Bases and limits to using 'degree.day' units

Degree.day units, for which there are also several synonymous terms, are often used in agronomy essentially to estimate or predict the lengths of the different phases of development. The physiological and mathematical bases upon which they are founded are, however, sometimes forgotten, resulting in questionable interpretations. Such is particularly the case for anything relating to variations in the temperature thresholds which enter into the calculation of these degree.day sums. Without seeking to draw up a synthesis of the extremely numerous works published in the field, this review article sets out to present the basic principles of the degree.day unit notion as well as the limits of its use. On this last point, we will particularly emphasise the influence of the non-linearity of the temperature response of the method used in determining the threshold temperature as well as the pertinence of the temperature taken into account in studying the phenomenon. Several practical conclusions are drawn from this review article.

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