The heat-unit system, involving the sum of daily mean temperatures above a given base temperature, is used with processing pea (Pisum sativum L.) to predict relative maturity during the growing season and to schedule planting dates based on average temperature data. The Quebec pea processing industry uses a base temperature of 5 °C to compute growing-degree days (GDD) between sowing and maturity. This study was initiated to verify if the current model, which uses a base temperature of 5 °C, can be improved to predict maturity in Quebec. Four pea cultivars, `Bolero', `Rally', `Flair', and `Kriter', were grown between 1985 and 1997 on an experimental farm in Quebec. For all cultivars, when using a limited number of years, a base temperature between 0.0 and 0.8 °C reduced the coefficient of variation (cv) as compared with 5.0 °C, indicating that the base temperature used commercially is probably not the most appropriate for Quebec climatic conditions. The division of the developmental period into different stages (sowing until emergence, emergence until flowering, and flowering until maturity) was also investigated for some years. Use of base temperatures specific for each crop phase did not improve the prediction of maturity when compared with the use of an overall base temperature. All years for a given cultivar were then used to determine the base temperature with the lowest cv for predicting the time from sowing to maturity. A base temperature from 0 to 5 °C was generally adequate for all cultivars, and a common base temperature of 3.0 °C was selected for all cultivars. For the years and cultivars used in this study, the computation of GDD with a base temperature of 3 °C gave an overall prediction of maturity of 2.0, 2.4, 2.2, and 2.5 days based on the average of the absolute values of the differences for the cultivars Bolero, Rally, Flair, and Kriter, respectively.
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