The COUNCE scale is a developmental scale recently proposed for rice and that is being used in Brazil. However, the duration of developmental phases of this scale in degree-days is not available for Brazilian irrigated rice cultivars. Thus, the objective of this study was to determine the thermal time of some developmental phases of the COUNCE scale in irrigated rice cultivars grown in Southern Brazil using different calculation methods and cardinal temperatures throughout the crop cycle. A field experiment was carried out in Santa Maria, RS, Brazil, during two years (2003/2004 and 2004/2005), with five sowing dates in each growing season. The experimental design was completely randomized with four replications. Each replication was a 12-liter pot with 10 plants. The cultivars used were IRGA 421, IRGA 416, IRGA 417, IRGA 420, BR-IRGA 409, BRS 7 TAIM, and EPAGRI 109. The date of R2 (colar of the flag leaf visible) and R9 (all grains with brown hulls) stages of the COUNCE scale were recorded on the main culm of five tagged plants per pot. Using the cardinal temperatures of 11 °C, 28 °C, and 40 °C, the thermal time of the emergence-R2 phase was 700-800 °C day for the very early cultivar, 1000-1400 °C day for early cultivars and 1300-1700 °C day for intermediate and late cultivars, and the thermal time of the R2-R9 phase was 400-700 °C day with no relationship with the developmental cycle of the cultivar.
temperature; phenology; developmental stages; degree-days
