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1. China National Rice Research Institute, Hangzhou 310006, China; 2. Zhejiang Provincial Center of Agricultural Technology Extension, Hangzhou 310020, China

Abstract With the development of China’s social economy, transfer of rural labors, and rise of labor costs, the rice production technologies are changing from manual operation to mechanical operation. The core of mechanical rice production is mechanical planting, while the key to mechanical planting is the seedling raising. On the basis of analyzing and studying the problems and experience of traditional mechanical transplanting of rice, a new overlapped tray seedling raising model was introduced. Main features of this model: substrate seedling raising and overlapped seedling tray. The seedling trays for seeding line are overlapped, and the overlapped trays are moved into the seedling room with controlled temperature and humidity. The temperature of the seedling room is controlled at 30-32℃, and the humidity of the seedling room is controlled at above 90%. After about 48 h, when the seedling height reaches 0.5 cm, move the seedling trays to the nursery. This model consists of one seeding center (seedling raising center) and N nurseries, to realize 1+N seedling raising for mechanical transplanting of rice. One seeding center can provide seedlings for several hundred to several thousand hectares of mechanical transplanting of rice, and provide a new model for social services. This model could improve the quality of seedlings, increase the seedling survival rate by about 20%, reduce the seedling raising costs by 15%-20%, reduce the seedling raising risks, and greatly increase the utilization rate of the site and equipment of seedling raising center.

Key words Rice, Mechanical transplanting, Seedling raising, Overlapped tray

1 Introduction

With the development of China’s social economy, transfer of rural labors, and rise of labor costs, it is urgent to transform and upgrade the rice production technologies. The rice industry is gradually changing from decentralized operation to large-scale operation, from manual operation to mechanical operation, from single production to the production and socialized service combined production[1-3]. In rice production, farming and harvesting have basically realized mechanization, but the mechanization level of planting is relatively low. There are many problems and high technical requirements for mechanical planting of rice. The core of mechanical rice production is mechanical transplanting of seedlings, while the key to mechanical transplanting of seedlings is the seedling raising.

The seedling raising soil for mechanical transplanting has experienced the mud seedling raising, dryland seedling raising, and gradually developed to substrate seedling raising[4-10]. The seedling raising model is changing from traditional "mud + small greenhouse" and "dryland soil + greenhouse" to large-scale overlapped tray seedling raising model. Traditional mechanical transplanting of seedling raising generally has the problems of not uniform seedling emergence, low seedling emergence rate, serious diseases and insect pests of seedlings, poor root system development, poor seedling mat formation, and low seedling quality. These problems lead to high seedling leakage rate, poor uniformity, low quality of mechanical transplanting, low and unstable yield, and restrict the development and application of mechanical transplanting technology. In recent years, in view of the problems existing in the mechanical transplanting technology, the researchers have undertaken extensive studies about the model of seedling raising for mechanical transplanting, developed the mud seedling raising, greenhouse seedling raising, and degradation film seedling raising, and improved the seedling raising effects of different planting seasons[11-15]. The double harvest rice mechanical transplanting is planting method with great difficulty. Thus, the rice production urgently needs the modern, advanced, and practical innovation in seedling raising for mechanical transplanting. During the seedling raising, if the temperature and seedling raising soil (substrate) humidity are not suitable, it will lead to not uniform seedling emergence and low seedling survival rate; abnormal climate such as low temperature and high temperature and failure to hold the temperature and uncover the film in time may lead to death or burn of seedlings; too dry or too wet seedling raising soil will restrict the seedling emergence; too high amount of fertilizer in seedling raising soil will harm the seedlings, and excessive dosage or not uniform seedling strengthening agent will harm the seedlings[11,13,15]. Especially, there are many problems in seedling raising for early season rice[13, 15-16].

In view of problems in seedling raising for mechanical transplanting, we analyzed technical requirements for large-scale production and socialized services of rice production. Through many years of study of seedling raising model for mechanical transplanting, we innovated upon the overlapped tray seedling raising model for mechanical transplanting and developed the auxiliary facilities, equipment, and seedling raising technologies. With several years of experiment and demonstration, this seedling raising model has shown better practical effect, is highly accepted and favored by agricultural technology extension personnel and numerous rice farmers.

2 Characteristics of overlapped tray seedling raising model for mechanical transplanting of rice

In the seedling raising of mechanical transplanting, the process from sowing to seedling emergence has high requirements for equipment and technology. Then, many problems occur in the rice production. In the seedling raising process, rice farmers do not have technology, leading to low quality of seedlings and poor mechanical transplanting effect, and seriously restricting the application of mechanical transplanting technology. For this, the process from sowing to seedling emergence adopts.

To this end, planting to the emergence stage using factory based operation, overlapped tray seedling emergence, to create seedling emergence temperature, humidity and oxygen environment, to ensure that the uniform and strong emergence of seedlings. This stage mainly includes seed soaking, disinfection, germination, seedling bed soil treatment or substrate, flow-line sowing, overlapped tray, tray conveying, temperature and moisture preservation, etc. From the sowing to seedling emergence, all are completed in the factory, to realize factory based seedling raising production. When the length of the seedlings reaches 0.5 cm, the trays are conveyed to the seedling raising site. The seedling site can be wet seedling plate, seedling raising greenhouse, paddy field dry nursery, and factory vertical seedling raising. One seedling raising factory can provide seedlings for many seedling raising sites, to realize "1 + N" seedling raising model. This seedling raising model consists of one seedling raising center plus N nurseries. By taking advantage of the rice seedling raising centers established with the policy support in recent years, and making full use of the facilities, equipment and technology of the seedling raising center, this model increases the equipment utilization efficiency. In various rice varieties such as early season rice, continuous cropping late rice, and single harvest late rice, centralized seedling raising can be implemented, to greatly expand the seedling supply, increase the service capability, realize large-scale, specialization and intensification of mechanical seedling raising of rice in China.

3 Advantages of overlapped tray seedling raising model for mechanical transplanting of rice

The overlapped tray seedling raising model is a new model of modern agricultural production. It promotes socialized services for seedling supply for mechanical transplanting and accelerates the extension of the mechanical transplanting technology. Compared with the traditional seedling raising model, this model is characterized by following advantages. (i) High seedling quality. This advantage is reflected in high emergence rate (increasing by 20% compared with the mud seedling raising), uniform emergence, robust seedlings, strong root system packing ability, and many white roots. After seedlings grow to transplantation state, the mechanical transplanting effect is good. (ii) Low seedling raising risks and costs. In traditional seedling raising of mechanical transplanting, there is problem of rotting of seeds and seedlings, and death of seedlings, leading to insufficient seedlings for mechanical transplanting, failure to transplant seedlings in time, and seriously influencing the rice production area and yield. However, the overlapped tray seedling raising model is free from adverse climate. And standardized seedling raising greatly reduces the risks of seedling raising. Using the large-scale centralized overlapped tray seedling raising, the equipment utilization rate and labor efficiency are increased, and the seedling quality is improved. It has proved that the seedling raising cost drops by 15%-20% compared with the traditional seedling raising. (iii) Increase in seedling raising ability, seedling supply ability, and seedling raising equipment utilization efficiency. Compared with the traditional seedling raising model, the overlapped tray in dark room has the seedling tray holding capacity increasing by around 20 times, and the indoor emergence management time can be shortened to 2 d from 4-5 d, so it greatly increases the seedling raising ability of the seedling raising site. Using the overlapped tray seedling conveying, it changes the seedling conveying method, so the transport is more convenient. Besides, the seedlings can be conveyed through overlapped trap, to realize long-distance seedling supply, so the transport cost is greatly reduced. The centralized seedling raising of overlapped tray seedling raising center adopts modern and advanced equipment and facility. Through daily sowing and supply of seedling tray, the equipment utilization efficiency is increased.

In addition, during the traditional seedling raising of mechanical transplanting, it needs many labors, while the overlapped tray seedling raising model adopts mechanical operation, so it greatly increases the labor productivity. The new model greatly increases the seedling raising ability, avoids repeated construction and investment of equipment for the seedling raising center, saves and increases the agricultural facility land and equipment input efficiency. The new model adopts modern equipment, large-scale seedling raising, advanced technology, sowing and emergence technology in place, stable emergence, realizing the standardization of rice transplanting, improving the technical availability rate, ensuring the quality and quantity of rice seedlings for mechanical transplanting. Therefore, it is favorable for stabilizing and increasing the level of mechanical transplanting.

4 Technology of overlapped tray seedling raising model for mechanical transplanting of rice

4.1SeedtreatmentConsidering the local ecological conditions, planting system, planting season, production mode and other factors, according to the previous and next crops for rotation, it is necessary to ensure rice varieties with safe and uniform rice ears. For double-harvest rice area, it is necessary to pay attention to proper combination of early season rice and continuous cropping late rice growth period, to realize high yield of mechanical transplanting of double harvest rice. For common rice varieties, the germination rate of seeds should be higher than 90%; for hybrid rice varieties, the germination rate of seeds should be higher than 85%. Seeds should be sun dried on fine days for 1-2 d, to increase the seed germination energy and germination rate, then seeds are selected using saline water or clean water. Besides, prochloraz + imidacloprid, phenamacril, Fludionil can be used to soak and disinfect seeds, to prevent diseases and insect pests such as bakanae disease and nematode, etc. The soaking time is determined based on the type of seeds and temperature. Generally, the southern early rice varieties are soaked for 2-3 d, the single harvest and continuous cropping hybrid late rice varieties are soaked for about 2 d, and the southern single harvest rice varieties are soaked for about 3 d. Due to the low temperature during soaking, the northern japonica rice generally needs to be soaked for 5-7 d. When seeds are about to germinate, it is able to sow after spreading for drying.

4.2MechanicalsowingMechanical sowing adopts special seedling trays special for mechanical transplanting. First, prepare the seedling raising soil or substrate. Dryland soil that has been fertilized or acidified or seedling raising substrate can be used to ensure safety of seedling raising and breed strong seedlings. Second, sow at the proper time. For southern early season rice, sowing can be carried out in March when it is warm, and the seedling age is 25-30 d; for southern single harvest rice, sowing is generally undertaken in the late May or the beginning of June, and the seedling age is 15-20 d; for southern continuous cropping late rice, the sowing period should be properly arranged according to the harvest time of the early rice, and the seedling age is generally 15-20 d. For seedling raising of overlapped tray mechanical transplanting, it is necessary to choose the seedling tray special for seedling emergence in dark room, adopt auxiliary precise sowing flow line, auxiliary overlapped tray mechanism, and automatic loading device. Before sowing, mechanical commissioning should be carried out. It is necessary to adjust the sowing quantity, bed soil laying quantity, soil coverage quantity, and water sprinkling amount, and complete the tray placing, soil laying, compressing, watering, sowing, and soil covering operations in one time. The sowing quantity should be properly determined according to the rice varieties, the season and the tray size. For the southern conventional double harvest rice, generally 100-120 g/tray (about 30 cm tray), and 450 trays for one hectare of paddy field, while for 23 cm tray, the sowing quantity should be reduced properly according to the area. For hybrid rice, the sowing quantity may be reduced properly according to the rice growth characteristics. For example, the sowing quantity for single harvest hybrid rice is 60-80 g/tray (about 30 cm tray).

4.3SeedlingemergenceindarkroomforoverlappedtrayFor flow line sown trays, adopt double-sided grid plate, overlap and stack the trays, and place 6-8 trays on each plate. Generally, one overlap includes 20-25 trays, and place one tray of soil or timber on the top to cover. Use forklift to convey the overlapped trays to the seedling emergence dark room with controlled temperature and humidity (30-32℃ temperature and above 90% humidity), and complete the emergence in the dark room. Place the seedling raising trays in the dark room for about 48 h. After the seed germination (the bud is about 0.5 cm long), use the forklift to move the plate out, and convey and supply the seedlings to each seedling raising site. Generally, every seeding raising center has 2-3 seedling emergence rooms to satisfy the requirements of daily sowing and emergence.

4.4SeedlingraisingmanagementIt is necessary to convey the seedlings with bud length of 0.5 cm bred in the seedling raising center to each seedling raising site, and carry out decentralized seedling raising according to actual situations of each seedling raising site. For early season rice, seedlings should be placed in plastic greenhouse or it is necessary to put up sheds above the seedling plates to keep temperature and humidity; for single harvest rice and continuous cropping late rice, seedling trays may be placed directly in paddy field, or put in pest-preventive sheds if conditions permit. During the seedling raising, it is necessary to properly manage the seedlings. For southern areas, the temperature is low, so it is necessary to cover plastic film to keep warm, and control the temperature within the greenhouse at 22-25℃ (but not higher than 30℃ and not lower than 10℃), and keep ventilation in time to prevent seedling rotting or burning. Besides, it is necessary to prevent and control diseases and insect pests during the seedling growth, especially the prevention and control of dripping disease and bakanae disease. In the middle and late paddy fields, it is necessary to set up the shading nets on the trays according to the weather conditions to prevent rainwater. For southern early rice, when the seedling age is generally 25-30 d, and the leaf age is 3.1-3.5 leaves, and the seedling height is 12-18 cm, the mechanical transplanting can be carried out; for single harvest rice and late rice, when the seedling age is generally 15-20 d, and the leaf age is 3.5-4.5 leaves, and the seedling height is 12-20 cm, the mechanical transplanting can be carried out.

5 Facilities for overlapped tray seedling raising model for mechanical transplanting of rice

The overlapped tray seedling raising model for mechanical transplanting of rice should be provided with proper facilities according to several key processes including seed treatment, soil preparation, seedling raising and sowing, overlapped tray conveying, and seedling emergence. Specifically, the equipment for seed treatment includes seed awner, seed soaking and germination pool, seed bag, seed basket, seed centrifugal dehydrator, the substrate seedling raising is preferred. If the local soil is used, the equipment for soil preparation includes soil pulverizer and mixer. The facilities for seedling raising and sowing includes seedling raising and sowing flow line, seed lifter, and auxiliary overlapped tray machine. The seedling emergence room equipment mainly includes intelligent dark room for seedling emergence, heating system, humidifier, and intelligent temperature and humidity control system. And the tray conveying equipment includes seedling tray holder, forklift, and seedling conveyor. Through many years of research and development, China National Rice Research Institute in cooperation with Hangzhou Fengzhu Agricultural Technology Company developed the overlapped tray seedling raising model for mechanical transplanting of rice and established the construction scheme.