Rice reproductive development stage thermal time and calendar day intervals for six US rice cultivars in the Grand Prairie, Arkansas, over 4 years

The rice crop's reproductive developmental timing in days and thermal time is needed for effective modelling, research interpretation and management of the crop. To obtain these data, a field study was conducted at Stuttgart, Arkansas, USA in 2007, 2008, 2009 and 2010. The study utilised data collected from randomised complete block design field experiments with three replications and six rice lines in each of the years. Averaged across years and cultivars, the degree-day-10 (DD10) intervals (thermal time units with a base temperature of 10 degrees C) for Reproductive Stages R3, R4, R5, R6, R7 and R8 were 21, 30, 19, 48, 70 and 189 degrees C-day, respectively. The average intervals in calendar days for R3, R4, R5, R6, R7 and R8 were 2.3, 3.3, 2.3, 6.0, 4.5 and 26.7 days, respectively. For R4 and R5, cultivar rankings differed over the 4 years with cultivar differences being mostly small, non-significant or inconsistent. For R6, the cultivar Cypress had either the longest or among the longest intervals. For R7, the medium grains had the longest or among the longest intervals. For R3 and R8, cultivar differences were significant with no significant year by cultivar interactions. For the R3 intervals, the primary difference was between Bengal and the five other lines. For R8, the intervals in both days and DD10 were least for Cypress, followed by Wells, followed by LaGrue and XL723 followed by the medium grains Bengal and Jupiter which had the longest intervals for R8. Consequently, the R3 interval could be generalised to five of the six lines in the study while R4, R5, R6 and R7 intervals could be generally applied with some caution. The R8 intervals were different among lines and grain types. These differences should not be ignored. The extremely short R8 interval for Cypress is likely associated with its high head rice yields across a range of environments compared to other long-grain rice cultivars and hybrids in the USA The utilisation of the rice reproductive growth stage intervals can potentially improve analysis and interpretation of field plot research, model predictions and management of the rice crop.