Coated Urea Materials for Improving Yields, Profitability, and Nutrient Use Efficiencies of Aromatic Rice

Abstract Intensive cultivation and introduction of input‐responsive high‐yielding varieties with application of major nutrients in rice–wheat rotation of Indo‐Gangetic plains (IGPs) lead to multiple nutrient deficiencies. A survey of Indian soils has shown that 40% are deficient in available zinc (Zn), 33% in sulfur (S), and 33% in boron (B). Studies have indicated that application of these nutrients with major nutrients can improve the crop productivity. Keeping the importance of aromatic rice in view, coated‐urea materials and their effects on rice yields, nitrogen (N), and Zn content in different parts and input economics are evaluated. Three field trials are conducted on aromatic rice to test boron‐coated urea (BCU), sulfur‐coated urea (SCU), and zinc‐coated urea (ZnCU) in 2013 and 2014. Results indicate that the highest yields are obtained with 0.5% BCU, 5.0% SCU, and 2.5% ZnCU as zinc sulfate heptahydrate. These treatments increase grain yield by 13%, 25%, and 17.9% over prilled urea (PU). Moreover, 0.5% BCU, 5% SCU, and 2.5% ZnCU register the highest N, S, and Zn contents in bran, husk, grain, and straw. Coated‐urea materials also improve use efficiencies and harvest index of N and Zn over PU. The findings of this study suggest that 0.5% boron, 5.0% sulfur, or 2.5% zinc‐coated urea show improvement in returns and benefit–cost ratio in aromatic rice of western IGPs.


Introduction
Rice (Oryza sativa L.) is the most important food crop not only in Asia but also in the entire world as it feeds almost half of increased rice dry matter yields by 55-68% [13,14] and doubled N recovery over PU. [15] However, very limited literature is available on effect of application of graded dose of SCU on grain yield, nutrient use efficiency, and input economics in the rice crop.
In India, ≈33% of soil samples collected from different locations were deficient in B. [16] Boron-deficient soils include those which are inherently low in B, calcareous and coarse textured soils, and those high in clay content. Therefore, application of B in these soils significantly improves plant growth, yield traits, and yield of crops. Soil application of B improved crop growth and grain yield in maize. [17,18] Similarly, application of BCU significantly increased grain yield and N recovery efficiency in spring wheat. [19] The field experiments on BCU, SCU, and ZnCU were set up with the aim to study the response of aromatic rice to varying Zn, B, and S-coated urea levels besides estimating the use efficiencies of applied coated urea/fertilizers and economic evaluation of different coated fertilizer materials.

Rice Yield and Yield Attributes
Application of prilled urea (PU) significantly increased the leaf area index (LAI) at 65 d DAT as compared to absolute control. Similarly, BCU at 0.5% (1.40 kg B ha −1 ), 0.4% (1.12 kg B ha −1 ), and 0.3% (0.84 kg B ha −1 ) increased the LAI ( Table 1). Application of SCU produced significantly higher LAI over PU and absolute control, and the highest LAI was recorded at 5.0% level (15.7 kg S ha −1 ). Urea coating with 2.5% Zn (zinc oxide (ZnO)) resulted in highest LAI and was similar to other treatments except PU and absolute control. Application of 0.5% BCU produced longest panicle being at par with BCU materials but significantly longer than PU and absolute control. Different SCU and PU treatments recorded similar panicle length and the longest was achieved with 5.0% SCU. In case of ZnCU, the highest panicle length was observed in 2.5% ZnCU (zinc sulfate heptahydrate (ZnSHH)) and was identical to other Global Challenges 2019, 3,1900013 Table 1. Effect of boron-coated urea, sulfur-coated urea, and Zn-coated urea materials on yield parameters and yields in aromatic rice. Means in a column with at least one letter common are not statistically significant using Fisher's least significant difference. concentrations except 0.5% ZnCU (ZnO), PU, and absolute control. Application of 0.3-0.5% BCU significantly increased grain weight panicle −1 as compared to 0.1 and 0.2% BCU, PU, and absolute control. Coating of urea with 5.0% S being at par with 3 and 4% SCU and increased grain weight panicle −1 as opposed to 1.0 and 2.0% SCU, PU, and absolute control. Application of 2.5% ZnCU (ZnSHH or ZnO) produced the heaviest panicle over other coating materials, PU, and absolute control. BCU, SCU, and ZnCU did not increase 1000 grain weight compared to PU but the weight improved compared to absolute control. Among the BCU treatments, 0.5% BCU recorded the highest grain and straw yield and the figures were superior to 0.1 and 0.2% BCU, PU, and absolute control. SCU (3.0-5.0%) produced significantly more grain and straw yield over PU and absolute control. Coated urea with 2.5% ZnSHH gave the highest grain and straw yield which was similar to that obtained with other ZnCU treatments excluding 0.5 and 1.0% ZnCU, PU, and absolute control (Table 1).

Nitrogen, Sulfur, and Zinc Concentrations in Different Rice Parts
The highest concentration of nitrogen was detected in bran followed by grain, straw, and husk, respectively. Application of 0.2 to 0.5% BCU enhanced the nitrogen concentration in grain, bran, husk, and straw as compared to PU and absolute control ( Table 2). On an average, 0.5% BCU increased N concentration in grain by 12% over PU treatment. Similarly, 5.0% SCU significantly improved N concentration of grain, bran, and husk over rest of the treatments with the exception of 4.0% SCU. Nitrogen concentration in straw increased significantly with the application of 1.0 to 5.0% SCU than PU and absolute control. Among ZnCU treatments, the highest N concentration in grain, bran, husk, and straw was recorded with 2.5% ZnCU (ZnSHH) Global Challenges 2019, 3,1900013  and it was significantly more than the figures obtained with PU, 0.5, 1.0, and 1.5% ZnCU, and absolute control. Application of PU significantly increased the S concentration in grain and bran over absolute control. However, S concentration in husk and straw was similar in PU and absolute control. Coating of urea with 2.0 to 5.0% S significantly improved S concentration in grain over PU and absolute control. Moreover, application of 3.0 to 5.0% SCU increased S concentration in bran, husk, and straw over PU. Among different treatments of SCU, the highest concentration of S in grain, bran, husk, and straw was registered in 5.0% SCU followed by 4.0% SCU (Figure 1).
Zinc concentration in grain, husk, and straw increased significantly with the application of PU compared to absolute control. Application of 1.0 to 2.5% ZnCU (ZnSHH or ZnO) further improved Zn concentration in grain, bran, husk, and straw over PU treatment. The highest Zn concentration in grain, bran, husk, and straw was registered with the application of 2.5% ZnCU (ZnSHH) closely followed by 2.5% ZnCU (ZnO) and 2.0% ZnCU treatments. Concentration of Zn in grain was 24.7% greater with 2.5% ZnCU (ZnSHH) over PU treatment. Among different sources, ZnSHH-coated urea was superior to ZnO-coated urea with respect to improvement in Zn concentration in grain, bran, husk, and straw (Figure 2).

Nitrogen Use Efficiency
The highest agronomic efficiency (AE N ) was achieved with 0.5% BCU and was significantly superior to PU and BCU (0.1-0.2%). Recovery efficiency (RE N ) increased from 34.8% (with PU) to 63.2% with 0.5% BCU, while the highest partial factor productivity (PFP N ) was recorded in 0.5% BCU and it was at par with 0.3-0.4% BCU treatment. With respect to SCU, all treatments recorded similar values of PFP except PU. SCU (4.0-5.0%) recorded significantly higher RE N than PU, 1.0, 2.0, and 3.0% SCU ( Table 2).
Among ZnCU treatments, the highest PFP N was registered with 2.5% ZnSHH and was at par with rest of the treatments except 0.5 and 1.0% ZnCU and PU. RE N and AE N also increased with 2.5% ZnSHH, but similar AE N was observed in 2.0% ZnCU. Coating of urea with 2.5% ZnCU almost doubled the recovery of N over PU (Table 2).

Zinc Use Efficiency
The highest RE Zn was found with 1.5% ZnCU along with ZnSHH and it was similar to 1.0 and 2.0% ZnCU (ZnSHH) treatments. RE N increased by 27.4 and 23.9% with 1.5 and 1.0% ZnSHH coated urea, respectively, over 0.5% ZnO coating, while the highest AE Zn was recorded in 1.0% ZnSHH coated urea treatment. The ZnCU and PU treatments did not differ from each other with respect to Zn harvest index (HI Zn ) but were significantly superior to the absolute control. Among treatments, the highest PFP Zn was registered with 0.5% ZnCU either through ZnSHH or ZnO and it was significantly superior to the rest of the treatments ( Table 3).

Economics
The cost of inputs for BCU ranged from US$ 4.67 to 23.30 ha −1 for 0.1 to 0.5% BCU (   gross return, net return, and benefit:cost ratio were recorded in ZnSHH-coated treatment compared to ZnO-coated treatments.

Discussion
Application of BCU (0.3-0.5%) contributing 0.84-1.40 kg B ha −1 increased rice yield, N uptake, and N use efficiencies (PFP N , RE N , and AE N ) as compared to 0.1-0.2% BCU, PU, and control. However, 0.5% BCU had the highest net returns and benefit:cost ratio ( Table 4). The recommended range of B is 0.30 to 2 kg B ha −1 for B-deficient Indian soils [20] and the amount of B supplied by 0.5% BCU (1.40 kg B ha −1 ) was within that range. This experiment also demonstrated that urea application with B enhances N concentration in grain, bran, husk, and straw of rice. It is reported that B and N have a positive interaction that might have helped in increasing N uptake. [19,21] Since N uptake is directly proportional to RE N , an increase in N uptake by rice resulted in corresponding increase in RE N . The maximum gross and net returns were obtained with 0.5% BCU, which were 12.9 and 23.9% higher than uncoated PU, respectively. These experimental data substantiate the fact that application of 0.5% BCU is a promising strategy for rice production especially in boron-deficient soils. Sulfur fertilization particularly by SCU in cereal-cereal rotations will guarantee consistent availability of S. [18] A number of researchers have already reported positive response to N and S fertilization in cereals. [1,[22][23][24] There was a significant improvement in PFP N , RE N , and AE N with SCU as compared to PU (Table 2) as reported earlier by the authors of this manuscript. [1] Using SCU as source of N and S might have increased N as well as S concentrations, which increased their uptake in Global Challenges 2019, 3,1900013 Table 4. Cost involved in the coating of boron, sulfur, and zinc onto prilled urea and economic evaluation for one hectare aromatic rice crop. Means in a column with at least one letter common are not statistically significant using Fisher's least significant difference.  grain and straw. Herein, SCU application increased rice yields compared to PU alone (Table 1). This study shows that application of 5% SCU (supplying 14.1 kg S ha −1 , at application of 130 kg N ha −1 ) enhanced rice productivity, net returns, and benefit:cost ratio. Morris [25] reported that S recommendation for cereals varies from 10 to 40 kg ha −1 and therefore 5% SCU supplied sufficient S to the crop and increased RE N over PU. The coated urea materials improved Zn content in different rice parts, that is, grain, bran, husk,, and straw which is important for nutritional quality of food and fodder. Results of this study indicate that coated fertilizers with Zn sources significantly improved rice yields as compared to PU (Table 1). Prasad et al. [26] reported that farmers in African and other developing countries are not adding Zn to soils due to unavailability and higher cost. The yield penalty due to Zn deficiency has been reported in several crops in Asian countries like India, Pakistan, and China, and Australia. [27] In India, several studies suggested that Zn fertilization increases productivity and profitability of rice and other cereals. [18,28,[29][30][31][32][33][34][35] Among rice parts, Zn concentration decreased in the order bran > straw > husk > grain, indicating that brown rice are much denser in Zn than polished rice grain. Thus, to overcome Zn malnutrition, considering the higher Zn accumulation in the bran, brown rice consumption especially in Asia and Africa could be recommended. [5] ZnSHH (2.5%) resulted in highest N and Zn uptake in grain, husk, bran, and straw, which was due to increased grain and straw yields, and increased concentration therein. However, Zn coating onto PU differ Zn concentration and their uptake in rice parts between sources with the same level of N input. The mobility of Zn in soil varied among sources which influences Zn concentration and consequently their uptake in different plant parts. In fact, ZnSHH is relatively more water soluble than ZnO in soil which influenced Zn uptake in rice grain parts. [5,18,31] In this study, ZnCU along with 2.5% ZnSHH led to the highest N concentration in rice grain, husk, bran, and straw which might be due to slow release of N-coated fertilizers that ultimately increased N uptake. [28,18,36] Application of 2.5 and 2% ZnSHH resulted in significant increment in RE N , PFP N , and AE N over PU owing to positive improvement in use efficiencies of N with ZnCU due to more rice yield and N uptake. The highest RE Zn was recorded in 1.5% ZnCU with ZnSHH, while the highest AE Zn and PFP Zn were recorded in 1.0 and 0.5% ZnSHH-coated urea (Table 3). Zn use efficiencies are high at lower application rates owing to its rapid adsorption over soil organic matter and clay minerals, and subsequent slow desorption. [37,38] Similarly, Zn-coated fertilizers would also permit farmers to use Zn along with N in Zn deficient conditions. Among Zn sources, ZnO is easier to coat because it forms a good emulsion with oil. [26] On the contrary, ZnSHH is a widely used inorganic source of Zn due to its solubility and easier market availability. Overall, coating of urea prills is an option to improve Zn content in rice parts and increase rice yields over PU.

Conclusion
Coating of urea with different concentrations of B, S, and Zn improves the growth, productivity, and profitability of aromatic rice. BCU, SCU, and ZnCU had beneficial effects in increasing N and Zn concentrations in bran, husk, grain, and straw. Coating of urea with Zn could be used as an effective alternative for ferti-fortification of Zn in rice grain to reduce Zn deficiency in human beings. Urea coating with 0.5% BCU, 5% SCU, or 2.5% ZnCU resulted in the maximum benefits and increased N as well as Zn use efficiencies.
Prilled Urea Coating Procedure: Urea-coated materials with different levels of Zn, S, and B were prepared as per the procedure described by Pooniya et al. [18] Coated material was prepared just before transplanting of rice. The outlay involved in coating of these urea materials based on prevailing Indian market prices (US $ ha −1 ) during that period is given in Table 4.
Yield Attributes and Plant Nutrient Analysis: The rice crop was harvested using sickles as soon as the grain matured after leaving the border area, that is, 0.5 m from all the corners of each plot. Ten panicles from each plot were selected and their length was measured. The crop was threshed using plot thresher. Data were recorded on LAI at 65 DAT, panicle length, grain weight panicle −1 , 1000 − grain weight, and yields. To calculate grain weight panicle −1 , ten panicles (selected previously) were threshed and individual grain weight was pooled to determine the average value. For grain yield estimation, moisture content was adjusted at 14% and the straw yield was recorded after sun drying. The recorded yields were expressed in Mg ha −1 . The input-cost relationships (US $ ha −1 ) for the rice crop is shown in Table 4. The collected plant samples were sundried followed by drying in hot air oven at 65 ± 5 °C, and ground and passed through 40 mesh sieve in a Macro-Wiley Mill. Samples of 0.5 g dry matter were taken from different parts for N and Zn analysis. Samples were analyzed following Kjeldahl digestion as described by Prasad et al. [43] Zn content in rice dry matter was determined by a di acid digestion method using atomic absorption spectrophotometry (AAS). [43] The N or Zn uptake was computed by multiplying their respective concentrations by the mass of rice dry matter. Total N or Zn uptake was calculated by summing up (grain + straw uptake) of N or Zn.
Nitrogen Use Efficiencies: Nitrogen use efficiencies, viz., AE N , RE N , and PFP N were calculated as suggested by Pooniya and Shivay [28] and Pooniya et al. [18] AE kg grain increased per kg N applied Yf Yc /Na N ) ( RE % of N taken up by a crop NUf NUc /Na 100 PFP kg grain per kg N applied Yf/Na where Yf and Yc are the yields (kg ha −1 ) in fertilized and control (no fertilizer) plots, respectively; NUf and NUc are the amounts of N taken up by a rice crop in fertilized and control plots, respectively, and Na refers to the amount of N applied (kg ha −1 ). Zinc Use Efficiencies: Zinc use efficiencies, viz., AE Zn , RE Zn , PFP Zn , and HI Zn were calculated as suggested by Pooniya and Shivay [28] AE kg grain increased per kg Zn applied Yf Yc /Zna Zn ) ( where Yf and Yc are the yields (kg ha −1 ) in fertilized and control (no fertilizer) plots, respectively; ZnUf and ZnUc are the amounts of Zn taken up by a rice crop in fertilized and absolute control plots (no N and no Zn), respectively; and Zna refers to the amount of Zn applied (kg ha −1 ). ZnUg and ZnUg + s are the amounts of Zn uptake in rice grain and grain + straw, respectively. Statistical Analysis: The experimental data were investigated statistically using analysis of variance (ANOVA) to determine treatment effects. [49] Fisher's least significant difference (LSD) was used as a post hoc mean separation test (P < 0.05) using Proc GLM in SAS 9.3 software. The Fisher's test was used when the ANOVA was significant.