MEASUREMENT AND PREDICTION OF PHOSPHORUS TRANSPORT FROM SWINE MANURE AT THE WATERSHED SCALE

Livestock production facilities are coming under increased scrutiny with regard to transport of phosphorus (P) from fields receiving animal manures. The objective of this study was to measure and simulate how swine manure management affects P export from a row crop watershed. Stream water sampling was conducted for one year at 14 locations within the Tipton Creek watershed in central Iowa. Data on soil P concentrations and manure production were also collected. Geographic Information System (GIS) data layers (topography, soils, land use, and land cover) were prepared to facilitate simulations using the Agricultural Non-Point Source model (AGNPS). The average dissolved reactive (DRP) and total phosphorus (TP) concentrations in stream water from 41 sampling dates were 0.14 and 0.21 mg L-1, respectively. Total P export from the Tipton Creek watershed from April 1, 2000 to April 1, 2001 was estimated at 10.6 metric tons (Mg) for an average loss of 0.52 kg P ha-1 Y-1. This loss represents approximately 2.8% of the estimated applied P (swine manure and commercial fertilizer). Four rainfall events during the 2000 growing season and snowmelt in March 2001 were responsible for 91% of the P transport from the watershed. Using animal inventory numbers and standard P excretion values, swine manure was estimated to supply approximately 35% of the land-applied P. AGNPS simulations of the 4 major storm events from 2000 were used to predict future P transport from an assumed 5% annual increase in swine production and subsequent increase in P application to soils of the watershed. These simulations indicated that, without any changes in current management, a 40% increase in P transport in such runoff events could occur in the next 5 years. However, a combination of diet manipulation and use of high available-P feedstuffs and/or phytase enzyme has been shown to reduce P excretion by as much as 50%. Broad adoption of these practices would likely prevent any increase in P production from swine facilities and potentially reduce overall manure P production.