Optimizing N fertilizer use for sesame under rain fed and irrigation conditions

Inappropriate use of fertilizers is one of the major production constraints in sesame. Studies to optimize the use of N fertilizer on sesame were conducted at Humera Agricultural Research Center (HuARC) under rain fed and irrigation conditions. Thirteen (13) N doses were evaluated in a Randomized Complete Block Design (RCBD) during 2016, 2017 and 2018. In the rain fed condition; results showed higher number of days to 50% flowering on the higher N application. The 31-121.5 kg N ha have scored higher seed yield, where the lower (18 kg N ha) and higher (156 kg N ha) N doses scored lower seed yield. In the irrigation condition; higher N application rate prolonged the flowering time to some extent and days to 90% maturity was reduced as N rate increased. Heavier seed weights and maximum seed yield were recorded on 64 and 75 kg N ha and the lower N doses scored low seed yield. Agronomic efficiency of N fertilizer (N-AE) was higher on 64 kg N ha under both growing conditions. The partial budget analysis revealed that 64 kg N ha (rain fed) and 75 kg N ha (irrigation) had the higher net profit, MRR and residual ranking. Therefore, for the greater sesame productivity 64 kg N ha has to be recommended for rain fed and 64 to 75 kg N ha for the irrigated sesame.


Introduction
Sesame (Sesamum indicum L.) is one of the most ancient oil seed crop in the tropics, where Ethiopia is considered to be the center of cultivation [1,2]. Origin of sesame has been a matter of discussion for a long but it was presumed to have originated in Africa and later spread to Asia [3]. Sesame is an important source of high quality oil and protein [4,5], the oil has excellent stability due to the presence of natural antioxidants (sesamolin and sesamin) [6] and these lignans protect from oxidative rancidity. Sesame, the chief source of "queen oil" has the capacity to set seed and yield remarkably well under high temperatures, it has a deep taproot for extracting moisture and nutrient from lower soil layers and succeeding on as little as 200 -400 mm of rain during the growing season [4]. In Ethiopia, sesame is a main source of foreign currency of the agricultural commodities next to coffee. Inappropriate use of fertilizers is one of the major production constraints in sesame [7,8]. Fertilizer applications to sesame is crucial for better yield and high seed quality [9]. HuARC [10] reported that application of 100 kg N/ha produced higher seed yield under rain fed conditions. Moreover, higher seed yield (1315.09 kg/ha) was recorded by application of 150 kg N/ha, while lower seed yield (554.17 kg/ha) recorded from 50 kg N/ha [11]. In Ethiopia, application of 120 kg/ha NPK plus 50 kg urea (42 kg N/ha) gave 32% yield increment over zero fertilization under rain fed condition [10]. Other study in the area witnessed that higher seed yield (744.3 kg/ha) was recorded from application of 100 kg/ha DAP and 50 kg/ha urea (41 kg N/ha) in Humera [12]. Under irrigation condition, application of 100 kg N/ha produced higher seed yield Bahar et al. (2011). Moreover, higher seed yield (1315.09 kg/ha) was recorded by application of 150 kg N/ha [7]. In the other case, higher seed yield of sesame (769 kg/ha) was recorded when 44 kg N/ha was applied [9]. El-Sherif [8] concluded that application of 142 kg/ha N was economical under irrigation condition. In general, Zenawi and Mizan [13] reviewed that the optimum N fertilizer application for the potential areas varies from 50-100 kg N/ha based on the soil type and moisture availability status. However, 41 kg N/ha is the common practice in the study area (potential area of sesame) where the world experience is 50-100 kg N/ha. Therefore, objective of the study was to optimize N fertilizer use for sesame under rain fed and irrigation conditions.

Description of the Study Area
A field experiment at Humera Agricultural Research Center (HuARC) western Tigray, which is located at latitude of 14°15' N, longitude of 36 o 37' E and elevation of 608m to optimize N fertilizer use in sesame was conducted. The agro-ecology of the location is described as hot to warm semiarid plain with mean temperature of 29°c, annual mean rainfall of 500 mm and chromic vertisol.

Treatments and Experimental Design
The field experiment was done in Randomized Complete Block Design (RCBD) with three replications in 2016, 2017 and 2018 production seasons. The experimental area included five rows with the length of five meter. The path between blocks and plots were 2 m and 1m, respectively. The treatments consisted of 13 N doses; 18, 29.5, 41, 52.5, 64, 75.5, 87, 98.5, 110, 121.5, 133, 144.5 and 156 kg N ha−1. Seeding was done in rows with row spacing of 40 cm and seedlings were thinned to achieve 10 cm space between plants when the plants attained a height of 5 cm. The 41 kg N/ha was used as standard check (research recommendation for rain fed [12] and blanket recommendation for irrigated sesame). Setit-2 variety from HuARC was used as a test variety, 38 kg P/ha and 7 kg S/ha were applied from NPS blended fertilizer. The NPS fertilizer was applied during plating and urea (N doses) were applied in two splits (1/2 during planting and the remaining at flower initiation). The field was ploughed and harrowed three times to obtain fine tilt, thereafter marked into plots. Early January and early July were the planting dates for the irrigated and rain fed sesame, respectively. Ridge and furrow were prepared for the irrigated sesame and seed were planted in mid pint of the ridges. Surface irrigation technique was used to water the irrigated sesame. All agronomic practices were applied to all experimental units uniformly. Border rows and 0.5 m from both sides of each row were used as buffer zones to minimize the effects of external factors. About 500 g composite top soil samples (0−20 cm) were collected using auger before planting to determine physicochemical properties of the soil of the experimental sites (Table 1). Average rainfall and temperature data of study site were presented ( Figure 1).

Data collection
Phenological data: 50 % flowering and 90% maturity were the two important phenological traits recorded.
Growth parameters: number of branches per plant and plant height were recorded.
Yield and yield components: number of capsules per plant, number of seeds per capsule 1000 seed weight and seed yield were collected. Agronomic N use efficiency was also calculated.
Nitrogen use efficiency can be measured in a variety of ways; the most basic level is agronomic efficiency (AE) of fertilizer application [20], which is measured as the increase in seed yield divided by the rate of application (eq1). Agronomic efficiency (AE) is expressed as the additional amount of economic yield per unit of nutrient applied [21].
Where, N-AE= agronomic use efficiency of nitrogen, F=yield of fertilized plot, C=yield of unfertilized or lower fertilized treatment.

Data Analysis
Collected data for the different traits of the field experiment was analyzed using computer statistical software Genstat-18. Means were compared using Duncan's Multiple Range Test (DMRT), at P level of 0.01 probability level.

Economic Analysis
Economic analysis was conducted using partial budget analysis as described by CIMMYT [22].
Fertilizer cost, labor cost (weeding, harvesting and threshing) and adjusted seed yield were used for the economic analysis. Seed yield was reduced by 10% in the analyses to minimize the effect of the small plots managed by researchers compared to the farmer's management as guided by CIMMYT [22]. All costs and benefits (eq 2-5) were calculated on a hectare basis in the Ethiopian currency, Birr.
The following equations were used: Where MRR is the marginal rate of return, NB is net benefit ha −1 for each treatment, and TCV is the total variable costs ha −1 for each treatment.
Dominance is a procedure of selection of profitable treatments from the range tested. The marginal rate of return (MRR) was calculated from the non-dominated treatments (eq 4). MRR denotes the return per unit of investment for the different managements tested in the field.
Residual was calculated from the minimum return and net benefits (eq 5). Following the analysis, treatments with the higher Residuals and MRR were recommended to farmers.

Results
The Analysis of Variance (ANOVA) results shown in Table ( (Table 2).

Physical and Chemical Properties of the Soil
Analysis of soil revealed that the total content of N, P and OC in the soil were very low below the critical levels (0.08%, 7.19 ppm and 1.01%, respectively). The soil pH was high (7.75) ( Table   1).

Weather Conditions
Long term monthly average temperature and rain fall of the experimental area was 29 O C and 500 mm, respectively ( Figure 1). There were no precipitations at all in the irrigated season and the minimum temperature was lower in the irrigated season compared to the rain fed season, while the maximum temperature was higher in the irrigated season ( Figure 1).

Effect of N fertilizer on sesame phenological traits
N fertilizer has showed statistical variation on the phenlogical traits of sesame (  Figure 2).

Effect of N fertilizer on growth parameters of sesame
Number of branches and plant height showed significant variation among the N doses tested under rain fed and plant height under irrigation condition (  (Table 4).

Effect of N fertilizers on sesame yield and yield components
The ANOVA showed that there was a significant variation among the treatments on number of capsules and seed yield ( Table 2). Maximum numbers of capsules (~62) were recorded on the 64 and 75.5 kg N ha -1 applied plots, while 18 and 29.5 kg N ha -1 scored the lower capsules. Lower seed yield (712 kg ha -1 ) was recorded from the lowest N rate (18 kg N ha -1 ), where the higher sesame seed yield (914-1092 kg ha -1 ) was harvested from the N fertilizers 52.5-121.5 kg N ha -1 (Table 3). Numbers followed with similar letters have not significant difference.
Regarding the seed yield across years; minimum seed yield was recorded from the higher and lower N doses (18, 144.5 and 156 kg N ha -1 ) and the maximum was from 75.5 kg N ha -1 in 2016 production year (Figure 3). In the second year (2017), 87.5 kg N ha -1 followed by 64 kg N ha -1 has recorded the maximum seed yield, whereas 18 kg N ha -1 scored the lowest seed yield.
Higher seed yield was also recorded from 75.5 kg N ha -1 and the lower seed yield was harvested under the lowest N application rate (18 kg N ha -1 ) (Figure 3).

Figure 3 Seed yield responses of the N doses under rain fed conditions across years
The ANOVA showed that there was a significant variation among the N doses on number of capsules, thousand seed weight and seed yield ( Table 2). Heavier seed weights (3.6-3.9 g) were recorded on 64, 75.5, 87 and 98.5 kg N ha -1 , while the lighter seeds were recorded on the lower N application rates (Table 4). Maximum numbers of capsules (~63) were recorded on the 64 and 75.5 kg N ha -1 applied plots, while 18 and 29.5 kg N ha -1 scored the lower (33) capsules. The higher seed yield was harvested on the 64 and 75.5 kg N ha -1 , where the lower seed yield was on the 18 kg N ha -1 (Table 4). The seed yield responses in each of the irrigation seeasons was presented in figure (4). Higher  and treatment ( Figure 5). In the rain fed condition, the higher AE (5.9) was calculated from 64 kg N ha -1 and the lower (0.6) was from the higher N application rate (156 kg N ha -1 ). In the irrigation, the maximum AE (6.1) was recorded from 64 and 75.5 kg N ha -1 , while the lower (0.9) was on the higher N dose ( Figure 5).

Results for economic analysis
Results of the N doses under rain fed condition indicated that 64 kg N ha -1 had the higher net profit (40320 Birr ha -1 ), about 5168 Birr ha -1 increment was achieved in comparison to the 41 kg N ha -1 ( Table 5). The 64 kg N ha -1 has a corresponding marginal rate of return (616.3%) and it was supported by the residuals, which was ranked as first compared to the other treatments (Table 6). In the irrigation condition; the 75.5 kg N ha -1 had higher net profit (32579 Birr ha -1 ), it has achieved about 9683 Birr ha -1 increment compared to the blanket recommendation (41kg N ha -1 ) ( Table 5). In addition, the 75.5 kg N ha -1 has higher corresponding marginal rate of return (84.6%) and also maximum residuals, which was ranked number one (Table 6).

Discussion
Days to 50% flowering showed an increment as N dose increased ( Figure 2). This result might be due to the nitrogen availability in the vegetative growth that might lead the plant to vigorous growth than to flower initiation. In addition, days to 90% maturity somewhat reduced as N rate increased up to 133 kg N ha -1 in the rain fed condition ( Figure 2). Haruna [23] noted that unfertilized plot was flowering 14% earlier than the 100 kg N/ha treated plot in moisture deficit areas. Similar results were reported that higher (69 kg N ha -1 ) nitrogen application has delayed flowering compared to the unfertilized ones in the drought prone parts of north Ethiopia [12]. On the other side, an experiment carried out in higher rainfall areas witnessed that emergence date, flowering date (52 days) and maturity date (89) were not affected by N application rates [5]. It can be concluded that, in the drought prone areas and/or in higher temperature areas, unfertilized sesame may flower earlier than the fertilized one. About 6-8 days difference was observed on 50% flowering between the rain fed and irrigated sesame, which the later one was flowered earlier. This might be because of the higher temperature in irrigation season compared to the rain fed season or months (Figure 1). Branch numbers and plant heights were higher in the higher N  [24]. Similarly, plant height was significantly increased by N rate (60 kg N ha -1 ) as per the report by Haruna [23].
Therefore, it can be concluded that in potential environments, application of higher dose of urea can increase the growth parameters of sesame.
The field experiment conducted in HuARC witnessed that application of 64 kg N ha -1 produced 27% capsule number and 15% seed yield increments compared to the previously recommended N rate (41 kg N ha -1 ) by Mizan et al [12] under rain fed (Table 3). In the irrigation, 64 and 75.5 kg N ha -1 have 25% and 30% seed yield increments, respectively, over the blanket recommendation (41 kg N ha -1 ) ( Table 4). This result aligns with studies conducted on application rates that 50 kg N ha-1 produced significantly higher number of capsules (83.8) and seed yield (888 kg ha-1 ) compared with the lower rate and untreated plots [25]. Higher seed yield was obtained when 75 kg N ha -1 dose was applied, where the lower and higher doses (0 and 133 kg N ha -1 ) recorded lower seed yield [26]. Similarly, application of 80 kg N ha -1 produced 752 kg ha -1 seed yield [27]. Furthermore, in the higher rainfall areas with lithosols higher seed yield was recorded when 75 kg N ha -1 was applied [28]. Haghighatnia et al [26] reported that higher yield was obtained from 50 kg N ha -1 compared to 160 kg N ha -1 and Blal et al [27] reported also higher seed yield of sesame was harvested when 92 kg N ha -1 was applied rather than 160 kg N ha -1 application. Field experiments conducted under irrigation condition in Egypt reported that seed yield were increased significantly when 92 kg N ha -1 applied rather than 156 kg N ha -1 [27].
Zenawi and Mizan [13] reviewed that application of nitrogen fertilizer for sesame could differ based on growing environments (considering the edaphic and climate conditions) 46 up to 100 kg N ha -1 . Agronomic efficiency of nitrogen fertilizer (N-AE) indicated that the 64 kg N ha -1 in comparison to 41 kg N ha -1 has about 0.5 and 1.7 kg additional amount of economic seed yield per a kg of N applied under rain fed and irrigation, respectively, and 75.5 kg N ha -1 has also scored 1.6 kg of seed yield per applied kg of N under irrigation ( Figure 5). N-AE of the higher N doses was below 1 kg kg -1 , this might be caused by the vigorous and leafy growth during the vegetative stage that prolonged flower initiation. Besides the agronomic efficiency results of the partial budget analysis also clearly confirmed that the additional capital invested in 64 kg N ha -1 under rain fed condition scored greater net profit and MRR. Besides both 64 kg N ha -1 and 75.5 kg N ha -1 had the higher net profit and MRR than all the other treatments under the irrigation condition.

Conclusion
Higher no. of days to 50% flowering was on the 156 kg N ha-1 and the lower days on 18 N ha -1 .
The both growing conditions. The partial budget analysis also revealed that 64 kg N ha -1 and 75.5 kg N ha -1 had the higher net profit, MRR and residual ranking under rain fed and irrigation respectively. Therefore, for the maximum sesame productivity 64 kg N ha -1 for rain fed and 64 to 75.5 kg N ha -1 should be recommended for irrigated sesame.
Funding: This study was supported financially by Tigray Agricultural Research Institute, Humera Agricultural Research Center.