Grazing management effects on sediment and phosphorus losses from streambanks

Agricultural lands are the leading cause of nonpoint-source pollution in the United States. Previous studies indicate that grazed lands can have more impact on sediment and phosphorus (P) losses to surface waters than cropland or other agricultural lands. The purpose of this research was to determine the effects of different grazing managements on streambank erosion and potential P losses. In 2004, three treatments, including rotational stocking, continuous stocking with limited stream access, and continuous stocking with full stream access were established in six adjacent pastures along Willow Creek at a research farm in Marshall County, Iowa. Erosion measurements were taken monthly from May to November in 2005 and 2006. Soil samples were collected, and total P, Mehlich III P, water-extractable P, and bulk density were analyzed by horizon with the intent of calculating the amount of P that potentially entered the stream at this site. Phosphorus levels did not vary significantly among treatments but were higher in the A horizons than in the C horizons. Although net erosion treatment differences were found in both years, these differences were not consistent and no trend emerged. Erosion/deposition activity was greatest in the continuous stocking with unrestricted access (CSU) treatment. In the four months that there 16 was a significant difference among treatments, the CSU treatment had the highest activity, denoting the most bank instability. The CSU treatment also had consistently higher potential and water-extractable P losses and movements (flux) than either the rotational stocking (RS) or the continuous stocking with restricted stream access (CSR) treatments. During the 2005 grazing season, the CSU treatment had a potential P loss of 24.2 g/m2 from the streambanks, as well as a flux of 86.3 g/m 2 in the system. In 2006, a much drier year, the CSU treatment had a potential P loss of 1.9 g/m2 and a P flux of 41.9 g/m 2 from the streambanks. Results from this study suggest that grazing management may have an effect on erosion/deposition activity, but did not have an effect on streambank erosion.