水稲の打込み式代かき同時土中点播技術の開発

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  • Development of a New Technology for Direct-Seeding of Rice Using a Seed-Shooting Seeder Combined with a Paddy Harrow
  • スイトウ ノ ウチコミ シキ シロカキ ドウジ ドチュウ テンパ ギジュツ ノ カイハツ

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1. Introduction Rice production cost in Japan is at the highest level in the world. After the GATT Uruguay Round of Agriculture (1993), Japan accepted the minimum access of rice and will accept the customs rule in the near future. Japanese rice farmers must reduce their costs and advance labor-saving work practices in order to deal with the world free-trade system. The technology of direct-seeding of rice has been adopted in the United States and other countries with low rice-production cost. In Japan, rice transplanting technology was established at the end of the1960's because it was suited to the conditions of the Japanese culture, economy, and society at that time. Since it is very difficult to reduce the cost and advance the labor-saving work practices in the current transplanting culture, a direct-seeding system should be introduced to Japanese rice farmers. However, the area of rice production under direct-seeding in Japan has been decreasing in the last 25 years after a 1974 maximum of 55,280 ha. That area was 8,911 ha in 2000, just 0.5% of the whole area of rice cultivation. Therefore, in the 1990's the Ministry of Agriculture, Forestry and Fisheries of Japan started developing some new technologies of directseeding system, with the aim of reducing rice-production cost. There are many types of known and available types of technology for direct-seeding of rice, but we decided to develop a new technology, a "seed-shooting seeder of rice combined with a paddy harrow," in short, "the seed-shooting seeder" or "the seed-shooting technology" as described in this paper. The reasons for this development are as follows. 1) A submerged direct-seeding system with puddling has the advantages of higher feasibility in rainy weather, effective weed control, and less leakage of water, over the direct-seeding system on dry land. 2) The most important factor in submerged direct-seeding is improving the unstable seeding depth of rice. However, the depth is stabilized in the proper range for seed germination (5 to 15mm) by the seed-shooting seeder. 3) Seed-shooting technology can produce a pattern of growing rice similar to the one produced in the transplanting method. 2. Seed-shooting technology features The seed-shooting seeder is composed mainly of a seed hopper, a seed roll, and a saw-toothed disc. The seeder is fitted on the rear of a paddy harrow that is mounted on a tractor. The seed roll and the saw-toothed disc are driven by direct-current motors (40W, 130W) powered by the tractor's battery. The revolutions of the motors are continuously changed and are set by a manual dial. The rice seeds used in this technology are coated with calcium peroxide (CaO_2), which assists seed germination in puddled soil. Several coated seeds are fed intermittently by the seed roll to the edge of the saw-toothed disc through a seed-supply tube. The seeds are hit by the rotating saw-toothed disc (varying from 100 to 1500rpm), and then shot from the seedshooting tube into the puddled soil. The seeder is called a "shotgun seeder" because of its seed-shooting performance. Specifications of the seeder were determined by laboratory tests. Its main features are as follows. 1) The saw-toothed disc is 190mm in diameter and 20mm in thickness, and has 32 surrounding teeth. Elastic foamrubber is the most suitable material for the disc. 2) The space between the disc case and the tip of the saw-tooth should be adjusted from 1 to 3mm. 3) Both the seed-supply tube and seed-shooting tube are fitted on the disc case. The lowest position of the seed-supply tube should be 10 mm above the horizontal centerline of the disc. 4) The seed-shooting tube should be 200mm in length, 30mm in internal width, and 25mm in internal depth. The fitting angle of the tube to the disc case should be 10 to 15 degrees. In order to measure the performance of the saw-toothed disc, we shot coated seeds into agar (viscosity : 50dPa・s). The seeds depth in agar was proportional to the seed velocity (i.e., disc revolution). This linear relationshop was also observed when testing puddled soil. We improved the ordinary fluted roll, producing a new cell roll with three cells that achieved a more accurate seed-feeding interval, an essential for hill-seeding. The new cell roll has another feature in that the bottom of the cell vibrates a slightly to remove adhering CaO_2 powder. 3. Seeding performance Important factors in the seed-shooting technology were both the seeding depth and the pattern of seeds within the seeding spot. 1) Seeding depth Seeding depth influences not only the seed emergence rate, which is the most important factor in directseeding culture, but it also affects lodging tolerance. The seed velocity right after being hit by the disc was measured by a high-speed camera in a laboratory test. The relation between seed velocity and the seeding depth in agar was investigated as well. The viscosity (dPa・s) of puddled soil was measured, instead of its hardness. Instead of seeding depth, we measured germination depth, which is easier to measure in real paddy fields. The results were as follows. (1) The coefficient of variation (CV) of the seed velocity was 15%, and this velocity was generally equal to the peripheral velocity of the disc. (2) When seeds were shot into agar (viscosity 50dPa・s), the seeding depth was proportional to the seed velocity. The average depth was 32mm (seed velocity 12m/s) and its CV was 37%. (3) Seeding depth in the paddy field was influenced by the seed velocity and the hardness of the puddled soil. The hardness influenced seeding depth more than the seed velocity. Seed velocities of 6 to 8m/s for soft soil (under 150dPa・s) and 10 to 12m/s for harder soil (above 150dPa・s) gave good results of proper seeding depth. (4) The CV of germination depth was about 50% in the hill-seeded spot. There were four to five seeds in the proper depth range (5 to 15mm) in the soil, or about 70% of all shot seeds. 2) Pattern of seeds in the hill-seeded spot The dimensions of the elliptical hill-seeded spots were 6 to 8 cm (major axis) and 4 to 5cm (minor axis). These were measured in both the paddy field tests and the laboratory tests. The minor axis was fixed by the width of the seed-shooting tube. The major axis was influenced by both the time needed to release several seeds from the cell, and the period of time needed to feed the seeds to the saw-toothed disc. In this case, a shorter time is preferable because it leads to a smaller major axis. In addition, the major axis was influenced by the work speed (the traveling speed of the tractor) and soil conditions (hardness, etc.). We confirmed that the average major axis ranged from 5 to 10cm in standard working conditions. 4. Seeding in a paddy field In order to improve the farm operation and the performance of the seeder, we investigated the puddling method and the work efficiency of the seeder in paddy field tests. Some parts of the paddy harrow were redesigned, and a marker and several other devices were fabricated. The highest performance of the seeding operation was achieved when the puddled soil was prepared with proper hardness at the point right after the paddy harrow. Certain operating conditions such as work speed, harrow revolution, puddling depth of the harrow, revolution of the saw-toothed disc needed to be set in the proper range. The paddy harrow was redesigned to increase the work speed and to provide puddling to reduce labor. Consequently, it was possible to realize the maximum work speed (1.0m/s) with a proper settings of paddy harrow revolution and hardness of puddled soil. We fabricated a big disc-type marker (287mm in diameter) because the existing markers were not effective in the soft puddling soil for the seed-shooting technology. The marker performance was confirmed in a paddy field test. The new marker has been on the market as a seeder option since 1998. Two operators with different skill levels carried out work efficiency testing in two paddy fields. Their average efficiency was 2.8h/ha. This was similar to that of the drill-seeder available on the market but inferior to the broadcast type seeder (such as a radio-controlled helicopter). The efficiency may be improved by increasing the work speed and the workwidth of the seeder. 5. Characteristics of seed germination and lodging tolerance 1) Seed emergence rate Seed emergence rate in this technology was influenced by factors such as the variety of rice, water -control management of the paddy field, the seeding depth, and the temperature. Seeding depth and temperature greatly influenced the seed-emergence rate of the submerged direct seeding of rice. Therefore, seed emergence tests were carried out using pots in an incubator to eliminate the influence of seeding depth (5 to 20mm) and temperature. Seeding depth was varied from 5 to 20mm in the seed-shooting technology. We concluded that the seeding depth should be 10 to 15mm in a warm area (like Kyushu) and 5 to 10mm in a cool area (like Tohoku and Hokkaido). The main target rate of seed emergence is more than 70% in submerged direct-seeding. It was clearly shown that the seed-shooting technology is able to achieve the target with the proper seeding conditions. Increased lodging tolerance of rice was one of the biggest advantages of seed-shooting technology. We developed a device for measuring the lodging tolerance of hill-seeded rice and compared it with the lodging tolerance of broadcast rice and hill-seeded rice. It was shown that the new device was superior to the existing device, although the measuring procedure was rather complicated. Comparison tests of lodging tolerance (dyn/stalk) of broadcast rice and hill-seeded rice were performed in a paddy field. The results clearly showed that the hill-seeded rice was superior to the broadcast rice. 6. Practical tests in paddy fields 1) Questionnaire distributed throughout Japan Practical tests of the seed-shooting technology were carried out in 1997 at 40 prefectural agricultural experiment stations throughout Japan. We gathered data concerning the seeding performance in different types of paddy fields using a questionnaire. The results were as follows. (1) Varieties of rice used for the seed-shooting technology were those with good taste such as Koshihikari, whose germination characteristics were inferior in direct-seeding culture. (2) The planting rate of dry seeds was about 10g/m^2 in Hokkaido, and 3 to 4g/m^2 in other areas. (3) The average work speed was 0.52m/s, and the maximum was 0.8m/s. We concluded that the speed range of 0.5 to 0.7m/s was appropriate considering both work efficiency and work accuracy. (4) Normal seed velocity was 6 to 8m/s in cool areas, and 8 to 12m/s in warm areas. The average germination depth was 7.3mm (CV =43%). (5) The seed-shooting spots major axis was 8.1cm (CV =31%), and the minor axis was 4.7cm (CV =31%). (6) Average yield of rice (brown rice) was 481g/m^2, which was about 10% less than that of the transplanting method. The results of the questionnaire proved that the seed-shooting technology was practical for rice farmers in Japan. 2) Practical test in Fukuoka-Ken We carried out a practical test of seed-shooting technology in Fukuoka-Ken. The conditions of the seeding were : work speed, 0.5m/s ; seed velocity, 12m/s ; and planting rate for dry seed, 3.0g/m^2 ; following the standard conditions for a warm area. The results were : germination depth, 10mm ; major axis of hill-seeded spot, 7.1cm ; rate of established seedling, more than 80% ; yield of rice, 520g/m^2 (nearly equal to that of the transplanting method). The possibility of reducing costs and labor in rice production with the seed-shooting technology was studied using the data from practical field tests in Fukuoka-Ken and Yamagata-Ken. We demonstrated that the cost was reduced to about 90% and the labor to 80 to 90% compared to that of the transplanting method.

水稲作の省力化・低コスト化・軽労化を進めるため, 日本型直播稲作技術の開発が農林水産省を中心に1990年代より始められた。著者等は, 移植様式に近い安定した栽培管理が可能で, 良食味品種にも適用できる直播技術として打込み式代かき同時土中点播技術を開発した。本技術のキーテクノロジーは打込み式代かき同時土中点播機である。本機の第1の特徴は, 代かきハローで仕上げ代かきを行いながら, 開発した鋸歯形打込みディスクで数粒の種子を打撃・加速し, 10m/s前後の速度で代かき土壌中に打込むことである。それによって5 ～ 15 mmの良好な播種深さが得られ, 安定した出芽率を確保することができる。第2の特徴は, 開発した点播用播種ロールで数粒の種子を鋸歯ディスクに間欠的に供給することにより楕円形状の多粒点播が可能となったことである。生育の中期・後期には株形成が進み耐倒伏性が強化されるため, 肥培管理等が容易になる。全国20の道県での実証試験アンケート調査と現地実証試験 (福岡県) による播種精度・生育管理・収量等のデータを収集した結果, 実用技術の見通しが得られ, 軽労化と10～20%の省力・低コストが期待できると結論づけられた。本機は1998年から3社の農業機械メーカーで市販化された。

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