Response of maize phenology and grain yield to various nitrogen rates and plant spacing at Bako, West Ethiopia

Maize (Zea mays L.) is one of the most impor tant cereal crops, widely grown in most parts of the world for various benefi ts of human beings. It is used as food, feed and raw materials for some industrial products [1]. In Ethiopia more than 9 million smallholder households grow maize more than any other crops at present [2]. It is one of the most strategic crops selected to achieve food security in this country [3]. Considering its important in food security, extensive research work is carrying out every year for further improvement its grain yield [4-6]. Nevertheless its productivity is still at a low profi le (3.2t/ha), because of poor agronomic practices, lack of improved seeds and climate variability [7]. Plant population and nutrient management are among key agronomic factors which determine growth, phenology and yield of maize and other fi eld crops.

Maize is an exhaustive feeder of nutrients; the chemical fertilizer especially nitrogen fertilizer is universally accepted as a key component to high maize yield [11], and if defi ciency occurs at tasseling and silking stages it may lead to virtual crop failure. Application of nitrogen as well as increase in its rate induced earliness of tasseling and silking stages [19][20][21][22]. In contrast to tasseling and silking stages, increasing nitrogen rate application signifi cantly increased number of days to physiological maturity [11,16,21,23].
Crop yield tend to increase with increasing the nitrogen rate.
Grain yield of maize increased signifi cantly with increasing in N rate [11,[24][25][26][27]. N-fertilization provided suffi cient nutritional requirements for the maize plants to rapidly grow and hence promoted its grain production [6].
Thus in order to obtain maximum yield per unit area application of adequate nitrogen fertilizer rate with appropriate plant density is crucial task especially for maize crop which is high infl uenced by these factors.
Considering the above facts, the present study was undertaken to elucidate the effects of nitrogen rates and plant spacing on phenology and grain yield of maize under main cropping season.

Description of experimental site
The experiment was conducted at Bako research site

Experimental materials
Intermediate maturing maize hybrid BH546 was used for the study. This hybrid gives 8500-9500 and 5500-7000kg ha -1 grain yield on-station and on-farm experiments, respectively BNMRC, 2014. Nitrogen fertilizer in the form of urea (46% N) and phosphorous fertilizer in the form of Triple Superphosphate (46% P 2 O 5 ) were used for the experiment.

Treatments and experimental design
The treatments comprised of six rates of nitrogen (0kg N ha -1 , 23kg N ha -1 , 46kg N ha -1 ,69kg N ha -1 ,92kg N ha -1 and 115kg N ha -1 ), fi xed inter row spacing of 75cm, and three intra row spacing of 20cm, 30cm and 40cm. The treatment (75×30cm spacing with 92kg N ha -1 ) was used as a standard check whereas 0kg N ha -1 was used as negative control. The experiment was arranged in Randomized Complete block Design (RCBD) in factorialcombination with three replications. The gross plot size was 4.8m×3.0m (14.4m 2 ) with row length of 4.8m, and net plot size 4.8m×1.5m (7.2m 2 ) was used for harvesting. The treatments were randomly assigned to the experimental unit within a replication. The replications were separated by 2m wide space.

Experimental procedures
Land prepared, three times, using tractor plough from

Crop data collection
Days to 50% tasseling Days were counted from sowing to the day when 50% of the maize plants shed their tassels in each plot. Days to 50% silking It was recorded as the number of days require from planting to when 50% of the maize plant showed extrusion of silks in each plot. Both days to 50% tasseling and silking were determined by similar way of visual observation.

Statistical analysis
Analyses of variances for the data recorded were conducted using the SAS version 9.3. Least Signifi cant Difference (LSD) test (5%) was used for mean separation if the analysis of variance indicated the presence of signifi cant treatment differences.

Effect of various nitrogen levels on fl owering date of maize
The analysis of variance revealed highly signifi cant (P<0.01) effects of applied N rates on the days to 50% tasseling and signifi cant (P<0.05) effect on the days to 50% silking of maize (Table 1). However, the interaction of N rates and plant spacing was not signifi cant.
The tasseling period became increased as the nitrogen level decreased and the maximum days to 50% tasseling (82.44) were recorded under 0kg N ha -1 , but it was statistically similar with days to 50% tasseling (81.78) recorded under 23kg N ha -1 Citation: Golla Figure 1). The minimum days to 50% tasseling was obtained under application of 115kg N ha -1 . However, statistically similar results were also recorded under application of 46, 69 and 92kg N ha -1 ( Figure 1). But the tasseling period became shortest numerically as the nitrogen level increased. In addition to early tasseling, the plots received higher N rate were uniformly fl owered, while in the lowest nitrogen rate the maize crop remained delay to attained 50% tasseling even though a very few plants tasseled earlier.
Like that of days to tasseling, days to silking delayed with decreasing nitrogen level and increasing in plant density, which accompany with high competition. Mean values for nitrogen rates showed that days to 50% silking delayed by 2 days in a treatment with no N application compared to a treatment that fed with 115kg N ha -1 ( Figure 1). Thus the maximum days to 50% silking (84.67) were recorded in a plot with no N application. However, it was statistically similar with days to 50% silking recorded under a treatment of 23, 46 and 69kg ha -1 N-rates. The minimum days to 50% silking (82.67) were obtained under maximum N rate (115kg N ha -1 ) But statistically similar result were also obtained under application of 46, 69 and 92kg N ha -1 . However, there was a decreasing trend in days to 50% silking with increasing in nitrogen rates.

Effect of plant spacing on fl owering date of maize
The analysis of variance showed that a signifi cant (P<0.05) effects and highly signifi cant (P<0.01) effect of plant spacing on the days to 50% tassing and days to 50% silking respectively (Table 1). However, the interaction of N rates and plant density was not signifi cant ( Table 1). The days to 50% tasseling and silking tend to increasing with decreasing plant spacing. The maximum days to 50% tasseling (81.83) were recorded under the lowest plant spacing (20cm), while the lowest day to 50% tasseling (81.11) were noted under the highest plant spacing (40). However the days to 50% tasseling obtained under medium plant spacing (30cm) was statistically similar to both days to 50% tasseling obtained under 20cm and 40cm plant spacing (Figure 2).
Similarly maximum number of days to 50% silking (84.72) were recorded in a plot where the maize sown at plant spacing 20cm which corresponding to 66,666 plant ha -1 while the minimum days to 50% silking (82.94) were observed at the half plant density, the widest plant spacing of 40cm. The number of days to 50% silking for intra row spacing of 30cm were in between these two plant spacing (83.44) ( Figure 2) which indicates signifi cant increasing trend in days to 50% silking with the increasing in plant spacing.

Effect of various nitrogen levels on physiological maturity date of maize
The effects of nitrogen rates and plant spacing on 90% physiological maturity were signifi cant but their interaction had not shown any signifi cant effect. Increasing nitrogen rate signifi cantly (p<0.05) increased number of days to physiological maturity ( Table 1). The maximum days to attained 90% physiological maturity were recorded under maximum N rate (115kg N ha -1 ), but statistically similar days to 90% maturity were recorded under application 46, 69 and 92kg N ha -1 . The shortest day to attained 90% physiological maturity was obtained under no N application, but it was not signifi cantly different from days of 90% physiological maturity recorded under treatments those received 23, 46 and 69kg N ha -1 (Figure 3). However, as the nitrogen rates increased, the days to physiological maturity were also numerically increased.

Effect of plant spacing on physiological maturity date of maize
The maturity of the crop was also affected by the plant Citation: Golla (Figure 4). under the rate of 92kg N ha -1 application for the same intra row spacing (20cm). The minimum grain yield (6358.80) was obtained under 0kg N ha -1 at 20cm plan spacing, but statistically similar grain yield were obtained under application of 0 and 23kg N ha -1 at all plant spacing including 46kg N ha -1 application in case of 40cm intra row spacing (

Effect of different nitrogen rates and plant spacing on grain yield of maize
The ultimate goal of crop production is increasing economic yield. Both of the main effects (rate of nitrogen and plant spacing) and their interaction had a signifi cant (P<0.01) effect on grain yield of maize (Table 1).
Maximum grain yield ha -1 (10207.80kg) was achieved at the plant spacing of 20cm with the application of 115kg N ha -1 , but it was statistically similar with (9886.90kg) that produced

Discussion
The tasseling and silking date of maize increased with decreasing in nitrogen rate. The delaying of fl owering date in response N starvation has been previously acknowledged [8,11,19,22], but this research is the fi rst to report the effect of reported results opposite to ours. Imran, et al., [15], stated that increasing nitrogen level consistently increased days to 50% tasseling due to prolonging the vegetative growth period.
Like that of days to tasseling, days to silking were delayed with N starvation and high population density, which accompany with high competition. This decreasing in silking period in response to increasing in N rate might be attributing to rapidness in growth period and promoting silk extrusion.
These results are in consistent with the fi nding of Shrestha [11], who observed that the earlier days to tasseling at the treatment which received the more rate of nitrogen. Similarly, Dawadi and Sah [21] and Jassal, et al., [22]. Reported that application of nitrogen as well as increase in its rate induced earliness of tasseling and silking stages. However, there has been some controversy regarding the phenological parameters of maize.
Imran, et al., [15], Sharifi , and Namvar [16], stated that maize took more time to tasseling and silking in plots that received the highest rate of N. In general, the grain yield ha -1 was increased with the increase in N rate and with decreasing in plant spacing.
However, in the wider plant spacing (40cm), increase of nitrogen rate above 92kg Nha -1 was not effective in increasing the grain yield which can be attributed to the extra loss of nitrogen through leaching from root zone. The mean grain yield ha -1 was increased by 16.35% and 24.44% with reducing plant spacing from 40cm to 30 and 20cm under highest N rate (115kg Nha -1 ) respectively.
Compared to the standard control of the plant spacing 30cm (44444 plant ha -1 ) with the application of 92kg Nha -1 , the mean grain yield was increased by 834kg ha -1 or 8.90% when the maize hybrid sown at plant spacing of 20cm with application 115kg Nha -1 .
In this experiment, the grain yield per unit area had a multiplicative trend, and application of the highest N rate (115kg N ha -1 ) at the closer plant spacing (20cm) was produced the maximum yield, although statistically similar yield under application of 92kg N ha -1 in the same plant spacing was obtained. So, these results indicates an opportunity to increase grain yield with higher N application and lowest plant spacing beyond 115kg Nha -1 and 20cm intra row spacing.
The positive relationship between yield and narrow plant spacing was due to the high number of high number of plants per unit area. The increased in maize grain yield under decreased spacing might be due to effi cient utilization of available resources (nutrient water and light). Farnia, et al., [10], stated that plant defi cit per unit area prevents maximum usage of production parameters while excessive density can increase the competition and decrease the yield.
Higher grains yield at higher nitrogen levels might be due to the lower competition for nutrient and positive effect of N on plant growth and coincide synchrony of male and female fl owering.

Conclusion
The results obtained in this experiment showed that reduction in plant spacing and nitrogen starvation resulted delaying to attain 50% tasseling and silking while it speeding up maturity period. Maize hybrid BH-546 produced the maximum grain yield when the hybrid was sown at 20cm plant spacing with application of 115kg N ha -1 , but application of 92kg N ha -1 at the same plant spacing (20cm) was also produce similar result. Furthermore the experiment was indicated a multiplicative trend of grain yield with increasing N rate and decreasing plant spacing, so further modifi cation of N rates up ward and plant spacing down ward might further increased the grain yield.