For improving nitrogen (N) use efficiency and yield of rainfed lowland rice, N management and cultivar experiments were conducted at Ubon Ratchathani, Northeast Thailand. In the N experiment, Khao Dawk Mali 105 (KDML105), one of the leading cultivars in Northeast Thailand, was grown under different N management methods including different N rates in frequent split urea application and the use of organic matter and slow release fertiliser (SRF). In the cultivar experiment, 18 cultivars of different origins were compared for growth, yield and N uptake under irrigated and well-fertilised conditions. No water stress developed in any experiment. The leaf area index and dry weight increased and the harvest index decreased linearly with plant N uptake, irrespective of N management methods. Grain yield of KDML105 attained the maximum of 4 t ha-1 at the N uptake of about 40, 80 and 90 kg ha-1 at panicle initiation, heading and maturity, respectively. The frequent split N application and use of SRF improved the fertiliser N recovery efficiency (apparent fertiliser N uptake per unit applied N), although they did not substantially increase agronomic efficiency (increment of yield per unit applied N) compared with the results of previous workers. These results suggest that yield of KDML105 can be improved by optimising the plant N uptake through increased fertiliser N recovery efficiency. Among 18 cultivars tested, Urumamochi, Taichung65, Takanari, IR72 and Taino70 had higher yields despite their shorter growth durations compared with KDML105. Although the physiological efficiencies of N use in terms of plant dry matter production (dry matter produced per unit N uptake) were similar among all cultivars, those in terms of yield were higher in Taichung65, Takanari, IR72 and Taino70 than in KDML105 because of their higher harvest indices. Thus, these four cultivars are suggested to have two important characteristics required for cultivars adapted to rainfed lowland conditions in Northeast Thailand: the shorter growth duration to avoid late season drought and the higher physiological efficiency of N use for yield production.
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