Amongst the many different proxy indicators of physiological maturity used in the literature are: (1) grain moisture thresholds (e.g., 28% grain moisture; Espe et al., 2015), (2) number of days since planting or heading (e.g., 35 days following 50% heading; IRRI, 2013), and (3) the firmness of the grain (e.g., IRRI,2013; Moldenhauer and Slaton, 2015). The definition listed in the ORYZA(v3) training manuals (IRRI) is: “Physiological maturity isvisually identified when the grains on the lower portion of secondary and tertiary panicles harden and begin to lose their greencolor.” However, even this measure is uncertain as the development of grain on secondary and tertiary panicles will depend on tillering density and rate, which is closely linked to planting density (Hill, 2013). Due to this uncertainty, the date designated as physiological maturity could differ by 5–10 d, which gives rise to a large difference in grain filling time and Yp.