The feeding of imported concentrates in Irish dairy systems results in phosphorus (P) surpluses that contribute elevated levels of soil P as well as increased amounts of P in manures. However manipulation of concentrate composition can lower P content of diets without compromising animal performance and is therefore a mitigation option for P-sensitive catchments. Rainfall simulation studies conducted over a 12 month period investigated how varying the P content of cattle manure applied to grassland impacted on P concentrations in surface runoff. The study was based on manures from cows fed four levels of dietary P from a mixture of grass silage and concentrates. Twenty-five runoff plots (1 x 0.5 m) were laid out in a Latin Square design on a hillslope site (slope 5%) under permanent pasture with five replicates per treatment plus a control. Following surface applications of manure at a rate of 50 m3 Ha-1, rainfall simulations using deionised water were delivered at an intensity of 20 mm hr-1 on days 2, 9, 31 and 49 after application to examine the persistence of the P signal over time. Fractions of P measured in runoff were dissolved reactive P (DRP), dissolved unreactive P and particulate P (PP). In contrast to other published studies on the impact of diet manipulation in which PP dominated, in all slurry treatments DRP accounted for the majority of TP collected in runoff. While reduced P diets showed a clear potential to reduce P concentrations in manure impacted runoff, this effect was relatively short-lived. Strategies that lower the interaction between applied manure and runoff could have a greater impact on P losses compared to varying dietary P.