Phosphorus Removal in Forage Production Systems That Utilize Animal Excrements.

Land application of swine effluent (SE) can provide essential plant nutrients for forage crop production, but phosphorus (P) additions can potentially be detrimental to the environment.  A multi-year study was conducted to evaluate the P balance from surface applied SE under several forage species in the semi-arid southern Great Plains.  Swine effluent and urea nitrogen (UN) was applied to supply 0, 56, 168, and 504 kg N ha^-1 annually.  Soil P was measured initially in 1997 prior to any SE applications, then again in 2001 and 2006.  Soil P levels decreased from removal of forage biomass at a rate of 4.65 mg kg^-1 yr^-1 (r²=0.9416) at all N loading rates.  The significance of P removal over time extended dramatically, indicating that differences among treatments were increasing.  Phosphorus levels decreased in UN treatments from the lack of P additions; however in SE treatments, P levels were reduced at the lowest SE loading rates then raised slightly at the high N loading rates, due to the increased P loading from SE additions.  It was observed that at the 168 kg N ha^-1 loading rate that P additions were almost equivalent to P removal from the biomass removal; suggesting that forage productions systems may adequately minimize and offset P risks associated with current environmental concerns.