Submitted to: Proceedings of the National Conference on Grazing Lands
Publication Type: Proceedings
Publication Acceptance Date: 12/10/2003
Publication Date: N/A
Interpretive Summary: The amounts of sediment and phosphorus (P) in surface runoff from agricultural lands are of concern because of the potential for siltation and eutrophication of waterways. Because of current problems, it is likely that the U.S. Environmental Protection Agency will implement regulations to control the total maximum daily loads of nutrients in watersheds in the near future. At this time, there is limited information on total sediment and P losses in runoff from pastureland in the Midwest. Because aboveground biomass limits soil disruption caused by the impact of raindrops and roots hold soil particles, forages harvested at an appropriate height through suitable grazing management should promote water infiltration and minimize sediment and P loss in runoff water from pastures. The purpose of this study is to quantify the amounts of sediment and P in surface runoff from Iowa pasturelands managed by different systems, develop tools to monitor and control sediment and P transport from pastures, and develop best management practices for producers to control sediment and P losses, while optimizing forage productivity. After two years, we found that by practicing good forage management techniques, the amounts of sediment and nutrients coming off of pastureland can be controlled. Practices such as the use of buffer strips around waterways and rotational grazing of cattle can greatly reduce degradation of surface water resources. The results of this research will provide useful information to cattle producers, local environment groups, and Cooperative Extension and Natural Resources Conservation Service personnel.
Technical Abstract: In 2001 (year 1) and 2002 (year 2), three blocks of five 1-ac (0.4-ha) 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. (5 cm), rotational stocking to a residual sward height of 2 in. (5 cm), and rotational stocking to a residual sward height of 4 in. (10 cm). 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 beneath each paddock. Rainfall simulators dripped at a rate of 2.8 in./hr (7.1 cm/hr) over a 5.4-ft2 (0.5-m2) area for a period of 1.5 hours. Runoff was collected and analyzed for total sediment, total phosphorus, and total soluble phosphorus. Simultaneous to each rainfall simulation, ground cover, penetration resistance, surface roughness, slope, the contents of phosphorus 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. (10 cm) 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 (10 m) 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 Bray-1 P concentrations.