In calibration of crop development rates in ORYZA, the user specifies the dates of panicle initiation, 50% heading, and physiological maturity (i.e. the time at which there is no further increase in grain yield). These dates define the duration of vegetative and reproductive stages, and accurate simulation requires robust assessment of each these events. Of the three, 50% heading is the most clearly defined and easiest to observe. Given an accurate date of 50% head-ing, panicle initiation can be estimated and ultimately has relatively small impact on the model results. Physiological maturity, on the