There is increasing environmental pressure to reduce phosphorus (P) inputs to agricultural systems, including dairy systems. In a recent study (Ferris et al., 2007) dairy cow performance (milk yield, milk composition, food intake and fertility) was found to be unaffected when dairy cows were offered diets containing reduced levels of dietary phosphorus over a four-year period. However, if cows are offered diets containing inadequate dietary P levels for an extended period, bone P reserves may become depleted. The aim of this experiment was to examine the effect of reducing dietary P level on bone composition. Bone samples were collected from cows culled/removed from the study described by Ferris et al. (2007).