In the area of Saône-et-Loire, a previous study showed that the economic efficiency of suckling farms widely depended on the proportion of grass in the feeding of the cattle. The aim of this paper is to assess the variability of grass production among years on some suckling farms, using the ISOP model of prediction of grass production. The model was improved by applying more accurate input data (more accurate climatic and soil data) and it made it possible to get satisfying estimations of production, as compared with measurements from a farm network. Grass production depends on soil type (serving as a water reservoir), and on nitrogen fertilization (which is often low, as compared with crop practices). After validation, estimations were made for some types of farms. The diversity of soils on the farm was taken into account, and the grass production of the whole farm was calculated. The variability of the production among years made it possible for the livestock farmer to assess the risk of having insufficient supplies of forage.