DAS) following the recommendation by Oikeh et al. (2008). In 2005, thefirst split was applied on time, but the second split was delayed by 22 days becausethere was no rain. The crop had started showing visible signs of moisture stress beforethe second application. In 2006, application of the first split was delayed by 11 daysbecause of lack of rain, while the second split was applied on time, two days before a goodrainfall.
To protect the crop against damage from nematodes, termites and ants, carbofuran(insecticide/nematicide) was twice applied at the rate of 2.5 kg a.i. ha71, at seeding and at30 DAS. All plots were weeded three times by hoe weeding.
Plant sampling and measurements
At physiological maturity, data on grain and dry biomass yields were collected from a netarea of 6 m2. Grain yield was corrected to a 14% moisture basis. Plant height at maturityand yield components including tillers and panicles were counted from 10 randomly-selected panicles. Harvest index (HI) for dry matter defined as grain yield per unit total drymatter (DM), was also collected.
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Statistical analysis
Statistical analyses were carried out using the mixed model procedure with the restrictedmaximum likelihood method (REML) for variance estimates over years (SAS Institute2001). Fixed effects were year, N and density levels, and varieties while replications(blocks) were random effects. Where three-way interactions were significant (p 5 0.05)between main effects, simple effect differences were evaluated among treatments. Thestatistical significance of a given factor at different levels of the other factor(s) (simplemain effects) was obtained using the least square means (LSMEANS) SLICE option inPROC MIXED (SAS Institute 2001). Mean separation was performed using the SASLSMEANS test (probability of difference [PDIFF]) at p � 0.05.
Results
Amount and distribution of precipitation
Precipitation totalled 518 mm in 2005 during the growing season from June throughOctober, about 145 mm below normal. In 2006, even though the total rainfall (625 mm)was close to normal (612 mm) during the crop cycle from April to mid-August,distribution was more uneven than in 2005 or the long-term mean (Figure 1). In 2005, afterthe first application of N on 15 July, there was a long dry spell for more than a monthwhich coincided with the period of vegetative growth before maximum tillering (earlydrought stress). There was also limited rain during the grain-filling period (late droughtstress) in September, 2005 compared with the long-term average. In July/August 2006,during the stages of grain set and grainfilling, the crop was visibly stressed by limitedmoisture because rainfall was less than one-seventh of the normal amount (Figure 1).
Crop growth and dry matter yield
There were significant effects of N, plant density and variety on plant height at harvest,and interaction effects of these with year (Table 1). In 2005 the tallest plants were in the
Figure 1. Rainfall distribution during the growing seasons in 2005 and 2006 versus long-term meanat Niaouli, Benin Republic.
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Table1.
ResultsofANOVA
from
Mixed
Modelproceduresforplantheight,yields,andyield
componentsasinfluencedbyyear(Y
),Varieties(V
),N
and
plantdensity
(D)levels,2005and20061.
Probabilitylevel
ofF
Dry
matter
Sourceofvariation2
NDF
DDF
Grain
(Mgha7
1)
Straw
(Mgha7
1)
Plant
height(cm)
Tillers
(No.m
72)
Panicles
(No.m
72)
Harvestindex
(kggrain
kg7
1DM)
Year(Y
)1
4ns
**
ns
**
**
**
Nitrogen
(N)
312
ns
**
*ns
ns
***
NL
(1)
(12)
***
***
***
ns
ns
***
NQ
(1)
(12)
*ns
ns
ns
ns
ns
Y6
N3
12
*ns
**
ns
ns
**
Density
(D)
232
ns
****
***
***
**
Y6
D2
32
***
***
ns
ns
ns
N6
D6
32
ns
ns
ns
ns
ns
ns
Y6
N6
D6
32
ns
ns
ns
ns
ns
ns
Variety(V
)3
144
***
**
***
***
ns
***
Y6
V3
144
**
**
ns
**
***
N6
V9
144
**
ns
ns
ns
ns
***
D6
V6
144
ns
ns
ns
ns
ns
ns
Y6
N6
V9
144
ns
ns
ns
ns
**
***
Y6
D6
V6
144
***
ns
ns
Ns
ns
***
N6
D6
V18
144
ns
ns
ns
Ns
ns
ns
Y6
N6
D6
V18
144
ns
ns
ns
Ns
ns
ns
1Probabilitylevelsare
fixed
effects.NDF,numeratordegreeoffreedom;DDF,denominatordegreeoffreedom
ofcovariance
parameters;
2SubscriptL,linear;subscriptQ,
quadratic;
*, **, ***indicate
significantFvalues
atp5
0.05,0.01,0.001,respectively;nsindicatesF-testnotsignificantatp5
0.05.
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120N treatment while in 2006 the tallest were with 30N and 60N (Figure 2a). NERICA4provided the tallest plants in 2005 while in the following year WAB56-104 were the tallest(Figure 2c). In 2006, the tallest plants were those within the lowest plant density (D1)(Figure 2b).
There were no significant effects of year, N and plant density but the interaction ofyear 6 N significantly influenced grain yield (Table 1). In 2006, grain yield with 120N was2–3 times lower than grains obtained with other N levels in both years (Table 2). Grainyields were similar among the other N levels indicating limited response to N applicationin both years. Furthermore, grain yield was depressed (p 5 0.001) by increasing levels ofN (Figure 3). NERICA4 at 30N gave the best yield of 1.2 Mg ha71 (Figure 3).
Figure 2. Influence of year 6 N level (a); year 6 plant density (b); and year 6 variety (c) onplant height at physiological maturity, 2005 and 2006. D1 ¼ 30 6 30 cm (44 plants m72);D2 ¼ 20 6 20 cm (100 plants m72); and 25 6 5 cm (80 plants m72). The error bars indicatestandard error of the mean.
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The interaction of year 6 plant density 6 variety on grain yield was significant(p ¼ 0.001) (Table 1). Grain yield was significantly reduced at the highest plant density inthe three NERICA varieties in 2005 though these were not affected in 2006 (Table 3). Inboth years, NERICA4 and WAB 56-104 gave the highest grain yield at each plant density.
There were significant effects of year, N, plant density, variety, and the interactions ofyear 6 density and year 6 variety on straw yield (Table 1). Averaging across years,densities and varieties, straw yield was increased linearly (p 5 0.001) from 2.0 Mg ha71
with 0N to 2.4 Mg ha71 with 60N, and attained its peak (2.8 Mg ha71) with 120N,indicating increase in straw yield of 14–34% over 0N. The interaction of year 6 plantdensity indicated that in 2005, straw yields were similar among the three plant densities,but in 2006 the greatest straw yield was obtained in D3 treatment (Figure 4a). However,irrespective of the plant density used, straw yields were 1.5–2.2 times lower in 2006 than in2005. Furthermore, the interaction of year 6 variety revealed that straw yield was similaramong the four varieties in 2005, but in 2006, NERICA1 gave the highest straw yield(Figure 4b).
Components of yield
Tiller production was affected by year, density and variety while panicle production wasaffected by density and the interactions of year and variety, and nitrogen, variety and year
Table 2. Influence of year and N application on grain yield (Mg ha71), 2005 and 2006.
Nitrogen (kg ha71)
Year
2005 2006
0 0.87a 0.95a30 0.76a 1.00a60 0.72ab 0.75a120 0.85a 0.37bSE (DF ¼ 3) 0.121
Means followed by the same letters are not significantly different at p 5 0.05.
Figure 3. Influence levels of N and variety (averaged across year and density) on grain yield ofupland rice, 2005 and 2006. Standard error of the mean (DF¼9) ¼ 0.095.
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(Table 1). Mean number of tillers was 273 m72 and panicles was 228 m72 in 2005. Thecorresponding values for 2006 were 199 (tillers) and 135 (panicles) per m2, indicating adecrease of 27–41% in both traits in 2006 over 2005 values. The highest number of tillersand panicles were produced in D3 plots. Averaging across year, N and density, tillerproduction was similar (235–253 m72) in NERICA1 and NERICA2, but both varietiesproduced significantly more tillers than NERICA4 or WAB 56-104. The NERICAvarieties produced 10–18% more tillers (p 5 0.001) than the check, cv. WAB 56-104.
In 2006, with 0N, all the NERICA varieties produced 45–65 more panicles m72 thanWAB 56-104 (Table 4). However, with 120N treatment, panicle production wassignificantly reduced by more than 50% in NERICA1 and NERICA2 than in the othervarieties. There was no response of panicles numbers to applied N in 2005 among thevarieties and by NERICA4 and WAB 56-104 in 2006 (Table 4).
The harvest index was significantly influenced by year, N, density and variety, and theinteractions of year 6 N 6 variety and year 6 plant density 6 variety (Table 1). Ateach N level (2005) and at 30N and 60N (2006) NERICA4 gave the largest HI. In 2006, HIwas significantly lower in NERICA1 than NERICA2 (Table 5). Furthermore, with 0N or120N, there was no significant difference in HI between NERICA4 and WAB 56-104 in2006. Also, in 2006, HI in all varieties was significantly decreased by 11–91% with 60Nand 120N than with other N levels.
In 2005, at D1, NERICA4 gave 26% higher HI than the popular WAB 56-104(Table 6). Also, in both years, NERICA4 and WAB 56-104 gave significantly higher HIthan the other NERICA varieties at all densities. In both years, D3 significantly decreasedHI in all varieties (Table 6).
Discussion
The variable responses of the varieties to the main and simple effects of N and spacing(plant density) in the two-year study might have been caused by the seasonal differences inrainfall distribution and moisture availability. It has been estimated that 25% of the fieldsused for upland rice production are prone to yield reduction caused by drought (Kasukaet al. 2005). In the present study, the observed lower mean grain yield (51.0 Mg ha71)
Table 3. Influence of year, plant density and variety (averaged across N) on grain yield (Mg ha71),2005 and 2006.
Year Variety
Density (No. m72)3
D1 D2 D3
2005 NERICA1 0.79b1A2 0.49cB 0.48bB2005 NERICA2 0.79bA 0.70bA 0.53bB2005 NERICA4 1.22aA 1.04aA 0.95aB2005 WAB 56-104 0.87bA 0.93aA 0.85aA
2006 NERICA1 0.41bA 0.50bA 0.44cA2006 NERICA2 0.55bA 0.62bA 0.76bA2006 NERICA4 1.07aA 0.93aA 1.11aA2006 WAB 56-104 1.05aAB 0.81aC 0.98aBC
1Means within a column in a given year and plant density followed by the same lowercase letter are notsignificantly different at p 5 0.05. Test effects of SLICING by year 6 plant density; 2Means within a rowfollowed by the same uppercase letter are not significantly different at p 5 0.05. Test effects of SLICING byyear 6 variety; 3D1 ¼ 44 plants m72; D2 ¼ 100 plants m72; and D3 ¼ 80 plants m72.
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Table 4. Influence of year, N application and varieties (averaged across density) on panicleproduction, 2005 and 2006.
Year Variety
Nitrogen (kg ha71)
0 30 60 120
2005 NERICA1 236a1A2 202aA 220aA 234aA2005 NERICA2 244aA 233aA 237aA 273aA2005 NERICA4 199aA 225aA 240aA 231aA2005 WAB 56-104 231aA 187aA 237aA 215aA
2006 NERICA1 157abA 131aB 125aB 51.6bC2006 NERICA2 177aA 137aB 130aB 70.3bC2006 NERICA4 168aA 152aA 149aA 162aA2006 WAB 56-104 112bA 145aA 138aA 153aA
1Means within a column in a given year and N level followed by the same letter are not significantly different atp 5 0.05. Test effects of SLICING by year 6 N level; 2Means within a row followed by the same uppercaseletter are not significantly different at p 5 0.05. Test effects of SLICING by year 6 variety.
Figure 4. Influence of year 6 plant density (a); and year 6 variety (b) on straw yield of upland rice,2005 and 2006. D1 ¼ 30 6 30 cm (44 plants m72); D2 ¼ 20 6 20 cm (100 plants m72); and25 6 5 cm (80 plants m72). The error bars indicate standard error of the mean.
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with 60N and 120N compared with about 2.0 Mg ha71 mean grain yield with 0N reportedfor the forest agro-ecosystem of Nigeria by Oikeh et al. (2008) might have been due to thesevere drought that occurred just after the application of the first split of N, for over amonth and at grain-filling in 2005 and during kernel set and grain-filling periods in2006. Moreover, rice has been reported to be a notoriously drought-susceptible crop duepartly to its shallow root systems, and rapid stomatal closure and leaf senescence evenunder mild water stress (Bernier et al. 2008).
A previous study reported by Oikeh et al. (2008) showed that grain yields of somewidely adopted NERICA varieties were depressed at zero N (N deficiency) over applied Nin the humid forest agro-ecosystems of Nigeria. In the present study by contrast, grainyields were higher at zero N than at other N levels in NERICA1 and NERICA2, possiblybecause of the moisture stress that occurred at grain set and grain-filling in 2006. Becausethese varieties have in general, greater biomass production in 2006, they were the mostsusceptible to drought thus resulting in lower partitioning of dry matter to the grain as
Table 6. Influence of year, plant density and variety (averaged across N) on harvest index for drymatter (kg grain kg71dry matter), 2005 and 2006.
Year Variety
Density (No. m71)3
D1 D2 D3
2005 NERICA1 0.20b1A2 0.13bB 0.13bB2005 NERICA2 0.22bA 0.18bAB 0.14bB2005 NERICA4 0.29aA 0.25aA 0.24aB2005 WAB 56-104 0.23bA 0.23aA 0.22aA
2006 NERICA1 0.16bB 0.23cA 0.16cB2006 NERICA2 0.24bA 0.29bA 0.28bA2006 NERICA4 0.44aA 0.40aAB 0.37aB2006 WAB 56-104 0.43aA 0.38aA 0.33aB
1Means within a column in a given year and plant density followed by the same lowercase letter are notsignificantly different at p 5 0.05. Test effects of SLICING by year 6 plant density; 2Means within a rowfollowed by the same uppercase letter are not significantly different at p 5 0.05. Test effects of SLICING byyear 6 variety; 3D1 ¼ 44 plants m72; D2 ¼ 100 plants m72; and D3 ¼ 80 plants m72.
Table 5. Influence of year, N application and variety (averaged across density) on harvest index fordry matter (kg grain kg71 dry matter), 2005 and 2006.
Year Variety
Nitrogen (kg ha71)
0 30 60 120
2005 NERICA1 0.21b1A2 0.16bAB 0.16bAB 0.09cB2005 NERICA2 0.20bA 0.19bcA 0.14cA 0.18bA2005 NERICA4 0.28aA 0.26aA 0.25aA 0.24aA2005 WAB 56-104 0.25abA 0.22bA 0.21abA 0.23abA
2006 NERICA1 0.33bA 0.22dB 0.15dB 0.03bC2006 NERICA2 0.45aA 0.30cB 0.25cB 0.08bD2006 NERICA4 0.44aA 0.47aA 0.42aA 0.29aB2006 WAB 56-104 0.46aA 0.42bA 0.35bAB 0.30aB
1Means within a column in a given year and N level followed by the same lowercase letter are not significantlydifferent at p 5 0.05. Test effects of SLICING by year 6 N level; 2Means within a row followed by the sameuppercase letter are not significantly different at p 5 0.05. Test effects of SLICING by year 6 variety.
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indicated by lower HI with increasing levels of N. High HI has been associated withenhanced grain yield in some rice varieties tolerant to severe drought occurring at thereproductive stage (Lanceras et al. 2004). The limited moisture could have irrevocablyreduced grain endosperm starch and protein assimilation, as reported also for maize(Bauer and Carter 1986; Oikeh et al. 1998) resulting in limited or depressed response withmoderate to high N application. Furthermore, although not evaluated in this study,phytotoxicity of urea volatilization caused by lack of moisture to dissolve the applied ureaafter application might have in addition, contributed to the depressed growth and yieldparticularly with 120N as earlier reported (Oikeh et al. 2008). NERICA varieties havebeen developed as low-management rice plant types targeted for resource-limited,smallholder production systems (Dingkuhn et al. 1998), and their performance with30N thus confirmed the significant role they play in African smallholdersystems. Therefore, under the conditions of the study, 30N was optimum for the varieties.
The observed greater yield in NERICA4 with 30N could be attributed to its higherpotential for panicle production and greater ability to partition photosynthates moreefficiently to the grain as indicated by its high HI, compared to the other varietiesdespite the severe moisture stress in both years. This might indicate a greater potential totolerate both prolonged early and late season drought stress in addition to its potential totolerate low N in this coastal savanna agro-ecosystem. Our results corroborate the studyreported by Lanceras et al. (2004), who associated enhanced HI and panicle productionwith high grain yield under severe drought occurring at the reproductive stage ofrice. Yadav et al. (2002) also reported that high HI increased grain yield in their studyconducted with pearl millet under drought stress condition.
Moreover, high weed competitiveness has been associated with NERICA4 in arecent study carried out in the savannas of northeastern Nigeria (Ekeleme et al. 2007),making it the most suitable variety among the NERICA varieties evaluated, for thesmallholder resource limited farmers in the coastal savanna agro-ecosystem of BeninRepublic.
Previous studies conducted in the forest agro-ecosystem of Nigeria showed thatNERICA1 had a potential for midseason drought escape at low N because of itsdevelopmental plasticity, and potential for tolerance to low N as well as being responsiveto high N (Oikeh et al. 2008). However, in this study, with moderate to severe moisturestress particularly during the grain-filling period in 2006, this variety was the leastperforming variety with 120N. Nevertheless, the potential of NERICA1 to adapt to milddrought by exhibiting accelerated flowering at low N under mild drought stress, inaddition to its tolerance to low N and high N responsiveness (Oikeh et al. 2008), andits long grain and aromatic trait, might have contributed to the high uptake ofNERICA1 among smallholder farmers in northern Nigeria as reported by Spenceret al. (2006). Although NERICA1 has been released to farmers in Benin Republic, thecurrent study indicated that it is not the most suitable variety but NERICA4 under severedrought particularly, at moderate to high N levels, for the degraded soils (Terre de barre)of the coastal savanna agro-ecosystem of Benin Republic.
Rice yield response to plant density has often been reported to be significantlyinfluenced by the rate of N fertilizer application (Wells and Faw 1978; Nguu and De Datta1979). But in this study, there was lack of significant interaction of plant density andapplied N on grain yield among the varieties, possibly because of the limited moistureavailability in the study periods. However, this study corroborates an earlier report thatshowed no differences among treatment combinations of N and spacing on rice grain yieldin the sub-humid area of southeastern Nigeria where moisture availability was not a
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limiting factor (Ogah 2005). This might suggests that the production of each variety wouldnot require more N application at a higher plant density under the conditions of the study.
Unlike Ottis and Talbert (2005), who did not detect a rice density effect on panicledensity and observed that optimum panicle density was achieved at rice density rangingfrom as low as 72 to as high as 373 plants m72 among O. sativa varieties, our resultsshowed significant density effects on tiller and panicle densities. An optimum panicledensity of 263 plants m72 was obtained at D3. However, the observed higher panicledensity did not translate into greater yield at this density compared with the otherdensities.
On the contrary, there was depression in growth and yield of the NERICA varietieswhen seeds were drilled in D3 plots which indicated that this plant spacing, although goodfor the O. sativa variety (WAB56-104), is not suitable for the cultivation of varieties ofNERICA particularly in environments that are prone to frequent drought. This study alsoshowed that maintaining single plants at 5 cm within row in D3 plots might haveencouraged higher intraspecific competition among neighbouring plants from mutual leafshading, thus reducing light interception and CO2 assimilation, and consequently limitingrice yields among the NERICAs. A similar observation of intraspecific competitionlimiting rice yield had been reported for O. sativa by Wells and Faw (1978). However, indeference to the study of Ottis and Talbert (2005), it is possible that wider spacingarrangements in D1 and D2 plots with up to four plants stand71 allowed for higherphotosynthetically-intercepted active radiation (PAR) to be more efficiently intercepted bythe rice canopy due to reduced mutual leaf shading resulting in greater yields in NERICAvarieties in D1 and D2 than in D3 treatment.
Furthermore, the significantly higher panicle density in D3 plots over D1 and D2 plotsdespite the lower initial plant population of 80 plants m72 as against 100 plants m72 in D1plots, implied that tillering rather than the initial plant population accounted for the highnumber of panicles per unit area. These results are in contrast to the findings of Wells andFaw (1978) who reported that the initial rice population rather than tillering potentialaccounted for high panicle density in some O. sativa varieties.
The observed higher tiller density amongst NERICA varieties compared to theO. sativa (WAB56-104) is one of the reasons reported in a PVS study carried out insouthwestern Nigeria for the high preference by farmers of the NERICA varieties over theO. sativa varieties (Okeleye et al. 2006). It is therefore, recommended to sow varieties ofNERICA in environments prone to frequent unpredictable drought, since they are likelyto produce more reproductive tillers, but will generally perform best when dibble-seeded at20 6 20 cm or 30 6 30 cm with up to 4 plants stand71.
Conclusion
The current study has shown that low N should be used for NERICA varieties inenvironments prone to frequent unpredictable droughts, because moderate to high Nmight depress yield because the plants grown with moderate to high N would be moresusceptible to severe drought stress causing lower harvest index. There is also thepossibility that drought stress might cause depressed growth and yield due to phytotoxicityof urea volatilization resulting from lack of moisture to dissolve the applied urea withmoderate to high N. The use of wide spacing of 20 6 20 cm or 30 6 30 cm with up tofour plants stand71 was optimum for NERICA varieties because this would allow forhigher photosynthetically-intercepted active radiation (PAR) to be more efficientlyintercepted by the rice canopy resulting in greater grain yield than when drilled at
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25 6 5 cm with one plant stand71. NERICA varieties produced 10–18% more tillers thanthe check, cv. WAB 56-104. Furthermore, NERICA4 particularly at 30 kg N ha71, wassuperior to other varieties in grain yield possibly because of its high panicle productionand HI, suggesting that this variety had the greatest potential for tolerance to low N andsevere drought stress compared with the other varieties under the conditions of theexperiment.
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