ARS | Publication request: Dairy Diet Phosphorus and Rainfall Timing Effects on Runoff P from Land-Applied Manure

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 19, 2008
Publication Date: N/A

Interpretive Summary: Surface-applied dairy manure can increase phosphorus (P) concentrations in runoff, which may contribute to eutrophication of lakes and streams. The amount of dietary P fed to dairy cows and the timing of a rain event after land-application of manure may further affect runoff P losses. The objectives of this study were to examine dietary P supplementation effects on manure and runoff P concentrations from rain events occurring at different time intervals after manure application. We collected manure from dairy cows fed either a non-supplemented low P diet or a diet supplemented with either a mineral or organic source of P. We hand-applied manure onto 1-meter long pans packed with soil. Then we used a rainfall simulator to rain on the plots two, five, or nine days after manure application. Each time we collected 30 minutes of runoff water. Concentrations of total P in the runoff were about 30 to 40% higher in the P-supplemented diets than in the non-supplemented ones. Delaying rainfall from two to five days after manure application caused large decreases in the concentrations of total P (89%) and dissolved P (65%) in the runoff. The concentration of total P decreased more (29%) with delay of rain to nine days after manure application. This research showed that dairy farmers can reduce the nutrient pollution potential of manure application by lowering or eliminating P supplementation of feed and by timing manure application to occur when runoff-producing rain is not expected for five or more days.

Technical Abstract: 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 and the timing of a rain event after land-application of manure may further affect runoff P losses. The objectives of this study were to examine dietary P supplementation effects on manure and runoff P concentrations from rain events occurring at different time intervals after manure application. Manure was collected from dairy cows fed an unsupplemented low P diet (LP; 3.6 g P kg-1), a diet supplemented with dicalcium phosphate (High Inorganic P, or HIP; 4.4 g P kg-1), or a diet supplemented with an organic source, wheat middlings (High Organic P, or HOP; 4.6 g P kg-1). Manure was hand-applied onto soil packed pans at a rate equivalent to 56 Mg ha-1 on a wet weight basis (LP: 30 kg P ha-1, HIP: 49 kg P ha-1, HOP: 62 kg P ha-1). Runoff was collected for 30 minutes after runoff initiation from simulated rain events (30 mm h-1 intensity) two, five, or nine days after manure application. Dietary P effects on runoff total P (TP) concentrations were significant two days after manure application with the runoff P from HIP and HOP diets being approximately 46% and 31% greater than that of the LP diet. Total P concentrations in runoff from the HIP and HOP diets were numerically higher than that of the LP diet at five (26% and 66%) and nine days (22% and 19%) after application, but the differences were not statistically significant. Large decreases in runoff TP (89%) and dissolved reactive P (DRP; 65%) concentrations occurred with delay of rainfall from two until five days after application. Total P further decreased 29% when rain was delayed from five to nine days, but DRP did not change. The proportion of TP as DRP increased as the time between manure application and runoff increased (19, 62, and 86% at two, five, and nine days, averaged across diets). This research showed that reducing dietary P and extending the time between manure application and a rain event can significantly reduce concentrations of TP and DRP runoff.