Hiroshi Nakano

Ratooning is a cultivation technique that involves harvesting a second crop (referred to as a ratoon) originating from the stubbles of the first crop. Rice ratooning has been implemented in countries located in tropical and temperate regions. Based on recent studies conducted in southwestern Japan, this review introduces and discusses the effects of harvest time and cutting height of the first crop and the roles of leaf blades and nonstructural carbohydrates (NSC) in the stubbles of the first crop on grain yield to provide the essential information necessary for the development of rice production technologies using ratooning.

In rice (Oryza sativa L.) production, dry direct seeding is one of the most essential technologies to reduce labor input and to increase net income. Field experiments were conducted in southwestern Japan in 2019 and 2020 to determine the effects of nitrogen (N) application rate and timing on grain yield, lodging, grain appearance, and protein content in dry direct broadcast seeded rice under different weather conditions. In 2019, plants had larger source ability using the normal solar radiation at the tillering stage, regardless of N application timings. Plants with applied N at the reproductive stage produced the highest grain yield and the highest percentage of undamaged grains under high air temperature condition during the early ripening stage. In 2020, plants did not have larger source ability due to the lower solar radiation at the tillering stage. Plants with applied N at the reproductive stage produced the highest grain yield but produced the highest percentage of green immature grains. These results mean that solar radiation at the tillering stage may be important for increasing grain yield and quality in dry direct broadcast seeded rice. Therefore, since plants grown under normal solar radiation condition at the tillering stage may have large source ability, more N application at the reproductive stage may be recommended to increase grain yield. In contrast, since plants grown under low solar radiation condition at the tillering stage may have small source ability, less N application at the reproductive stage may be recommended to decrease the occurrence of green immature grains.

Rice (Oryza sativa L.) ratooning is one of the most essential production strategies to increase grain yield. Field experiments were conducted in southwestern Japan in 2021 and 2022 to determine the effects of planting date and cutting height of the first crop on grain yield. Grain yield of the second crop was higher at early planting date than at late planting date in 2021, but the yield difference was narrowed in 2022. The higher grain yield at early planting date resulted from the higher number of spikelets and percentage of filled spikelets due to the higher stubble nonstructural carbohydrate (NSC) content, leaf area index (LAI), and air temperatures. Grain yield of the second crop was higher at high cutting height than at low cutting height in 2021, but the yield difference was narrowed in 2022. The higher grain yield at high cutting height resulted from the higher number of spikelets due to the higher NSC content and LAI. In the weather conditions, grain yield had a strong positive correlation with the air temperature, which had positive correlations with the number of panicle and percentage of filled spikelets. In the stubble conditions, grain yield had a strong positive correlation with NSC content, which had positive correlations with the number of panicle and percentage of filled spikelets. Therefore, early planting, which secures high air temperatures, and high cutting height, which produces high NSC content, are recommended to increase grain yield of the second crop.