Phosphorus Feeding Strategies for Dairy: Effects on Manure P and P Cycles1

Most dairies in the U.S. continue to be land-based, that is, they produce most of their feed and use their land to recycle manure nutrients through crops. To remain economically viable, many dairies are increasing herd size and importing more feed. In these systems, the expanded importation of feed and fertilizer P has resulted in large positive P balances (Tunney, 1990; Bundy, 1998; Satter and Wu, 1999). In many areas of intensive livestock production, amounts of P produced in manure often exceed local crop requirements (Sharpley et al., 1993; Kingery et al., 1994). This can lead to the build-up of soil test P to levels much above what are needed for optimal crop yields. For example, repeated application of manure and fertilizer has increased the level of soil test P (Bray-1 P extraction) in Wisconsin from an average of 34 ppm in the 1968-73 period to 52 ppm in 1995-99 (Combs and Peters, 2000). A level of 25-35 ppm is considered more than adequate in all but sandy soils for alfalfa, corn and soybean production based on University of Wisconsin soil test recommendations. Clearly, alternative feed, fertilizer, manure and land management strategies are needed that reduce nutrient accumulation and loss from dairy farms. Excessive soil nutrient accumulation, transport of nutrients by surface and subsurface runoff, and subsequent pollution are pressing environmental challenges facing the livestock industry (USDA-USEPA, 1999). While much remains to be learned about the relationship between soil P levels and the potentia