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Effects of tied-ridge, nitrogen fertilizerand cultivar on the yield and nitrogenuse efficiency of sorghum in semi-aridEthiopiaWondimu Bayu a , Norman F.G. Rethman b & Piet S. Hammes ba Amhara Region Agricultural Research Institute , Bahir Dar ,Ethiopiab Department of Plant Production and Soil Science , University ofPretoria , Pretoria , South AfricaPublished online: 10 Aug 2011.

To cite this article: Wondimu Bayu , Norman F.G. Rethman & Piet S. Hammes (2012) Effectsof tied-ridge, nitrogen fertilizer and cultivar on the yield and nitrogen use efficiency ofsorghum in semi-arid Ethiopia, Archives of Agronomy and Soil Science, 58:5, 547-560, DOI:10.1080/03650340.2010.532488

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Effects of tied-ridge, nitrogen fertilizer and cultivar on the yield

and nitrogen use efficiency of sorghum in semi-arid Ethiopia

Wondimu Bayua*, Norman F.G. Rethmanb and Piet S. Hammesb

aAmhara Region Agricultural Research Institute, Bahir Dar, Ethiopia; bDepartment of PlantProduction and Soil Science, University of Pretoria, Pretoria, South Africa

(Received 21 May 2010; final version received 10 October 2010)

Moisture deficit, poor soil fertility and lack of improved varieties constrainedsorghum production in north-eastern Ethiopia. An experiment was conducted in2002 at Kobo and Sirinka in north-eastern Ethiopia to study the possible effects ofseedbed, nitrogen fertilizer and cultivar on the yield and N use efficiency (NUE) ofsorghum. The experiment was carried out in a split–split plot design with seedbed(tied-ridge vs. flatbed planting) as main plots, N fertilizer (0, 40 and 80 kg N ha71)as subplots and sorghum cultivars (Jigurti, ICSV111 and 76T1#23) as sub-subplots, with three replications. At Kobo, the seedbed by cultivar interaction affectedall parameters. Nitrogen fertilization increased biomass yield and NUE at bothlocations and grain yield at Sirinka. Cultivars showed different performance whereICSV111 and 76T1#23 were superior in grain yield, N uptake and concentration,N harvest index and NUE of grain (NUEg) compared with Jigurti. Thus, plantingICSV111 and 76T1#23 in tied-ridging and with N fertilization at Kobo and inflatbed and with N fertilization at Sirinka is recommended. This study revealedthat tied-ridging is not a solution in all areas where moisture deficiency is aproblem. Its effectiveness is affected by rainfall amount and soil type.

Keywords: nitrogen fertilizer; N use efficiency; sorghum; tied-ridge

Introduction

Sorghum (Sorghum bicolor L. Moench) is the major staple crop in the lowland areasof north-eastern Ethiopia. Its productivity is, however, largely constrained by watershortage and poor soil fertility. Sorghum in north-eastern Ethiopia is producedentirely under rainfed conditions where the rainfall is usually inadequate, short induration, poorly distributed and highly variable between and within seasons. In suchsituations, the contribution of the use of an on-farm water-harvesting technique(tied-ridging) would be immense in terms of efficiently utilizing the available waterfrom the rain and thus reducing the impact of drought on sorghum (Jensen et al.2003). Tied-ridges work by retaining rainwater within the root zone, therebyensuring crop survival during prolonged dry spells.

Soils in north-eastern Ethiopia are often deficient in nutrients, with nitrogenbeing the main limitation (Bayu et al. 2002). The use of commercial nitrogen

*Corresponding author. Email: wondimubayu@yahoo.com

Archives of Agronomy and Soil Science

Vol. 58, No. 5, May 2012, 547–560

ISSN 0365-0340 print/ISSN 1476-3567 online

� 2012 Taylor & Francis

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fertilizer for sorghum production is, however, generally low because of high cost andclimatic risks. Thus, N uptake and use by the crop is of fundamental importance tonitrogen economy in crop production. Therefore, agronomic techniques along withcultivar selection, which can improve crop N uptake and use, should be tailored intothe production system in order to improve sorghum productivity. Under suboptimalN fertilizer application, efficiency of N use can be improved through agronomicpractices that can improve water availability and through breeding and selection forgenotypes exhibiting greater N use efficiency. There may be genotypic differences inN uptake, efficiently utilizing the absorbed N, and N partitioning (Kamoshita et al.1998). High yielding sorghum cultivars that can efficiently use nutrients on poor soilscan contribute towards improved crop productivity on nutrient poor soils.

The use of improved technologies like tied-ridging, nitrogen fertilizer and impro-ved cultivars separately, as often done by farmers, could not realize benefits thatmight be obtained from the combined use of these production factors. Traditionally,the development approach has focused on the use of single elements of productionfactors such as improved cultivars, mineral nutrition or water conservation measures.However, substantial impacts can be realized through the integrated use of theseinputs. Therefore, this experiment was conducted with the objective of determiningthe single and combined effects of seedbed type, nitrogen fertilizer and cultivar on thegrowth, yield and nitrogen use efficiency attributes of sorghum.

Materials and methods

This experiment was conducted in 2002 at Sirinka (118 450 N, 398 360 E,1890 m.a.s.l.) and Kobo (128 90 N, 398 380 E, 1470 m.a.s.l.) in north-easternEthiopia. The soil type at both locations was a Eutric Vertisol. The 0–30 cm soilhorizon at Sirinka contains 57.5–60% clay, 32.5–35% silt and 7.5% sand, with a pHof 6.64 (1:2.5 in water), 1.39–1.68% organic C, 0.14–0.15% total N, 8.6–10.4 mgkg71 available (Olsen) P, 0.97 meq 100 g71 K, and cation-exchange capacity (CEC)of 33.3 meq 100 g71. Similarly, at Kobo the 0–30 cm soil horizon contains 45–47.5%clay, 42.5% silt and 10–12.5% sand, with a pH of 7.31–7.38 (1:2.5 in water), 1.20–1.22% organic C, 0.09% total N, 10.4–13.6 mg kg71 available (Olsen) P, 0.4 meq100 g71 K, and CEC of 43.2 meq 100 g71.

The experiment was arranged in a split-split plot design with two seedbed types(tied-ridge planting vs flatbed planting) as main plots, three N fertilizer levels (0, 40and 80 kg ha71) as subplots and three sorghum cultivars (Jigurti, ICSV111 and 76T1#23) as sub-sub-plots, with three replications. Jigurti is a late maturing local cultivar,whereas ICSV111 and 76T1#23 are improved early maturing cultivars. Tied-ridgeswere constructed, a week before planting, using oxen- and tractor-drawn implementsat Sirinka and Kobo, respectively. In all plots, P as triple superphosphate was bandapplied at planting at the rate of 20 kg P ha71. Urea was used as the source of N,which was applied in a band at 20 kg ha71 at planting and the remainder sidedressed at approximately the six to eight leaf stage of the crop.

The cultivars were hand drilled in 75 cm rows and thinned to an interplantspacing of 15 cm. Gross plot size was 5.25 m (6.0 m at Kobo) wide by 5 m long.Prior to planting, composite surface (0–15 cm) and subsurface (15–30 cm) soilsamples, from nine points across the experimental field, were collected andanalyzed for soil physicochemical properties following the procedure outlined byPage et al. (1982).

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Two rows at Sirinka and three rows at Kobo were hand-harvested for grain yielddetermination at maturity after discarding border rows. Grain yield was adjusted to12.5% moisture content. Plant samples for N uptake determination at harvest wereoven dried at 708C to constant weight. Grain and stover samples were groundseparately to pass through a 1 mm sieve. The N content of the samples wasdetermined using the micro-Kjeldahl method (Page et al. 1982). N uptake in thegrain and stover was estimated by multiplying their concentrations with grain yieldand stover biomass, respectively. Total aboveground biomass N uptake wascalculated by adding the uptake in the grain and stover.

The determination of NUE followed the definitions suggested by Traore andMaranville (1999). The term NUEb (N use efficiency for total aboveground biomassproduction, kg dry matter kg71 N) was defined as the total aboveground biomassdivided by total N amount in total aboveground biomass, and NUEg (N useefficiency for grain production, kg grain kg71 N), was defined as grain yield dividedby the total N amount in the grains. Nitrogen harvest index (NHI%) was calculatedas the ratio of grain N uptake to total aboveground N uptake. Harvest index (HI)was calculated as the ratio of grain yield to the aboveground biomass yield. Leaf areaindex (LAI) is the ratio of total leaf area of the plant divided by the surface area ofthe land occupied by the plant.

Analysis of variance was performed using the MSTATC statistical program(MSTATC 1989). Differences among treatment means were delineated usingDuncan’s multiple range test (p 5 0.05).

Results and discussion

Data on the growing season rainfall and maximum and minimum air temperatures,as well as the long-term average rainfall are presented in Table 1. Growing seasonrainfall at Sirinka was comparable with the long-term mean, whereas Kobo received524 mm of rainfall, *65 mm above the long-term average. Soil conditions wereexcessively wet in August and September. The greater and more intense seasonalrainfall at Kobo created run-off, which resulted in the silting up of tied-ridges.

Table 1. Monthly precipitation and air temperatures for the 2002 season and average long-term precipitation for the growing season at Sirinka and Kobo.

Sirinka Kobo

Precipitaion (mm)Temperature

(8C) Precipitaion (mm)Temperature

(8C)

Month 2002 Long-terma Max.b Min.c 2002 Long-terma Max.b Min.c

July 229.5 192.1 30.5 17.1 99.8 108.2 34.0 19.6August 280.6 266.3 27.8 15.7 296.0 200.4 30.9 17.6September 134.0 95.0 26.6 14.7 112.4 95.2 29.9 15.7October 12.0 57.7 26.6 12.4 16.0 43.7 30.5 13.0November 0.0 21.7 25.5 11.6 0.0 11.9 29.5 12.0

Total 656 633 524 459

Note: aLong-term average rainfall (1980–2002 for Sirinka and 1973–2002 for Kobo). bMaximum,cMinimum.

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Analysis of variance shows that the main effects of seedbed type are notsignificant for almost all parameters studied at both locations (Tables 2 and 3).Similarly, the interaction effects of seedbed type by N fertilizer, N fertilizer bycultivar, and seedbed type by N fertilizer by cultivar were not significant for mostparameters at both locations (Tables 2 and 3). The seedbed type by cultivarinteraction was significant for many parameters at Kobo, but was not significant formost parameters at Sirinka while the main effects of nitrogen fertilizer and cultivarare significant for most parameters (Tables 2 and 3).

Aboveground biomass

Aboveground biomass was significantly affected by the N fertilizer and cultivar atSirinka (Table 2) and by N fertilizer and by the seedbed type by cultivar interactionat Kobo (Table 3). At Sirinka, aboveground biomass increased from 8218 kg ha71

with no fertilizer application to 10,235 kg ha71 with the application of 80 kg Nha71. Differences between cultivars at Sirinka indicate that Jigurti producedsignificantly higher aboveground biomass (13155 kg ha71), which was 74% higherthan that produced by the lowest yielder cultivar 76T1#23 (Table 4).

At Kobo, aboveground biomass increased from 7843 kg ha71 with no fertilizerapplication to 8943 kg ha71 with the application of 80 kg N ha71 (Figure 1A).Under both tied-ridge and flatbed planting, Jigurti produced the highest totalaboveground biomass yield (Figure 1B). The greater biomass production under tied-ridge planting could possibly be due to improved moisture availability (Hulugalle1987). ICSV111 and 76T1#23 did not differ significantly in biomass productionunder both tied-ridge and flatbed planting.

At both locations, the application of nitrogen fertilizer 440 kg N ha71 did notsignificantly increase aboveground biomass. The greater aboveground biomass withN fertilization could be due to the increase in leaf area development and thusphotosynthetic potential with N fertilization. Increased leaf area development withN fertilization was observed at Sirinka where the average LAI values for 0 and 80 kgN ha71 at anthesis (72 days after emergence) were 1.81 and 2.16, respectively.

Grain yield

Significant differences in grain yield were observed between N fertilizer treatmentsat Sirinka (Table 2) and between the interaction effects of seedbed type and cultivarat Kobo (Table 3). At Sirinka, significantly higher grain yields were obtained withthe application of 40 and 80 kg N ha71 (Table 4) compared with no N application,although increasing N fertilizer application to 440 kg ha71 did not increase grainyield significantly. Increased grain yields due to N application could be ascribedto increased biomass production, improved harvest index and increased seed setwith fertilization. This could be justified by the increased number of seeds perplant and harvest index with fertilizer application (Table 4), as reported by Buahet al. (1998).

At Kobo, grain yield increased when using tied-ridge planting compared withflatbed planting for all the three cultivars, similar to the results of Nyakatawa (1996).The cultivar Jigurti with flatbed planting resulted in the lowest grain yield of 1145 kgha71. Under both tied-ridge and flatbed plantings, the early maturing cultivarsICSV111 and 76T1#23 gave better yields than Jigurti (Table 5).

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Table

2.

Analysisofvariance

table

fortheeff

ectofseedbed

type,

Nfertilizer

andcultivars

onsorghum

atSirinka(m

eansquare

values).

Sources

of

variation

df

BY

GY

HI

SNP

SNC

GNC

SNU

GNU

TNU

NHI

NUEb

NUEg

Seedbed

type(S)

115972928.9ns

686591.1ns

0.004ns

739908.1ns

0.002ns

0.050ns

81.1ns

318.7ns

721.6ns

0.000ns

0.8ns

56.2ns

Error

23284553.6

336940.5

0.001

105901.3

0.000

0.006

4.5

48.9

83.0

0.001

110.3

26.5

Nitrogen

(N)

220355869.5**

4875523.1**

0.003**

1396580.0*

0.017*

0.086*

205.0*

1518.0**

2744.6**

0.008**

2282.4**

36.5ns

S6

N2

316565.4ns

362261.9ns

0.001*

385381.5ns

0.003ns

0.031ns

1.4ns

230.7ns

266.3ns

0.003*

431.2ns

23.5ns

Error

82552295.6

472776.1

0.000

226276.7

0.003

0.019

35.9

96.2

225.0

0.001

128.5

19.3

Cultivar(C

V)

2188202885.7**

628117.3ns

0.229**

1987986.4**

0.051**

0.016ns

1121.1**

58.3ns

1162.2**

0.114**

21676.2**

887.1**

S6

CV

2133073.3ns

332169.6ns

0.000ns

201172.7ns

0.003ns

0.042*

3.7ns

35.2ns

20.7ns

0.002ns

95.9ns

33.5ns

N6

CV

4705498.3ns

299668.2ns

0.002ns

178420.0ns

0.002ns

0.010ns

9.9ns

27.5ns

57.3ns

0.004ns

128.0ns

78.2*

S6

N6

CV

42527.9ns

42646.9ns

0.001ns

17443.6ns

0.006ns

0.053**

18.3ns

122.4ns

132.5ns

0.005ns

667.0ns

24.1ns

Error

24

926440.0

303162.1

0.001

81742.1

0.004

0.012

38.9

89.2

160.5

0.004

495.5

21.5

Note:BY,abovegroundbiomass;GY,grain

yield;SNP,seed

number

per

panicle;HI,harvestindex;SNC,stover

nitrogen

concentration;GNC,grain

nitrogen

concentration;

SNU,stover

nitrogen

uptake;

GNU,grain

nitrogen

uptake;

TNU,totalnitrogen

uptake;

NHI,nitrogen

harvestindex;NUEb,nitrogen

use

efficiency

forbiomass;NUEg,

nitrogen

use

efficiency

forgrain.*,**andnsdenote

significantdifferencesatp�

0.05,p�

0.01andnonsignificantdifference,respectively.

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Table

3.

Analysisofvariance

table

fortheeff

ectofseedbed

type,

Nfertilizer

andcultivars

onsorghum

atKobo(m

eansquare

values).

Sources

of

variation

df

BY

GY

HI

TKW

SNP

SNC

GNC

SNU

GNU

TNU

NHI

NUEb

NUEg

Seedbed

type(S)

110447361.1ns

12102186.9ns

0.127ns

90.2ns

3102724.7**

0.136ns

0.057ns

714.31ns

4605.58*

1682.25ns

0.31ns

535.8ns

904.4ns

Error

21895877.6

743799.5

0.002

11.9

41677.3

0.095

0.165

258.59

119.86

115.05

0.02

444.9

173.6

Nitrogen

(N)

26809665.7**

497742.0ns

0.002ns

25.6*

340884.1ns

0.199**

0.235*

1296.21**

747.51*

4023.56**

0.01ns

2379.5**

140.3*

S6

N2

2816853.9ns

122922.6ns

0.010ns

11.3ns

173944.5ns

0.002ns

0.008ns

93.54ns

29.42ns

116.55ns

0.01ns

368.5ns

77.1ns

Error

8665547.3

274245.4

0.004

3.4

147781.6

0.018

0.050

77.01

157.71

213.87

0.00

131.4

25.1

Cultivar(C

V)

282093551.5**

5966439.5**

0.319**

321.0**

6398677.4**

0.116**

0.230**

2218.10**

3953.71**

307.47ns

0.41**

16647.2**

648.9**

S6

CV

210008760.9**

1380886.2**

0.001ns

15.4*

169825.4ns

0.014ns

0.055ns

99.08ns

540.10*

848.05*

0.05**

188.3ns

109.7*

N6

CV

41756186.9ns

449495.1ns

0.002ns

2.3ns

250832.1ns

0.026ns

0.074bs

218.02ns

228.26ns

651.66*

0.00ns

767.4**

31.9ns

S6

N6

CV

41088462.2ns

448195.5ns

0.006ns

1.2ns

134071.2ns

0.009ns

0.014ns

69.86ns

157.22ns

152.57ns

0.01*

242.3ns

62.1ns

Error

24

783263.0

203500.2

0.003

3.1

141296.7

0.016

0.033

104.97

115.96

239.70

0.00

144.3

23.8

Note:BY,abovegroundbiomass;GY,grain

yield;SNP,seed

number

per

panicle;HI,harvestindex;SNC,stover

nitrogen

concentration;GNC,grain

nitrogen

concentration;

SNU,stover

nitrogen

uptake;

GNU,grain

nitrogen

uptake;

TNU,totalnitrogen

uptake;

NHI,nitrogen

harvestindex;NUEb,nitrogen

use

efficiency

forbiomass;NUEg,

nitrogen

use

efficiency

forgrain.*,**andnsdenote

significantdifferencesatp�

0.05,p�

0.01andnonsignificantdifference,respectively.

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The early maturing cultivars ICSV111 and 76T1#23 produced the highest grainyield while having the lowest shoot dry mass, indicating the efficient translocation ofa large proportion of assimilates to the grain, whereas Jigurti, having the largestshoot dry mass, produced the lowest grain yield, indicating its poor ability in

Table 4. Effect of N fertilizer and cultivar on aboveground biomass yield (BY), grain yield(GY), harvest index (HI) and seed number per panicle (SNP) of sorghum at Sirinka.

Treatments BY (kg ha71) GY (kg ha71) HI SNP

N level (kg ha71)0 8218b 2523b 0.34b 1683b40 9810a 3260a 0.36a 2164a80 10235a 3528a 0.36a 2168aLSD0.05 1228 528.2 0.002 365.6

CultivarJigurti 13155a 3015a 0.23c 1637bICSV111 7569b 2978a 0.40b 2096a76T1#23 7540b 3318a 0.44a 2283aLSD0.05 662.2 ns 0.021 196.7Coefficient of Variation (%) 10 17 10 14

Note: Means within columns for each comparison followed by the same letters are not significantlydifferent at p � 0.05.

Figure 1. Effect of (A) N fertilizer and (B) seedbed by cultivar interaction on sorghumaboveground biomass at Kobo. Bars followed by the same letters are not significantly differentat p � 0.05.

Table 5. Seedbed type by cultivar interaction effect on sorghum grain yield (kg ha71) atKobo.

Cultivars

Seedbed type Jigurti ICSV111 76T1#23

Tied-ridge 2682bc 3120ab 3352aFlat 1145d 2681c 2488cLSD0.05 438.9

Note: Means followed by the same letters are not significantly different at p � 0.05.

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partitioning assimilates to the grain. The late maturing cultivar Jigurti at Kobo hadrelatively less time for carbon assimilation due to the risk of drought at grain fillingand, therefore, had lower grain yield. Grain yield at Kobo was closely andsignificantly associated with HI (r ¼ 0.99) indicating that grain yield differencesbetween cultivars could be attributed partly to differences in harvest index. Cultivardifferences in grain yield could also be attributed to differences in seed number perplant where the high-yielding cultivars had a greater number of seeds per panicle(Table 6). Cultivars also differed in seed size, with Jigurti having the largest seeds(Table 6). The larger seed size of Jigurti seems, however, to be counterbalanced bythe lower number of seeds and thus did not influence the final yield.

Nitrogen uptake

Significant differences in grain nitrogen uptake were observed between N fertilizertreatments at both locations and between the seedbed type by cultivar interactioneffects at Kobo (Tables 2 and 3). The results of N fertilizer effect indicate thatsignificantly higher grain N uptakes were recorded with the application of 40 and80 kg N ha71 (Figure 2A). According to Rao et al. (1991), fertilizer N can increase Navailability from the other N pools and/or stimulate root growth and thus increaseplant N uptake. The increased N uptake with N fertilization could also be explainedby the development of a larger and more effective root system, although root system

Table 6. Cultivar differences in harvest index (HI), seed number per panicle (SNP) andthousand kernel weight (TKW) at Kobo.

Cultivar HI SNP TKW (g)

Jigurti 0.17b 1715c 25aICSV111 0.39a 2243b 21b76T1#23 0.41a 2905a 17cLSD0.05 0.037 258.6 1.23CV (%) 18 16 8

Note: Means within columns for each comparison followed by the same letters are not significantlydifferent at p � 0.05. Coefficient of Variation (CV).

Figure 2. Effect of (A) N fertilizer at Sirinka and Kobo and (B) seedbed type by cultivarinteraction at Kobo on grain N uptake in sorghum. Means followed by the same letters are notsignificantly different at p �0.05.

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was not measured in this study, which would improve N recovery (Adu-Gyamfiet al., 1996).

At Kobo, the interaction effect of seedbed type by cultivar on grain N uptakeindicates significantly higher grain N uptake in the early maturing and high-yieldingcultivars (ICSV111 and 76T1#23) under both tied-ridge and flatbed planting (Figure2B). The highest grain N uptake of 71 kg ha71 was recorded for ICSV111 with tied-ridge planting, whereas the lowest grain N uptake of 23 kg ha71 was recorded forJigurti with flatbed planting. In Jigurti and 76T1#23, grain N uptake declined withflatbed planting compared with tied-ridge planting. The greater grain N uptake withtied-ridge planting could be due to the greater partitioning of N to the grain withgreater availability of moisture. The higher grain N concentration and grain Nuptake in ICSV111 and 76T1#23 could be due to a greater NHI and greater grainyield production. Results indicate that cultivars with high yield potentialaccumulated more N than the cultivar with a lower yield potential. Grain N uptakeis a function of grain yield and grain N concentration. Analysis of the log of grain Nuptake as a sum of the logs of grain yield and grain N concentration following themethod of Koutroubas and Ntanos (2003) showed that grain yield accounted for92% of the variation in grain N uptake among cultivars, whereas grain Nconcentration explained only 8% of the variation. From the limited data presentedhere it is suggested that selection for grain N uptake should be based primarily ongrain yield.

Significant differences in total nitrogen uptake were observed between N fertilizertreatments at both locations (Tables 2 and 3), between cultivars at Sirinka andbetween the seedbed type by cultivar interaction effect (Table 3) at Kobo. Data onthe N fertilizer effect indicate that significantly higher total plant N uptakes wererecorded with the application of 40 and 80 kg N ha71 (Figure 3A). Cultivars atSirinka differed in total N uptake in which Jigurti had significantly higher values(74 kg ha71) than ICSV 111 and 76T1#23, which had similar values (58 and 62 kgha71, respectively). The results of the seedbed type by cultivar interaction effect atKobo revealed that the three cultivars had statistically similar total N uptake undertied-ridge planting, but differed under flatbed planting. Under flatbed planting,ICSV111 and 76T1#23 had greater total N uptake than Jigurti (Figure 3B). The

Figure 3. Effect of (A) N fertilizer at Sirinka and Kobo and (B) seedbed type by cultivarinteraction at Kobo on total N uptake in sorghum. Bars followed by the same letter are notsignificantly different at p �0.05.

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highest and lowest total N uptakes of 101 and 76 kg ha71 were recorded for Jigurtiwith tied-ridge and flatbed planting, respectively.

Cultivar differences in N uptake could be due to differences in the resourcecapture system. Cultivars with an extensive root system will maintain nutrientuptake until maturity (Traore and Maranville 1999). Thus, the greater total Nuptake in the late maturing cultivar Jigurti could be due to the fact that it has moretime to take up N, similar to the findings of Sing et al. (1998), and perhaps has betterrooting depth and root distribution, which might have enabled plants to take up Nfrom different soil layers.

Stover and grain N concentration

Significant stover N concentration differences were observed between N fertilizertreatment and cultivar at both locations (Tables 2 and 3). Results on the effect of Nfertilizer on stover N concentration indicate that stover N concentration significantlyincreased with the application of N fertilizer. Stover N concentration increased from0.35 to 0.40% at Sirinka and from 0.52 to 0.73% at Kobo with the application of80 kg N ha71. Data on cultivar differences in stover N concentration indicate thatsignificantly higher stover N concentrations were recorded for ICSV111 (0.38 and0.67%) and 76T1#23 (0.41 and 0.69%). Jigurti, which yielded the highest biomass,had the lowest stover N concentration, which could be due to dry matter dilutioneffect, as suggested by Hons et al. (1986) for other sorghum cultivars. Thisobservation highlights the ability of Jigurti to accumulate greater stover biomass atlower shoot N concentration, a characteristic highly desirable in low-N environ-ments (Traore and Maranville 1999).

Significant differences in grain N concentrations were observed between Nfertilizer treatments at both locations, between cultivars at Kobo and between theinteraction effects of seedbed type by cultivar at Sirinka. Grain N concentrationincreased significantly from 1.29 to 1.42% at Sirinka and from 2.03 to 2.24% atKobo with N fertilization (Table 7). The observed increase in grain N concentrationwith N fertilization in this experiment was consistent with the findings of Traore

Table 7. Effect of nitrogen fertilizer and cultivar on stover (SNC) and grain (GNC) nitrogenconcentration.

Treatments

Sirinka Kobo

SNC (%) GNC (%) SNC (%) GNC (%)

N level (kg ha71)0 0.35b 1.29b 0.52b 2.03b40 0.34b 1.38ab 0.65a 2.22a80 0.40a 1.42a 0.73a 2.24aLSD0.05 0.042 0.106 0.103 0.171

CultivarJigurti 0.30b 1.39a 0.54b 2.04bICSV 111 0.38a 1.37a 0.67a 2.27a76T1#23 0.41a 1.34a 0.69a 2.17aLSD0.05 0.043 0.075 0.087 0.125CV (%) 18 8 20 8

Note: Means followed by the same letters are not significantly different at p � 0.05. Coefficient ofVariation (CV).

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and Maranville (1999). Cultivar differences in grain N concentration at Koboindicate that ICSV111 and 76T1#23 had significantly greater grain N concentration(2.27 and 2.17%, respectively) (Table 7). The higher grain N concentration in thehigh-yielding cultivars could be due to increased N absorption resulting fromincreased reproductive sink demand. The seedbed type by cultivar interactioneffect at Sirinka revealed that grain N concentration in Jigurti and ICSV111increased from 1.33 and 1.32% with tied-ridging to 1.46 and 1.42% with flatbedplanting.

A negative association between grain yield and grain N concentration has beenreported (Kamoshita et al. 1998), making the simultaneous improvement of thesetraits difficult. However, in our study, grain N concentration was not correlated withgrain yield, suggesting that cultivars exhibiting both high yield and high grain Nconcentration could be selected. Cultivars ICSV111 and 76T1#23 having both highergrain yield and high grain N concentration, for example at Kobo, support thisargument.

Nitrogen harvest index (NHI)

NHI differed between cultivars at Sirinka and between the seedbed type by cultivarinteraction effect at Kobo (Tables 2 and 3). Results on cultivar difference indicatethat ICSV111 and 76T1#23 had significantly higher NHI (0.70 and 0.72,respectively) compared with Jigurti (0.57). At Kobo also, NHI was significantlyhigher for ICSV111 and 76T1#23 under both tied-ridge and flatbed plantings (Table8). The highest NHI value of 74% was recorded for ICSV111 with tied-ridgeplanting, whereas the lowest NHI value of 28% was recorded for Jigurti with flatbedplanting. In all cultivars, NHI was higher with tied-ridge planting.

The lower NHI in Jigurti suggests that its lower NHI canceled out the advantageof greater N uptake. NHI was tightly and significantly correlated (r ¼ 0.99) with drymatter harvest index at both locations, indicating the association of N partitioningwith that of dry matter partitioning to the grain. This association is clearlydemonstrated by ICSV111 and 76T1#23, which had both higher NHI and HI. NHIalso had a significant negative association with number of days to maturity(r ¼ 70.99) and plant height at maturity (r ¼ 70.99) at Sirinka, which implies thattaller cultivars, which are usually late maturing, tend to partition less N to the grainthan early maturing short stature cultivars. In this study, a lower NHI was obtainedin the taller (281–294 cm) and late maturing [1459–1465 growing degree days(GDD)] cultivar Jigurti compared with the shorter (143–174 cm) and early maturing(1250–1289 GDD) cultivars ICSV111 and 76T1#23.

Table 8. Effect of seedbed type by cultivar interaction on NHI at Kobo.

Seedbed type

Cultivars

Jigurti ICSV111 76T1#23

Tied-ridge 0.55d 0.74a 0.71abFlat 0.28e 0.62cd 0.65bcLSD0.05 0.075

Note: Means followed by the same letters are not significantly different at p � 0.05.

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Nitrogen use efficiency

NUE for biomass production (NUEb) was significantly different between N fertilizertreatment and cultivar at Sirinka and between N fertilizer by cultivar interactioneffect at Kobo (Tables 2 and 3). Nitrogen fertilizer significantly influenced NUEb,which decreased significantly with increasing levels of N fertilizer. Values were159 kg dry mass kg71 N for the no fertilizer application, 149 kg dry mass kg71 N forthe application of 40 kg N ha71, and 137 kg dry mass kg71 N for the application of80 kg N ha71. The decrease in NUEb values with an increase in the levels of Nfertilizer agrees with the findings of Traore and Maranville (1999). According toBuah et al. (1998), this relationship generally occurs because plant N contentincreases proportionally more than dry matter production with increased fertilitylevels. Cultivar differences in NUEb revealed that the late maturing cultivar Jigurtihad significantly greater NUEb values (188 kg dry mass kg71 N) than ICSV111(132 kg dry mass kg71 N) and 76T1#23 (125 kg dry mass kg71 N). ICSV111 and76T1#23 did not significantly differ in NUEb. Jigurti derived its greater NUEb valuesfrom a more vegetative growth and lower N concentration in the shoot thanICSV111 and 76T1#23. At Kobo, NUEb was significantly different between the Nfertilizer by cultivar interaction effects where all cultivars produced the highestbiomass per unit absorbed N with no fertilizer application. NUEb with no Nfertilization ranged from 78 to 152 kg dry matter kg71 N, whereas with theapplication of 80 kg N ha71 it ranged from 71 to 107 kg dry matter kg71 N.

NUE for grain production (NUEg) was significantly different between Nfertilizer treatments at Kobo, between cultivars at Sirinka and between seedbedtype by cultivar interaction effects at Kobo (Tables 2 and 3). Results on N fertilizereffect on NUEg showed that a significantly higher NUEg value was recorded for nofertilizer application, although application of 40 and 80 kg N ha71 had similarvalues (Figure 4A). At Sirinka, NUEg was significantly different only betweencultivars where the highest significant NUEg values were recorded for 76T1#23 andICSV111, which had statistically similar values (55 and 52 kg grain kg71 N,respectively), whereas the lowest NUEg value was recorded for Jigurti (41 kg grainkg71 N). Results on the interaction effect of seedbed type by cultivar at Koborevealed that ICSV111 and 76T1#23 under both tied-ridge and flatbed planting hadsignificantly higher NUEg values (Figure 4B). The highest NUEg value of 35 kg

Figure 4. Effect of (A) N fertilizer and (B) seedbed type by cultivar interaction on nitrogenuse efficiency of grains (NUEg) at Kobo. Bars followed by the same letter are not significantlydifferent at p � 0.05.

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grain kg71 N was recorded for 76T1#23 with tied-ridge planting, whereas thelowest NUEg value of 14 kg grain kg71 N was recorded for Jigurti with flatbedplanting. For all cultivars, NUEg was higher with tied-ridge planting than withflatbed planting.

At both locations, the higher NUEg values for ICSV111 and 76T1#23 may relateto the high grain yield potential of these cultivars, which leads to a high reproductivesink demand for nitrogen. Buah et al. (1998) indicated that the physiologicalprocesses of carbohydrate partitioning and N metabolism are associated, thusgenotypes with differences in grain yield potential may have differences in Naccumulation and NUE. In this study, cultivars that had the highest grain yieldgenerally had the highest NUEg values, an indication of a positive relationshipbetween NUE and grain yield. This observation agrees with the results of Buah et al.(1998). The higher NUEg in ICSV111 and 76T1#23 may also be due to their higherNHI. In terms of NUEg, a 34 and 52% difference between the most and the leastnitrogen-efficient cultivars were found at Sirinka and Kobo, respectively. Thisindicates that success in increasing sorghum yield on poor N soils could be achievedby screening genotypes for NUEg.

Conclusion

The predominance of erratic rainfall and drought in most parts of north-easternEthiopia have refocused attention on the use of tied-ridging as a rainwater-harvesting technique to reduce drought risks in sorghum production. The results ofthis study, however, clearly demonstrated that tied-ridging at Sirinka was notbeneficial in a wet season like that of the study season. Therefore, alternative on-farm rainwater-harvesting techniques like, for example, conservation tillage andmulching, wherever they are feasible, need to be explored. At Kobo the use of tied-ridging with improved cultivars has improved the yield, nitrogen content, NHIand NUE.

As nitrogen fertilization increased both biomass and grain yields and NUEattributes, it is advisable that farmers in north-eastern Ethiopia should apply Nfertilizer to increase the yield and quality of sorghum. ICSV111 and 76T1#23 areimportant cultivars in north-eastern Ethiopia where farmers cannot afford to applylarge amounts of inorganic fertilizers, as these cultivars are N use efficient (NUEg)and give higher yields on nitrogen poor soils. The difference in NUEg betweenefficient and inefficient cultivars was large enough to indicate that success inincreasing sorghum yield on nitrogen poor soils could be achieved by screeninggenotypes for NUEg. NUEg did not show significant cultivar 6 N fertilizerinteraction effect and could be used as a selection criterion in developing newcultivars adapted to low soil fertility conditions.

From this study, it could be concluded that tied-ridging is not a panacea for amoisture-deficiency problem. Its effectiveness could be affected by rainfall amountand soil type as clearly seen by the difference in its performance at Kobo, which has arelatively lighter clay soil (45–47%) and lower amount of rainfall (524 mm), andSirinka, which has relatively clayey soil (58–60%) and higher rainfall (656 mm).Because of this, waterlogging was observed at Sirinka. Thus, whenever tied-ridging isdeemed to be used in sorghum production the soil type and rainfall amount of thearea needs to be given due consideration. To ensure the results the field experimentmust be repeated.

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