AMINA KHATUN1*, MD. KHAIRUL QUAIS1, HASINA SULTANA1, MD. KHAIRUL ALAM BHUIYAN2 , MD. ABU SALEQUE31-Rice Farming Systems Division, Bangladesh Rice Research Institut, Bangladesh2-Agronomy Division, Bangladesh Rice Research Institute, Bangladesh3- Coordinator for Advanced Studies and Research, Bangladesh Rice Research Institut, Bangladesh*Corresponding author E Corresponding author E Corresponding author E Corresponding author ECorresponding author ECorresponding author ECorresponding author E Corresponding author ECorresponding author ECorresponding author ECorresponding author ECorresponding author ECorresponding author ECorresponding author ECorresponding author ECorresponding author ECorresponding author E-mail:mail:mail: mail: aminabrri@gmail.comReceived: Received:Received:Received: Received:Received:Received: 7 July 2014uly 2014 uly 2014uly 2014uly 2014uly 2014uly 2014 Accepted: Accepted: Accepted: Accepted: Accepted: Accepted: Accepted: 25 November 201425 November 201425 November 201425 November 2014 25 November 201425 November 201425 November 201425 November 201425 November 201425 November 2014 25 November 201425 November 201425 November 201425 Abstract Lowland rice (Oryza sativa L.) in South Asia is under stress, as nitrogen removals by crops are higher than their replenishment through fertilizers. Limited information is available on optimizing nitrogen dose in lowland rice to turn out higher yield. The present investigation aimed to optimize nitrogen fertilization and it’s response to the growth and yield of lowland rice. We evaluated two modern rice varieties (BRRI dhan28 and BRRI dhan29) under six nitrogen rates ranging from 0 to 250 kg ha-1 during the dry season. Positive response of nitrogen fertilization was observed in tiller and dry matter production from the early growing stage in both varieties. Tillering increased progressively with the advancement of growth stage and reached at the peak within 60 days after transplanting (DAT) in all cases. BRRI dhan28 and BRRI dhan29 achieved the highest dry matter at 75 and 90 DAT with 250 kg N ha-1. The relationship between N application and N uptake by plants became quite evident at 30 DAT and beyond. N uptake started to increase from 45 DAT and showed a peak at 75 DAT in both the varieties. Nitrogen application significantly increased the grain yield of both varieties. The highest yield of 5.15 and 6.34 Mg ha-1 was obtained with 150 kg N ha-1 in BRRI dhan28 and BRRI dhan29, respectively. However, the nitrogen dose was optimized at 156 and 158 kg ha-1 for BRRI dhan28 and BRRI dhan29, respectively. Manuscript profile

Abstract Nitrogen (N) is commonly applied in equal split during different growth stages of rice. However, higher requirement of N during panicle initiation (PI) stage emphasizes relatively higher N dose at that particular stage. A field experiment was conducted during dry seasons (November-May) of two consecutive years from 2013 to 2015 to determine appropriate timing and amount of N application and its effect on yield and N use efficiency. Two modern rice varieties (BRRI dhan28 and BRRI dhan29) under six N management options viz. i) One third of N was applied at initiation of tillering (IT) stage + one third at active tillering (AT) stage + one-third at panicle initiation (PI) stage (N1)(Recommended practice); ii) One-half at IT stage + another-half at PI stage (N2); iii) One third at IT stage + two-third at PI stage (N3); (iv) One-fourth at IT stage + one-fourth at AT stage+ half at PI stage (N4); v) One-half at IT stage + another half at AT stage (N5); and vi) N-control (N6) were evaluated in randomized complete block design with three replications. Recommended practice and application of higher dose at PI stage gave similar yields in BRRI dhan28 and BRRI dhan29 in both the years. Agronomic use efficiency showed similar trend of results. Therefore, inadequate N application at early growth stages for any inevitable circumstances could be compensated by higher N application at PI stage. Manuscript profile