Bangladesh Agron. J. 2023, 26(1): 84-95 DETERMINATION OF ECONOMIC NITROGEN RATE FOR TRANSPLANTED AUS RICE VARIETIES OF BANGLADESH Munmun Akter1, Md. Khairul Alam Bhuiyan2 and Sheikh Muhammad Masum1* 1Department of Agronomy, Sher-e-Bangla Agricultural University, Dhaka-1207, Bangladesh 2Agronomy Division, Bangladesh Rice Research Institute *Corresponding author, Email: smmasum607@yahoo.com (Received: 24 July 2023, Accepted: 28 September 2023) Keywords: Rice, nitrogen, nitrogen uptake, nitrogen use efficiency, yield Abstract A field experiment was conducted at the Agronomy field, Bangladesh Rice Research Institute, Gazipur, Dhaka, from April to September 2019 to determine the economic nitrogen rates for popular transplanted Aus varieties. The experiment was carried out in a randomized complete block design (RCBD), with two factors. Factor A: three varieties as- BR26, BRRI dhan48, and BRRI dhan82; and Factor B: five levels of nitrogen rates as- 0, 40, 60, 80, 100 kg ha−1. The experimental data show the individual effect of variety and nitrogen (N) rate was significant in the case of N concentration, N uptake in grain and straw, and nitrogen harvest index (NHI). Overall, increasing N rate increases grain and straw's concentration irrespective of varieties. But higher N concentration and uptake were observed in the N rate of N60-N100 kg ha−1 in all varieties. Among the varieties, BRRI dhan82 observed higher N uptake (61.23 kg ha−1) in grain. Higher total N uptake was also observed in BRRI dhan48 at 80 kg N ha−1. NHI ranged from 55 to 72 % in different N levels, indicating 55 to 72% of the absorbed N translocated to the grains, and 45% to 32% remained in the dry matter within varieties. The estimated economic nitrogen dose for maximum yield was determined by regression analysis, and found that N rates of BR26, BRRI dhan48, and BRRI dhan82 were 97, 95, and 55 kg ha−1, respectively. The findings of this study indicated that the response of different N rates on three Aus varieties was linear up to 80 kg N ha−1 might be owing to better N uptake that made yield increase after that decreasing. Introduction Rice (Oryza sativa L.) belongs to the family Poaceae, it’s the most important cereal crop in the developing world and the main source of carbohydrates for the people of Bangladesh, and it is the staple food of more than half of the people in the world (BRRI, 2021). About 90% of annual rice is produced and consumed in Asia. The average yield in Asia is lower than the global mean yield (Haider, 2018). About 35-60% world's population derives most of its calories from rice (Tayefe et al., 2014). The possible way to meet this increased demand is by improving rice yield hectare−1 (Liu et al., 2016). Bangladesh is the 3rd largest rice-producing country in the world (USDA, 2020). About 75% area and over 80% of the total irrigated area of Bangladesh is covered by rice (Nasim et al., 2017). Bangladesh's Aus rice (summer rice) is a significant crop for drought-prone, low-water- requiring environments. Aus is usually planted in March-April and harvested in June-July, and the climate is practically combined with hot summer (March-May). Aus rice occupies about 12.53% of the total cultivable area from where modern varieties cover 10.67%, local varieties cover 1.86 %, and 7.49% of the total production comes from Aus rice, where modern varieties cover 6.87% and local varieties cover 0.62%. In Bangladesh, the current status of the total area and production of Aus rice is 1.08 million ha and 2.71 million MT (BBS, 2021). Economic nitrogen rate for transplanted aus rice 85 Nitrogen (N) fertilizer is required to grow and develop rice and grain production. N fertilizer is the main input in rice production and the optimum rate and profitability of management during the application and stability of the production system (Djaman et al., 2018). Improving leaf N concentration, photosynthetic rate, delaying leaf senescence, and increasing dry matter for grain filling, N helps to increase rice productivity (Hasegawa et al., 1994). During the Aus season, deep placement of fertilizer above 52 and 78 kg N ha−1 had no significant effects on grain yields but reduced N recovery (Islam et al., 2016). N can improve panicle size and grain weight and reduce spikelet sterility (Fageria, 2009). The application of N fertilizer in rice has also been reported to significantly increase the grain and straw N uptake and N use efficiency (Hassan et al., 2009). Balanced fertilization is more important for realizing potential grain yield and nutrient uptake than a single application of fertilizer, particularly N (Talashilker and Vimol, 1986). Farmers generally apply more fertilizers than the recommended amount with the idea that using N will always increase the yield, which can result in change, negatively affect the sustainability of the production system, and increase production costs (Fan et al., 2012). On the other hand, the application of excess N can result in groundwater pollution, increased production costs, reduced yields, environmental pollution (Djaman et al., 2018), and vegetative growth, which makes the plant susceptible to insects, pests, and diseases and ultimately reduces yield (Chamely et al., 2015). The maximum output of rice was recorded due to the application of 90 to 250 kg N fertilizer ha−1 (Meena et al., 2003). Nitrogen fertilizers are considered to have dominant influences on different agronomic characteristics of rice, such as plant height, tiller number, filled grains panicle−1, spikelet panicle−1, grain yield, straw yield, biological yield, harvest index, etc. Nitrogen also influences the interception of sunlight, leaf area index (LAI), crop growth rate, total dry matter production (TDM), and N uptake (NUP) to the plant. Under these circumstances, it is essential to determine the N-responsive stages of plant growth to ensure its maximum requirement to utilize the applied N effectively. Moreover, an economic N rate is required for the farmers to get maximum profit. Nitrogen fertilizer varies in different locations, ecosystems, and different varieties depending on the initial N status of the soil (Masum et al., 2008) A suitable combination of variety and rate of N is required for better yield. Therefore, the research objectives were to find the performance of T. Aus rice varieties in different N levels and the economic N rate of T. Aus rice varieties. Materials and Methods The experiment was conducted at the experimental farm of Bangladesh Rice Research Institute in Aus season (summer rice) during the period from April to August 2019. The experimental location's climate is subtropical and characterized by heavy rainfall from April to September and scanty rainfall from October to March, with an average annual rainfall of 2000 mm. The soil of the experimental site belongs to the order Inceptisols in the USDA soil classification system. Before the initiation of the experiment, soil samples from the top layer (20 cm) were collected and analyzed. The soil was silty clay loam in texture having pH 6.45, organic matter 1.54 (%), total N .014% (Kjeldahl digestion method), available P 24.65 ppm, exchangeable K 0.18 (me/100 g soil) and available S 19.55 ppm. Five rates (0, 40, 60, 80, and 100 kg ha-1) of N were tested on three high-yielding rice varieties (BR26, BRRI dhan48, and BRRI dhan82). Sprouted seeds were sown in the wet nursery bed on 10 April 2019. Proper care was taken to raise healthy seedlings in the seedbed. Weeding irrigation was done when was necessary. The experiment was laid out in 2 factors randomized complete block design with three replications. There were 45 unit plots in the experiment. Each replication was divided into 15 unit plots where the treatment combinations were allocated at random. The size of each unit plot was (2.5 m x 4.0 86 Masum et al. m =10 m2). The spacing between block to block and plot to the plot was 0.6 m and 0.4 m, respectively. The N was applied as urea in three equal splits: the first at the time of final land preparation, the second at maximum tillering stage and the third at panicle initiation. All the treatments received an equal amount of phosphorus (P), potassium (K), and sulfur (S) fertilizers and applied as basal. The application of fertilizer in the experiment is shown in Table 1. Table 1. Application of fertilizer in the experimental plot Treatment Fertilizer (kg ha−1) Application of fertilizer in the experimental plot (g 10 m⁻2) Total 1st installment 2nd installment 3rd installment N0 N0 0 0 0 0 N1 N40 87 29 29 29 N2 N60 130 43.3 43.3 43.3 N3 N80 174 58 58 58 N4 N100 217 72.3 72.3 72.3 TSP 53 53 53 0 0 MoP 82 82 82 0 0 Gypsum 28 28 28 0 0 Twenty days old seedlings were transplanted on 30 April 2019. In each plot, spacing (plant to plant and row to row) was 20 x 20 cm with two seedlings hill−1. The field was investigated occasionally to detect the visual difference between the treatment and any infestation by weeds, insects, and diseases to minimize considerable losses by pests. Experimental data collection began at 15 days after transplanting and continued until harvest. The necessary data on yield contributing characters were collected from twelve selected hills (2 x 2) x 3 from each plot in the field, and at the harvest plot, yield was harvested from a 5 m2 plot and adjusted to 14% moisture content expressed as t ha−1. After harvest, grain and straw were taken from respective plots, and all samples were oven-dried at 70oC for 72 h, weighed, ground, and then subsamples were taken for N determination. The standard micro-Kjeldahl procedure measured n content in the grains and straw (Bremner and Mulvaney, 1982). N uptake in grain and straw was calculated by following formulae. Nitrogen uptake by grain (kg ha-1) = % N in grain × Grain yield (kg ha-1 ) 100 Nitrogen uptake by straw (kg ha-1) = % N in straw × straw yield (kg ha-1 ) 100 The optimum N dose for the tested Aus rice variety was determined by regression of the grain yield with the N rates: Y = a + bN+ cN2 Where Y is rice yield (kg ha−1), N is nitrogen dose (kg ha−1), a is intercept (estimated yield without N application), b and c are coefficients, respectively. Differentiating Y with respect to N of the equation gives the N dose for the maximum yield. The optimum N dose for maximum yield is calculated by N= -b/2c, and the equation of economic N rate is x= (EN-b)/2c. Where, EN= pf/py. Here, pf indicates the price of N fertilizer (18 Tk kg−1 and py indicates the price of paddy (Tk kg−1). The data obtained for different characters were statistically analyzed with the computer- based software CropStat V. 7.2, and mean separation was done by the least significance difference test (LSD) at a 5% level of significance. Regression analyses were done to determine the economic N rates of the respective varieties. Economic nitrogen rate for transplanted aus rice 87 Results and Discussion Nitrogen concentration (N%) in plant parts during growth stage Nitrogen is the only nutrient that has not a common soil mineral source. Nitrogen in beneficial forms of plants is probably the most limited nutrient for plant growth. Most of plants' nutritional dynamics and supply rates are related to Model N. Nitrogen concentration (N%) was significantly influenced by different varieties used in the present study (Table 2). Table 2. Nitrogen concentration (%) of BR26, BRRI dhan48, and BRRI dhan82 as affected by nitrogen rates at BRRI farm, Gazipur BR26 N0 0.25 0.47 0.52 0.57 0.29 0.20 N40 0.34 0.84 0.89 0.90 0.67 0.48 N60 0.45 1.01 1.14 1.07 0.79 0.69 N80 0.50 1.16 1.52 1.18 0.83 0.79 N100 0.52 1.54 1.74 1.31 0.87 0.82 BRRI dhan48 N0 0.23 0.53 0.55 0.46 0.22 0.21 N40 0.26 0.76 0.92 0.84 0.59 0.59 N60 0.39 1.02 1.15 1.05 0.83 0.70 N80 0.43 1.31 1.58 1.15 0.88 0.81 N100 0.45 1.54 1.70 1.39 0.91 0.87 BRRI dhan82 N0 0.21 0.53 0.53 0.49 0.21 0.23 N40 0.29 0.88 0.90 0.80 0.55 0.52 N60 0.34 1.18 1.22 1.01 0.75 0.69 N80 0.46 1.41 1.71 1.09 0.83 0.76 N100 0.52 1.65 1.74 1.25 0.91 0.82 LSD (0.05) ns ns ns ns ns ns CV (%) 18.6 12.4 9.5 7.7 10.1 10.2 Note: ns= Non-significant; N0= 0 (Control), N1= 40 kg ha−1, N2 60 kg ha−1, N3= 80 kg ha−1, N4= 100 kg ha−1 At 15 DAT (days after transplanting), in BR26, the N concentration was recorded as 0.41. The results obtained from BRRI dhan48 showed the lowest N concentration (0.35). On 30 Treatment Days after transplanting 15 DAT 30 DAT 45 DAT 60 DAT 75 DAT Maturity Variety BR26 0.41 1.01 1.16 1.01 0.69 0.59 BRRI dhan48 0.35 1.03 1.18 0.98 0.69 0.63 BRRI dhan82 0.36 1.13 1.22 0.93 0.65 0.60 LSD (0.05) 0.52 0.98 ns 0.56 ns ns Nitrogen rate N0 0.23 0.51 0.54 0.52 0.24 0.21 N1 0.29 0.83 0.90 0.85 0.60 0.53 N2 0.39 1.07 1.17 1.04 0.79 0.69 N3 0.47 1.29 1.60 1.14 0.84 0.78 N4 0.49 1.58 1.73 1.32 0.89 0.84 LSD (0.05) 0.68 0.13 0.11 0.72 0.66 0.60 CV (%) 18.6 12.4 9.5 7.7 10.1 10.2 Variety × Nitrogen rate 88 Masum et al. DAT, BRRI dhan82 (1.13) recorded the highest N percentage and the lowest (1.01) from BR26. At 45 DAT, the highest N concentration was recorded by BRRI dhan82 (1.22) and the lowest (1.16) from BR26. At 60 DAT, the highest N concentration (0.98) was obtained from BRRI dhan48 and the lowest (0.56) from BRRI dhan82. At 75 DAT, the highest N concentration (0.69) was obtained from BRRI dhan48 and BR26 and the lowest (0.65) from BRRI dhan82. At maturity, the highest N concentration (0.63) was obtained from BRRI dhan48 and the lowest (0.59) from BR26. Nitrogen concentration in plant parts increased over time due to the application of nitrogenous fertilizer and plant demand. Thereafter it seems to decline trend towards maturity. Similar research findings were also reported by Zhang et al. (2009), Haque et al. (2015a), and Djaman et al. (2016). Different levels of N application showed statistically significant variations in N concentration (N%) of Aus rice at 15, 30, 45, 60, 75 DAT, and maturity (Table 2). At 15 DAT, the highest N concentration (0.49) was recorded by N100 (100 kg ha−1), closely followed (0.47) by N80 (80 kg N ha−1). The results obtained from N0 (0 kg ha−1) showed the lowest N concentration (0.23). On 30 DAT, the highest N concentration (1.58) was recorded by N100 (0 kg ha−1) and the lowest (0.51) from N0 (0 kg ha−1). At 45 DAT, the highest N concentration (1.73) was recorded by N100 (100 kg ha−1) and the lowest (0.54) from N0 (0 kg ha−1). At 60 DAT, the highest N concentration (1.32) was obtained from N100 (100 kg ha−1) and the lowest (0.52) from N0 (0 kg ha−1). At 75 DAT, the highest N concentration (0.89) was obtained from N100 (100 kg ha−1) and the lowest (0.24) from N0 (0 kg ha−1). At maturity, the highest N concentration (0.84) was obtained from N100 (100 kg ha−1) and the lowest (0.21) from N0 (0 kg ha−1). Perez et al. (1996), Cassman et al. (2003), and Djaman et al. (2018) reported that improvement in crop yields is attributed to the increase in fertilizer use, especially N fertilizer. The combination effect of variety and different N rates application had an insignificant influence on N concentration at different growth stages of the three varieties of Aus rice (Table 2) However, at 45 DAT, the highest N concentration (1.74) was observed from BRRI dhan82 and BR26 × N100, (100 kg ha−1). At maturity, the lowest N concentration (0.20) was found from the combination of BR26 x N0 (0 kg ha−1). Similarly, many researchers recorded N concentrations for different rice varieties with N rates (Peng et al., 2004; Djaman et al., 2016; Djaman et al., 2018). Nitrogen uptake by plants during growth stage Nitrogen uptake (NUP) was significantly influenced by different varieties used in the present study (Table 3) during 15, 30, and 60 DAT. At 15 DAT, the highest NUP was recorded by BR26 (3.85 kg ha−1). The results obtained from BRRI dhan48 showed the lowest NUP (2.99 kg ha−1). On 30 DAT, the highest NUP was recorded by BRRI dhan82 (26.29 kg ha−1) and the lowest (23.18 kg ha−1) from BR26. At 45 DAT, the highest NUP was recorded by BRRI dhan82 (53.67 kg ha−1) and the lowest (50.50 kg ha−1) from BR26. At 60 DAT, the highest NUP (63.58 kg ha−1) was obtained from BR26, and the lowest (56.26 kg ha−1) from BRRI dhan82. At 75 DAT, the highest NUP (56.33 kg ha-1) was obtained from BR26 and the lowest (52.66 kg ha−1) from BRRI dhan82. At maturity, the highest NUP (42.63 kg ha−1) was obtained from BRRI dhan48 and the lowest (42.05 kg ha−1) from BR26. Arthanari et al. (2007) reported that the response of rice to nutrient uptake may vary with varieties. Similar research findings were also reported by Koutroubas and Ntanos (2003) and Artacho et al. (2009). Different N levels showed statistically significant variations in the NUP of Aus rice at 15, 30, 45, 60, 75 DAT, and maturity (Table 3). At 15 DAT, the highest NUP (4.69 kg ha−1) was recorded by N100 (100 kg ha−1), closely followed (4.59 kg ha−1) by N80 (80 kg N ha−1). The results obtained from N0 (0 kg ha−1) showed the lowest NUP (1.67 kg ha−1). On 30 DAT, the highest NUP (35.97 kg ha−1) was recorded by N100 (0 kg ha−1) and the lowest (10.11 kg ha−1) from N0 (0 kg ha−1). At 45 DAT, the highest NUP (75.14 kg ha−1) was recorded by N100 (100 kg ha−1) and the lowest (21.51 kg ha−1) from N0 (0 kg ha−1). At 60 DAT, the highest NUP (82.34 kg ha−1) was obtained from N100 (100 kg ha-1) and the Economic nitrogen rate for transplanted aus rice 89 lowest (48.79 kg ha−1) from N0 (0 kg ha−1). At 75 DAT, the highest NUP (72.67 kg ha−1) was obtained from N100 (100 kg ha-1) and the lowest (17.85) from N0 (0 kg ha−1). Table 3. Nitrogen uptake (kg ha−1) of BR26, BRRI dhan48, and BRRI dhan82 as affected by nitrogen rates at BRRI farm, Gazipur Treatment Days after transplanting 15 DAT 30 DAT 45 DAT 60 DAT 75 DAT Maturity Variety BR26 3.85 23.18 50.50 63.58 56.33 42.05 BRRI dhan48 2.99 22.22 50.67 58.94 53.60 42.63 BRRI dhan82 3.22 26.29 53.46 56.26 52.66 42.36 LSD (0.05) 0.45 2.25 ns 3.63 ns ns Nitrogen rate N0 1.67 10.11 21.51 27.79 17.85 11.95 N1 2.38 17.45 37.47 48.79 45.75 32.12 N2 3.49 24.49 50.50 64.32 63.93 49.22 N3 4.54 31.47 73.09 74.74 70.78 58.62 N4 4.69 35.97 75.14 82.34 72.67 59.82 LSD (0.05) 0.59 2.91 4.56 4.68 5.46 4.35 CV (%) 18.1 12.6 9.2 8.1 10.4 10.6 Variety × Nitrogen rate BR26 N0 1.92 9.45 21.04 31.71 21.79 11.57 N40 2.82 17.72 36.89 52.71 51.19 30.32 N60 4.03 23.31 49.23 67.11 64.79 49.24 N80 5.02 29.73 69.81 82.70 71.95 59.19 N100 5.47 35.69 75.52 83.69 71.89 59.93 BRRI dhan48 N0 1.57 9.99 21.85 24.55 16.15 11.30 N40 1.99 15.73 38.11 47.49 44.54 33.95 N60 3.39 22.73 49.76 63.55 64.75 48.46 N80 4.09 28.66 70.86 74.70 71.58 59.17 N100 3.89 33.99 72.76 84.39 70.99 60.26 BRRI dhan82 N0 1.53 10.89 21.64 27.09 15.62 12.97 N40 2.30 18.91 37.42 46.18 41.51 32.09 N60 3.06 27.42 52.51 62.29 62.25 49.97 N80 4.51 36.04 78.59 66.80 68.79 57.49 N100 4.71 38.23 77.13 78.94 75.11 59.26 LSD (0.05) ns ns ns ns ns ns CV (%) 18.1 12.6 9.2 8.1 10.4 10.6 Note: ns= Non-significant; N0= 0 (Control), N1= 40 kg ha−1, N2 60 kg ha−1, N3= 80 kg ha−1, N4= 100 kg ha−1 At maturity, the highest NUP (59.82 kg ha-1) was obtained from N100 (100 kg ha−1) and the lowest (11.95 kg ha−1) from N0 (0 kg ha−1). Present research results indicate that total N uptake by rice plants increased with increased N rates up to a certain level, then decreased. Yesuf and Balcha (2014) also reported similar findings. Sharma and Mittra (1990), Paikaray et al. (2001), Jahan et al. (2014), Islam et al. (2015), and Hussain et al. (2016) reported that the use of an optimum dose of N might have helped for good vegetative growth and root system, which increased the higher N uptake by plants and hence increased yield and yield components of rice. The combination effect of variety and different N rate applications had a non-significant influence https://www.tandfonline.com/doi/full/10.1080/24749508.2020.1742509 https://www.tandfonline.com/doi/full/10.1080/24749508.2020.1742509 90 Masum et al. on NUP at different growth stages of the three varieties of Aus rice (Table 3). Therefore, at 60 DAT, the highest NUP (84.39 kg ha−1) was observed from BRRI dhan82 × N100 (100 kg ha−1), closely followed (83.69 kg ha−1) by N100 (100 kg N ha−1) from BR26. At 15 DAT, the lowest NUP (1.52 kg ha−1) was found from the combination of BRRI dhan82 × N0 (0 kg ha−1) closely followed (1.57 kg ha−1) by N0 (0 kg N ha−1) from BRRI dhan48. Similar research findings were also reported by Paikaray et al. (2001) and Jahan et al. (2014). Nitrogen concentration (N%) in grains Nitrogen concentration of grain yield was significantly influenced by different varieties used in the present study (Table 4). The highest N concentration (N%) of grain yield was recorded by BRRI dhan48 (0.93). The results obtained from BR26 showed the lowest N concentration (N%) of grain yield (0.82). Different N levels showed statistically significant variations in N concentration (N%) of grain yield of Aus rice (Table 4). Table 4. Nitrogen concentration (N%) and nitrogen uptake (kg N ha−1) of BR26, BRRI dhan48, and BRRI dhan82 as affected by nitrogen rates at BRRI farm, Gazipur Treatment N (%) in grain N (%) in straw N (%) in grain + straw N uptake grain (Kg ha-1) N uptake in straw (kg ha-1) N uptake (grain+ straw) (Kg ha-1) Nitrogen harvest index (%) Variety BR26 0.82 0.41 1.22 31.93 19.91 51.84 60.41 BRRI dhan48 0.93 0.34 1.27 43.31 19.03 62.34 68.15 BRRI dhan82 0.89 0.33 1.21 35.04 16.38 51.42 66.68 LSD (0.05) 0.58 0.22 ns 5.13 2.43 7.11 2.36 Nitrogen rate N0 0.52 0.26 0.78 14.96 10.13 25.09 59.43 N1 0.72 0.33 1.06 26.49 16.03 42.52 62.17 N2 0.99 0.37 1.36 45.37 20.47 65.83 68.52 N3 1.07 0.41 1.48 52.36 23.24 75.60 69.12 N4 1.08 0.43 1.51 44.61 22.35 66.95 66.15 LSD (0.05) 0.75 0.28 0.74 6.63 3.14 9.18 3.05 CV (%) 8.9 8.1 6.2 18.7 17.6 17.2 4.8 Variety × Nitrogen rate BR26 N0 0.48 0.29 0.76 12.51 10.10 22.61 55.38 N40 0.68 0.38 1.06 22.24 16.82 39.06 57.20 N60 0.89 0.42 1.31 37.58 21.86 59.44 63.01 N80 1.01 0.48 1.49 45.27 26.01 71.28 63.70 N100 1.02 0.47 1.49 42.03 24.77 66.80 62.75 BRRI dhan48 N0 0.61 0.26 0.87 17.89 10.69 28.58 62.84 N40 0.79 0.34 1.13 29.29 16.23 45.52 64.19 N60 1.01 0.34 1.36 53.39 20.82 74.21 71.89 N80 1.10 0.37 1.47 61.48 23.46 84.94 72.37 N100 1.12 0.39 1.52 54.49 23.96 78.46 69.44 BRRI dhan82 N0 0.47 0.22 0.69 14.48 9.60 24.08 60.06 N40 0.70 0.28 0.98 27.94 15.04 42.98 65.12 N60 1.06 0.34 1.40 45.14 18.72 63.85 70.67 N80 1.12 0.37 1.48 50.34 20.25 70.59 71.29 N100 1.10 0.41 1.51 37.29 18.31 55.59 66.24 LSD (0.05) ns ns ns ns ns ns ns CV (%) 8.9 8.1 6.2 18.7 17.6 17.2 4.8 Note: ns= Non-significant; N0= 0 (Control), N1= 40 kg ha−1, N2 60 kg ha−1, N3= 80 kg ha−1, N4= 100 kg ha−1 Economic nitrogen rate for transplanted aus rice 91 The highest N concentration (N%) of grain yield (1.08) was recorded by N100 (100 kg ha−1), which was closely followed (1.07) by N80 (80 kg N ha−1). The results obtained from N0 (0 kg ha−1) showed the lowest N concentration (N%) of grain yield (0.52). The combination effect of variety and different N rate application had no significant influence on the grain yield of the three Aus rice varieties (Table 4). The highest N concentration (N%) of grain yield (1.12) was observed from BRRI dhan48 × N100 (100 kg ha−1) and BRRI dhan82 × N80 (80 kg ha−1), and the lowest N concentration (N%) of grain yield (0.47) was found from the combination of BRRI dhan82 × N0 (0 kg ha−1). Nitrogen uptake by grains Nitrogen uptake of grain yield was significantly influenced by different varieties used in the present study (Table 4). The highest NUP of grain yield was recorded by BRRI dhan48 (43.31 kg N ha−1). The results obtained from BR26 showed the lowest NUP of grain yield (31.93 kg ha−1). Different N levels showed statistically significant variations in NUP of grain yield of Aus rice (Table 4). The highest NUP of grain yield (52.36 kg ha−1) was recorded by N80 (80 kg ha−1). The results obtained from N0 (0 kg ha−1) showed the lowest NUP of grain yield (14.96 kg ha−1). The combination effect of variety and different N applications had no significant influence on the NUP of grain yield of the three varieties of Aus rice (Table 4). The highest NUP of grain yield (61.48 kg ha−1) was observed from BRRI dhan48 × N80 (80 kg ha−1), and the lowest NUP of grain yield (12.51 kg ha−1) was found from the combination of BR26 × N0 (0 kg ha−1). Nitrogen concentration (N%) in straw The N concentration of straw was significantly influenced by the different varieties used in the present study (Table 4). The highest N concentration of straw yield was recorded by BR26 (0.41). The results obtained from BRRI dhan82 showed the lowest N concentration of straw (0.33). Different N levels showed statistically significant variations in terms of N concentration of straw of Aus rice (Table 4). The highest N concentration of straw yield (0.43) was recorded by N100 (100 kg ha−1). The results obtained from N0 (0 kg ha−1) showed the lowest N concentration of straw yield (0.26). The combination effect of variety and different N applications had no significant influence on the N concentration of straw of the three varieties of Aus rice (Table 4). The highest N concentration of straw (0.48) was observed from BR26 × N80 (80 kg ha−1), and the lowest N concentration of straw yield (0.22) was found from the combination of BRRI dhan82 × N0 (0 kg ha−1). Nitrogen uptake by straw Nitrogen uptake of straw yield was significantly influenced by different varieties used in the present study (Table 4). The highest NUP of straw yield was recorded by BR26 (19.91 kg ha−1). The results obtained from BRRI dhan82 showed the lowest NUP of straw yield (16.38 kg ha−1). Different N levels showed statistically significant variations in NUP of the straw yield of Aus rice at days after transplanting (Table 4). The highest NUP of straw yield (23.24 kg ha−1) was recorded by N80 (80 kg ha−1). The results obtained from N0 (0 kg ha−1) showed the lowest NUP of straw yield (10.23 kg ha−1). The combination effect of variety and different N applications had no significant influence on the NUP of the straw yield of the three varieties of Aus rice (Table 4). The highest NUP of straw yield (26.01 kg ha−1) was observed from BR26 × N80 (80 kg ha−1), and the lowest NUP of straw yield (9.60 kg ha−1) was found from the combination of BRRI dhan82 × N0 (0 kg ha−1). 92 Masum et al. Nitrogen harvest index N accumulation in grains (N accumulation in grains plus dry matter/ straw) explain the N harvest index. The N harvest index was significantly influenced by different varieties used in the present study (Table 4). The highest N harvest index was recorded by BRRI dhan48 (68.15%). The results obtained from BR26 showed the lowest N harvest index (60.41%). Different N levels showed statistically significant variations in terms of the N harvest index of Aus rice (Table 4). The highest N harvest index (69.12) was recorded by N80 (80 kg ha−1). The results obtained from N0 (0 kg ha−1) showed the lowest N harvest index (59.43). The combination effect of variety and different N rates had no significant influence on the N harvest index of the three varieties of Aus rice (Table 4). The highest N harvest index (72.37%) was observed from BRRI dhan48 × N80 (80 kg ha−1), and the lowest N harvest index (55.38%) was found from the combination of BR26 × N0 (0 kg ha−1). Relationship between grain yield and grain N uptake and grain yield with total N uptake Grain yield of varieties and N uptake in grain and total N uptake were significantly and linearly related (R2 = 0.8863** and 0.8619**) (Figure 1 “A, B”), indicating that higher grain yield would be due to higher N uptake. Nitrogen use efficiency is largely influenced by grain yield, N fertilizer input, and N uptake (Qiao et al., 2012). Fig. 1. Relationship between grain yield and grain N uptake and grain yield with total N uptake (**= significant at 1% level). Determination of optimum and economic of N doses The variation of grain yield of BR26, BRRI dhan48, and BRRI dhan82 at different N rates was determined through regression equation (Figure 2). Differentiating the quadratic equation of yield response concerning applied different N doses, the optimum N rate appeared as 99, 95 and 57 kg ha−1 for BR26, BRRI dhan48, and BRRI dhan82, respectively. Considering economic N rate would be 97, 93, and 53 kg ha−1 for BR26, BRRI dhan48, and BRRI dhan82, respectively. Fageria and Santos (2014) also found efficient and moderately efficient rice varieties in N use efficiency, and none were grouped as inefficient. A B Economic nitrogen rate for transplanted aus rice 93 Fig. 2. Grain yield responses to N fertilization and determination of optimum and economic N doses in different Aus varieties during Aus 2019 at BRRI, Gazipur (*=significant at 5% level) Conclusion The economic N rates calculated from the quadratic regression equations were 97 kg ha−1, 95 kg ha−1, and 55 kg ha−1 for BR26, BRRI dhan48, and BRRI dhan82, respectively. Therefore, the N fertilizer requirement of Aus rice crops should be based on variety, and the inherent capacity of soil N supply to improve Aus rice yield furthermore minimize the excessive use of chemical 94 Masum et al. fertilizer. 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