green-maize potential of hybrid and open-pollinated cultivars at varying levels of applied nitrogen:...

Green-maize potential of hybrid and open-pollinated cultivars at varying levels of applied

nitrogen: relationship with grain yield

SK Kim1,3, VO Adetimirin2*, ST Yoon1,4, MA Adepoju1 and BA Gbadamosi1

1International Institute of Tropical Agriculture, Oyo Road, Ibadan, Nigeria. 2Department of Agronomy, University of Ibadan, Ibadan, Nigeria.

Present address: 3International Agricultural Research Institute (IARI), Kyungpook National University, Taegu, South Korea. 4Department of International Agricultural Development, Dankook University, Chungchong Namdo,

South Korea.*To whom correspondence should be addressed ([email protected]; [email protected])

Abstract Two sets (white- and yellow-kernelled) of six cultivars of fi eld-maize, each com-prising three hybrids and three open-pollinated (OP) varieties, were evaluated for green-maize productivity under three levels of nitrogen fertilisation (0, 60 and 120 kg ha–1) at two locations in southwestern Nigeria. The relationship between green-maize and grain yield was also investigated. Numbers of marketable ears and marketable-ear yield were signifi cantly correlated. Green-maize traits and grain yields showed different responses to increased nitrogen fertiliser application: grain yield was signifi cantly greater at 120 kg ha–1 than at 60 kg ha–1 whereas marketable-ear yield was not signifi cantly changed by the higher rate. The hybrids showed higher green-maize and grain yield responses than the OPs to fertiliser application. At 60 kg ha–1 nitrogen application, marketable-ear yield averaged 0.620 kg m–2 and 0.567 kg m–2 for white hybrids and OPs, respectively, and 0.576 kg m–2 and 0.439 kg m–2 for yellow hybrids and OPs. Marketable-ear yield was signifi cantly correlated with grain yield for both the white and yellow cultivars. Regressions of grain yield on marketable-ear yield were signifi cant and such regression equations can thus be used to estimate market-able-ear yield potential from grain yield data. Copyright © 2008 John Wiley & Sons, Ltd

Key words: green-maize, grain yield, marketable-ear yield, nitrogen fertilisation

Introduction

Maize is the most important cereal crop in sub-Sahara Africa: in 2005, maize production in sub-Sahara Africa was estimated at 50.7 million tonnes of grain from 26.9 million hectares (FAO 2007). In many countries in Africa, physiologically immature fi eld-maize, known as green-maize, is harvested and consumed (after roasting or boiling as ‘corn on the cob’), pro-viding a major source of calories. In a study of fi ve fi eld-maize genotypes including early and late varieties, Osanyintola et al. (1992) reported that the best time to harvest green-maize was 20 days after silking, when kernel sugar content, succulence and tenderness were highest: this timing corresponds to the early to late milk stage. Empirical estimates of maize harvested

Tropical ScienceTrop. Sci. 2007, 47(4), 149–158Published online 25 February 2008 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/ts.208

150 SK Kim et al.

Copyright © 2008 John Wiley & Sons, Ltd Trop. Sci. 47: 149–158 (2007) DOI: 10.1002/ts

and consumed green in Africa are not readily available, but Fajemisin (1983) estimated that 60% of the maize grown in Nigeria is consumed green. Green-maize is also popular in other countries of West Africa, as well as in Central, East and Southern Africa.

In production systems where there are two annual growing seasons separated by a short dry season, as in the forest zone of West Africa, high humidity prevails during the short intervening dry season when the maize matures, and this does not favour the natural drying needed when maize is produced for grain. Artifi cial drying is uneconomic due to its high-energy requirement and the associated high cost. Thus, the prevailing humidity makes the fi rst season’s crop more suitable for production of green-maize than for grain production. In this and other production systems, green-maize is usually the fi rst farm produce to reach the market after the preceding dry season, and it therefore serves to break the hunger gap (Kim 1997). Its availability at a time when other crops are not ready for harvest, and the resulting higher profi t margin (Alimi and Alofe 1993), are other factors responsible for its popularity. Nitrogen, the most limiting element in tropical soils, has a profound infl uence on maize pro-ductivity. While there is concern in developed countries about pollution of groundwater with nitrates due to high fertilisation rates (Hirel et al. 2001), application of fertiliser in Africa, where most farmers are resource-poor, is usually sub-optimal. Optimum fertiliser application is required for commercially profi table production of cereal crops.

Despite the popularity of green-maize, estimates of maize yield from farms and experi-mental fi elds across the African continent are often reported as grain yield. This presupposes that grain yield potential can be substituted for green-maize potential. Maize produced for grain is harvested at physiological maturity, which is attained at about 53 to 61 days after silking (Hillson and Penny 1965). There appears to be no previous report on whether geno-types that optimise grain yield also show the same superiority with respect to green-maize yield, especially since the maturing maize kernels serve as a physiological sink beyond the time when they would be harvested for green-maize.

The International Institute of Tropical Agriculture (IITA) has developed many hybrid and open-pollinated (OP) maize cultivars with high adaptation and resistance to the major biotic constraints in West and Central Africa. The objectives of this study were to evaluate, under varying levels of nitrogen fertilisation, the green-maize potential of elite hybrid and OP maize cultivars, and to investigate the relationship between green-maize yield and grain yield.

Materials and methods

The experiment was conducted in 1992 at Ibadan (7°26′N 3°54′E, 150 m above sea level, 1250 mm annual rainfall, 75% mean relative humidity (r.h.) in the derived savanna zone and Ikenne (6°52′N 3°42′E, 60 m above sea level, 1450 mm annual rainfall, 80% mean r.h.) in the forest zone, representing the two ecological zones of southwestern Nigeria. The soil types were Gambari-series alfi sol at Ibadan and Alagba-series alfi sol at Ikenne. Pre-plant total soil nitrogen in both locations was below 1.0–1.5 g kg–1, the critical range for maize in Nigeria (Agboola 1972).

Relationship between green-maize and grain yield 151


Two sets of six maize genotypes developed at the IITA were used for the study: one set of white-kernelled cultivars and one set of yellow cultivars. Each set consisted of three hybrids and three OP cultivars. Each set was planted out in a split-plot design with four rep-lications. Sub-plots consisted of four-row ridges, each 5 m long. Row- and hill-spacing were 0.75 m and 0.30 m, respectively. Nitrogen levels and cultivars were randomised by main plots and sub-plots, respectively. Three levels of nitrogen were studied: 0 kg ha–1, 60 kg ha–1 (wholly as a basal application) and 120 kg ha–1 (half as a basal application and the other half applied 28 days after planting). The nitrogen source was calcium ammonium nitrate. Maize was planted on 22 April in both Ibadan and Ikenne. Two maize seeds were planted per hill, and thinned to one plant per hill 14 days after planting. To control weeds, Primextra (atrazine + metolachlor) and Gramozone (paraquat) were applied pre-emergence at the rate of 5 l ha–1 and 4 l ha–1, respectively.

Two of the four rows in each sub-plot were harvested as green-maize 20 days after silking, and the remaining two rows were harvested at physiological maturity for grain-yield deter-mination. Data collected for green-maize included total number of green ears, number of marketable ears and marketable-ear yield (without husk). Ears considered marketable had a minimum of 250 fi lled kernels. Grain moisture was determined on randomly selected ears from rows harvested for grain. Grain yield was computed on the basis of 80% shelling per-centage and 150 g kg–1 moisture (Kim and Adetimirin 1997). Analysis of variance was carried out using SAS software (SAS Institute, Cary, North Carolina). Signifi cantly different treat-ment means were separated using the Duncan’s multiple range test. A single degree of freedom orthogonal contrast was used to compare the two groups of cultivars (hybrids vs OPs).

Results and discussion

Initial analysis showed that, for both the white- and the yellow-kernel cultivar sets, location effects were not signifi cant and also that there were no signifi cant effects of interaction of location × nitrogen or location × cultivar. This is not surprising, since the climate at the two locations was similar and since the soil type at both sites was alfi sol with pre-plant nitrogen levels below the critical range for maize in Nigeria. The data from the two locations were therefore pooled for all statistical analyses reported here.

Mean marketable-ear yield and grain yield were, respectively, 6200 kg ha–1 and 3527 kg ha–1 for the white hybrids and 5667 kg ha–1 and 3080 kg ha–1 for the white OPs. Equivalent respective values for the yellow cultivars were 5763 kg ha–1 and 3312 kg ha–1 for hybrids and 4385 kg ha–1 and 2867 kg ha–1 for OPs.

The white hybrids had bigger and more uniform ears than the white OPs. Orthogonal comparison indicated signifi cant differences (p < 0.05) between the white hybrid and white OP groups for total number of green ears, marketable-ear yield and grain yield, but there was no signifi cant difference between the two groups for number of marketable ears (Tables 1 and 2). Across nitrogen levels and cultivars, the total number of green ears and marketable-ear yield of the white hybrids were, respectively, 5.8% and 9.4% higher than those of the

152 SK Kim et al.


Table 1. Numbers of green-maize ears (total ears and marketable ears) of hybrid and open-pollinated (OP) white maize cultivars at three rates of nitrogen application in southwestern Nigeria

Group Cultivar Nitrogen application rate (kg ha–1)

0 60 120 Mean Rank 0 60 120 Mean Rank

Total green ears (no. m–2) Marketable green ears (no. m–2)

Hybrid 8766-12 4.13 4.67 4.73 4.51 bc (3) 1.67 3.67 3.87 3.07 b (5)Hybrid 8505-5 4.53 4.93 4.93 4.80 a (1) 2.73 3.93 4.33 3.67 a (1)Hybrid 9021-18 4.53 4.47 4.60 4.53 ab (2) 1.87 3.80 3.80 3.16 b (3)

Mean of hybrids 4.61 A 3.30 A

OP IK83 TZSR-W 4.07 4.53 4.80 4.47 bc (4) 2.27 3.53 4.13 3.31 ab (2)OP TZPB-SR 4.13 4.33 4.80 4.42 bc (5) 2.00 3.40 3.87 3.09 b (4)OP EV 8443-SR 3.87 4.40 4.33 4.20 c (6) 2.00 3.27 3.73 3.00 b (6)

Mean of OPs 4.36 B 3.13 A

Mean per N application rate 4.21 b 4.56 a 4.70 a 2.09 c 3.60 b 3.96 a

Cultivar means followed by the same lower-case letter (abc) in the same column are not signifi cantly different at p < 0.05. Group means followed by the same upper-case letter (AB) in the same column are not signifi cantly different at p < 0.05. Means of all cultivars for each N application rate followed by the same letter (within each measure: total or marketable) are not signifi cantly different at p < 0.05.

Table 2. Marketable-ear yield (green-maize) and grain yield of hybrid and open-pollinated (OP) white maize cultivars at three rates of nitrogen application in southwestern Nigeria



Marketable-ear yield (kg m–2) Grain yield (kg m–2)

Hybrid 8766-12 0.373 0.693 0.700 0.589 a (3) 0.170 0.393 0.423 0.329 ab (3)Hybrid 8505-5 0.387 0.773 0.833 0.665 a (1) 0.164 0.437 0.499 0.367 a (1)Hybrid 9021-18 0.320 0.747 0.753 0.607 a (2) 0.152 0.474 0.463 0.363 a (2)


OP IK83 TZSR-W 0.307 0.640 0.720 0.556 a (6) 0.133 0.390 0.432 0.320 b (4)OP TZPB-SR 0.333 0.640 0.767 0.580 a (4) 0.132 0.354 0.400 0.295 b (6)

OP EV 8443-SR 0.300 0.640 0.753 0.565 a (5) 0.114 0.386 0.426 0.309 b (5)

Mean of OPs 0.567 A 0.308 B


Cultivar means followed by the same lower-case letter (ab) in the same column are not signifi cantly different at p < 0.05. Group means followed by the same upper-case letter (AB) in the same column are not signifi cantly different at p < 0.05. Means of all cultivars for each N application rate followed by the same letter (within each yield measure: marketable ear or grain) are not signifi cantly different at p < 0.05.



white OPs. Similarly, mean grain yield of the hybrids was 14.5% higher than that of the OPs. These results showed the white hybrids to be superior to the white OPs for yield of both green-maize and grain.

Nitrogen application to the white cultivars signifi cantly (p < 0.05) increased yields of both green-maize and grain. Application of 60 kg ha–1 gave increases of 7.6% in total numbers of green ears, 72.3% in numbers of marketable ears, 104.6% in marketable-ear yield (an increase of 3.52 t ha–1) and 180.0% in grain yield (an increase of 2.61 t ha–1), over the results from the non-fertilised treatment. A further increase in nitrogen application rate to 120 kg ha–1 caused a signifi cant (p < 0.05) additional increase of 9.9% in numbers of marketable green ears (Table 1) but did not increase marketable-ear yield signifi cantly (Table 2). However, the increased nitrogen application resulted in a signifi cant (p < 0.05) further increase in grain yield (8.6% more than the yield at 60 kg ha–1), though the limited benefi t of this yield increase would be unlikely to justify the increased costs of applying the additional fertiliser.

Differences among white cultivars were signifi cant for all traits except marketable-ear yield. Grain yields of the three white hybrids were, however, not signifi cantly different from each other, nor were those of the three white OP cultivars. At each of the nitrogen rates used, the cultivar with the highest green-maize and grain yield was always a hybrid while the lowest was always an OP cultivar. The cultivar × nitrogen rate interaction was signifi cant for market-able-ear yield and grain yield, indicating that the maize cultivars responded differently to nitrogen application rates for these traits.

In the set of yellow-kernelled maize cultivars, orthogonal comparison showed that the differences in mean performance of yellow hybrids and yellow OP cultivars were signifi cant (p < 0.05) for all traits (Tables 3 and 4). Across nitrogen levels and cultivars, means for hybrids exceeded those for OP cultivars by 14.4% for total number of green ears, by 17.9% for number of marketable ears, by 31.4% for marketable-ear yield and by 15.5% for grain yield. As with white kernel cultivars, the yellow hybrids had more uniform ears than the yellow OP cultivars.

All green-maize traits and grain yields of yellow cultivars at 60 kg ha–1 nitrogen applica-tion were signifi cantly (p < 0.05) higher than those in the unfertilised treatment. The increase was 21.0% for total numbers of green ears and 104.4% (an increase of 3.16 t ha–1) for mar-ketable-ear yield. The increases in numbers of marketable ears and grain yield, respectively, of 48.6% and 103.4% (an increase of 1.79 t ha–1) observed in the yellow cultivars were con-siderably lower than those obtained with the white cultivars following similar levels of nitrogen application. Green-maize traits for yellow cultivars at 120 kg ha–1 nitrogen applica-tion were not signifi cantly different from those at 60 kg ha–1. As with the white cultivars, the increase in nitrogen application from 60 to 120 kg ha–1 resulted in a signifi cant (p < 0.05) further increase in grain yield (an additional 12.1% compared with the 60 kg ha–1 treatment) of the yellow cultivars, though this would probably not give suffi cient economic benefi t to justify the cost of the additional fertiliser application.

The interaction of variety × nitrogen rates was signifi cant for marketable-ear yield and grain yield of the yellow cultivars, as indicated by changes in ranking of the cultivars at dif-ferent application rates in Table 4. There were no signifi cant differences among the three

154 SK Kim et al.


yellow hybrids for any of the green-maize traits or for grain yield. However, TZEE-Y, an early-maturing OP cultivar, produced signifi cantly lower numbers of ears and yields of green-maize and grain than the other yellow cultivars.

Numbers of marketable ears, marketable-ear yield and grain yield were signifi cantly (p < 0.01) correlated with one another, with correlation coeffi cients of 0.96 to 0.99 for the white cultivars, and 0.80 to 0.93 for the yellow cultivars (Table 5). For the white cultivars, the numbers of green ears were not signifi cantly correlated with the other green-maize traits and grain yield but signifi cant correlations were found between the numbers of green ears and the other traits in yellow cultivars. The regression equation of grain yield (Y) on market-able-ear yield of green-maize (X), both in kg ha–1, was Y = –922.3 + 0.711 X (R2 = 0.98) for the white cultivars, and Y = 305.8 + 0.543X (R2 = 0.80) for the yellow cultivars. The regres-sion equation Y = –929.7 + 0.214X (R2 = 0.93) describes the relationship between market-able-ear yield in kg ha–1 (Y) and number of marketable ears per hectare (X) for white cultivars, while Y = –2613 + 0.250X (R2 = 0.86) describes the relationship between these traits for yellow cultivars.

This study provides information on the relationship between green-maize and grain yield in fi eld-maize. The different hybrids of the same kernel colour used in the present study exhibited similar yield potential either as green-maize or as grain, and the ranking of geno-types in respect of these traits was thus of limited consequence. The high R2 values of the regression equations describing the relationship between green-maize (number and yield of

Table 3. Numbers of green-maize ears (total ears and marketable ears) of hybrid and open-pollinated (OP) yellow maize cultivars at three rates of nitrogen application in southwestern Nigeria



Total green ears (no. m–2) Marketable green ears (no. m–2)

Hybrid 8522-2 3.60 4.40 4.73 4.24 ab (2) 2.60 3.80 3.93 3.44 ab (2)Hybrid 8644-27 3.87 4.13 4.40 4.13 abc (3) 2.20 3.53 3.80 3.18 abc (3)Hybrid 8644-31 3.87 4.73 4.47 4.36 a (1) 2.93 3.73 3.73 3.47 a (1)


OP ACR85 TZSR-Y 3.40 4.47 4.07 3.98 bc (4) 1.87 3.80 3.33 3.00 c (5)OP Suwon-1-SR 3.60 4.13 3.87 3.87 c (5) 2.80 3.53 2.87 3.07 bc (4)OP TZEE-Y 2.67 3.53 3.67 3.29 d (6) 1.87 2.80 2.80 2.49 d (6)

Mean of OPs 3.71 B 2.85 B

Mean per N application rate 3.50 b 4.23 a 4.20 a 2.38 b 3.53 a 3.41 a

Cultivar means followed by the same lower-case letter (abcd) in the same column are not signifi cantly different at p < 0.05. Group means followed by the same upper-case letter (AB) in the same column are not signifi cantly dif-ferent at p < 0.05. Means of all cultivars for each N application rate followed by the same letter (within each measure: total or marketable) are not signifi cantly different at p < 0.05.



marketable ears) and grain yield indicate that green-maize yield could be reliably estimated from grain yield and vice-versa, using these equations. Although harvesting for green-maize occurs four to six weeks before physiological maturity, grain yield determination is a more convenient measure for comparison, because cultivars are harvested for grain at the same time whereas, for green-maize, cultivars are harvested at different times depending on the date when 50% or 75% silking is attained. This is possibly why grain yields are widely

Table 5. Correlation coeffi cients (r) among green-maize traits and grain yield of white and yellow groups of cultivars across all nitrogen application rates (n = 18)

Kernel colour No. of marketable ears Marketable ear yield Grain yield

No. of green ears WhiteYellow

0.44NS

0.91**0.28 NS

0.88**0.27 NS

0.72**No. of marketable ears White

Yellow0.96**0.93**

0.96**0.80**

Marketable ear yield WhiteYellow

0.99**0.89**

** Signifi cant at p < 0.01. NS Not signifi cant.

Table 4. Marketable-ear yield (green-maize) and grain yield of hybrid and open-pollinated (OP) yellow maize cultivars at three rates of nitrogen application in southwestern Nigeria



Marketable-ear yield (kg m–2) Grain yield (kg m–2)

Hybrid 8522-2 0.387 0.687 0.720 0.598 a (1) 0.212 0.386 0.380 0.326 a (2)Hybrid 8644-27 0.353 0.713 0.727 0.598 a (2) 0.166 0.423 0.376 0.322 a (4)

Hybrid 8644-31 0.353 0.633 0.613 0.533 ab (3) 0.168 0.354 0.472 0.331 a (1)


OP ACR85 TZSR-Y 0.247 0.693 0.633 0.525 b (4) 0.179 0.386 0.400 0.322 a (3)OP Suwon-1-SR 0.260 0.600 0.513 0.458 c (5) 0.142 0.357 0.413 0.304 a (5)OP TZEE-Y 0.213 0.380 0.407 0.333 d (6) 0.172 0.205 0.327 0.235 b (6)

Mean of OPs 0.439 B 0.287 B


Cultivar means followed by the same lower-case letter (abcd) in the same column are not signifi cantly different at p < 0.05. Group means followed by the same upper-case letter (AB) in the same column are not signifi cantly dif-ferent at p < 0.05. Means of all cultivars for each N application rate followed by the same letter (within each yield measure: marketable ear or grain) are not signifi cantly different at p < 0.05.

156 SK Kim et al.


reported as the main measure of maize production, even in areas where green-maize is more popular, such as the forest ecological zone of West Africa. A standard practice in green-maize marketing is the sorting of harvested ears, in which ears with poorly fi lled kernels are rejected as unmarketable. Harvested ears are rarely weighed, either before or after sorting. The number of marketable ears is, therefore, the most practical index of green-maize productivity, and the results of this study show that it can reliably be used to estimate marketable-ear yield.

The nitrogen application rate of 120 kg ha–1 at which increased response was obtained for grain yield in this study is similar to the rate of 100 kg ha–1 reported by Kogbe and Adediran (2003) for the derived savanna in Nigeria. However, previous studies in sub-Sahara Africa have not reported yields of green-maize, and they have therefore not addressed nitrogen fer-tiliser requirements for this crop. A major fi nding of the present study, conducted in forest and derived savannah zones, is that the responses of green-maize traits and grain yields to nitrogen fertiliser rates are different. The responses of green-maize traits to the higher nitro-gen application (120 kg ha–1) were weak and non-signifi cant in white cultivars, and negligible in yellow cultivars, whereas grain yields showed a moderate and signifi cant response to this higher fertiliser rate in white and yellow cultivars. Although the gain in grain yield at the higher rate in this study was probably not suffi cient to justify the increased cost of the addi-tional application of fertiliser, the practical implication of these results is that calculations of optimal rates of nitrogen fertiliser to be applied to maize should take into consideration whether the crop is to be used for fresh consumption or for grain.

In many countries of sub-Saharan Africa, OP cultivars are more widely grown than hybrids. The results of this study indicate that the hybrids evaluated were more productive in terms of both green-maize and grain than the OP varieties. The higher productivity of hybrids was manifested at nitrogen application rates of 0 and 60 kg ha–1 for marketable-ear yield and at all three nitrogen application rates for of grain yield. The concept of nitrogen-use (N-use) effi ciency, defi ned as crop yield per unit of nitrogen (Moll et al. 1982), has hitherto been examined in maize only in terms of grain yield. However, the principle can be extended to green-maize yield. In Africa, where fertilisers are not readily available and affordable, the best cultivars would be those which, in addition to being N-use effi cient under nitrogen-stress conditions, are responsive to nitrogen fertilisation. Presterl et al. (2003) noted that N-use effi cient cultivars would be especially benefi cial for low-input production systems. For green-maize production, examples of such cultivars in the present study are white hybrids 8505-5 and 9021-18 and yellow hybrids 8644-31 and 8522-2. In relation to grain maize, Akintunde et al. (1993) and Kogbe and Adediran (2003) also reported higher N-use effi ciency for hybrids compared with OPs in various ecological zones in Nigeria. N-use effi ciency is the product of nitrogen uptake effi ciency and nitrogen utilisation effi ciency (Gallais and Hirel 2004). These two components of N-use effi ciency are infl uenced by: root morphology and extension; biochemical or physiological mechanisms regulating nitrate uptake and assimilation; and redistribution and transport of nitrate to different plant parts (Beauchamp et al. 1976; Pollmer et al. 1979; Rizzi et al. 1993; Gallais and Hirel 2004). The hybrids used in this study need to be studied in greater detail with respect to these components of N-use effi ciency.



The white hybrids had higher green-maize yields and grain yields than the yellow culti-vars. In Africa, more resources have been devoted to the breeding of white-kernel cultivars because white maize used to be preferred over yellow maize in most African countries (Kim et al. 1985). Seed companies have, since 1984, marketed two of the hybrid cultivars tested in this study, namely 9021–18 (previously coded 8321–18) and 8644–27, marketed as Oba Super-1 and Oba Super-2, respectively. Among several factors that have encouraged seed companies to promote these cultivars are: their uniformly big ears; tolerance to abiotic stresses such as drought and low soil nitrogen; and better resistance than the widely grown OPs to major biotic constraints (Kim et al. 1993). The high green-maize yields of 8505–5 and 8644–31 are indicative of their potential for commercialisation. TZEE-Y, an early OP variety, showed the lowest green-maize potential among the cultivars tested. However, this variety is ready as green-maize two weeks ahead of other cultivars, and the higher price it commands at that time may compensate to some degree for the low marketable-ear yield.

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