Impact of Nitrogen Application Rates on Upland Rice Performance, Planted under Varying Sowing Times

Application of suitable nitrogen (N) fertilizer application rate (NR) with respect to sowing time (ST) could help to maximize the performance and productivity of upland rice in Southern Thailand. The 2-year experiments were conducted in the sheds to evaluate the agronomic responses of the upland rice genotype, Dawk Pa-yawm, under various combinations of NR and ST between 2018-2019 and 2019-2020 aimed at obtaining sufficient research evidence for the improved design of long-term field trials in Southern Thailand. As with the initial research, four NR were applied as N0 with no applied N, 1.6 g N pot(-1), 3.2 g N pot(-1) and 4.8 g N pot(-1), and experiments were grown under three ST including early (ST1), medium (ST2) and late sowing (ST3). Results from the experiments indicate that the application of 4.8 g N pot(-1) resulted in maximum grain yield under all ST in both years. However, a maximum increase in grain yield was observed under ST2 by 54-101% in 2018-2019 and by 276-339% in 2019-2020. Maximum grain N uptake of 0.57 and 0.82 g pot(-1) was also observed at NR 4.8 g N pot(-1) under ST2 in both years, respectively. Application of NR 4.8 g N pot(-1) resulted in the highest N agronomic efficiency (NAE), nitrogen use efficiency (NUE) and water use efficiency (WUE). However, the performance of yield and yield attributes, N uptake, N use efficiencies and WUE were declined in late sowing (ST3). Significant positive association among yield, yield attributes, N uptake and WUE indicated that an increase in NR up to 4.8 g N pot(-1) improved the performance of Dawk Pa-yawm. The results suggest that the application of 4.8 g N pot(-1) (90 kg N ha(-1)) for upland rice being grown during September (ST2) would enhance N use efficiencies, WUE and ultimately improve the yield of upland rice. However, field investigations for current study should be considered prior to general recommendations. Moreover, based on the findings of this study, the importance of variable climatic conditions in the field, and the variability in genotypic response to utilize available N and soil moisture, authors suggest considering more levels of NR and intervals for ST with a greater number of upland rice genotypes to observe variations in field experiments for the precise optimization of NR according to ST.