Rice seeds were collected from stocks kept in a refrigerator. Seeds of each line were immersed in just enough water to cover them. Water was changed every day for 7 days (May 2 to May 8) during germination stimulation. Seedlings were grown in special boxes (30 × 15 × 3 cm) for approximately 20 days in a greenhouse: two seeds were planted in each cell (2 × 2 × 3 cm) in the box on May 11 and 12 and watered twice a day (07:00 and 12:00). Seedlings were then individually transplanted into a paddy field (120 m2: 6.0 × 20.0 m) of the University Farm on June 8. The paddy field received 4.0 kg of basal fertilizer containing nitrogen, phosphorus, and potassium (weight ratio, nitrogen : phosphorus : potassium = 2.6 : 3.2 : 2.6). Except for the period of mid-season drainage (July 10 to July 17), the water level was maintained at 5–7 cm above ground to prevent seedlings from submersing. A herbicide (Joystar L floable) was applied on June 20 to kill weeds growing uncontrollably and the water was then kept at a high enough level to cover the weeds for 1 week. Koshihikari, Koshihikari carrying Jukkoku_sd1, Koshihikari carrying IR8_sd1, Koshihikari carrying d60, Koshihikari carrying d60 and Jukkoku_sd1, and Koshihikari carrying d60 and IR8_sd1 were grown in 4 m2 plots (2 × 2 m) (two instances per line). Basal fertilizer was applied to give 0.43 g/m2 nitrogen, 0.53 g/m2 phosphorus, and 0.53 g/m2 potassium. After ripening, 10 plants per genotype were sampled twice to assess the following traits: panicle heading time, culm length, internode interval, and panicle length.