Method and experiment of longitudinal seedling feeding with intermittent grid like seedling mechanical transplanting

Rice is one of the most important food crops, with an annual planting area of approximately 30 million hectares in China. Hybrid rice accounts for about 50% of the national rice planting area, serving as a critical guarantee for the country's food security. However, the mechanized planting of hybrid rice has long been a challenge in the development of rice production mechanization and has not yet been fully overcome. To address this issue, a key approach is to adopt low seeding rates (50–65 g/tray) to cultivate robust blanket seedlings and achieve precise planting of 1–3 seedlings per hill by accurately controlling the seedling pickup amount of the transplanter. Nevertheless, due to the instability of longitudinal seedling feeding in transplanters, existing mechanized blanket seedling transplanting technologies have not yet fully achieved this goal. Currently, transverse seedling feeding in transplanters is primarily achieved using spatial double spiral cam mechanisms to ensure fixed-frequency (18, 20, or 24 times) quantitative delivery, which is accurate and reliable. In contrast, longitudinal seedling feeding relies on intermittent motion mechanisms to drive star wheels or ratchets, which in turn drive rubber-toothed seedling delivery belts. This process uses friction and the gravitational force of the seedlings to push the entire seedling mass toward the seedling pickup area. However, during longitudinal seedling feeding, factors such as machine vibration, the quality of the seedling blanket, and the moisture content of the seedling blocks can cause the seedlings to slide or become compressed and deformed in the seedling box. This leads to inaccuracies in longitudinal seedling feeding and subsequently affects transplanting quality. This study addresses the instability of longitudinal seedling feeding during mechanical transplanting of hybrid rice blanket seedlings. Based on the principle of forced seedling feeding with intermittent grids, a longitudinal seedling feeding method for mechanical transplanting of intermittent grid blanket seedlings is proposed. First, an intermittent grid blanket tray was developed to cultivate machine-transplanted blanket seedlings with intermittent grids (referred to as intermittent grid blanket seedlings). Second, using EDEM software and RecurDyn dynamics software, a physical model of the blanket seedling root-soil composite and the longitudinal seedling delivery belt of the transplanter was constructed. Combined with the mechanical parameter characteristics of blanket seedlings, a simplified DEM-MFBD dynamics model for longitudinal seedling feeding during mechanical transplanting of hybrid rice intermittent grid blanket seedlings was established, and longitudinal seedling feeding simulation experiments were conducted. Response surface methodology was employed to analyze the relationship between key parameters of the intermittent grid blanket tray and seedling feeding performance indicators, and tray parameters were optimized based on field experiments. Experimental results indicate: (1) Based on EDEM software, RecurDyn software, and experimental testing and analysis, a discrete element model of the root-soil composite body for hybrid rice blanket seedlings and a simplified model of the longitudinal seedling feeding components of the transplanter were constructed. Furthermore, a DEM-MFBD coupled simulation model for the dynamics of the longitudinal seedling feeding process was established, laying a foundation for exploring methods to improve the stability of longitudinal seedling feeding;(2) An intermittent grid seedling tray was designed, and a regression model between the longitudinal feeding stability and the main design parameters of the interrupted grid was developed. The optimal parameters for the intermittent grid were determined as follows: number of grids: 4, bottom inclination angle: 84°, and bottom length: 3.8 mm. Simulation studies indicated that the blanket seedlings cultivated in trays with this parameter combination significantly enhanced the stability of the longitudinal seedling feeding;(3) Field experiments and simulation results demonstrated that the optimized intermittent grid hybrid rice blanket seedlings exhibited higher operational stability and better transplanting quality during the actual planting process. Field tests confirmed that the simulation model was consistent with the field results, demonstrating reliable accuracy. Comparative analysis between traditional seedling trays and the intermittent grid trays showed that the blanket seedlings cultivated using the intermittent grid trays reduced the influence of their intrinsic parameters, decreasing the coefficient of variation (CV) of longitudinal feeding by an average of 9.29 percentage points. The average qualified uniformity rates for hills with 1-3 seedlings and 1-4 seedlings were increased by 6.68 and 7.65 percentage points, respectively. The designed intermittent grid seedling tray significantly improved the stability of the longitudinal seedling feeding. The research findings provide a valuable reference for achieving hill-drop transplanting with fewer seedlings per hill in hybrid rice cultivation.