Effect of planting date on accumulated temperature and yield of ratooning rice

【Objective】 The effect of temperature on plant development is often characterized by degree days or accumulated temperature. In this paper, we investigate how planting date affects accumulated temperature and the consequence for the yield of two ratooning rice varieties. 【Method】 The experiment was conducted in a rice field in Changsha, central Hunan province. It included two rice varieties: Y Liangyou 900 (YLY900), which has a long growth period, and Y Liangyou 911 (YLY911), which has a moderate growth period. The seeds of both varieties were sown on March 15 (B1), March 20 (B2), March 25 (B3), March 30 (B4), April 5 (B5), or April 10 (B6) to establish a planting date gradient. In each treatment, we measured physiological parameters including growth duration, yield component, and seed-setting rate to evaluate the physiological development of the crops. 【Result】 The average annual growth period of YLY911 was 215 days. Its first and regeneration seasons were eight and ten days shorter, respectively, than those of YLY900. Delaying the planting date shortened the vegetative growth period of the first season and reduced the total growth period of both seasons in both varieties. The variation in grain yield in the first and regeneration season, as well as annual yield of YLY911, followed a parabolic relationship, peaking in B2. Conversely, the yield of YLY900 decreased steadily as planting date was delayed. Yields in the first and regeneration season, as well as annual yield, were consistently higher in YLY911 than in YLY900. The primary factors affecting grain yields were the number of ears per unit area and the seed-setting rate. With delay in planting dates, the annual accumulated temperature for both varieties first increased and then declined, peaking in B2. The accumulated temperature in the first and annual seasons for YLY900 was significantly higher than for YLY911, while the accumulated temperature in the regeneration season was lower for YLY900 than for YLY911. Such differences in the regeneration season between the two varieties were statistically insignificant (p>0.05). 【Conclusion】 For ratooning rice in our studied area, the optimal planting date was March 15 for varieties with long growth period and March 20 varieties with moderate growth period.