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TECHNICAL REPORTS

Surface Water Quality

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

^a VT Agency of Agric., Food, and Markets, Montpelier, VT
^b USDA-ARS, Dairy Forage Research Center, Marshfield, WI
^c Fox Hollow Consulting, LLC, Columbus, OH
^d formerly, College of Agric. and Life Sci., Univ. of Vermont, Burlington, VT

^* Corresponding author (bill.jokela{at}ars.usda.gov).

Received for publication December 29, 2007. 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.

Abbreviations: DRP, dissolved reactive P • HIP, high inorganic phosphorus diet • HOP, high organic phosphorus diet • LP, low phosphorus diet • TP, total phosphorus • WEP, water-extractable phosphorus