Maize (Zea mays L.) is the world’s widely grown cereal crops and primary staple food crop in many developing countries [1]. Globally, it was known as “Queen” of cereals because it has the highest genetic yield potential among the cereals www.syngenta.co.in/corn. Maize is an important grain crop of the world and it ranks 3^rd after wheat and rice in area basis and total production. 

In Ethiopia the result of the year 2015/16, meher season post-harvest crop production survey indicate that the total land areas of about 12,486,270.87 hectares were covered by grain crops out of which 79.88% (9,974,316.28 hectares) was under cereals and maize covered 16.91% (2,111,518.23 hectares) and gave 7151712.25 tons of grain yields [2]. 

Maize is one of the most important crops in Ethiopia in general and in Oromia region in particular. In this region, maize grain was produced on 368,453.46 hectares, which accounts 52.57 % of land covered by grain crop with a yield productivity of 1296 kg ha^-1[2]. In Jimma zone out of the total area allocated for cereal crop 451,467.03 hectare of land maize accounts 33.8% (152,697.09) hectare of land is covered by maize by private farmer [2]. Despite the large area under maize cultivation the national and as well as the zonal maize productivity is lower than the nationwide [3].

Past research, efforts in Ethiopia resulted in the development, release of open-pollinated, and hybrid varieties for different agro-ecologies of the country [4]. However, the national average yield, 3387 kg ha^-1 is still far below the world average 5.5 tons ha^-1[2]. The yield of maize is less in Ethiopia (3387 kg ha^-1) when compared to other countries like Egypt (8000) kg ha^-1, Italy (8899) kg ha^-1, Canada (10000) kg ha^-1, Argentina (8290) kg ha^-1and China (5890) kg ha^-1[3]. The yield gap is attributed to a number of factors like frequent occurrence of drought, declining of soil fertility, poor agronomic practice, limited use of input, poor seed quality, disease, and others [5].

Yield is a function of genotypes, environments, and crop management. Fertilizer management is crucial for maize cultivation [6]. Among the fertilizers, N is very important because this element is responsible on major activities for growth and development of maize crop [7]. According to Guo et al. [8], N is the most yield-restraining nutrient in crop production globally. Maize being the heavy feeder crop, a balanced dose of organic and inorganic application of fertilizer is needed for increased productivity.

Fertilizer recommendations in Ethiopia are based either on nationwide blanket recommendation or extrapolating results of a specific research center [9], irrespective of inherent soil properties of specific areas. Consequently, it is impossible to use single recommendation for all conditions, because the optimum fertilizer requirement varies depending on environmental factors such as soil fertility, moisture supply, genotype, planting date, planting pattern etc. [10]. 

Reports by IFPRI [11] also emphasized lack of site-specific fertilizer recommendations as among the major challenges of soil fertility and crop production in Ethiopia. This is the evidence that a wide range of N-fertilizer rates for maize were reported for various places of Ethiopia. Farmer of study area is widely cultivating the conventional maize (CM) varieties such as Shone, BH661, and BH554 etc. These conventional maize varieties are deficient in two essential amino acids, lysine, and tryptophan. For this reason currently different organization are working toward production of quality protein maize (QPM) varieties for their enhanced nutritional quality [12]. 

It is assumed that production of quality protein maize with appropriate nitrogen fertilizer rate is one way to increase maize yield and achieve nutritional requirements. The maize grower in the study area did not start using quality protein maize seed at wider scale except those which are, used in different trial sites by various organizations such as Agricultural Research center, Universities, and non-governmental organization.

Therefore, the current experiment was conducted having the following objectives:

• To determine the best combination of nitrogen fertilizer rate and maize hybrids, that yields optimum production

• To evaluate the agronomic performance of each maize hybrids under investigation in response to N-fertilizer application in the given agro-ecological setting

Description of Study Area

The study was conducted in Jimma Agricultural Research Center (JARC) under irrigated condition in 2016/17. The Center was one of Ethiopian Institute of Agricultural Research Center (EIAR) located in Manna woreda, Jimma zone, Oromia Region. It was located at 365 km south west of Addis Ababa and has an altitude of 1753 m a.s.l. The area has maximum and minimum temperature of 28 ⁰c and 9 ⁰c respectively. The climate is humid tropical with bimodal rainfall, ranging from 1200 to 2800 mm [13]. In normal years, the rainy season extends from February to early October. The dominant soil type of the study area is dominated and characterized as Cambsol and Nitsol, respectively [14]. Maize (Zea mays L.), Tef (Eragrostis tef), Sorghum (Sorghum bicolor), Coffee (Coffea arabica), and Enset(Enset ventricosum) are the most important crop dominantly growing in the study area.Soil Physico-

Chemical Properties of Study Area

Selected physical and chemical properties of the soil were analyzed from the surface composite soil sample (0-30 cm) taken from the experimental field before planting as indicated in Table 1. The soil in study area has apH value of 5.53, which is moderately acidic. According to FAO [15] classification, the soil pH categorized as extremely acidic if soils falls below 4.6, strongly acid if pH ranges between 4.6-5.5 and moderately acidic if pH is between 5.6-6.5. Organic matter content of soil in the study area was 6.11 %. According to Berhanu [16] the organic matter content of the soil was high. Berhanu [16] classified soils with >5.20, 2.6-5.2, 0.8-2.6 and <0.8% organic matter content as high, medium, low and very low, respectively.

Total Nitrogen of soil in study area was found to be 0.24%, which is classified as moderate according to Tekaligne.

Exchangeable Potassium was 1.96 (cmol (+) kg^-1soil which is considered as high. According to the classification of Metson, 0–0.1, 0.1–0.3, 0.3–0.7, 0.7–2.0, >2 which are categorized as very low, low, moderate, high, and very high respectively. Available Phosphorus was 10.67 mg P kg^-1soil that is medium P content. According to Olsen et al. [18] p rating (mg kg^-1 soil), p content of <3 is very low, 4 to 7 is low, 8 to 11 is medium, and >11 is high. The cation exchange capacity of soil in study area was 25.2 cmol (+) kg^-1 which is high CEC. According to Landon classification CEC of <6, 6-12, 12-25, 25-40, >40 cmol (+)/kg very low, low, moderate, high and very high. In addition, soil texture of study area was 14.9% silt, 47.4% clay, and 37.7%, which is classified under textural class of clay.

Table 1: Selected Physical and Chemical Property of Soil in Study Area

CEC = Cation Exchange Capacity

Experimental Materials

The most important experimental material that was used during the experiment is two-quality protein maize (BHQPY545 and MH138Q) and one conventional hybrid (shone) that is widely used by farmers in study area. BHQPY545 was released in 2008 by Bakko Agricultural Research Center and adapted to an altitude of 1000-1800 m a.s.l. Its yield potential was 8.0-9.5 and 5.5-6.5 t/ha on research station and farmers field respectively. MH138Q was released by Melkasa Agricultural Research Center in 2012 and adapted to an altitude of 1000-1800 m a.s.l. Its yield potential was 7.5-8.0 and 5.5-6.5 t/ha on research station and farmers field, respectively [12]. Urea (46% N) and triple super phosphate (20% P₂0₅) were used as sources of N and P, respectively.

Treatments and Experimental Design

The experiment has four nitrogen level (0 50, 100 and 150 kgha^-1designated as N₀, N₅₀, N₁₀₀, and N₁₅₀and three maize Hybrid, BHQPY545, MH138Q and shone (PHB39G19) designated as H₁, H₂, and H₃, respectively. The experiment was laid in a randomized complete block design (RCBD) double folded in a 3 by 4 factorial arrangement with three replications. The gross size of each plot was 4m x 3.75m which has gross area of 15 m² and net area of (3m x 3m )9m² with total six rows of plants and among which, four was central rows from which important data were collected. Spacing between blocks and plots was1.5 m each. The spacing between rows and plants were 75 cm and 25cm, respectively Table 2.

Table 2: Treatment Combination between Nitrogen Fertilizer Rate and Maize Hybrids

N = Nitrogen Fertilizer and H = Hybrid Variety

Experimental Area Management

The experimental site was oxen ploughed and left for 7 days exposed to the sun, then second and third plough was also done by oxen then crushed by man power and any trash was cleared from an area, and leveled out to maintain a well leveled seed bed and then followed by ridge preparation between rows. Individual plot size was 3.75 × 4 m consisting of six ridges and then plots was grouped to six blocks each with six plots. The planting was performed on February3, 2017 and harvested on July 15, 2017. Sowing was done manually on the ridge, two seeds of maize per hill was planted to ensure uniform emergence of seed at75 cm spacing between row and 25 cm between plants and later thinned to one plant to maintain maximum population limit. The plots were irrigated immediately after sowing and thereafter at intervals of 7 days and twice a week during flower initiation and silk formation.

The application of nitrogen fertilizer was in three-splits in accordance to the treatment rate. i.e. at time of planting, knee height and booting stage due to high evaporation capacity and leaching capacity of nitrogen fertilizer under irrigated agriculture and phosphorus at rate of 69 Kg ha^-1. Plot was then hand weeded as much as needed until crop was fully matured and all the data is completed.

Crop Data Collection and Measurements

Growth Parameter

• Ear Length (EL): This was recorded in cent meters from the measure average ten randomly taken ears per net plot area at harvest and measured in centimeter

• Plant Height (Ph): Plant height was measured in centimeter as the length of maize from ground level to the point where the tassel starts to branch. Five plants were sampled randomly from the net plot and their mean was recorded as plant height (cm)

• Leaf Area (La): All available leaves of five plants per net plot were considered for measurement at 50% silking stage and their leaf length and width were measured and the LA was calculated using methods described by McKee as: Leaf area (LA) = Leaf length (cm) x Maximum width of leaf (cm) x 0.733

• Leaf Area Index (Lai): This was calculated as the ratio of total leaf area per five plants (cm²) divided by the area of land occupied by these plants

Yield and Yield Components

• Number of Ears Per Plant (Nepp): This was recorded from the count of all existing ears from a given plant from net plot area at harvest

• Number of Kernels Per Ear (NKPE): It was recorded from five randomly taken ears from net plot area

• Hundred-Kernel Weight (HKW): Measured in gram and100kernels was counted from a bulk of shelled grains of each net plot area of the treatment, weighed using sensitive balance, and adjusted to 12.5% moisture status

• Grain Yield (GY): This was determined from the net harvestable area, the yield adjusted to 12.5% moisture content, and the result was converted to kg ha^-1

• Above Ground Dry Biomass Yield (AGDBMY): This was recorded from five plants at harvest by drying in oven for 24 hrs at 84⁰Cto a constant weight and the result converted to kg ha^-1 basis

• Harvest Index (HI): The harvest index was calculated as the ratio of grain yield to the above ground dry biomass yield of the same expressed in percent

Statistical Data Analysis

Data were subjected to analysis of variance using general linear model (GLM) procedures of SAS 9.1.3. The differences between treatment means was compared using Least Significant Difference (LSD) test at 5% level of significance when the ANOVA showed the presence of significant difference.

Growth Parameter

Plant Height (PH): The analysis of variance revealed that the main effect of nitrogen fertilizer rate and maize hybrids produced highly significant difference (p<0.01) on PH. However, the interaction effect between nitrogen fertilizer and maize hybrids turned insignificant.

Accordingly, significantly tallest plant 249.4 cm was obtained from the application of 150 kg ha^-1 nitrogen although it was in par with 100kg ha^-1 of N application 241.7cm and the shortest 210.2cm from the treatment that received 0 kg ha^-1. However, there was no statistically significant difference in PH for the maize treated with nitrogen rate of 100 and 150 kg ha^-1 Table 3. As nitrogen fertilizer rate increase from 0-50, 0-100, and 0-150 kg ha^-1 plant height was improved by 7.11, 13.03, and 15.72 %, respectively. The increase in plant height with respect to increased nitrogen fertilizer application attributed to maximum vegetative growth of the plants under higher nitrogen availability. This is due to the increase in cell elongation as nitrogen is essential for plant growth process including improvement in chlorophyll content that is responsible for dark green color of stem and leaves, which enhance vigorous vegetative growth, branching and tillering [20].

This result was in conformity with the finding of Mitiku and Asnakech [21] reported that plant height significantly increases from 347.33 cm to 360.66 cm as nitrogen rate increase from 0 kg ha^-1 to 92 kg ha^-1. Adamu et al. [22] reported significant differences in maize plant height after varying nitrogen fertilizers levels, the control treatment showed the lowest plant height while the highest level of nitrogen was related with the longest plant height. 

Similar result was also reported by Masresha [23] who observed that as nitrogen rate increase from 0 kg ha^-1 to 96 kg ha^-1 plant height increase from 209 cm to 228 cm. In contrast to present study, Sadeghi and Bahrani reported that increase in nitrogen fertilizer rate had no significant effect on plant height. Considering the main effect of hybrids, MH138Q was found to be significantly shortest and hybrid Shone was the tallest having 238.54cm and 221.96 cm PH, respectively Table 3. PH of Shone and BHQPY545 were found at par Table 3. The present result was in agreement with the finding of Ibrahim et al. [24] who observed that presence of significant difference in plant height among crops varieties and concluded this difference was due to differences in crops genetic constitution to growth indices.

The difference in plant height of maize hybrids might be related to inherent genetic factor [25]. Enujeke [26] also concluded that differential growth of maize with respect to plant height observed among the varieties attributed to differences in genetic characteristics of the individual varieties, including growth rates.

Table 3: Main Effect of Nitrogen Fertilizer Rate and Maize Hybrids on Plant Height (PH) Under Irrigated Condition

*Means in Columns Followed by the Same Letters Are Not Significantly Different at 5% Level of Significance, PLH = Plant Height, LSD = Least Significant Difference at 5% Probability Level, CV (%) = Coefficients of Variation in Percent

Leaf Area (LA)

Leaf area (LA) and leaf area index (LAI) are very important plant growth parameters because effectiveness of photosynthesis depends on large and efficient assimilating area, an adequate supply of solar energy and CO₂ and favorable environmental conditions [27]. Analysis of variance for LA (p<0.01) showed that presence of highly significant difference due to main effect of nitrogen fertilizer rate. The main effect of maize hybrids and the interaction effect of nitrogen fertilizer rate and maize hybrids was not significant.

Accordingly, significantly largest LA 6833.4 cm² were obtained from the highest nitrogen fertilizer application rate of 150 kg ha^-1) and the smallest LA 4501.6 was obtained from the ni treatment. Table 4 In general, for nitrogen fertilizer there was significant increase in LA at each level of nitrogen fertilizer rate. Compared to the control treatment the average leaf area was increased by 14.81, 27.25, and 34.12 % up on application of 50, 100, and 150 kg N ha^-1, respectively. From this result, it is evident that increased leaf area under increased nitrogen rate application attributed to increased rate of nitrogen fertilizer promotes the large number of green leaf per plant and vigorous vegetative growth in crop plant [28]. Likewise, plants deficient in nitrogen fertilizer showed smaller number of green leaf per plant. According to Devi et al. [29], nitrogen affects plant growth and productivity by helping the crop to have a better root growth and establish vigorous root system enabling the plant to mobilize soil moisture and nutrients more efficiently, alter leaf area photosynthetic capacity through increased plant height and girth growth and secure better canopy structure.

The present result was in agreement with the finding of Imran et al. [30] report that increasing nitrogen application from zero to210 kg ha^-1, leaf area increased from 1973 cm² to 2757 cm² in maize linearly and significantly. Likewise, Debebe [31] also report that that as nitrogen rate increase from controlled to 105 kg ha^-1plant height, ear height, leaf area and leaf area index of maize increased significantly. Moges [28] also observe that smallest (6605.30 cm²) from the control treatment and the largest leaf area (7222 cm²) from application of 128 kg ha^-1 nitrogen fertilizer.

Table 4: Main Effect of Nitrogen Fertilizer Rate on Maize Leaf Area (LA) and Leaf Area Index (LAI) Under Irrigated Condition

Means in Columns Followed by the Same Letters Are Not Significantly Different at 5% Level of Significance, LA = Leaf Area, LAI = Leaf Are Index, LSD = Least Significant Difference at 5% Probability Level, CV (%) = Coefficients of Variation in Percent

Leaf Area Index (LAI)

The ratio of average plant leaf to the area covered by the same plant (leaf area index) is an important growth index that indicate average leaf exposed to sun radiation per unit of area covered by the plant [32]. Under constant ground area, leaf area index is directly proportional to the leaf area. The analysis of variance at revealed that the presence of highly significant difference (p< 0.01) in LAI in response to the main effect of nitrogen fertilizer rate. However, the main effect of hybrids and, the interaction effect of nitrogen fertilizer rate and maize hybrids was not significant. 

The highest leaf area index 3.6 is recorded from the highest nitrogen rate of nitrogen fertilizer (150 kg ha^-1) and the smallest small leaf area index 2.4 is obtained from the control treatment Table 4. An increasing trend in LAI was observed with increased N application rates. The increase in LAI was possibly due to the improved leaf expansion in plants by giving optimum nitrogenous fertilizers. Similar to this finding, Haghighi et al. [33] reported an increasing trend in LAI on maize due to an increase in N fertilizer application rates. In line with the finding of Dinh et al. reported that increasing N application from zero to 240 kg ha^-1increased leaf area index of maize from 3.23 to 4.65. In the same way Jasemi et al. [34] concludes that higher LAI associated with the rate of nitrogen fertilizer application.

Yield Component

Number of Ear per Plant (NEPP): Analysis of variance for NEPP (p<0.01) indicate that highly significant effect nitrogen fertilizer rate. The main effect of hybrids and the interaction effect of nitrogen fertilizer rate and maize hybrids were not significant. Accordingly, the lowest 0.99 and the highest 1.35, NEPP was obtained from 0 and 150 kg ha^-1 nitrogen fertilizer, respectively Table 5. 

In this study, there was consistent increment in number of NEPP with increasing nitrogen fertilizer rate. The increased NEPP with increasing application rate of nitrogen fertilizer might be due to better availability of nutrient for translocation of photosynthetic product to the final sink while the lowest NEPP at the lowest dose of nitrogen was due to deficient of nutrient that resulted in unproductive ears. Amanullah [35] conclude that successive increment in NEPP with increasing rate of nitrogen fertilizer application might be due increased nitrogen levels which in turn increased the yield attributes initiation and uptake other micro nutrient by enforcing the plant to have high of root. This increase translocation of photosynthetic materials from source to sink.

In line with the previous findings, Masresha [23] reported highest number of ear per plant of maize (1.39) and the lowest number ear per plant (1.18) from 96 Kgha^-1and zero kg ha^-1nitrogen fertilizer application, respectively. Similar result was also reported by Alizade et al. who found highest number of ears per plant, grain yield, biological yield, grain weight, number of grains per ear, number of grains per row, and harvest index with the application of 250 kg ha^-1 nitrogen fertilizer.

Table 5: Main Effect of Nitrogen Fertilizer Rate on Number of Ear per Plant (NEPP) of Maize under Irrigated Condition

Means in Columns Followed by The Same Letters Are Not Significantly Different at 5% Level of Significance. NEPP = Number of Ear per Plant, LSD = Least Significant Difference at 5% Probability Level, CV (%) = Coefficients of Variation in Percent

Ear Length (EL)

Ear length was an important yield component that directly influenced the magnitude of maize yield. Analysis of variance for EL (p<0.01) showed the presence of highly significant difference on NEPP due to the main effect of maize hybrids and the nitrogen fertilizer. The interaction between maize hybrids and nitrogen fertilizer rate was not significant.

Accordingly, the longest EL 15.93 cm was obtained from application of 150 kg ha^-1 and the shortest EL 13.70 obtained from 0kg ha^-1 Table 6. Generally, there was an increase in ear length as the nitrogen rate increase from zero to 150 kg ha^-1.

Characters linked to ear length were positively influenced by nitrogen fertilization because nitrogen fertilizer influenced the division and expansion of cell and photosynthetic process [36]. In agreement with this study, Raouf and Reza [37], and Turgut [38] observed significant difference in ear length in response to different nitrogen fertilizer rate application.

Similar result was also reported by Maral et al. [20] who observed the presences of significant increase in ear length of maize from 10.17 to 15.69 cm due to increasing level of nitrogen fertilizer from 50 to 200 kg ha^-1, respectively. More ever, the research finding from Okumura et al. [36] and Santos et al. [39] proved the existence of significant difference in maize EL up on the application of varied rate of nitrogen fertilizer. In contrast to the present result, Cruz et al. [40] and Fernandes et al. [41] report that EL was not significantly influenced by nitrogen fertilizer rate.

The data regarding the EL of maize hybrids reveal that longest EL 15.72 cm is obtained from the hybrid shone and the shortest EL 14.14 were obtained from the hybrids, MH138Q Table 6. It is observed that significantly different EL between maize hybrids might be genetic factor. In line with the present finding, Min N [42] reported that differential cob length among maize genotypes attributed to genetic factor. Abuzar et al. [43] and Kandil also reported significant differences in EL among different varieties of maize.

Table 6: Main Effect 0f Nitrogen Fertilizer Rate and Maize Hybrids on Ear Length (EL) and Number of Kernel per Ear (NKPE) of Maize under Irrigated Condition

Means in Columns Followed by the Same Letters Are Not Significantly Different at 5% Level of Significance, NKPE = Number of Kernel per Ear EL = Ear Length, LSD = Least Significant Difference at 5% Probability Level, CV (%) = Coefficients of Variation in Percent

Number of Kernel Per Ear (NKPE)

The analysis of variance (p<0.01) revealed that the main effect due to nitrogen fertilizer rate and maize hybrids on the NKPE was highly significant. However, the interaction effect of nitrogen and the maize hybrids was not significant. 

The highest NKPE 569.90 and lowest 476.82 obtained from zero and 150 kg ha^-1nitrogen fertilizer application, respectively. The application of N fertilizer at the rate of 

50 and 100 kg ha^-1produced statistically in significant result in NKPE. Kernel number was improved by 7.06, 9.09, and 16.33 % with increment of nitrogen fertilizer rate from zero to 50,100, and 150 kg ha^-1, respectively Table 6. The increase in NKPE of maize in the N fertilized probably may be attributed to less competition among maize plants for nitrogen, under better nitrogen supply and the plants therefore accumulated more biomass and partitioned more dry matter into the reproductive parts (ears) because of its higher LAI [35]. 

Similar findings were also reported by Geremew [44] reported that highest number of ear per plant, grain rows per ear, ear diameter, ear length and thousand kernel weight was recorded from application of 184 kg N ha ^-1. 

The data analysis also revealed that maize hybrid had significant (p<0.05) effect on NKPE. BHQPY545 had significantly highest NKPE 540.3 than MH138Q which had the lowest 496.3 NKPE, but was in par Shone hybrid Table 6. The difference in NKPE of maize hybrid might be attributed to the difference in genetic makeup among the hybrids [45]. In agreement with the finding, Raouf et al. [46] observed that number of grains per ear of maize was significantly varied among maize varieties.

Hundred Kernel Weight (HKW)

The analysis of variance for HKW (p<0.05) indicated both the main effect of nitrogen fertilizer and maize hybrids as well their interaction effect was found insignificant. In agreement with this finding, Kwaga [47] found a non-significant value of hundred kernel weight among different maize genotype. Likewise, Gooding and Davies [48] reported non-significant effect of nitrogen fertilizer application rate on thousand kernels weight. However, Matusso [49] reported that existence of significant difference in thousand-grain weight of maize among different rate of nitrogen fertilizer.

Yield and Harvest Index

Grain Yield (GY): Maize grain yield is a complex trait of different yield contributing factor including the number of ear per plant, ear length, number of row per ear, number kernel per ear and hundred-kernel weight. The analysis of data showed that the main effect of nitrogen fertilizer application and the use of different maize hybrids to be varied highly significant at p<0.01 for GY. However, the interaction between maize hybrids and nitrogen fertilizer rate was not found significant. 

Accordingly, significantly highest GY 7950.5 Kg ha^-1 were obtained from the application of 150kg ha^-1 nitrogen fertilizer while significantly lowest 4094.7 Kg ha^-1 was obtained from 0kg ha^-1. It was observed that increasing rate of nitrogen fertilizer from zero to 50, 100 150 kg ha^-1improved grain yield by 41.6%, 44.5%, and48.5%, respectively Table 7. This might due to the application of nitrogen fertilizer at higher rate which promote kernel filling and reduce the number of bare ear in maize [50]. The higher in yield and yield components of maize under higher N rates over the lower nitrogen rates was attributed to the increase in leaf area index and total dry matter accumulation, increase in NUE and harvest index [35].

Qahar and Ahmad observe the minimum grain yield of 3011 kgha^-1 and highest grain yield of 8013 kg ha^-1from control and 350 kgha^-1 nitrogen fertilizer application, respectively. Similar trend in yield differences across different nitrogen levels have been reported by Zeidan et al. [51]. 

Zelalem [52] also reported that N application increased the grain yield of maize and application of 100 kg Nha^-1 gave the highest grain yield of 5852 kgha^-1as compared to the control treatment (0 kg ha^-1) nitrogen fertilizer. Concerning the main effect of hybrids the highest grain yield 7100.9Kg ha^-1 was obtained from the hybrids BHQPY545 which was at par with Shone hybrid and the lowest grain yield 5778Kg ha^-1 was obtained from MH138Q. GY of shone and BHQPY545 significantly exceeds GY of MH138Q by 16.85% and 18.64%, respectively Table 7.

The difference in maize hybrids in their yield performance might be related to the inherent genetic difference among the hybrids. In agreement to this finding of Akram et al. [53], Sampath et al. [54] and Sani et al. [55] reported variations in yield and yield components among different maize cultivars. Ragheb and Rassy conclude that hybrids generally differ from each other in grain yield due to genetic factors and the different physiological performance. The physiological factors include extended root system with more root hairs to absorb more nutrients and the canopy architecture to intercept more photosynthetic light.

Table 7: Main Effect of Nitrogen Fertilizer Rate and Maize Hybrids on Grain Yield (GY) and Above Ground Dry Biomass Yield (AGDBMY) of Maize under Irrigated Condition

Means in Columns Followed by the Same Letters Are Not Significantly Different at 5% Level of Significance.GY = Grain Yield, AGDBMY = Above Ground Dry Biomass Yield, LSD (0.05) = Least Significant Difference at 5% Probability Level, CV (%) = Coefficients of Variation in Percent

Above Ground Dry Biomass Yield (AGDBMY)

Analysis of variance for AGDBMY (p<0.05) indicated that significant effect of maize hybrids and highly significant (p<0.01) effect of nitrogen fertilizer rate. However, interaction between nitrogen fertilizer rate and maize hybrids was not significant.

In this regards the significantly maximum AGDBY 14084 kg ha^-1 was obtained from the application of 150 kg ha^-1 nitrogen fertilizer, which was actually at par with AGDBMY obtained from the application of 50 and 100 kg ha^-1N fertilizer, and the minimum AGDBY 8732 kg ha^-1 was obtained from the 0 kg ha^-1 nitrogen fertilizer application Table 7. The improvement in above ground dry biomass production with increased nitrogen rate might due the fact that application of nitrogen fertilizer at high rate promote highest leaf area which in increase the amount of light captured that in one way or another influence the amount of photo assimilate production [56].

In line with this finding, Moges [28] observe that increase in biological yield with increase rate of nitrogen fertilizer might due to better crop growth rate, LAI and accumulation of photo assimilate due to maximum days to maturity by the crop, which ultimately produced more biological yield. The result also depicted the presence significant difference among maize hybrids. The minimum AGDBMY 10546 kgha^-1 and the maximum 13344 kgha^-1 were obtained from MH138Q and BHQPY545, respectively. BHQPY545 and conventional hybrid, Shone were statistically at par. Table 7. Wasaya et al. [57] and Mir et al. [58] reported that, maize genotype have different genetic potential and showed a substantial different response to biological yield.

Harvest Index (HI)

Harvest index shows the physiological capability of a plant to change the part of photosynthesis to final yield. Higher harvest index indicates a larger percentage of total dry matter transformed into final kernel yield. Analysis of variance for HI (p<0.05) indicated significant effect of nitrogen fertilizer rate. However, the main effect of maize hybrids and the interaction effect between nitrogen fertilizer rate and the maize hybrids were not significant.

Accordingly, significantly the lowest HI 0.48 and the highest HI 0.57 was obtained from 0 and 150kg ha^-1nitrogen fertilizer, respectively. Nitrogen rate of 50, 100, and 150 kg ha^-1 were statistically at par Table 8. In this study, it was observed that increasing harvest index with increasing nitrogen fertilizer rate. The increase in harvest index with increasing rate of nitrogen fertilizer might be due to increases in nitrogen fertilizer application rate over the control which promotes proportional increment in both grain yield and biological yield of maize. In agreement to the current result, Mir et al. [58] recorded that significantly lowest (38.81%) and highest (48.18%) HI, from the control and 225 kg ha^-1 nitrogen fertilizer application, respectively. 

Likewise, Sabir et al. [59] also observe that higher harvest index and grain yield from higher nitrogen rate per hectare. In contrast to this result, Wasaya et al. [57] reported insignificant difference in harvest index of maize among different levels of nitrogen fertilizer. However, this result was not in conformity with the finding of Kidist [60] who observes a decreasing harvest index with increasing rate of nitrogen fertilizer application.

Table 8: Main Effect of Nitrogen Fertilizer Rate on Maize Harvest Index (HI) Under Irrigated Condition

Means in Columns Followed by the Same Letters Are Not Significantly Different at 5% Level of Significance, HI = Harvest Index, LSD = Least Significant Difference at 5% Probability Level, CV (%) = Coefficients of Variation in Percent

Correlation Analysis

The correlation among the hybrid yield and yield components indicates that all yield components are positively correlated with the grain yield and one another Table 9. NEPPL, EL, NKPE, and AGDBMY were highly correlated (p<0.01) with grain yield with r² = 0.442**, r² = 0.667**, r² = 772**, andr² = 0.464**, respectively. Similarly, AGDBMY was also significantly correlated with NEPPL, EL and NKPE with r² = 0.382^*, r² = 0.619^**and r² = 0.574^**, respectively.

This indicated that the increase or the decrease in yield component could influence the final grain yield positively or negatively based on the magnitude of the change. In agreement with the previous finding of Shaban et al. [61] reported that maize yield components are positively correlated with the final grain yield and the magnitude of yield is heavily dependent on the factor of yield component. Similarly, Lamin [62] report significant correlation between the maize yield and yield component. Likewise, the findings of this study were also reported by other researchers [36,40,63].

Table 9: Correlation Analysis among the Yield Component, Grain Yield and Harvest Index of Maize Hybrids under Varied Rate of Nitrogen Fertilizer

Note: * And ** Are Significant and Highly Significant at 0.05 and 0.01 Respectively, NEPP = Number of Ear Per Plant, NKPE = Number of Kernel Per Ear EL = Ear Length, HKW = Hundred Kernel Weight, GY = Grain Yield, AGDBMY = Above Ground Dry Biomass Yield, Ns = Non-Significant

Development of appropriate nutrient supply packages and best performing genotypes in specific area is an important towards sustainable agriculture and one way of increasing crop productivity and as well as increase farmer income. Among crop most yield limiting nutrient, nitrogen is primary macro- nutrient, which is needed by the plant to complete their life cycle. Therefore, the present study wasundertaken in order to evaluate the response of medium maturing maize hybrid to different nitrogen fertilizer rateunder irrigated condition at Melko, Jimma zone. A factorial combination of consisting of four nitrogen fertilizer rate 0, 50, 100 and 150 kg ha^-1 and three maize hybrids one conventional hybrid maize widely used by farmer in the study area, Shone and two quality protein maize hybrids, BHQPY545 and MH138Q were tested under randomized complete block design (RCBD) in 2017.

The result show that almost for all except for hundred kernel weight, nitrogen fertilizer rates and maize hybrids significantly affect phenological, growth, yield and yield component parameter and the interaction between nitrogen fertilizer rate and maize hybrids for all parameter was insignificant.

Significantly highest grain yield, (7950.5 kg ha^-1) was obtained from the highest rate of nitrogen fertilizer (150) kg ha^-1 and while the lowest grain yield is obtained from the control treatment. With regards grain yield of maize hybrids highest grain yield (7100.9 kg ha^-1) is obtained from BHQPY545 and the lowest grain yields (5778 kg ha-1) were obtained from quality protein hybrid, MHQ138. The highest AGDBMY (14084 kg ha^-1) recorded from 150 kg ha^-1 and the lowest (8732) kg ha^-1was obtained from the control treatment. However, there is no observed statistically significant difference between nitrogen fertilizer rate of 50, 100, and 150 kg ha^-1 on above ground dry biomass production.

On other hand maize hybrid, BHQPY545 has the highest AGDBMY of 13344 kg ha^-1 and the hybrid, MH138Q has the lowest AGDBMY of 10546 kg ha^-1 compared to the rest hybrids. Significantly highest harvest index of 0.57 is recorded from the 150 kg ha^-1 and the lowest harvest index 0.48 obtained from the control treatment. Correlation analysis between yield and yield component also reveal that grain yield was significantly correlated with the number of ear per plant, ear length number of kernel per ear and hundred kernel weights. 

Generally, application of 150 kg ha^-1 nitrogen fertilizer was observed significantly give better agronomic performance in the study area. Since the current work is a single season and single location work further work should be done in multiple location and multi-season including the rain fed production in order to reach at conclusive recommendation of current results. 

Aknowledgement

Above all, I would like to thank and glorify the Almighty Allah, who gave me health and strength that I required to complete this research and fulfill all aspirations of my life. It is my great pleasure to express my sincere appreciation and special gratitude to my Major advisor, Dr. Tadele Amdemariam and Co-advisor, Dr. Tesfa Bogale for their valuable advice, sustained and educated guidance, enthusiastic collaboration and critical reviewing of the research proposal and the Thesis manuscript

I would like to thanks Mr.Ashenafi Ayano who is the director of Jimma Agricultural Research Center (JARC) for allowing me to do my Thesis work within the Center and use Center’s required facilities through my stay. I am also highly indebted to all staff members of Jimma Agricultural Research Center. Last but not least, I am grateful to my family, especially, my wife Muslima Abatemam, my mother Leyla Abamecha, and my father Abarashad Abagisa for their encouragement, loving support and patience in all my life’s endeavor.

1. Kandil.E. “Response of some maize hybrids (Zea mays L.) to different levels of nitrogenous fertilization.” Journal of Applied Sciences Research, vol. 9, no. 3, 2013, pp. 1902–1908.

2. Central Statistical Agency (CSA). Agricultural sample survey report on area and production of major crops private peasant holdings, Meher season. Vol. 1, Addis Ababa, Ethiopia, 2016.

3. United States Department of Agriculture (USDA). Report on world agricultural production. Washington, USA, 2017.

4. Worku, Mosisa, et al. “Advances in improving harvest index and grain yield of maize in Ethiopia.” East African Journal of Sciences, vol. 1, no. 3, 2009, pp. 74–79.

5. International Maize and Wheat Improvement Center (CIMMYT). Second semi-annual progress report for the QPM development project for the Horn and East Africa. July–December 2003, 2004.

6. Baral, B.R. et al. “Growth and yield response of hybrid maize (Zea mays L.) to phosphorus levels in sandy loam soil of Chitwan Valley, Nepal.” International Journal of Environmental Science, vol. 4, no. 2, 2015, pp. 147–156.

7. Jat, M. et al. “Indian Journal of Fertilizer.” Indian Journal of Fertilizer, vol. 9, no. 4, 2013, pp. 80–94.

8. Guo, C. et al. “Application of controlled release urea in rice: reducing environmental risk while increasing grain yield and improving nitrogen use efficiency.” Communications in Soil Science and Plant Analysis, vol. 47, 2016, pp. 1176–1183.

9. Magezia, Waga. “Effects of methods and rates of phosphorus fertilizer application and planting methods on yield and related traits of maize on soils of Hawassa area.” Innovative Systems Design and Engineering, vol. 2, no. 4, 2011, pp. 315–335.

10. Kogbe, J. and Adediran, J. “Influence of nitrogen, phosphorus and potassium application on the yield of maize in the savanna zone of Nigeria.” African Journal of Biotechnology, vol. 2, no. 10, 2003, pp. 345–349.

11. International Food Policy Research Institute (IFPRI). Maize value chain potential in Ethiopia: constraints and opportunities for enhancing the system. Addis Ababa, Ethiopia, 2010.

12. Teklewold, Adefris et al. Quality protein maize (QPM): a guide to the technology and its promotion in Ethiopia. CIMMYT, Addis Ababa, Ethiopia, 2015.

13. Jimma Agricultural Research Center (JARC). Meteorological data report. 2016.

14. Food and Agriculture Organization of the United Nations (FAO). Provision of soil map of Ethiopia. Land Use Planning Project, Addis Ababa, Ethiopia, 1994.

15. Food and Agriculture Organization of the United Nations (FAO). FAO fertilizer and plant nutrition bulletin: guide to laboratory establishment for plant nutrient analysis. Rome, Italy, 2008.

16. Debele, Berhanu. The physical criteria and their rating proposed for land evaluation in the highland region of Ethiopia. Ministry of Agriculture, Addis Ababa, Ethiopia, 1980.

17. Day, P.R. “Hydrometer method of particle size analysis.” Methods of soil analysis, edited by C.A. Black, Agronomy Part II, no. 9, American Society of Agronomy, 1965, pp. 562–563.

18. Olsen, S.R. et al. “Estimation of available phosphorus in soil by extraction with sodium bicarbonate.” USA Circular no. 939, 1954.

19. Jackson, M.L. Soil chemical analysis. Prentice-Hall of India, New Delhi, 1967.

20. Maral, M. et al. “Effects of nitrogen fertilizer and plant density management in corn farming.” Journal of Agricultural and Biological Science, vol. 2, 2012, pp. 133–137.

21. Woldesenbet, Mitiku et al. “Effect of nitrogen fertilizer on growth, yield and yield components of maize (Zea mays L.) in Decha District, southwestern Ethiopia.” International Journal of Research – Granthaalayah, vol. 4, no. 2, 2016, pp. 95–100.

22. Adamu, U.K. et al. “Growth response of maize (Zea mays L.) to different rates of nitrogen, phosphorus and farmyard manure in Morogoro Urban District, Tanzania.” American Journal of Experimental Agriculture, vol. 9, no. 2, 2015, pp. 1–8.

23. Mitiku, Masresha. Response of maize (Zea mays L.) to application of mineral nitrogen fertilizer and compost in Godere District, Gambella Region, southwestern Ethiopia. Thesis, Haramaya University, 2014.

24. Ibrahim, K. et al. “Growth indices and yield of tomato (Lycopersicon esculentum Karst.) varieties as influenced by crop spacing at Samaru.” Proceedings of the 18th HORTSON Conference, vol. 1, 2000, pp. 40–47.

25. Abbas, M. et al. “Maize response to split application of nitrogen.” Journal of Agriculture and Biology, vol. 5, 2003, pp. 19–21.

26. Enujeke, E. “Effects of variety and spacing on growth characters of hybrid maize.” Asian Journal of Agriculture and Rural Development, vol. 3, no. 5, 2013, pp. 296–310.

27. Jain, T.C. and Misra, D.K. “Methods of estimating leaf area in croplands.” Indian Journal of Agriculture, vol. 11, 1966, pp. 280–283.

28. Asefa, Moges. Effect of nitrogen fertilizer rates and plant densities on yield and yield-related traits of maize under irrigation in Southern Tigray, Ethiopia. Thesis, Haramaya University, 2015.

29. Devi, I.S. et al. “Character association and path coefficient analysis of grain yield and yield components in double crosses of maize.” Crop Research, vol. 21, 2001, pp. 355–359.

30. Imran, S. et al. “Effect of nitrogen levels and plant population on yield and yield components of maize.” Advanced Crop Science Technology, vol. 3, 2015, p. 170.

31. Degu, Debebe. Response of hybrid maize to nitrogen and phosphorus in Gedeo Zone, Southern Ethiopia. MSc thesis, Haramaya University, Haramaya, Ethiopia, 2010.

32. Boote, K.B. et al. “Potential uses and limitations of crop models.” Agronomy Journal, vol. 88, 1996, pp. 704–716.

33. Haghighi, B.J. et al. “Evaluation of the effects of biological fertilizer on physiological characteristics and yield and its components of corn (Zea mays L.) under drought stress.” American Journal of Agricultural and Biological Science, vol. 5, 2010, pp. 189–193.

34. Jasemi, M. et al. “Effect of planting date and nitrogen fertilizer application on grain yield and yield components in maize.” American-Eurasian Journal of Agricultural and Environmental Science, vol. 13, 2013, pp. 914–919.

35. Amanullah.M. “Source and rate of nitrogen application influence agronomic N-use efficiency and harvest index in maize (Zea mays L.) genotypes.” Maydica, vol. 59, 2014, pp. 80–89.

36. Okumura, R.S. et al. “Influence of different nitrogen levels on growth and production parameters in maize plants.” Journal of Food, Agriculture & Environment, vol. 9, nos. 3–4, 2011, pp. 510–514.

37. Raouf, S. and Reza. “Response of maize (Zea mays L.) cultivars to different levels of nitrogen fertilizer.” Journal of Food, Agriculture & Environment, vol. 7, nos. 3–4, 2009, pp. 518–521.

38. Turgut, I. “Effects of plant populations and nitrogen doses on fresh ear yield and yield components of sweet corn (Zea mays saccharata Sturt.).” Turkish Journal of Agriculture and Forestry, vol. 24, 2004, pp. 341–348.

39. Santos, P.G. et al. “Evaluation of the agronomic performance of corn hybrids in Uberlândia, MG, Brazil.” Pesquisa Agropecuária Brasileira, vol. 37, 2002, pp. 597–602.

40. Cruz, S.C.S. et al. “Nitrogen fertilization for corn cultivated under a no-tillage system in the state of Alagoas, Brazil.” Revista Brasileira de Engenharia Agrícola e Ambiental, vol. 12, 2008, pp. 62–68.

41. Fernandes, F.C.S. et al. “Levels and nitrogen use efficiency of six maize cultivars.” Revista Brasileira de Milho e Sorgo, vol. 4, 2005, pp. 195–204.

42. Min N.P. “Evaluation of hybrid and OPV maize varieties for grain yield and agronomic attributes under farmers’ field conditions at Dukuchhap.” Nepal Agricultural Research Journal, vol. 9, 2009.

43. Abuzar, M.R. et al. “Effect of plant population densities on yield of maize.” Journal of Animal and Plant Sciences, vol. 21, no. 4, 2011, pp. 692–695.

44. Taye, Geremew. Effect of nitrogen and phosphorus on growth, yield and yield components of maize at Nejo, West Wollega, Ethiopia. MS thesis, Haramaya University, Haramaya, Ethiopia, 2010.

45. Muhammad, T. et al. “Comparative yield performance of different maize (Zea mays L.) hybrids under local conditions of Faisalabad, Pakistan.” Journal of Life and Social Sciences, vol. 6, no. 2, 2008, pp. 118–120.

46. Raouf, S.S. et al. “Effect of population density on yield and yield attributes of maize hybrids.” Research Journal of Biological Sciences, vol. 4, no. 4, 2009, pp. 375–379.

47. Kwaga, Y. “Effect of time of nitrogen application on the performance of maize (Zea mays L.) varieties at Mubi, northern Guinea savanna of Nigeria.” American Journal of Research Communication, 2014.

48. Gooding, M.J. and Davies, W.P. Wheat production and utilization. CAB International, Wallingford, UK, 1997.

49. Matusso, P. “Growth and yield response of maize (Zea mays L.) to different nitrogen levels in acid soils.” Academic Research Journal of Agricultural Science and Research, vol. 4, no. 2, 2016, pp. 35–44.

50. Kelbesa, Kena. Effect of nitrogen rates and inter-row spacing on growth, yield and yield components of maize (Zea mays L.) at Nejo, Western Ethiopia. MSc thesis, Haramaya University, Haromaya, 2015.

51. Zeidan, M.S. et al. “Effect of N-fertilizer and plant density on yield and quality of maize in sandy soil.” Research Journal of Agriculture and Biological Science, vol. 2, 2006, pp. 156–161.

52. Bekeko, Zelalem. “Effect of nitrogen and phosphorus fertilizers on some soil properties and grain yield of maize (BH-140) at Chiro, Western Hararghe, Ethiopia.” African Journal of Science, vol. 8, no. 45, 2013, pp. 5692–5697.

53. Akram, M. et al. “Performance of autumn planted maize (Zea mays L.) hybrids at various nitrogen levels under salt-affected soils.” Soil and Environment, vol. 29, 2010, pp. 23–32.

54. Sampath, O. et al. “Evaluation of genotypes and nitrogen levels for yield maximization in rabi maize (Zea mays L.).” International Journal of Innovative Research and Development, vol. 2, 2013, pp. 314–318.

55. Sani, B.M. et al. “Grain yield and yield components of quality protein maize genotypes as influenced by irrigation and plant population in the Nigerian savannah.” Journal of Agricultural Science, vol. 6, 2014, pp. 166–172.

56. Jalilian.J and Delkhoshi.H. “How much leaves near the ear contribute on yield and yield components of maize?” Cercetări Agronomice în Moldova, vol. XLVII, no. 2, 2014.

57. Wasaya, A., et al. “Response of maize to tillage and nitrogen management.” Journal of Animal and Plant Science, vol. 22, no. 2, 2012, pp. 452–456.

58. Mir, F. et al. “Effect of nitrogen fertilizer on yield and yield component of maize.” International Journal of Agriculture and Biosciences, vol. 6, no. 1, 2017, pp. 18–20.

59. Sabir, M.A. et al. “Effect of nitrogen phosphorus on yield and quality of two hybrids of maize (Zea mays L.).” Journal of Agriculture, vol. 38, 2000, pp. 339–346.

60. Abera, Kidist. Growth, productivity and nitrogen use efficiency of maize (Zea mays L.) as influenced by rate and time of nitrogen fertilizer application in Haramaya District, Eastern Ethiopia. MSc thesis, Haramaya University, Haramaya, Ethiopia, 2013.

61. Shaban.M, et al. “Response of yield and yield components of maize (Zea mays L.) to different biofertilizers.” International Journal of Advanced Biological and Biomedical Research, vol. 1, no. 9, 2013, pp. 1068–1077.

62. Lamin.B. Nitrogen utilization efficiency, growth and yield response of maize (Zea mays L.) to integrated application of mineral nitrogen and cattle manure. Thesis, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana, 2016.

63. Gomes, R.F. et al. “Effect of levels and timing of nitrogen application on agronomical traits of no-till corn.” Revista Brasileira de Ciência do Solo, vol. 31, 2007, pp. 931–938.