Planting date and variety have a profound effect on the grain yield of ratoon crops. However, the optimum planting date for different varieties has not been fully investigated in ratoon rice systems in Japan. A field study was conducted in southwestern Japan in 2022 and 2023 to identify key traits of varieties suitable for different planting dates in high-yielding rice ratooning systems by evaluating the effects of planting date (early vs. late: mid-April vs. mid-May) and variety (eight varieties) on grain yield. In early planting, the grain yield of the second crop was positively correlated with spikelet number, but not with the percentage of filled spikelets, 1,000-grain weight, or air temperatures during the growth stages of the second crop. These findings indicate that, due to global warming, air temperatures during the growth stages of the second crop are not substantially lower in early planting, and that the percentage of filled spikelets and 1,000-grain weight do not decline significantly. Thus, varieties with higher spikelet numbers may achieve higher grain yield in early planting. However, in late planting, the grain yield of the second crop was positively correlated with spikelet number, the percentage of filled spikelets, 1,000-grain weight, and air temperatures during the growth stages of the second crop, and negatively correlated with the growth duration of the first crop. These findings suggest that, despite global warming, air temperatures during the growth stage of the second crop tend to be lower in late planting, resulting in reductions in the percentage of filled spikelets and 1,000-grain weight. Consequently, early-maturing varieties with higher spikelet numbers and stronger grain-filling ability may achieve higher grain yield in late planting.
Nakano et al. (2026) studied this question.