Abstract:
Nitrogen fertilizer is a major input required for cereal crop production in Ethiopia and its efficient use is important for the economic sustainability of cropping systems. Furthermore, the dynamic nature of N and its susceptibility for loss from soil-plant systems creates a unique and challenging environment for its efficient management especially on Vertisols. Developing suitable nitrogen fertilizer management practices is important strategy to improve productivity, quality and nitrogen use efficiency of bread wheat grown on Vertisols. Therefore, three field experiments were designed to determine the effects of N rate, time of application, method of application and the response of bread wheat varieties to nitrogen fertilizer on growth, yield, grain quality and nitrogen use efficiency of bread wheat for two consecutive years (2014 and 2015) on Vertisols under rainfed conditions in the central highlands of Ethiopia. Wheat variety, named Menze was used as a test crop for the first two experiments. In all three experiments the N rates used were 0, 120, 240 and 360 kg N ha-1. This can be followed by the time of application and method of application treatments. The five times of application include (T1= ½ at sowing and ½ at tillering; T2= all at tillering; T3= ½ at tillering and ½ at booting; T4= 1/3 at sowing, 1/3 at tillering and 1/3 at booting, T5= ¼ at sowing, ½ at tillering and ¼ at booting) plus one control. The methods of application are (M1= broadcasting the fertilizer in two equal splits at sowing and tillering; M2: banding half of the nitrogen fertilizer at a depth of 3-5 cm at sowing and the rest half applied at tillering by side dressing; M3: banding half of the nitrogen fertilizer at a depth of 5-7 cm and the rest half applied at tillering by side dressing; M4: banding half of the fertilizer N at a depth of 7-9 cm and the rest half at tillering by side dressing; and M5: banding the whole fertilizer at a depth of 7-9 cm at sowing) plus one control. The third experiment included three wheat varieties namely; Menze, Tsehay and ET-13. The experiments were evaluated in a randomized complete block design in a factorial arrangement with four replications. The N rate and time of application experiment revealed that most yield components increased with increasing rates of nitrogen to the level of 360 kg ha-1. Split application of nitrogen three times; ¼ at sowing, ½ at tillering and ¼ at booting also produced the highest value for most of the yield components. The interaction effect showed the highest grain yield (6436 kg ha-1) in response to the application of 360 kg N ha-1 in three split doses of ¼ at sowing, ½ at tillering and ¼ at booting which was in statistical parity with the application of 240 kg N ha-1 with the same split application. The maximum grain protein (12.8%), wet gluten (33.5%) and zeleny sedimentation values (40.6 ml) were recorded at the rate of 360 kg N ha-1 in 2015. The split application of N three times (¼ at sowing, ½ at tillering and ¼ at booting) resulted in the highest grain protein (12.3%), wet gluten (32.1%) and zeleny sedimentation value (38.7 ml) in 2014. The nitrogen use efficiency indices in this experiment decreased with increased N rate from 120 to 360 kg ha-1. The split application of N three times increased grain nitrogen uptake, total nitrogen uptake and the different nitrogen use efficiency traits. The net benefit (59072.1 Ethiopian Birr) and marginal rate of return (12565.73%) were obtained at N rate of 240 kg ha-1 when applied in three splits of ¼ at sowing, ½ at tillering and ¼ at booting. In general, it is noticeable that the application of nitrogen beyond 240 kg ha-1 did not influence xxiii most yield components and grain yield significantly. It could be concluded that application of 240 kg N ha-1 in three splits doses of ¼ at sowing, ½ at tillering and ¼ at booting should be considered in N application management in order to decrease environmental risk and farming cost. The N rate and method of application experiment revealed that the highest number of kernels per spike (47.51), number of spikes m-2 (295.92), and grain yield (5116.11 kg ha-1) were recorded in the 2015 growing season with the application of 360 kg N ha-1 which were statistically similar with the application of 240 kg N ha-1 for all these parameters. The application of nitrogen only once at sowing at a depth of 7-9 cm produced the highest number of kernels per spike (47.85), number of spikes m-2 (295.74),and grain yield (5147.22 kg ha-1). The interaction effect showed that the highest grain nitrogen concentration (2.43%) was recorded in the year 2014 with the application of 360 kg N ha-1 and the highest grain nitrogen uptake (122.56 kg ha-1) was recorded in 2015 with the application of 360 kg N ha-1. Method of N application did not significantly affect grain quality parameters of wheat. Nitrogen fertilizer had a consistent increasing effect on quality of wheat except grain starch. The nitrogen use efficiency traits decreased with increasing rates of N fertilizer in both growing seasons. The application of 240 kg N ha-1 produced a net benefit of 50731.25 Ethiopian Birr with a marginal rate of return of 1199.16%. In general, plots treated with 240 kg N ha-1 and N fertilizer application at a depth of 7-9 cm only once at sowing resulted in highest grain yield, accepTable quality coupled with the best economic benefit and reduce environmental risk. The N rate and variety experiment indicated that the highest number of spikes was produced by variety ET-13 in both growing years. Variety Menze produced the maximum number of kernels per spike (52.4) at N rate of 360 kg ha-1. The highest straw yield (12677 kg ha-1) was produced by variety ET-13 at N rate of 360 kg ha-1 which was statistically at par with the application of 240 kg N ha-1. Moreover, significantly the highest grain yield was produced by variety Menze (5718.32 kg ha-1) and Tsehay (5535.09) in the year 2015 at N rate of 360 kg ha-1 which was statistically at par with 240 kg N ha-1. Variety Tsehay produced the highest quality parameters (grain protein, wet gluten, zeleny sedimentation value) at N rate of 360 kg ha-1 in both growing years. Variety Tsehay had the highest grain and straw N content of 2.7% and 0.35%, respectively, at N rate of 360 kg ha-1. Higher values of nitrogen use efficiency parameters were recorded at lower N rate (120 kg ha-1) for all the varieties under the study. The highest nitrogen recovery efficiency was produced by variety Tsehay. The economic analysis showed varieties ET-13 and Tsehay produced the highest net benefit and marginal rate of return at a rate of 120 kg N ha-1 whereas variety Menze required 240 kg N ha-1. In general, it can be concluded that a high yield of high quality wheat requires the application of medium nitrogen rate (240 kg ha-1) within experiments, while application beyond this rate is not economically efficient. Nitrogen use efficiency was found to be greater at low nitrogen rate (120 kg ha-1) and decreased with increased N application.