Nitrogen use efficiency in F1 hybrid, improved and local cultivars of rice (Oryza sativa L.) during different cropping seasons.

This study was conducted in order to determine the affects of N fertilizer levels (0, 60, 90 and 120 kg N per ha) on dry matter accumulation (DM) and grain yield in F1 hybrid rice cultivar (BoiTapSonThanh), improved cultivar (Khangdan 18) and local cultivar (KhauSuu) in both Spring and Autumn cropping seasons. As increasing amount of N fertilizer was applied in both cropping seasons, DM and other related characters such as number of tiller and leaf area index (LAI) were much higher in the F1 hybrid than in those of the improved and local cultivars at most active tillering and flowering stages. In contrast, leaf N content were much higher in the improved and local cultivar than that in the F1 hybrid as N levels were increased at the flowering stage in both cropping seasons. There were no significant differences in the SPAD reading or specific leaf area in all rice cultivars under all N fertilizer levels. When N increased from 90 to 120N in both cropping seasons, grain yield significantly increased in the F1 hybrid, whereas it was not found in improved or local cultivars. In all rice cultivars, nitrogen use efficiency (NUE) for the grain yield was higher in the Spring season than in the Autumn season. In both cropping seasons, NUE was higher in the F1 hybrid than in other cultivars under all N levels. As N fertilizer increased from 60 to 120N, the NUE for grain yield was decreased in the improved cultivar (from 21.20 to 16.62 in Spring season and from 7.97 to 2.39 in Autumn season) and in local cultivar (from 30.35 to 10.99 in Spring season and from 6.86 to 2.42 in Autumn season), whereas it was maintained in the F1 hybrids (31.36 - 27.89 in Spring and 17.61 - 19.68 in Autumn season). The higher levels of NUE found in the F1 hybrid rice than in other cultivars in both cropping seasons was attributed to both the higher number of panicles and spikelets per panicles. Thus, hybrid rice can contribute to sustainable and ecologically benificial agricultural practice in Vietnam using low-input cultivation