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Abstract. An experiment was conducted to examine the effect of deep-rooted perennial grasses on the water and nitrogen economy of 3 mature pasture communities with different botanical compositions but the same fertiliser history. One pasture was dominated by volunteer naturalised pasture grasses (Eleusine tristachya and Danthonia spp.) (termed ‘degraded’), another was phalaris (Phalaris aquatica) dominant (phalaris), and a third was dominated by phalaris into which white clover (Trifolium repens) had been recently sown (phalaris–white clover). Two replicates of each pasture type were grazed continuously over 4 years with young weaner sheep changed each year. Measurements of hydrology, nutrient cycling, botanical composition and animal production were made in order to quantify the sustainability characteristics of each of the pasture types. Data are summarised as absolute measures at various points in time and also as trends over time. The ranking of standardised treatment measures was then summed to provide an index of sustainability with or without a weighting assumed to be representative of the relative importance of various layers of sustainability viewed from the perspective of a hypothetical ‘typical’ grazier. The results show that the phalaris–white clover treatment was substantially more sustainable, in both ecological and economic terms, than either of the other treatments. The unweighted index for the phalaris–white clover pasture was 3.61 compared to 2.08 and 1.98 for the phalaris and ‘degraded’ pastures, respectively.