In order to establish a hierarchy among the main factors of the variations of the digestibility of forages (species, physiological stage of the sward, growth conditions, fertilisation or management practices, etc.) it is necessary to integrate them into a single model. Such a model, showing the variations of the digestibility of standing herbage during re-growth, is presented here, strengthened by data from ecology and from eco-physiology. This model is based on the representation of the plant as being made up of two parts : one, called metabolic, comprises the metabolically active tissues (of which the organic matter is practically wholly digestible), and the other, called structural, which comprises the supporting tissues (of which the digestibility decreases with time). The digestibility of the plant depends then on that of the structural part (varying with age, the nitrogen nutrition and the species) and on the proportion of this part in the total bio-mass. The digestibility of the young tissues depends on the species sown or on the functional type of the grass (in the case of permanent pastures), and on the level of the nitrogen nutrition; the residual length of the leaf sheaths gives an indication of the intensity of use. A model is also proposed for the reproductive shoots. The validity of the model rests upon numerous experimentations. Its use presupposes that the digestibility of the various species at the vegetative stage be known, as well as the dates of the different phenological stages.