To improve water use efficiency (WUE) of border irrigation, an integrated approach of tensiometric regulated irrigation that considered both crop response (yield and quality) and water losses was investigated. The study was carried out on plurispecific and permanent grasslands on a Mediterranean plain (Crau region in France) with high quality hay production. The soil water potential (Ψtotal) was used to schedule irrigation and diagnose mild water deficits. Neutron probe measurements were used to quantify the soil water deficit experienced by the plants based on the fraction of transpirable soil water (FTSW). The dynamic of above-ground dry matter production and botanic composition were analysed with reference to the FTSW. The results show the extreme sensitivity of grassland to a mild soil water deficit between two irrigation dates. Across the range of soil water deficit observed in these experiments, both hay production and botanical composition were closely related to the average FTSW. Above-ground biomass rapidly stopped increasing when the FTSW reached 60%. The botanical composition was also significantly changed by water deficits. The concordance between the soil water measurements and grassland response implies that the regulated deficit irrigation based on the soil water potential or FTSW can be used to optimize border irrigation efficiency when the trade-off between water saving and profit losses by mild water deficit is not easy to manage