Phosphates represent one of the primary sources of water pollution in Northern Ireland, an issue of particular concern in view of existing EU legislation, such as the Water Framework Directive. Inputs of phosphorus (P) to agricultural systems contribute to this problem, with dairy farms often implicated as a potential source of P pollution. Part of this problem can be attributed to the fact that large quantities of P are imported onto dairy farms in concentrate feeds which tend to be high in P (7.3 g P/kg DM in a survey of 35 winter concentrates for dairy cows). However, recent research has indicated that the P content of dairy cow rations can be reduced without any negative effect on animal performance (Wu and Satter, 2000). Nevertheless, the rations offered in this study were very different from those commonly offered within grassland based dairy systems in the UK. Consequently a study was established to examine the effect of reducing dietary P levels on cow performance, with the study being conducted over two successive lactations. The long-term nature of the study was deemed necessary as cows can mobilise bone phosphorus reserves for milk production when dietary phosphorus levels are inadequate.