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NUE; UREA stabil; Profitability; Yield

Nitrogen is the most limiting plant nutrients in general and in the tropics particular where the fluctuation of temperature and precipitation scorches the organic matter, which is a large reserve of organically bounded nitrogen which later converted into inorganic forms and utilized by plants with the deed of soil biota. Hence, high rate of organic matter decomposition severely depletes soil nutrients especially soil nitrogen and usually manifested in the poor performance of test crops in Ethiopia without any supply of inputs.

Different studies released recommendations since nineteen seventy especially on nitrogen and phosphorus although it is blanket recommendation. Since then, the associated yield obtained from a given pieces of land decreases without the application of chemical fertilizers witnessing that there is a need for applying sufficient amount of nitrogen in the cropping systems in Ethiopia. Therefore, the fertilizer import has been progressively increased for the last decade (Agricultural Input and Marketing Directorate, Ministry of Agriculture (MoA), Central Statistics Agenecy (CSA), 2010 & 2011, African Fertilizer, FAO). Parallel to this, the national fertilizer consumption started at nineteen seventy and gradually increased through nineteen eighty, nineteen ninety five and two thousand four and two thousand five which stood at 950, 43 200, 250 000, and 323 000 tons respectively.

Figure 1: Maize yield (kg ha^-1), total production (x10⁴ tons), and area coverage (x10⁴ ha) in Ethiopia from 1993-2019. Adapted from the FAOSTAT database.

Based on the International Fertilizer Association’s (IFA) suggestion fertilizer application rate, nutrient uptake close to 4.3 million tons of nitrogen, phosphate, and potash is required to produce this much maize, wheat, barley, and sorghum. However, CSA’s farm management practice data from two thousand three/ two thousand four to two thousand ten / two thousand eleven shows that the actual amount of fertilizer applied was 1.6 million tons, and when considering that a limited amount of nutrients from the fertilizer is actually taken up by the crops, more than 2.7 million tons of nutrients from the soil. A large portion of the uptake of nitrogen, which is the most important micronutrient for crop growth, is coming from the soil, given the fact that insufficient amounts applied.

Sieling reported that the mineral fertilizer NUE for winter wheat decreases with increasing N levels. Hatfield and Prueger (2004) also confirmed that the efficiency of N use by a crop depends upon the response of water and N availability during the growing season.

Accordingly, in Ethiopia urea as a source of nitrogen and DAP as a source of nitrogen and phosphorus applied as a high-grade fertilizer in order to obtain optimum harvest. One of the major challenging issues especially in high rainfall and waterlogged area in connection with these are the loss of nitrogen through ammonium volatilization or fixation by clay minerals. On the other hand, in low moisture area, its low solubility and toxic effect burns the root so as to reduce its growth and the performance of the crops grown under this condition also challenges the use of these fertilizers. The national average yield of rice is about 2.8t ha^-1 (CSA, 2018), which is lower compared to the world average productivity of 4.6 t ha^-1 (FAOSTAT, 2018).

Nitrogen is the major yield-limiting plant nutrient in the rice production system of northwestern Ethiopia as stated by [1]. UREAstabil is one of the slow-releasing nitrogen fertilizers and hydrolysis of urea is reduced by the presence of N-(nbutyl) thiophosphoric- triamide (nBTPT) that slows down urease activity of urea hydrolysis thereby improve recovery of nitrogen applied. This fertilizer application belived to increase crop productivity. However, the soil, crop management practice, availability of water and climate condition affects urease inhibitor (nBTPT) [2].

UREA^Stabil is a concentrated nitrogen fertilizer that can be applied as a granular for field crops as well as liquid fertilizer through irrigation water for the orchard. Besides, it supposed to have basic advantage of having a combination of rapidly soluble, well absorbable nitrogen with urease inhibitor that helps to improve nitrogen penetration to plant roots by restraining the sorption and fixation of NH4+ in the surface soil layer, which slows the effect of this nitrogen form down. In another way it helps to reduce its losses due to ammonia volatilization into the atmosphere during surface application. However, this product has never been tested in our context [3].

Therefore, this research is proposed: - To determine the effects of UREA^Stabil on the enhancement of nitrogen utilization efficiencies of the rice crop. To evaluate the influence of UREA^Stabil on the growth and yield of rice under waterlogged conditions [4].

Description of the study area

An experiment on UREA^Stabil and conventional urea nutrients were conducted on rain fed lowland rice in Fogera district. The study at Fogera area is situated at 11 °54.4’46.3’’N to 11 °57’03.0’’N latitude and 37 °41’23.9’’E to 37 °42’32.2’’ E longitude at elevation range of 1787-1812 meter above sea level. The study site received mean annual rainfall of 1219 mm with annual average minimum and maximum temperature of 12.75°C and 27.37°C, respectively [5].

Plant material

The rice variety X-jigna has rain fed lowland ecosystem adaptability and good characteristics of cold tolerance. The morphology of the variety is unerect leaf angle and unerect flage leaf angle. The genotype has intermediate type of panicle and purple color of the apex. The biomass response is promising and highly preferred by the producers with the right agronomic management practice. In addition to the high biomass and yield performance white color grain of X-jigna variety chosen by the rice producing farmers [6].

Soil characteristics

The soil in Fogera immeasurably require nitrogen fertilizer and without N fertilizer yield is not expected but other macro nutrients like P & K deficiency not highly observed [7]. The experimental soil type was found to be vertisols and the textural class was clay loam. The PH (H₂O) of the soil was 5.89 and soil organic carbon was 1.2% [8].

Experimental design and treatments

The treatments were comprised UREA^Stabil and conventional urea at recommended rates of nitrogen sources. Therefore, a total of eight rates were used to evaluate the efficiencies of UREA^Stabil under waterlogged and high rainfall conditions. The treatments set-up was (without any external application of N (control=0N), recommended N from conventional urea 91kgha^-1 (30kgha^-1 at planting & 61kgha^-1 tillering stage), recommended N from UREA^Stabil (91kgha^-1), half of the recommended N from UREA^Stabil (45.5 kgha^-1), recommended N from UREA^Stabil (45.5 kgha^-1 at planting & 45.5 kgha^-1 tillering), half more than the recommended N from UREA^Stabil (136.5kgha^-1, 45.5 kgha^-1 at planting & 91 kgha^-1 tillering), half more than the recommended N from conventional urea (136.5kgha^-1, 45.5 kgha^-1 at planting & 91 kgha- 1 tillering), half more than the recommended N from UREA^Stabil 136.5 kgha^-1) once at planting. The treatments were laid out in Randomized Complete Block Design (RCBD) and replicated three times. The gross size of the experimental plots was 3m x 4m consisting of 15 rows planted at a spacing of 20 cm apart with the seeding rate of 100 kg ha ^-1 X-jigna rice variety. The net plot area was made by excluding the left and right outer rows and a plot length of 0.5 m from the top and bottom sides of the plot. The final net plot size was thus 2.6m x 3m [9].

Data collected

Data on plant height, panicle length, number of total tillers per m2, number of fertile panicles per m2, thousand seeds weight, grain yield, straw yield and harvest index were collected timely from the net plot areas following their respective standard measuring methods and procedures. The rice grain yield and thousand seeds weight were adjusted at 14% standard moisture content [10].

Data analysis

All collected data were subjected to analysis of variance (ANOVA) using SAS software version 9.0 (SAS-Institute, 2003). Mean separation was done by using Least significance difference (LSD) method at probability levels of P< 0.01 and P< 0.05 depending on the ANOVA results. Statistical analysis of the grain yield and NUE data were also accomplished by standard analysis of variance (ANOVA) [11].

Partial budget analysis

A method of organizing experimental data and information about the costs and benefits of various alternative treatments. A partial budget analysis methodology is a way of computing the total costs that vary and the net benefits of each treatment in an on-farm experiment. Includes the average yields for each treatment, adjusted yields and gross benefit (based on the field price of the crop). It also incorporates all the costs that vary for each treatment (CIMYYT (1988) [12-15].

The N use efficiency of mineral N fertilization was calculated by equation:

Agronomic N use efficiency

Where F and C represent Fertilized and Control plots respectively. NUE can be calculated as the ratio between the amount of fertilizer N removed with the crop and the amount of fertilizer N applied. It can be expressed in %.

The Analysis of variance indicated that plant height was significantly (P<0.05) affected by UREA^Stabil and Conventional urea. The highest plant height (83.6cm) was recorded from the split application of 136.5kg N ha^-1 from UREA^Stabil splited at planting (45.5 kg N ha^-1) and tillering stage (91 kg N ha^-1) while the lowest plant height (70.7cm) was recorded from the control without the application of nitrogen fertilizer (Table 1). However, from the conventional urea fertilizer the plant height was recorded 80.8cm from the application of 136.5 kg N ha^-1 and splited at planting (45.5 kg N ha^-1) and 91kg N ha^-1 tillering stage [16-19].

Panicle length was not significantly affected by conventional urea and UREA^Stabil fertilizer application where as the number of total tillers were highly significantly (P<0.01) affected. The highest number of tiller (260.7per m2) was found from the application of UREA^Stabil (136.5kgha- 1) at planting and tillering stage where as the lowest number of tiller (154.7 per m2) was found from the control without application of nitrogen fertilizer. Similar results reported as rice is a unique crop with an indeterminate tillering potential, and the actual tillering number is easily influenced by nutrients availability, planting density and variety. Split application of conventional urea fertilizer (136.5 kgha^-1 at planting (45.5 kgha^-1 and 91 kgha^-1 at tillering stage) had resulted 245.7 tiller number per m2 [20].

The analysis of variance for the number of fertile grains showed that significantly (P<0.05) affected by UREA^Stabil and conventional urea fertilizer. The highest number of fertile grains (248.0 per m2 ) was found from 136.5 kg N ha^-1 of UREA^Stabil in split application 45.5kgha- 1 at planting and 91kg N ha^-1 at tillering stage where as the lowest number of fertile grain (147.3) was attained from the control without the application of nitrogen fertilizer. However, from conventional urea through the application of 136.5 kg N ha^-1, 45.5 kg N ha^-1 planting and 91kg N ha^-1 at tillering stage 226.0 fertile grains per m2 were produced. The disadvantage of urea fertilizer is that considerable amounts of N can be lost from through volatilization which may result in very low N fertilizer use efficiency [21-22].

The grain yield was highly significantly (P<0.01) affected by both UREASabil and conventional urea fertilizer N source applications. However, Concerning UREA^Stabil fertilizer application the highest grain yield (3.55 t ha^-1) was found from the application of 136.5 kg N ha^-1 from the split application while from the conventional urea fertilizer application 3.14 t ha^-1 grain yield was produced at 136.5 kg N ha^-1 split application at planting (45.5 kgha^-1) and tillering stage (91kgha^-1). Improved grain and straw yields at the higher rates of N nutrient may be attributed to the fact that application of fertilizer for crop uptake and translocation to sink thereby expressing superior crop growth and development [23-25].

Straw yield was highly significantly (P<0.01) affected by UREA^Stabil and conventional urea N sources. The straw yield was affected both by sources and rates of N fertilizer. However, the highest straw yield (5.79 t ha^-1) was recorded from 136.5 kg N ha^-1 from UREA^Stabil and from the conventional urea fertilizer 5.67 t ha^-1 was recorded as split application of 136.3 kg N ha^-1 (45.5 kgha^-1 planting and 91 kg N ha^-1 at tillering stage) of rice (Table 1).

Table 1: Combined mean effects of UREA^Stabil and conventional urea fertilizer sources and rates on growth and yield of rice for two consecutive years (2015-2016) in Fogera districts, northwest Ethiopia.

Thousand seed weight was not significantly (P<0.05) affected by UREA^Stabil and conventional urea treatments even if the grain yield is significantly different between treatments [26].

The rice harvest index was highly significantly (P<0.001) affected by the UREA^Stabil. The highest HI (44.6) was shown from 136.5 kg N ha^-1 from UREA^Stabil source of nitrogen applied once at planting and the lowest (26.0) from 45.5 N as the source of UREA^Stabil followed by 91 kg N ha^-1 (41.7 HI) from conventional urea split application among nitrogen source of fertilizers. Similar results observed at the highest harvest index from the report of. Higher grain yields in the fertilizer treatments were associated with higher harvest index [27].

The analysis of Agronomic Efficiency (AE) for the nitrogen sources and rates indicates that the maximum AE 30.55 was exhibited at 136.5 kg N ha^-1 from UREA^Stabil source of nitrogen split application (45.5kgha- 1 basal and 91kgha^-1 tillering stage), then the AE reduce to 15.53 at 136.5 kg N ha^-1 from conventional urea N source split application (45.5 kgha- 1 at planting and 91 kgha^-1 at tillering stage) and 15.68 AE at 136.5 kg N ha^-1 from UREA^Stabil source once application. The AE becomes high 30.55, 25.93, 24.62 at 45.5 kg N ha^-1 from UREA^Stabil , 91kg N ha^-1 from UREA^Stabil (45.5/45.5), 91 kg N ha^-1 from conventional urea (30/90) respectively (Table 2).

Table 2: Agronomic Efficiency (AE) of Nitrogen.

The lower agronomic efficiency at the highest N rates in the current experiment indicates that emphasis should be given to efficient nitrogen application methods. The split application method even for slow release N fertilizer is highly essential for water logged areas like Fogera because that from the current experiment the AE is higher at split application of slow release N source than from conventional urea N sources and real time of N management to reduce denitrification and loss of nitrogen fertilizer to efficiently use by the plant. AE N is usually higher at low N rate than at high N rate [28-30].

Following the CIMYYT (1988) partial budget analysis method, grain and straw yield adjustments, computations of total variable costs (TVC), gross benefits (GB) and net benefits (NB) were accomplished (Table 3) [31-33]. Dominance analysis was conducted after arranging the treatments in their order of TVC. A treatment will be contepleted as dominated if it has higher TVC but lower NB than a previous treatment with lower TVC and higher NB (Table 4). Non-dominated treatments were taken out and marginal rate of return (MRR) was computed (Table 5). According to the CIMYYT (1988) partial budget analysis, treatments revealing the minimum or more MRR (>100%) will be considered for the comparison of their NB. Highest NB (Birr 47,356 ha^-1) with acceptable level of MRR (478.3%) was observed at 136.5kg N ha^-1 split application of UREA^Stabil (45.5/91). Split application of 136.5 kg N ha^-1 from UREA^Stabil source 45.5 kg N ha^-1 as basal and 91kg N ha^-1 tillering stage is the most profitable rate and source to be recommended for low land rice production of Fogera area [34, 35].

Table 3: Results of grain yield and straw yield adjustments, total variable cost, gross and net benefits analysis.

Table 4: Results of dominance analysis.

Table 5: Results of Mariginal Rate of Return (MRR) analysis.

Application of different rates of slow release UREA^Stabil and conventional urea fertilizer strongly affected the grain yield of rice. Parallel to the grain yield, the straw yield is highly necessary for cattle feed in Fogera area. Slow release UREA^Stabil nitrogen fertilizer sturdily influenced the straw yield. In waterlogged areas, the application of highly mobile nitrogen fertilizer beyond the optimum increase the lose and decrease the final out put yield. From the study split application of 136.5 kgha^-1 conventional nitrogen (as basal 45.5 kgha^-1 & 91kgha^-1 tillering stage) reduce net benefit by increasing the total variable cost. The split application method even for slow release N fertilizer is highly essential for water logged areas like Fogera because that from the current experiment the AE is higher at split application of slow release N source than from conventional urea N sources and real time of N management and to efficiently use by the plant. This further revealed slow release UREA^Stabil nitrogen fertilizer enables to enhance the nitrogen utilization efficiency by reducing denitrification, leaching and ammonia volatilization. In waterlogged conditions of rice production, slow release UREA^Stabil nitrogen source of fertilizer has strong and promising effect on the growth and yield and yield components rice.

Based on the results of the present study both biological and partial budget analysis revealed that the highest grain yield and economic profitability was exhibited from 136.5 kg N ha^-1 applied in the form of UREA^Stabil in split application 45.5 kg N ha^-1 as basal and 91 kg N ha^-1 tillering stage for water logged areas of Fogera and similar agroecologies in Ethiopia.

Further research work for the improvent of nitrogen use efficiency of rice on slow release UREA^Stabil nitrogen sources of fertilizer in combination with micro nutrients also recommended.

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