The risk of significant phosphorus loss from a vegetative treatment area (VTA) is very low. Typically, only 3 to 5% of the phosphorus excreted by livestock is transported with runoff from the feedlot. Most phosphorus remains in the open lot and is harvested with solids during lot cleaning. Of that which is transported with runoff, much of this will settle out in the settling basin, never reaching the vegetative treatment area (VTA). The risk of overloading a VTA is generally small.
It may be possible to see some overloading of the headlands of a VTA with P. Soil monitoring will provide a good indicator of possible overloading of the headlands. However, overloading of the headlands will provide minimal risk for P loss from a VTA. Permanently vegetative areas such as a VTA typically have very low soil loss (primary means of P transport).
Nitrogen leaching is generally a greater concern than phosphorus runoff from a VTA. USDA Meat Animal Research Center has carefully monitored for nutrient concentrations in a VTA over many years with a focus on nitrogen. In a 2005 paper, it was reported that: ”The estimated total nitrogen load entering the VTA was equivalent to or less than the total nitrogen load removed by the hay crop harvested from the VTA. No water was measured exiting the VTA, either by deep percolation or by direct release, during the four-year study period. As a result, the discharge water from the basin was effectively used for hay crop production. . . . Soil analyses in these zones indicated that surface soil NO3-N levels, particularly closest to the discharge tubes, had increased. Currently nitrogen is contained near the surface and has not started to infiltrate deeper into the VTA soil.”
Source: ASABE Technical Library
Author: Rick Koelsch, University of Nebraska
