Accurate prediction of temperature and daylength effects on development is essential for ensuring that key transitions of vegetative and reproductive growth phases occur at the correct time. Early models predicted crop development poorly, and it was well into the 1980s and 1990s before robust modules for phenology (Hadley et al. 1984; Ritchie, Godwin & Otter-Nacke 1985a, 1985b; Kiniry & Bonhomme 1991; Grimm et al. 1993, 1994; Ritchie et al. 1998; Jamieson et al. 2007) were created. Even now, the most important param- eters to set when modelling a new cultivar are those affecting crop development and phenology.
Timing of key events such as floral initiation, anthesis and physiological maturity are usually predicted by integrating a developmental rate, R, over time. Models usually assume that a potential rate of development, Rpot, is modified by indi- vidual environmental factors,
R=Rpot ∗f(T,P,V...Z)
where T is the primary temperature effect, P is the effect of photoperiod and V to Z optionally describe effects of vernali- zation or of specific abiotic stresses that may accelerate or delay development. The vernalization effect, if present, varies with temperature but operates independently of the main temperature effect. R is integrated over intervals defined by the two stages that delimit a given phase. The duration of the phase may be expressed in various types of units but most often is expressed in thermal time (TT) or a proxy such as leaf number.