Quantifying Phosphorus Levels In Landscapes Associated with Bahiagrass-Based Pastures and Beef Cattle Operation In Subtropics.

Relatively little information exists regarding possible magnitudes of P losses from grazed pastures. Whether or not P losses from grazed pastures are significantly greater than background losses and how these losses are affected by soil, forage management, or stocking density are not well understood. Critical to determining environmental balance and accountability is an understanding of P excreted, P removal by plants, and acceptable losses of soil P within the manure management and crop production systems and export of P off-farm. We hypothesized that properly managed cow-calf operations would not be major contributors to excess loads of extractable P in surface and ground water. To verify our hypothesis, we examined the comparative concentrations of total P among soils, forage, surface water and groundwater beneath bahiagrass-based pastures with cow-calf operations. Soil samples were collected at 0-20, 20-40, 40-60, and 60-100 cm across the pasture’s landscape (top slope, TS; middle slope, MS; and bottom slope, BS) in the fall and spring of 2004 to 2006, respectively. Forage availability and P uptake of bahiagrass were also measured from the TS, MS, and BS. Bi-weekly groundwater and surface water samples were taken from wells located at TS, MS, and BS and from the run-off/seepage area (SA). Overall, there was no buildup of soil P. There had been no movement of extractable total P into the soil pedon since average degree of phosphorus saturation (DPS) in the upper 20 cm was 21% while DPS at 60-100 cm was about 3%. Our livestock operations contribute negligible concentrations of P to groundwater (0.7 mg/L) and surface water (0.6 mg/L). Our results indicate that current pasture management including cattle rotation in terms of grazing days and current fertilizer application (inorganic + manures + urine) for bahiagrass pastures offer little potential for negatively impacting the environment. Properly managed livestock operations contribute negligible loads of P to shallow groundwater and surface water.