Abstract:
Maize (Zea mays L) is an important food security crop widely grown in Hararghe Zones, eastern Ethiopia. However, the average yield of the crop is low due to several biotic and abiotic production constraints, among which low soil fertility and moisture stress are the major factors. As a result, three field experiments were conducted in the Haramaya District during the 2019/20 and 2020/21 main cropping seasons. The first objective was to investigate the response of two hybrid maize varieties (BHQPY 545 and MHQ 138) and one composite open-pollinated maize variety (Raare-1) to six nitrogen (N) fertilizer rates (0, 23, 46, 69, 92, and 115 kg N ha–1 ). The second objective was to investigate the response of a maize variety (BHQPY 545) to N fertilizer rates and the timing of application. The third objective was to evaluate the effects of levels of supplementary irrigation and N fertilizer rates on maize yield and yield components. In the first experiment, there were 18 treatments comprising a combination of three maize varieties, and six N fertilizer rates (0, 23, 46, 69, 92, and 115 kg N ha-1 ). The experiments were laid out as a randomized complete block design (RCBD) in a factorial arrangement with three replications. In the second experiment, there were 21 treatments comprising six fertilizer rates (0, 23, 46, 69 92, and 115 kg N ha–1 ) and four timing of application (all at tillering; ½ at sowing + ½ at tillering; 1/3rd at sowing + 1/3rd at tillering +1/3rd at tasselling; ¼ th at sowing + ½ at tillering +¼ th at tasselling). The experiments were laid out as a randomized complete block design in a factorial arrangement with three replications. In the third experiment, there were 24 treatments consisting of four levels of supplementary irrigation (SI), namely: (0 [rainfed], 50%, 75%, and 100%) of the crop water requirement, assigned to the main plots whereas, six N fertilizer rates (0, 23, 46, 69, 92 and 115 kg N ha–1 ) were assigned to the sub-plots. The experiments were laid out as a randomized complete block design in a split-plot arrangement with three replications. Data were collected on growth parameters, yield components, and yield. Results from the first experiment revealed that most of the studied variables were affected by the main effects of year of growing, N application rates, and maize variety. Plant height, leaf number plant–1 , leaf area, and leaf area index grown in 2019/20 exceeded the values of plants grown in 2020/21 by about 8.4%, 5.4%, 25.8%, and 28.1%, respectively might be associated with rainfall variability. The optimum leaf area index (4.1), effective ear plant–1 (2.2), ear diameter (4.7 cm), grain rows ear–1 (15.1), number of grains row–1 (41.3), 1000-grain weight (346.6 g), and harvest index (45.3%) were recorded at 92 kg N ha–1 application. The optimum stover yield (8.8 t ha–1 ) and grain yield (8.4 t ha–1 ) were recorded for variety BHQPY 545 at 69 kg N ha–1 . The highest grain yield obtained from variety BHQPY 545 at 92 kg N ha– 1 exceededthe grain yield of MHQ 138 and Raare-1 at this N fertilizer rate by about 56% and 67%, respectively. The variety BHQPY 545 produced the maximum net profit (138,222 Ethiopian birrs ha-1 ) at 115 kg N ha-1 , N agronomic efficiency (79 kg kg–1 ), and marginal rate of return (302%) at 69 kg N ha-1 . It is concluded that the BHQPY 545 variety produced the optimal stover, grain yields, the highest net benefit, and marginal rate of return in response to the application of 69 kg ha–1 . In the second experiment, growth parameters, yield components, and grain yield of maize significantly (p ≤ 0.01) responded to the year of growing, N fertilizer rates, and timing of application. For almost all variables studied, significantly higher values were obtained for the cropping year of 2019/20 than in 2020/21. N rates at 92 kg ha–1 produced maximum ear diameter (4.6 cm), xix number of grain rows ear–1 (14.5), 1,000-grain weight (365.5g), harvest index (46.7%), and N agronomic efficiency (58.5 kg kg–1 ). The optimum stover yield of 9.2 t ha–1 was obtained in response to the application of 69 kg N ha–1 in three splits of ¼ that sowing, ½ at tillering, and the other ¼ th at tasselling, and the lowest mean stover yield of 5.7 t ha–1 was obtained from the treatments that received 23 kg N ha–1 with all types of split application. The highest grain yield of 9.41t ha–1 was obtained in response to 92 kg N ha–1 in three splits of ¼ th at sowing, ½ at tillering, and ¼ th at tasselling, which surpassed the yield obtained from this rate of N in two splits of ½ at sowing and ½ at tillering by about 43%, which was statistically comparable to 8.48 t ha–1 grain yield obtained from the same rate of N fertilizer in three splits of 1/3rd at sowing, 1/3rd at tillering, and 1/3rd at tasselling, which outperformed the two splits by about 29%. The lowest grain yields were obtained from the treatments that received 23 kg N ha–1 with all timing of split application as well as 46 kg N ha–1 with all N doses applied at tillering as well as with the N dose applied in two splits of ½ at sowing and ½ at tillering. The maximum net profit (162,492.9 Ethiopian birr ha–1 ), and marginal rate of return (1,033%) were obtained from 92 kg N ha–1 in three splits of ¼ th at sowing + ½ N at tillering + ¼ th at tasselling, and in three splits with 1/3rd at sowing, 1/3rd at tillering, and the remaining 1/3rd at tasselling, respectively. It is concluded that applying 92 kg N ha–1 in three splits of 1/3rd at sowing, 1/3rd at tillering, and 1/3rd at tasselling was found to be the most economical rate and timing of nitrogen application in the study area. In the third trial, 69 kg N ha–1 generated the optimum plant height, leaf area, and leaf area index, increasing them by about 22%, 31%, and 33%, respectively, when compared to 0 kg N ha–1 . Likewise, plants that received 92 kg N ha–1 produced optimum values of the number of effective ears plant–1 , cob diameter, cob length, and grain rows cob–1 , which increased by about 150%, 65%, 97%, and 38%, respectively, as compared to plots with no N application. Similarly, 75% ETc supplementary irrigation (SI) resulted in the optimum plant height, leaf area, leaf area index, cob diameter, and cob length, which increased by about 19%, 26%,24%, 34%, and 50%, respectively, when compared to only rain-fed plots but did not differ significantly from plots that received 100% ETc SI. Plants that received 75% ETc SI with 69 kg N ha-1 application had significantly increased stover yield by about 90%, grain number cob–1 by about 183%, 1000-grain weight by about 44.4%, and grain yield by about 155% (three-fold) compared to only rain-fed with 69 kg N ha−1 application. A maximum grain yield of 9.6 t ha–1 was recorded in response to the application of 100% ETc SI with 92 kg N ha–1 , which was in statistical parity with the grain yield obtained in response to the application of 100% ETc SI with 69 and 115 kg N ha–1 , as well as 75% SI with 69, 92, and 115 kg N ha–1 . On the other hand, 75% ETc SI coupled with 92 kg N ha–1 resulted in the highest water productivity of 1.15 kg ha–1 m –1 but did not differ significantly from plots that received 69 and 115 kg N ha–1 , and 100% ETc SI with 69, 92, and 115 kg N ha–1 . The 100% ETc SI with 92 kg N ha–1 generated the maximum net benefit (223,488.5 Birr ha–1 ), whereas the highest marginal rate of return (473%) was obtained from 69 kg N ha–1 with 75% ETc SI. Therefore, it is concluded that applying 69 kg N ha–1 with 75% ETc SI was found to be the most cost-effective rate of nitrogen fertilizer and supplementary irrigation in the study area.