Dairy Diet Phosphorus and Rainfall Timing Effects on Runoff Phosphorus from Land‐Applied Manure

Surface‐applied dairy manure can increase P concentrations in runoff, which may contribute to eutrophication of lakes and streams. The amount of dietary P fed to dairy cows ( Bos taurus ) and the timing of a rain event after manure application may further affect runoff P losses. The objective of this study was to examine dietary P supplementation effects on manure and runoff P concentrations from rain events occurring at different time intervals after manure application. Manure from dairy cows fed an unsupplemented low P diet (LP; 3.6 g P kg −1 ) or a diet supplemented with either an inorganic (HIP; 4.4 g P kg −1 ) or an organic (HOP; 4.6 g P kg −1 ) source was hand‐applied onto soil‐packed pans at 56 wet Mg ha −1 Thirty min of runoff was collected from simulated rain events (30 mm h −1 ) 2, 5, or 9 d after manure application. Total P (TP) concentrations in runoff from HIP and HOP diet manure from the 2‐d rain were 46 and 31% greater than that of the LP diet. Runoff P concentrations from high P diets were numerically higher than that of the LP diet at 5 and 9 d after application, but differences were significant only for dissolved reactive P (DRP) at 5 d. Large decreases in runoff TP (89%) and DRP (65%) concentrations occurred with delay of rainfall from 2 d until 5 d. The proportion of TP as DRP increased as the time between manure application and runoff increased. Results showed that reducing dietary P and extending the time between manure application and a rain event can significantly reduce concentrations of TP and DRP in runoff.