MEASUREMENT AND PREDICTION OF PHOSPHORUS TRANSPORT FROM SWINE MANURE AT THE WATERSHED SCALE
2003
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.
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