Bangladesh Agron. J. 2022, 25(2): 109-117 EFFECT OF NITROGEN DOSES IN BORO RICE GROWN WITH VARIABLE LEVELS OF FLOATING DUCKWEED A. Hossain1, H.M.M.T. Hossain2, A.K.M.R. Amin2, and M.H. Mahmud3 1Bangladesh Institute of Nuclear Agriculture, 2Department of Agronomy, SAU Dhaka-1207, Bangladesh, 3Project Implementation Unit-BARC, National Agricultural Technology Program-Phase-II Project, BARC, Dhaka Corresponding E-mail: alif.bina21@yahoo.com (Received: 11 December 2022, Accepted: 29 January 2023) Keywords: Nitrogen dose, boro rice, floating duckweed, yield Abstract An experiment was carried out at the Agronomy Research Farm of Sher-e-Bangla Agricultural University, Dhaka during the period from November 2018 to June 2019 to study the nitrogen requirement of boro rice grown with floating duckweed. The experiment was laid out in a Factorial Randomized Complete Block Design with three replications. The treatments were duckweed: viz. D0 = Control (0 g duckweeds m-2), D1 = 200 g duckweeds m-2, D2 = 400 g duckweeds m-2 and D3 = 600 g duckweeds m-2) and nitrogen treatment: N1 = 45 kg N ha-1, N2 = 90 kg N ha-1 and N3 = 180 kg N ha-1). Different doses of duckweeds showed significant influence on most of the parameters of which D2 (400 g duckweeds m-2) treatment the best performance on growth and yield parameters of rice. Among different doses of nitrogen, N2 (90 kg N ha-1) gave the best results on growth and yield parameters of rice. In case of combined effect, D2N2 showed the highest number of tillers hill-1 (16.40), dry weight hill-1 (48.97 g), number of effective tillers hill-1 (14.73) and number of grains panicle-1 (189.50). This treatment combination also showed the highest number of filled grains panicle-1 (183.90), panicle length (25.83 cm), 1000 -grain weight (25.13 g), seed yield (7.24 t ha-1), straw yield (8.47 t ha-1), biological yield (15.71 t ha-1) and harvest index (46.09%). On the other hand, D0N3 gave the least results on these parameters. Findings revealed that application of 400 g duckweeds m-2 with 90 kg N ha-1 showed the superiority over other treatment combinations to produce higher grain yield of boro rice. Introduction Globally, agriculture is currently facing unprecedented challenges for nourishing the increasing population without devastating the environment (Chen et al., 2014; Zhang et al., 2015). In Bangladesh, area covered by rice was 11.39 million hectares with the production of 35.05 million metric tons (BBS, 2018). Boro rice is one of the most important rice crops for Bangladesh with respect to its high yield and contribution to rice production. Nitrogen is a major essential plant nutrient and a key input for increasing crop yield. Rice plants require a large amount of nitrogen at the early and mid tillering stage to maximize the number of panicles. Optimum dose of nitrogen fertilization plays a vital role in growth and development of rice plant. Rice grain yield was recorded significantly highest between ranges of 90-250 kg ha-1 nitrogen application (Marazi et al., 1993; Daniel and Wahab, 1994; Bali et al., 1995). A significant increase in tillering (Hussain et al., 1989; Meena et al., 2003) with increase in nitrogen supply was observed. GM is generally eco-friendly, economically viable and renewable for sustainable agriculture. 110 Hossain et al. Over the last 40 years, research regarding duckweed mainly focused on phytoremediation and nutrient recovery from wastewater and for animal feedstock and the production of biofuels, due to its high growth rate, high biomass yield, excellent nutrient uptake ability, and tolerance to high nutrient levels (Cheng et al., 2002; Mohedano et al., 2012). So far, few studies have examined the influence of duckweed on NH3 volatilization (Zimmo et al., 2003; Li et al., 2009; Sun et al., 2015), and only Li et al. (2009) reported that duckweed cover combined with urea could effectively increase rice yield at 90 and 180 kg N ha−1. Meanwhile, the current Chinese agriculture systems are highly fertilized, few studies have comprehensively accessed the N balance of urea combined with duckweed in current intensive rice cropping systems. Therefore, a field experiment was conducted with the aims to increasing N use efficiency with floating duckweed on boro rice yield. Materials and Methods The experiment was carried out at Sher-e-Bangla Agricultural University farm, Dhaka, Bangladesh during the period from November 2018 to June 2019. The geographical location of the site is 90°33´E longitude and 23°77´N latitude which belongs to “The Modhupur Tract”, AEZ-28. The experiment was laid out in Factorial Randomized Complete Block Design (RCBD) with three replications. The experiment was comprised of duckweed at four levels, D0 = Control (0 g duckweeds m-2), D1 = 200 g duckweeds m-2, D2 = 400 g duckweeds m-2 and D3 = 600 g duckweeds m-2, and nitrogen at three levels, N1 = 45 kg N ha-1, N2 = 90 kg N ha-1 and N3 = 180 kg N ha-1. Healthy seeds of boro rice var. BRRI dhan28 was used as test crop. After final preparation of land with recommended fertilizer dose, 40 days old seedlings with 25 cm × 15 cm spacing was maintained. After collection of duckweeds from sources those were kept in a cistern in research field for purification and refreshing. When field was ready then measured in several packets previously determined weight after water removal. Duckweeds were introduced in the target plots after two days of planting seedlings. Water level of plots maintained in below plots ridges to control duckweeds in desired plots. All intercultural and plant protection measures were taken to all the plots. The first plant height and number of tillers hill-1 were measured at 25 DAT and continued up to harvest with 20 days interval. Data on yield components were collected from the sample plants of each plot. The significant differences among the treatments were judged at 5% level of probability by using Least Significant Difference (LSD) with a computer operated program named MSTAT-C. Results and Discussion Plant height Significant difference was found among the treatment on plant height of rice at all growth stages due to application of duckweed at different rates, different doses nitrogen and its combination (Table 1). Results revealed that the maximum plant height (23.19, 61.41 and 97.49 cm at 30, 60 DAT and at harvest, respectively) was found from the treatment D2 which was statistically same with D1 at 60 DAT but at harvest it was significantly different from other treatments. The lowest plant height (20.28, 52.82 and cm at 30, 60 DAT and at harvest, respectively) was found from control treatment D0. The maximum plant height (23.06, 60.69 and 97.28 cm at 30, 60 DAT and at harvest, respectively) was found from the treatment N3 which was statistically similar with N2 whereas the lowest plant height (20.21, 54.68 and 89.14 cm at 30, 60 DAT and at harvest, respectively) was found from the treatment N1. The result obtained from the present study was followed by the findings of Adhikari (2018) and Chamely and Islam (2015). The highest plant height (24.43, 63.99 and 100.90 cm at 30, 60 DAT and at harvest, respectively) was found from the treatment combination of D2N3 whereas the lowest plant height (21.77, 55.59 and 92.73 cm at 30, 60 DAT and at harvest, respectively) from the treatment combination of D0N1. Effect of Duckweed and Nitrogen on Growth and Yield of Boro Rice 111 Table 1. Plant height and number of tillers hill-1 of rice as influenced by duckweed and nitrogen and their combination Treatments Plant height (cm) Number of tillers hill-1 30 DAT 60 DAT At harvest 30 DAT 60 DAT At harvest Effect of duckweed (D) D0 20.28 b 52.82 d 88.35 c 6.256 c 10.03 b 10.81 c D1 22.12 ab 59.78 a 94.29 b 7.333 b 13.09 a 14.44 ab D2 23.19 a 61.41 a 97.49 a 9.289 a 14.16 a 14.76 a D3 21.16 ab 56.78 c 92.86 b 6.978 bc 10.95 b 13.06 b LSD(0.05) 2.41 2.30 1.87 0.92 1.40 1.50 Effect of nitrogen (N) N1 20.21 b 54.68 b 89.14 b 7.125 11.91 b 13.06 b N2 21.80 ab 57.72 ab 93.32 ab 7.875 13.60 a 14.89 a N3 23.06 a 60.69 a 97.28 a 7.392 10.66 c 11.85 b LSD(0.05) 2.08 5.45 7.92 NS 1.22 1.30 Combined effect of duckweed and nitrogen D0N1 21.77 a-c 55.59 a-c 92.73 ab 6.00 e 9.913 e-g 10.22 de D0N2 19.94 bc 52.80 bc 88.30 ab 6.87 de 10.93d-g 12.41 b-d D0N3 19.14 c 50.08 c 84.03 b 5.90 e 9.253 g 9.800 e D1N1 22.18 a-c 60.67 a-c 94.77 ab 6.20 de 13.07b-d 14.27 ab D1N2 23.21 a-c 62.43 ab 98.11 ab 6.60 de 14.40 ab 15.99 a D1N3 20.96 a-c 56.23 a-c 89.98 ab 9.20 bc 11.80 c-f 13.07 bc D2N1 23.63 ab 61.46 ab 96.59 ab 9.77 b 14.19 a-c 14.54 ab D2N2 21.49 a-c 58.79 a-c 94.93 ab 11.43 a 16.17 a 16.40 a D2N3 24.43 a 63.99 a 100.9 a 6.67 de 12.13 b-e 13.33 bc D3N1 21.43 a-c 55.97 a-c 93.62 ab 6.53 de 10.47 e-g 13.20 bc D3N2 22.81 a-c 60.73 a-c 97.33 ab 6.60 de 12.91 -d 14.76 ab D3N3 19.24 c 53.63 a-c 87.63 ab 7.80 cd 9.46 fg 11.21 c-e LSD(0.05) 4.17 10.90 15.83 1.60 2.43 2.59 CV (%) 11.35 11.16 10.03 12.67 11.90 11.53 In a column means having similar letters) arc statistically identical and those having dissimilar letter(s) differ significantly as per 0.05 level of probability D0 = Control (0 g duckweeds m-2), D1 = 200 g duckweeds m-2, D2 = 400 g duckweeds m-2, D3 = 600 g duckweeds m-2, N1 = 45 kg N ha-1, N2 = 90 kg N ha-1, N3 = 180 kg N ha-1 Number of tillers hill-1 Significant effect was found on number of tillers hill-1 of rice among the treatment at all growth stages due to application of duckweed at different rates, different doses nitrogen and its combination (Table 1). Results revealed that the maximum number of tillers hill-1 (9.29, 14.16 and 14.76 at 30, 60 DAT and at harvest, respectively) was found from the treatment D2 which was statistically similar with D1 at 60 DAT and at harvest whereas the lowest number of tillers hill-1 (6.26, 10.03 and 10.81 at 30, 60 DAT and at harvest, respectively) from control treatment D0. The highest number of tillers hill-1 (7.88, 13.60 and 14.89 at 30, 60 DAT and at harvest, respectively) was found from the treatment N2 which was significantly different from other treatments. The lower number of tillers hill-1 (7.39, 10.66 and 11.85 at 30, 60 DAT and at harvest, respectively) was found from the treatment N3 which was statistically identical with N1. More or less similar result was also observed by Karim (2019), Adhikari (2018) and Chamely and Islam (2015) also found that N had significant effect on number of tillers hill-1. The maximum number of tillers hill-1 (11.43, 16.17 and 16.40 at 30, 60 DAT and at harvest, respectively) was found from the treatment combination of D2N2 which was statistically similar with the treatment combination of D1N1, D1N2, D2N1 and D3N2. The lowest number of tillers hill-1 (5.90, 9.25 and 9.80 at 30, 60 DAT and at harvest, respectively) was found from the 112 Hossain et al. treatment combination of D0N3 which was statistically similar with the treatment combination of D0N1 and D3N3. Number of non-effective tillers hill-1 Significant effect was found on number of non-effective tillers hill-1 of rice among the treatment due to application of duckweed at different rates, different doses nitrogen and its combination (Table 2). The highest number of non-effective tillers hill-1 (4.25) was found from control treatment D0 which was significantly different from other treatments. The lowest number of non- effective tillers hill-1 (1.76) was found from the treatment D2. The highest number of non- effective tillers hill-1 (3.45) was found from the treatment N3while lower number of non-effective tillers hill-1 (2.26) was found from the treatment N2 followed by N1. The result obtained from the present study was followed with the findings of Karim (2019) and Adhikari (2018). In treatment combinations, lower number of non-effective tillers hill-1 (1.40) was found from D2N2 which was statistically similar with D1N2 and D2N1. Effective tillers hill-1 Number of effective tillers hill-1 of rice among the treatment was found significant due to application of duckweed at different rates, different doses nitrogen and its combination (Table 2). The maximum number of effective tillers hill-1(13.02) was found from the treatment D2 followed by D1 whereas the lowest number of effective tillers hill-1 (7.41) from control treatment D0. Higher number of effective tillers hill-1 (11.67) was found from the treatment N2 followed by N1 whereas the lowest number of effective tillers hill-1 (8.97) from N3. Karim (2019) and Chamely and Islam (2015) also found significant effect of N on number of effective tillers hill-1 of rice which supported the present study. Higher number of effective tillers hill-1 (14.73) was found from the treatment combination of D2N2 which was statistically similar with D1N2 whereas the lowest number of effective tillers hill-1 (6.40) was found from the treatment combination of D0N3 which was statistically similar with D0N1 and D3N3. Number of grains panicle-1 Number of grains panicle-1 of rice among the treatment due to application of duckweed at different rates, different doses nitrogen and its combination were found significant (Table 2). Higher number of grains panicle-1 (179.40) was foun from the treatment D2 which was statistically identical with D1 whereas the lowest number of grains panicle-1 (132.60) from control treatment D0. The maximum number of grains panicle-1 (169.70) was found from the treatment N2 which was statistically similar with N1 whereas the lowest number of grains panicle-1 (149.80) from the treatment N3. The result obtained from the present study was corroborates with the findings of Karim (2019), Adhikari (2018) and Chamely and Islam (2015). The maximum number of grains panicle-1 (189.50) was found from the treatment combination of D2N2 which was statistically similar with the treatment combination of D1N1, D1N2, D2N1, D2N3 and D3N2. The lowest number of grains panicle-1 (121.50) was found from the treatment combination of D0N3 which was statistically similar with D0N1, D0N2 and D3N3. Number of filled grains panicle-1 Significant variation was found on number of filled grains panicle-1 of rice among the treatment due to application of duckweed at different rates, different doses nitrogen and its combination (Table 2). The maximum number of filled grains panicle-1 (170.00) was found from the treatment D2 which was statistically identical with D1 whereas the lowest number of filled grains panicle-1 (110.10) from control treatment D0. The highest number of filled grains panicle-1 (156.60) was found from the treatment N2 whereas the lower number of filled grains panicle-1 (129.30) from the treatment N3 which was statistically identical with N1. Similar result was also observed by Chamely and Islam (2015) and Adhikari (2018). The maximum number of filled grains panicle-1 (183.90) was found from the treatment combination of D2N2 which was statistically similar with Effect of Duckweed and Nitrogen on Growth and Yield of Boro Rice 113 the treatment combination of D1N2, D2N1 and D2N3. Lower number of filled grains panicle-1 (93.18) was found from the treatment combination of D0N3 which was statistically similar with the treatment combination of D0N1. Number of unfilled grains panicle-1 Statistically significant variation was found on number of unfilled grains panicle-1 of rice among the treatment due to application of duckweed at different rates, different doses nitrogen and its combination (Table 2). The highest number of unfilled grains panicle-1 (22.53) was found from control treatment D0.The lowest number of unfilled grains panicle-1 (9.35) was found from the treatment D2 which was significantly different from other treatments. Chamely and Islam (2015) also found similar result with the present study. The lowest number of unfilled grains panicle-1 (13.08) was found from the treatment N2 whereas higher number of unfilled grains panicle-1 (20.51) was found from the treatment N3 followed by N1. The lowest number of unfilled grains panicle-1 (5.62) was found from the treatment combination of D2N2 which was significantly different from other treatment combinations whereas the maximum number of unfilled grains panicle-1 (28.29) was found from the treatment combination of D0N3 followed by D0N1 and D3N3. Table 2. Yield contributing parameters of rice as influenced by duckweed and nitrogen and also their combination Treatments Yield contributing parameters No. of non- effective tillers hill-1 No. of effective tillers hill-1 No. of grains panicle-1 No. of filled grains panicle-1 No. of unfilled grains panicle-1 Panicle length (cm) 1000- grain weight (g) Effect of duckweed (D) D0 4.25 a 7.41 d 132.6 c 110.1 c 22.53 a 18.89 c 20.94 b D1 2.40 c 11.87 b 171.6 a 155.7 a 15.81c 23.48 a 23.05 ab D2 1.76 d 13.02 a 179.4 a 170.0 a 9.35 d 24.58 a 24.49 a D3 3.11 b 8.58 c 152.8 b 133.5 b 19.30 b 21.29 b 22.31 b LSD(0.05) 0.31 1.05 16.89 15.34 1.73 2.07 2.13 Effect of nitrogen (N) N1 2.93 b 10.02 b 157.7 ab 141.1 b 16.65 b 22.01 ab 22.88 ab N2 2.26 c 11.67 a 169.7 a 156.6 a 13.08 c 23.29 a 23.82 a N3 3.45 a 8.972 c 149.8 b 129.3 b 20.51 a 20.88 b 21.40 b LSD(0.05) 0.27 0.91 14.62 13.28 1.49 1.80 1.84 Combined effect of duckweed and nitrogen D0N1 4.53 a 7.57 fg 131.1 de 108.8 ef 22.26 b 18.90 ef 21.40b-d D0N2 3.43 bc 8.25 ef 145.3 c-e 128.3 de 17.03 cd 20.61 c-f 22.07a-d D0N3 4.80 a 6.40 g 121.5 e 93.18 f 28.29 a 17.17 f 19.37 d D1N1 2.47 e 11.30 cd 170.7 a-c 155.0 bc 15.69 de 23.73a-d 23.03a-d D1N2 1.53 g 13.80 ab 180.2 ab 167.4 ab 12.79 e 24.55 ab 24.45 a-c D1N3 3.20 b-d 10.52 d 163.7 a-c 144.8b-d 18.96 c 22.15b-e 21.67a-d D2N1 1.67 fg 12.80 bc 178.1 ab 168.4 ab 9.723 f 24.15 a-c 24.67 ab D2N2 1.40 g 14.73 a 189.5 a 183.9 a 5.623 g 25.83 a 25.13 a D2N3 2.20 ef 11.53 cd 170.4 a-c 157.7 a-c 12.71 e 23.75a-d 23.67 a-c D3N1 3.03 cd 8.42 ef 151.0b-d 132.1 c-e 18.94 c 21.27b-e 22.40a-d D3N2 2.67 de 9.89 de 163.7 a-c 146.8b-d 16.86 cd 22.17b-e 23.63 a-c D3N3 3.62 b 7.44 fg 143.7 c-e 121.6 de 22.09 b 20.43 d-f 20.91 cd LSD(0.05) 0.54 1.82 29.25 26.57 2.99 3.59 3.69 CV (%) 11.13 10.49 10.86 11.02 10.53 9.62 9.59 In a column means having similar letters) arc statistically identical and those having dissimilar letter(s) differ significantly as per 0.05 level of probability 114 Hossain et al. D0 = Control (0 g duckweeds m-2), D1 = 200 g duckweeds m-2, D2 = 400 g duckweeds m-2, D3 = 600 g duckweeds m-2, N1 = 45 kg N ha-1, N2 = 90 kg N ha-1, N3 = 180 kg N ha-1 Panicle length Panicle length of rice among the treatment due to application of duckweed at different rates, different doses nitrogen and its combination was found significant variation (Table 2). The maximum panicle length (24.58 cm) was found from the treatment D2 which was statistically identical with D1 whereas the lowest panicle length (18.89 cm) from control treatment D0. The maximum panicle length (23.29 cm) was found from the treatment N2 which was statistically similar with N1 whereas the lowest panicle length (20.88 cm) from the treatment N3. The maximum panicle length (25.83 cm) was found from the treatment combination of D2N2 which was statistically similar with the treatment combination of D1N1, D1N2, D2N1 and D2N3. The lowest panicle length (17.17 cm) was found from the treatment combination of D0N3 which was statistically similar with the treatment combination of D0N1, D0N2 and D3N3. Weight of 1000 grain Significant variation was found on 1000- grain weight of rice among the treatment due to application of duckweed at different rates, different doses nitrogen and its combination (Table 2). The maximum 1000- grain weight (24.49 g) was found from D2 which was statistically similar with D1 whereas the lowest 1000- grain weight (20.94 g) was found from control treatment D0 which was statistically identical with D3. The maximum 1000- grain weight (23.82 g) was found from the treatment N2 which was statistically similar with N1 whereas the lowest 1000 grain weight (21.40 g) from the treatment N3. The result obtained from the present study was following with the findings of Chamely and Islam (2015). The maximum 1000- grain weight (25.13 g) was found from the treatment combination of D2N2 which was statistically similar with the treatment combination of D0N2, D1N1, D1N2, D1N3, D2N1, D2N3, D3N1 and D3N2. The lowest 1000- grain weight (19.34 g) was found from the treatment combination of D0N3 which was statistically similar with the treatment combination of D0N1 and D3N3. Grain yield Significant effect was found on seed yield of rice among the treatment due to application of duckweed at different rates, different doses nitrogen and its combination (Table 3). Higher grain yield (6.24 t ha-1) was found from the treatment D2 which was statistically identical with D1 whereas the lowest grain yield (3.87 t ha-1) was found from control treatment D0. The maximum grain yield (6.08 t ha-1) was found from the treatment N2 which was followed by N1. The lowest grain yield (4.39 t ha-1) was found from the treatment N3 which was significantly different from other treatments. Gewaily and Adel (2018) was also found a linear increase in grain yield with continuous rate increase of nitrogen from 0 to 220 kg ha-1. The maximum grain yield (7.24 t ha-1) was found from the treatment combination of D2N2 which was statistically similar with D1N2. The lowest seed yield (3.40 t ha-1) was found from the treatment combination of D0N3 which was statistically similar with the treatment combination of D0N1, D3N1 and D3N3. The result obtained on seed yield of rice was conformity with the findings of Yao and Zhang (2017) that increased N use efficiency with duckweed and also found that rice cultivation using urea combined with duckweed achieved higher rice yield by 9–10%. Straw yield Significant effect was found on straw yield of rice among the treatment due to application of duckweed at different rates, different doses nitrogen and its combination (Table 3). The maximum straw yield (7.71 t ha-1) was found from the treatment D2 which was statistically Effect of Duckweed and Nitrogen on Growth and Yield of Boro Rice 115 similar with D1 whereas the lowest straw yield (6.60 t ha-1) was found from control treatment D0. The highest straw yield (7.77 t ha-1) was found from the treatment N2 which was significantly different from other treatments. Biological yield Significant effect was found on biological yield of rice among the treatment due to application of duckweed at different rates, different doses nitrogen and its combination (Table 3). The maximum biological yield (13.64 t ha-1) was found from the treatment D2 which was statistically identical with D1. The lowest biological yield (10.47 t ha-1) was found from control treatment D0 which was statistically identical with D3. The highest biological yield (13.84 t ha-1) was found from the treatment N2 which was significantly different from other treatments. The lowest biological yield (11.92 t ha-1) was obtained from the treatment N3 which was statistically identical with N1. Chamely and Islam (2015) and Karim (2019) also found similar result with the present study. The maximum biological yield (15.71 t ha-1) was found from the treatment combination of D2N2 which was statistically similar with the treatment combination of D1N2 and D2N1. The lowest biological yield (9.51 t ha-1) was found from the treatment combination of D0N3 which was statistically similar with the treatment combination of D0N1. Table 3. Yield parameters of rice as influenced by duckweed and nitrogen and their combination Treatments Yield parameters Grain yield (t ha-1) Straw yield (t ha-1) Biological yield (t ha-1) Harvest index (%) Effect of duckweed (D) D0 3.87 c 6.60 b 10.47 b 36.90 b D1 5.89 a 7.27 ab 13.05 a 44.54 a D2 6.24 a 7.71 a 13.94 a 44.61 a D3 4.64 b 6.85 b 11.50 b 40.16 ab LSD(0.05) 0.61 0.69 1.30 4.46 Effect of nitrogen (N) N1 5.01 b 6.99 b 11.92 b 41.34 N2 6.08 a 7.77 a 13.84 a 43.52 N3 4.39 c 6.57 b 10.96 b 39.80 LSD(0.05) 0.53 0.60 1.13 NS Combined effect of duckweed and nitrogen D0N1 3.75 gh 6.46 cd 10.21 fg 36.76 c D0N2 4.46 efg 7.24 b-d 11.70 c-g 38.09 bc D0N3 3.40 h 6.10 d 9.51 g 35.84 c D1N1 5.84 cd 7.17 b-d 12.68 c-e 44.83 ab D1N2 6.95 ab 8.02 ab 14.97 ab 46.39 a D1N3 4.87 d-f 6.63 cd 11.51 c-g 42.41 a-c D2N1 6.16 bc 7.55 a-c 13.71 a-c 44.94 ab D2N2 7.24 a 8.47 a 15.71 a 46.09 a D2N3 5.31 c-e 7.10 b-d 12.41 c-f 42.82 a-c D3N1 4.30 e-h 6.76 cd 11.07 d-g 38.83 a-c D3N2 5.65 cd 7.34 a-c 13.00 b-d 43.52 a-c D3N3 3.98 f-h 6.45 cd 10.43 e-g 38.12 bc LSD(0.05) 1.06 1.20 2.25 7.72 CV (%) 12.09 9.93 10.87 10.98 In a column means having similar letters) arc statistically identical and those having dissimilar letter(s) differ significantly as per 0.05 level of probability D0 = Control (0 g duckweeds m-2), D1 = 200 g duckweeds m-2, D2 = 400 g duckweeds m-2, D3 = 600 g duckweeds m-2, N1 = 45 kg N ha-1, N2 = 90 kg N ha-1, N3 = 180 kg N ha-1 116 Hossain et al. Lower straw yield (6.57 t ha-1) was found from the treatment N3 which was statistically identical with N1. The maximum straw yield (8.47 t ha-1) was found from the treatment combination of D2N2 which was statistically similar with the treatment combination of D1N2, D2N1 and D3N2. Lower straw yield (6.10 t ha-1) was found from the treatment combination of D0N3 which was statistically similar with the treatment combination of D0N1, D1N3, D3N1 and D3N3. Harvest index Significant effect was found on harvest index of rice among the treatment due to application of duckweed and combined effect of duckweed and nitrogen, but non-significant in nitrogen application at different doses (Table 3). Higher harvest index (44.61%) was found from the treatment D2 which was statistically similar with D1 and D2 whereas the lowest harvest index (36.90%) from control treatment D0. The highest harvest index (43.52%) was found from the treatment N2 whereas the lowest harvest index (39.80%) from N3. Karim (2019) and Chamely and Islam (2015) was also found similar result. The maximum harvest index (46.09%) was found from the treatment combination of D2N2 which was statistically identical with the treatment combination of D1N2. Treatment combinations, D1N1 and D2N1 were similar with D2N2. The lowest harvest index (35.84%) was found from the treatment combination of D0N3 which was statistically identical with D0N1. Conclusion Considering the above results, it may be concluded that (400 g duckweeds m-2) application showed the superiority over other treatments to produce higher grain yield of rice. Nitrogen (90 kg N ha-1) application showed higher rice yield over other nitrogen treatments. Application of 400 g duckweeds m-2 with 90 kg N ha-1 performed the best among the treatments. The trial is confined in one location with one season resulr so, more research is needed in different agro- ecological zones (AEZ) of Bangladesh for regional adaptability and other performances. References Adhikari, J. 2018. Effect of nitrogen fertilizer and weed management on the yield of transplant aman rice. J. Bangladesh Agril. Univ. 16(1): 12–16. Bali, A.S., M. Siddique, B.A. Ganai, H.V. Khan and K.N. Singh. 1995. Response of rice (Oryza sativa) genotypes to nitrogen levels under transplanted conditions in Kashmir valley. Indian J. Agron. 40(1): 35-37. BBS. 2018. Bangladesh Bureau of Statistics. Statistical Year Book of Bangladesh. Stat. Div. Min. Planning. Dhaka. Chamely, S.G. and N. Islam. 2015. Effect of variety and nitrogen rate on the yield performance of boro rice. Progress. Agric. 26 (1): 6-14. Chen, X., Z. Cui, M. Fan, X. Yan, N. An, W. Zhang, and F. Zhang. 2014. Producing more grain with lower environmental costs. Nature. 514: 486-489. Cheng, J., B. A. Bergmann, J. J. Classen, A.M. Stomp, and J.W. Howard. 2002. Nutrient recovery from swine lagoon water by Spirodela punctata. Bioresour. Technol. 81: 81-85. Daniel, K.V. and K. Wahab. 1994. Levels and time of nitrogen in semi dry rice. Madras Agric. J. 81(6): 357-358. Gewaily, E.E. and M.G. Adel. 2018. Effects of nitrogen levels on growth, yield and nitrogen use efficiency of some newly released Egyptian rice genotypes. Open Agriculture. 3: 310–318. http://refhub.elsevier.com/S0378-4290(17)30650-0/sbref0040 Effect of Duckweed and Nitrogen on Growth and Yield of Boro Rice 117 Hussain, T., G. Jilani, and A. Ghaffar. 1989. Influence of rate and time of nitrogen application on growth and yield of rice in Pakistan. Int. Rice. Res. Newsl. 14(6): 18. Karim, M. 2019. Effect of Different Doses of Urea on the Yield of Boro Rice Varieties in Haor Areas of Bangladesh. Asian Pl. Res. J. 3(2): 1-9. Li, F., Z. Wang, J. Dai, Q. Li, X. Wang, C. Xue, H. Liu and G. He. 2015. Fate of nitrogen from green manure, straw, and fertilizer applied to wheat under different summer fallow management strategies in dry land. Biol. Fert. Soils. 51: 769-780. Li, H., X. Liang, Y. Lian, L. Xu and Y. Chen. 2009. Reduction of ammonia volatilization from urea by a floating duckweed in flooded rice fields. Soil Sci. Soc. Am. J. 73: 1890-1895. Marazi, A.R., G.M. Khan, K.H. Singh and A.S Bali. 1993. Response of rice (Oryza sativa) to different N levels and water regimes in Kashmir Valley. Indian J. Agric. Sci. 63(11): 726- 727. Meena, S.L., S. Surendra, Y.S. Shivay and S. Singh. 2003. Response of hybrid rice (Oryza sativa) to nitrogen and potassium application in sandy clay loam soils. Indian J. Agric. Sci. 73(1): 8- 11. Mohedano, R.A., R.H.R. Costa, F.A. Tavares and P. Belli-Filho. 2012. High nutrient removal rate from swine wastes and protein biomass production by full-scale duckweed ponds. Bioresour. Technol. 112: 98-104. Sun, H.J., H.L. Zhang, J. Min, Y.F. Feng and W.M. Shi. 2015. Controlled-release fertilizer, floating duckweed, and biochar affect ammonia volatilization and nitrous oxide emission from rice paddy fields irrigated with nitrogen-rich wastewater. Paddy Wat. Environ. 14: 105-111. Yao, Y. and M. Zhang. 2017. Duckweed (Spirodela polyrhiza) as green manure for increasing yield and reducing nitrogen loss in rice production. Field Crops Res. 214: 273–282. Zhang, X., E.A. Davidson, D.L. Mauzerall, T.D. Searchinger, P. Dumas and Y. Shen. 2015. Managing nitrogen for sustainable development. Nature. 528: 51-59. Zimmo, O.R., N.P. Van-der-Steen and H.J. Gijzen. 2003. Comparison of ammonia volatilisation rates in algae and duckweed-based waste stabilisation ponds treating domestic wastewater. Water Res. 37: 4587-4594. http://refhub.elsevier.com/S0378-4290(17)30650-0/sbref0120 http://refhub.elsevier.com/S0378-4290(17)30650-0/sbref0120 http://refhub.elsevier.com/S0378-4290(17)30650-0/sbref0115 http://refhub.elsevier.com/S0378-4290(17)30650-0/sbref0135 http://refhub.elsevier.com/S0378-4290(17)30650-0/sbref0135 http://refhub.elsevier.com/S0378-4290(17)30650-0/sbref0175 http://refhub.elsevier.com/S0378-4290(17)30650-0/sbref0175 http://refhub.elsevier.com/S0378-4290(17)30650-0/sbref0175 http://refhub.elsevier.com/S0378-4290(17)30650-0/sbref0175 http://refhub.elsevier.com/S0378-4290(17)30650-0/sbref0175