Runoff Phosphorus in a SMALL Rotationally-Grazed Pasture in Georgia with NO History of Broiler Litter Application.

Pastures are sources of phosphorus (P) into water sources and can contribute to eutrophication and impairment. Close to 4 million ha of land in the Southern Coastal Plain and the Southern Piedmont in eastern USA is used for pasture and hay production. We present an 11-yr (1999 to 2009) of dissolved reactive P (DRP) and total P (TP) concentration and load data from a 7.8 ha rotationally-grazed pasture (W1) near Watkinsville, in the Georgia Piedmont. Cattle numbering 21 to 224 (mean 91) grazed W1 on 69 occasions for 1 to 71 days (mean 19.2). Annual rainfall was 72-494 mm below the average in 7 of 11 years. Approximately 58% of the months had below average (deficit) and 42% had average or above average (non-deficit) rainfall. Twenty runoff events were recorded during the deficit and 54 during the non-deficit months. Nutrient analysis was performed in 43 to 47 runoff event samples. Event flow-weighted concentration (FWC; mg L^-1) ranged from 0.381 to 7.065 for DRP (mean 1.930) and 0.364 to 7.591 for TP (mean 2.41). Event load (kg ha^-1) ranged from 0.000 to 0.635 for DRP (mean 0.104) and 0.000 to 0.546 for TP (mean 0.114). The mean DRP to TP ratio for FWC and load was 0.82. The FWC for DRP and TP was greater with cattle in than out of W1 (p = 0.026; means 3.139 vs 1.855 mg L^-1 for TP). Cattle presence did not increase loads (p ≥ 0.400). The FWC for DRP and TP were similar between deficit and non-deficit periods (p > 0.2), but loads were 3 to 4 times greater during the non-deficit than the deficit period (p < 0.05). Although the management had limited the export of potentially greater P amounts, the results point to the need for further reduction.