Abstract
The growth, yield and N use efficiency of rice under different sizes of urea super granule (USG) and planting spacing management were studied. The objectives of the study were to evaluate the effect of different sizes of USG and plant spacing on inbred and hybrid rice, and to determine optimum rate of USG and plant spacing for higher yield of rice in dry (Boro rice) and wet (T. aman rice) seasons. The field experiment was laid out in a split-plot design where variety was placed in the main plot, N management in the split plot and plant spacing in the split-split plot. Hybrid Hira 99-5 was tested in both Boro and Aman seasons, BRRI dhan29 was grown in Boro season and BRRI dhan41 in Aman season. There were seven rates of urea supergranule (USG) and prilled urea (PU) for both boro and T. aman rice. Three plant spacings 25 cm x 15 cm, 20 cm x 15 cm and 20 cm x 15 cm were used for both rice crops. Considering N management, USG @ 3.6 g/4 hills performed significantly better considering all parameters including crop yield in Boro season. In case of plant spacing, the 25 cm × 15 cm spacing both BRRI dhan29 and Hira 99-5 gave higher grain yield in Boro season than the other spacings. In Boro season, BRRI dhan29 performed better with grain yield in 8.23 t ha1 by using USG @3.6 g/4 hills with the plant spacing 25 cm x 15 cm in 2007 and 7.96 t ha ̈1 in 2008. Hira 99-5 produced the highest grain yield with using USG @ 3.6 g/4 hills by following the plant spacing 25 cm × 15 cm, the yield being 10.74 t ha1 in 2007 and 8.62 t ha' in 2008. In Aman season, BRRI dhan41 gave higher grain yield with using USG @ 1.8 g/4 hills and plant spacing of 25 cm x 15 cm and recorded grain yields of 5.06 t ha' and 5.47 t ha1 in two years, respectively. On the other hand, Hira 99-5 gave the highest grain yield with using USG @ 1.8 g/4 hills and 25 cm x 15 cm plant spacing in both years and it was 4.79 t ha' in 2006 and 6.12 t ha' in 2007. The use of USG for wetland rice cultivation is highly promising since it positively influences N use efficiency as well as increasing of organic matter content in soil through producing more root dry matter. In Boro season, highest amount of grain N uptake was observed both in BRRI dhan29 and Hira 99-5 using USG @ 3.6 g/4 hills and 25 cm x 15 cm plant spacing which was recorded 102.05 kg ha' and 202.99 kg ha1 in 2007 and 92.34 kg ha" and 168.78 kg ha' in 2008, respectively. In Aman season, the highest amount of N uptake by grains was found in BRRI dhan41 with USG @ 1.8 g/4 hills and 25 cm × 15 cm plant spacing and it was 46.05 kg ha1 in 2006 and 48.68 kg ha1 in 2007. Initial soil N was 30.80 kg ha'' in the experimental plots considering the furrow soil in the land but it was increased to 33-44 kg ha after harvest of last experimental crop in case of using USG @ 3.6 g/4 hills with 25 cm x 15 cm plant spacing. Consider the interaction of variety, size of USG and plant spacing it was observed that Hira 99-5 and BRRI dhan29 produced the highest amount of root dry matter with using 3.6 g USG/4 hills and 25 cm x 15 cm plant spacing in 75 DAT mentioning 3,829.32 kg ha' and 3,280 kg ha' in 2007 and 2,635 kg and 3,160 kg ha' in 2008, respectively. In the case of Aman season, BRRI dhan41 and Hira 99-5 produced highest amount of root dry matter-3,840 kg ha' and 1,957 kg ha' in 2006 and 3,688 kg ha1 and 1,984 kg ha' in 2007, respectively with using 2.7 g USG/4 hills and 25 cm x 15 cm plant spacing in 75 DAT. It was found from the Boro season experiments that BRRI dhan29 produced 25-32.5% more grain yields using USG @ 3.6 g/4 hills with 25 cm x 15 cm plant spacing than prilled urea and Hira 99-5 produced 44-170% more grain yields following the same treatments with saving 30 kg l g N ha in case of BRRI dhan29 and 20 kg N ha' for Hira 99-5. Similarly in Aman season, BRRI dhan41 and Hira 99-5 produced 7-62% and 39-66% more grain yields over control with saving of 45 kg N ha' in case of both varieties.