Determining the Best Planting Density in Mechanized Transplanting of Rice Compared to Manual Transplanting in Conditions of High Seedling Age

It is important to determine the appropriate planting density in the conditions of using old seedlings of paddy rice. For this purpose, a field experiment was conducted as a randomized complete block design at Qaemshahr, Northern Iran, in four replications and in two years, 2020 and 2021. Plant densities consisted of manual-traditional planting of up to 11 hills/m2 (with unequal planting spaces and common in the region) as control and manual-equidistant planting with 25 hills/m2 (with planting spaces of 20 × 20 cm and recommended by the Rice Research Institute of Iran) and machine-planting with 15.9, 20.8 and 20.8 hills/m2 (with planting spaces of 30 × 21, 30 × 16 and 30 × 12 cm, respectively). The paddy rice seedlings aged 45 d in manual-planting and 35 d in machine-planting. The results showed that the number of days from transplanting to flowering and flowering to maturity for machine-planting densities were greater than those of the control and 25 hills/m2 in manual-equidistant planting. The interaction effect of year × planting density was significant only for the flag leaf length and plant height. The greatest panicle length and flag leaf length were obtained when the rice plants were sown in a density of 25 hills/m2 in manual-equidistant planting. The greatest panicles/m2 and the percentage of grain-bearing spikelets were obtained when rice plants were sown by the machine. The maximum grain yield was obtained from planting density of 25 hills/m2 in manual-equidistant planting (8022 kg/ha).  The planting densities of 15.9, 20.8 and 27.8 hills/m2 in machine planting with 8034, 8236 and 8168 kg/ha grain yield ranked next as they out-yielded the control by 19%, 19%, 21% and 21%, respectively. It was concluded that machine planting with planting densities of 15.9, 20.8 and 27.8 hills/m2 is advantageous, at least when using old paddy rice seedlings.