Impacts of Cattle Grazing Management on Sediment and Phosphorus Loads in Surface Waters
In 2001 (year 1) and 2002 (year 2), three blocks of five 1-ac paddocks were grazed by beef cows on hills at the Iowa State University Rhodes Research and Demonstration Farm to determine the effects of grazing management on phosphorus (P) and sediment runoff from pastureland. Grazing management treatments included an ungrazed control, summer hay harvest with winter stockpiled grazing, grazing by continuous stocking to a residual sward height of 2 in., rotational stocking to a residual sward height of 2 in., and rotational stocking to a residual sward height of 4 in. At four times (late spring, mid-summer, early autumn, and early the subsequent spring) in each year, rainfall simulations were conducted at 6 sites within each paddock and 6 sites in a buffer zone down slope of each paddock. Rainfall simulators dripped at a rate of 2.8 in./hr over a 5.4- ft2 area for a period of 1.5 hours. Runoff was collected and analyzed for total sediment, total P, and total soluble P. Simultaneous to each rainfall simulation, ground cover, penetration resistance, surface roughness, slope, the contents of P and moisture of the soil, sward height and forage mass were measured. Losses of sediment, total P, and total soluble P were greater from grazed paddocks than ungrazed paddocks in year 1. However, in year 2, losses of sediment, total P, and total soluble P from paddocks grazed by rotational stocking to a sward height of 4 in. or harvested as hay during the summer and grazed during winter did not differ from ungrazed paddocks. In both years, losses of sediment, total P, and total soluble P from the buffer area immediately or 30 ft below the paddocks were lower than within the paddocks. Of the physical measurements, the proportion of ground cover was most highly related to sediment loss. Soil Bray-1 P concentrations did not differ between treatments, but were related to the losses of total and total soluble P. Results imply that sediment and phosphorus losses in pasture runoff may be reduced by managing rotational stocking to maintain adequate sward height and/or using vegetative buffer strips along pasture streams. Such management practices are particularly important in pastures on soils with high P concentrations.