The grazing systems are an essentiel component of these volcanic highlands (1000-2000m). Our investigations consist on building up diagnosis tools and decision rules for a sustainable control of these fragile grazed ecosystems composed of native and sown pastures. The use of a systemic approach allows us to analyse the inter-relationship between the vegetation dynamic and the agricultural practices through three biological indicators which relate to one hundred paddocks in six cattle farms. Concerning the vegetation dynamics, it seems that some management practices lead to a non-reversable degradation stage. As regards the forage system, the measure of sward-height and estimation of the volume of available herbage show a poor relationship between management practices of pastures and the biological seasonal rhythm. The minerai nutritional composition of vegetation illustrates that fertilizing practices are not adapted to the particular nature of soif (especially Phosphorous availability).