Livestock grazing effects on phosphorus cycling in watersheds

Elevated phosphorus (P) loading of wetlands, streams, lakes, and reservoirs can occur from nonpoint sources such as grazing of uplands, wet meadows, and palustrine wetlands. Erosion caused by livestock grazing or any activity will increase the total P load in streams; however, herbivores can also harvest P from forage and export a significant amount of P from the watershed. Some land managers fail to recognize that the P taken up by plants will continue to cycle through soil and water. Dissolved P or P attached to soil particles suspended in water are the primary vectors of P movement in a watershed. Herbivores add another vector with more opportunities to export P from the watershed. Using best management practices such as rotational grazing, buffer strips next to wetlands, and proper irrigation management should reduce overland flow and streambank erosion. Livestock grazing should harvest and remove a significant amount of P from the ecosystem by incorporation into bone and tissue mass of growing animals and beef export from the basin. The Phosphorus Uptake and Removal from Grazed Ecosystem (PURGE) model uses three separate methods to estimate P retention in cattle, and using limits of the input variables, predicted a range from 4 to 50 Mg P could be removed annually from 17,700 ha of pasture in the Cascade Reservoir watershed in west-central Idaho. With proper grazing management, cattle should be part of a long-term solution to P loading and improvement of water quality in Cascade Reservoir