Methods are available to predict phasic development of spring and obligate winter wheat but less progress has been made with facultative winter wheat. This study reports equations to predict the double ridge and anthesis stages of facultative winter wheat, based on daily mean temperature, vernalizing temperature and photoperiod. Phasic development of facultative winter wheat is hastened when the seed or vegetative crop is vernalized. However the crop still develops even if there is no vernalization, in contrast to the response of obligate winter wheat, which can remain vegetative indefinitely in the absence of vernalization.

The equations are based on separate linear responses of daily developmental rate to mean temperature, vernalizing degrees and photoperiod. The three functions are multiplied and the equation includes five parameters: threshold temperature, vernalizing maximum temperature, vernalizing minimum temperature, critical photoperiod and a rate constant. The parameters are estimated by fitting the model to observations with a non-linear least-squares procedure. The observations are of dates of seedling emergence, double ridge and anthesis of 21 crops of the variety Oxley, grown in multiple sowing dates in field environments at latitudes ranging from 10 to 65°. The times from seedling emergence to anthesis ranged from 61 to 110 days. The model fitted the data with a standard error of 4 days and predicted independent data with a similar error. It was necessary to separately estimate the phases, with the vernalization function needed before double ridge but not after. The parameters are statistically significant, are not intercorrelated and the values of those related to temperature and photoperiod seem physiologically reasonable. The model is a simple and accurate method to predict development in simulation models and as a stand-alone method for scheduling operations such as grazing.