Research progress on precision variable-rate seeding technology and equipment for maize

Maize is one of the most important cereal crops in the world. Maize production is of great significance to alleviate the world food crisis. The maize seeding rate can be adjusted during variable-rate seeding according to the spatial heterogeneity of the farmland environment, which is an important means to break through the bottleneck of maize yield improvement and improve resource utilization efficiency. In this review, the variable-rate seeding of maize was summarized from three aspects: the accurate acquisition of soil fertility indicators, the optimal seeding rate decision-making, and the precise control of seeding rate. The research status was also outlined on the rapid detection of soil fertility indicators using near earth and remote sensing. The spatial interpolation methods of soil attributes and farmland management zoning methods commonly used to accurately express the spatial distribution of soil fertility indicators were summarized. An emphasis was put on the research status and the advantages and disadvantages of variable-rate seeding decision-making methods based on farmland management zones and models. Based on the research ideas of developed countries and combined with the national conditions of China, three seeding decision-making methods based on fuzzy reasoning, field experiments, and machine learning were proposed. The structural composition and working principle of variable-rate seeding control systems were summarized in the developed countries. The research progress was reviewed on the electric drive seed-metering system in the variable-rate seeding process, in terms of the operating speed measurement, seed-metering device driving, seeding delay, and the compensation. The research gap was determined in the variable-rate seeding technology between developed countries and China. The research priorities and development suggestions were proposed for China in the field of variable-rate seeding of maize in the future. Developed countries were carried out the extensive and in-depth research on dynamic detection of soil fertility indicators around spectral analysis, electromagnetic induction, and current voltage four terminal. A variety of dynamic detection equipment was also developed for the soil fertility indicators. However, China is still in the theoretical research stage of in-situ dynamic detection of soil fertility indicators at present. There was few relevant equipment to realize the dynamic detection. Multi indicators high-precision collaborative detection using multi-sensor fusion has been the development trend of dynamic detection of soil fertility indicators. The Kriging and k-means clustering with the geo-statistics were the most studied and applied methods for the soil attribute spatial interpolation and farmland management area division, respectively. In terms of optimal seeding rate decision-making, the seeding rate decision-making on farmland management zone was still occupied the leading position in the field of seeding rate decision-making, due to its simplicity and strong regional adaptability. With the deepening of research on the mechanism of soil fertility affecting maize seeding rate and the continuous accumulation of experimental data for seeding rate decision-making, model-based precision seeding rate decision-making methods will be expected to gradually develop in the future. The variable-rate seeding control system with a controller as the core is the carrier for implementing variable seeding operations and is mainly composed of a human-machine interface, controller, satellite positioning system, communication unit, and seeding unit. Developed countries have realized the various variable-rate seeding control systems with different architectures. The main research was conducted on the electric drive seed-metering control system from the aspects of operation speed detection, seed-metering device driving, and control system architecture optimization. But there was no the core technology of variable-rate seeding in China at present. Much attention can be put on the dynamic detection of soil fertility indicators and the decision-making of the best maize seeding rate. The precision metering and electric drive metering control system of maize can be developed for a maize precision seeding unit with the high seeding quality and response speed for variable seeding operations.