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Hanrahan, Laura Patrice

Plant and Soil Science

2004

MS

Agricultural runoff from manure applications is one source of phosphorus pollution in lakes and streams. Commonly, dietary P levels in the dairy ration have been higher than the recommended values, which can increase P loading potential in runoff. In this study the dietary P level in the dairy cow rations was altered so that one diet met the recommended National Research Council (NRC) level and two diets were supplemented with inorganic or organic forms of P to higher levels typical of those fed in Vermont. Two rain simulation experiments were done to assess the dietary P effects on manure and runoff P. The first examined the effect of time from manure application to rainfall (2,5, or 9 days), and the second evaluated the effect of different manure storage methods (slurry, solid pack, and unamended). In the first experiment, dietary P effects on runoff total P (TP) concentrations two days after manure application were significant, with the high inorganic P (HIP) and high organic P (HOP) diets being approximately 31 % and 23% greater than the low organic P (LOP) diet. Total P concentrations in runoff from the HIP and HOP diets were still higher than the LOP diet at five (21 % and 40%) and nine days (18% and 15%) after manure application despite the lack of significant differences. A substantial decrease in runoff TP (89%) and dissolved reactive P (DRP) (65%) concentrations occurred with delay of rainfall from two until five days after manure application. Total P decreased 29% more as rain was delayed from five to nine days and DRP had no further decreases. The proportion of TP as DRP increased across all diets as the time between manure application and runoff increased. By nine days the DRP:TP ratio had reached 86%, signifying that the majority of runoff P losses were readily available for algal uptake. In the second experiment, the LOP diet significantly reduced runoff TP and DRP concentrations relative to the two high P diets. Runoff TP concentrations from the solid manure were less in the HIP diet than the HOP diet, otherwise the source of supplementation was not significant for the simulated storage treatments. More importantly, the solid treatment had lowered total solids and longer time until runoff initiation. Therefore, if the duration of a rain event is less than the time it takes to initiate runoff from a solid manure, runoff P losses may be significantly less compared to a slurried manure. Our studies have shown that P concentrations in runoff following manure application were reduced by lowering the P content of diets and by delaying a rain event, and that storing manure in slurry and/or solid pack form reduced P runoff concentrations.