Nonpoint-Source Pollutant Load Reductions Associated with Livestock Exclusion

Cattle (Bos taurus) grazing on unimproved pastures can be a significant, yet often overlooked, source of pollutants to surface waters, especially when the cattle have unlimited access to streams in the pastures. Livestock exclusion from streams has been demonstrated to reduce sediment and possibly nutrient yield from streams draining pastures. The purpose of this study was to evaluate the effects of excluding dairy cows from, and planting trees in, a 335-m-long and 10to 16-mwide riparian corridor along a small North Carolina stream. Analysis of 81 wk of pre-exclusion and 137 wk of post-exclusion fencing data documented 33, 78, 76, and 82% reductions in weekly nitrate + nitrite, total Kjeldahl nitrogen (TKN), total phosphorus (TP), and sediment loads, respectively, from the 14.9-ha pasture area adjacent to the fenced section of stream. Statistical analyses by t-tests and analysis of variance suggested that the reductions in mean weekly loads post-fencing were significant (P < 0.05) for all pollutants except nitrate + nitrite. Thus, the results indicated that livestock exclusion and subsequent riparian vegetation establishment was effective at reducing pollutant export from an intensively grazed pasture. C grazing on unimproved pastures is a widespread and important agricultural management practice throughout the USA. These pastures can be significant, yet often overlooked, sources of pollutants to surface waters, especially when they are stocked near or above their carrying capacity. Heavy stocking often results in decreased vegetative cover and consequently increased runoff and erosion. Because pastures tend to be located in wetter land along streams or on steeply sloping land, pollutants from pastures can be washed relatively easily and quickly to surface waters. A major source of pollutants in pastures is associated with livestock in and around perennial and intermittent streams (Line et al., 1998). Nutrients from cattle wastes deposited close to a stream are much more likely to enter the surface water and cause degradation of the water resource. Studies of grazed rangeland areas indicate that cattle with unlimited access to streams contribute to streambank alteration and retreat (Marlow et al., 1987). minimize pollution from pastures and rangeland, livestock exclusion fencing has been proposed (Davis et al., 1991). The fencing keeps livestock away from streambanks, thereby preventing the mechanical breakdown of banks by livestock hooves and facilitating the establishment of a vegetative filter along the streams. The vegetative filter can help reduce the movement of sediment and nutrients from source areas on uplands to the stream (Daniels and Gilliam, 1996). Livestock exclusion North Carolina Cooperative Extension Service, North Carolina State Univ., Box 7637, Raleigh, NC 27695. Received 20 Sept. 1999. *Corresponding author ([email protected]). Published in J. Environ. Qual. 29:1882-1890 (2000). fencing has been shown, through 13 yr of monitoring, to reduce sediment yield from a beef cow pasture by up to 40% (Owens et al., 1996). Alternatively, simply providing an off-stream water supply without fencing was shown to reduce sediment from streambank erosion by 77% and concentrations of total nitrogen and phosphorus by more than 50% in a cow-calf pasture; however, this study was relatively short-term and included only limited monitoring of streams (Sheffield et al., 1997). Thus, while relatively large reductions in sediment yield as a result of livestock exclusion fencing have been documented by a few studies, reductions in nitrogen and phosphorus yields are much less certain. The goal of this study was to evaluate the effectiveness of the livestock exclusion best management practice (BMP) in reducing the sediment, nitrogen, and phosphorus loading rates from a dairy cattle pasture in the Piedmont region of North Carolina. Additionally, the effect of installing an alternate watering system without fencing was evaluated. This study is part of a larger watershed project, the Long Creek 319 National Monitoring Program Project, which has the objective of evaluating the effectiveness of a variety of BMPs. The Long Creek Project is one of 30 projects in the USEPA’s National Monitoring Program, which has a goal of evaluating sources, transport, and control of nonpointsource pollution in variety of settings throughout the USA (Osmond et al., 1997). DESCRIPTION OF THE STUDY SITE This study was conducted in a 56.7-ha watershed that contained residential homes (4.0 ha), apartments (0.9 ha), small businesses (2.0 ha), and farmstead (4.4 ha) along its periphery, but was predominantly pasture for dairy cows and replacement heifers (Fig. 1). Because the nonpasture areas were less than 20% of the watershed and the management of these areas remained relatively constant throughout the entire period of monitoring, the effects of these land uses were considered insignificant, except to increase the absolute loads during both the preand post-BMP periods. The exception to this was that a small apartment complex, shown as the square shaded area at the bottom of Fig. 1, was constructed just before the beginning of the start of monitoring and may have had some effect on the results. The 56.7-ha watershed was divided into an upper pasture (41.8 ha upstream of Site D) that was lightly grazed by 75 100 heifers and calves and a lower pasture (14.9 ha between Sites D and E) that was heavily grazed by adult cows. The dairy farm has been at this location for at least 100 yr; therefore, there is a long history of grazing on these pastures. In both pastures the livestock had unlimited access to the small stream draining the pastures that was the only source of water Abbreviations: BMP, best management practice; NO~and NO3-, nitrite + nitrate nitrogen; TKN, total Kjeldahl nitrogen; TP, total phosphorus; TSS, total suspended solids; TS, total solids.