Spatial distribution of livestock concentration areas and soil nutrients in pastures

Abstract

Livestock concentration areas can be significant point sources of nutrient pollution. Our objective was to determine the spatial distribution of livestock concentration areas in pastures at the farm scale, along with the distribution of soil nutrients at the individual livestock concentration area scale. We georeferenced and measured the size of all livestock concentration areas in cool-season grass-legume pastures on five farms (four grazing dairies and a beef cattle farm) in Maryland, Pennsylvania, and New York during two years. Soil of selected concentration areas on each of the farms was sampled to 0 to 5 and 0 to 15 cm (0 to 2 and 0 to 6 in) depths to compare nutrient levels with paired unaffected areas of the pasture. On one farm, we sampled two concentration areas more densely (20 to 25 samples, 0 to 5 cm depth along each of five 100 m [328 ft] transects) to measure spatial distribution of soil nutrients. The transects were arranged radially to encompass variation both up and downslope. We installed runoff plots at three locations on and near the two concentration areas to measure nutrients in surface water runoff from simulated rainfall. On the five farms, concentration areas occurred most frequently at paddock gates (38% of sites). Although fewer in number, concentration areas at feeding sites were often larger than those at gates or other locations and accounted for most (48%) of the area affected by livestock congregation. Most concentration areas were small (median area 100 m² [1,076 ft²]), isolated (median distance, 61 m [200 ft] from a water body), and surrounded by vegetation. Intensive sampling on one farm showed that soil within 20 to 40 m (66 to 132 ft) of concentration areas was enriched in phosphorus, which contributed to higher phosphorus concentration in the runoff from simulated rainfall compared with the rest of the pasture. Pastures used as holding and feeding areas with highly elevated soil nutrients and no surrounding vegetation to filter runoff represented a direct threat to surface water quality. Many concentration areas, however, were surrounded by vegetation, which would mitigate this risk.