Dairy Diet Phosphorus Effects on Phosphorus Losses in Runoff from Land-Applied Manure

Phosphorus losses from land-applied manure can contribute to nonpoint source pollution of surface waters. Dietary P forms and levels influence P concentrations in animal manures and may affect P losses from land-applied manure. The objective of this study was to determine the effects of dairy diet P concentration on P losses in runoff from land-applied manure. Dairy manures from two dietary P levels were applied (24 May 1999) at 56 wet Mg ha -1 to a silt loam soil, to provide 40 and 108 kg P ha -1 , respectively. The high P diet manure was also applied at 21 wet Mg ha -1 (40 kg P ha -1 ) to provide a P rate equivalent to the low P diet manure. Plots were subjected to simulated rainfall (75 mm h -1 ) just prior to corn (Zea mays L.) planting in June and again after harvest in October 1999. Runoff was analyzed for dissolved reactive P (DRP), bioavailable P (BAP), total P (TP), and sediment concentration. Natural runoff from the same plots was collected from November 1999 through July 2000 and analyzed for DRP. At equivalent manure rates, DRP concentration in June runoff from the high P diet manure was =10 times higher (2.84 vs. 0.30 mg L -1 ) than the low P diet manure, and four times higher (1.18 vs. 0.30 mg L -1 ) when applied at equivalent P rates. Phosphorus concentrations in October runoff and November to July natural runoff were lower (0.02 to 1.69 mg L -1 ), but treatment effects were the same as for the June runoff. These results show that excessive addition of inorganic P to dairy diets increases the potential for P loss in runoff from land-applied manure, even at the same P application rate. Diet P effects on potential losses in runoff from land-applied manure should be considered in P-indexing and nutrient management planning.