Effects of grazing management on sediment and phosphorus runoff

In spring 2001, grazing was initiated on pastures at the Iowa State University Rhodes Research Farm to determine the effects of grazing management on nutrient and sediment loss from pastureland. Treatments included an ungrazed control, summer hay harvest with winter stockpiled grazing, continuous stocking to a residual height of 2 in., rotational stocking to a residual height of 2 in., and rotational stocking to a residual height of 4 in. In three months (June, August and October) during the year, rainfall simulations were conducted at 6 sites within each paddock and 6 sites in a buffer zone down slope from each paddock. Rainfall simulators dripped at a rate of 2.8 in./h over a 5.4-ft(2) area for a period of 1.5 h. Amounts of rainfall and runoff were recorded at 10-min. intervals during the sampling period. Runoff was collected and analyzed for total sediment, total P, and dissolved P. Simultaneous to each rainfall simulation, measurements were taken of ground cover, penetration resistance, surface roughness, slope, the contents of P and moisture of the soil, and the sward height and mass of forage. Slope and forage treatment in paddocks did not significantly affect the concentration and amount of sediment in runoff. However, the concentrations and amounts of total P in runoff from paddocks that were grazed were greater than paddocks that were ungrazed or harvested for hay during summer. The concentrations and amounts of total P in runoff were lower in paddocks that were rotationally stocked to a height of 4 in. than in those that were rotationally stocked to 2 in. The amounts of sediment and total P in runoff from paddocks that were grazed were greater than their respective buffers.