Degree-day requirement for heading and maturity of three most popular rice varieties in Sri Lanka as influenced by location and season

Growing degree-days (DD- °C days) required to complete growth phases in plants is generally considered as a constant. However, this practice has recently been questioned. Therefore, we tested the DD requirement for the growth phases of three Sri Lankan rice varieties, Bg300, Bg352 and Bg358. These rice varieties are widely cultivated in Sri Lanka across years, seasons and locations. When averaged across seasons, years and locations, the DD required for heading and maturity of Bg300 were 1,051 and 1,543 °C days, while those of Bg352 were 1,098 and 1,627, and Bg358 were 1,203 and 1,698 °C days, respectively. Moreover, DD required for heading and maturity varied among locations. Number of days required for heading and ripening correlated negatively, i.e., reducing ripening duration by 8.4 h due to the delay in heading by one day. Further, DD required for heading and maturity were positively correlated with the mean seasonal minimum and maximum temperatures. Therefore, crop duration could not be lowered as expected under warming climates. Hence, genotype × environment dependent DD requirements of rice varieties need to be considered when making agronomic and policy decisions in both regional and seasonal scales, and increasing the precision of phenology predictions.