Non-point source pollution of surface water by nutrients such as phosphorus can degrade water quality for drinking, recreation and industry. When excess nutrients accumulate in lakes and reservoirs, water quality issues such as algal blooms often result. Because agriculture has been identified as a source of non-point phosphorous pollution, there has been a strong push to identify and manage farm sources of phosphorus runoff. On dairy farms, possible sources of this runoff include cropland, grazed pastures and outside cattle holding areas such as feedlots, barnyards and overwintering lots. In the United States, research on phosphorous loss due to runoff from grazed pastures has been limited.
Physically monitoring phosphorous loss from farms is an expensive, lengthy process. Simulation models are potentially a more rapid, cost-effective way to estimate phosphorous loss from farms. Agriculture Research Service soil scientist Peter Vadas, who works at the U.S Dairy Forage Research Center in Madison, worked with a team of USDA scientists to develop the Annual Phosphorous Loss Estimator (APLE) spreadsheet, which predicts the phosphorous lost through runoff for diverse types of farms and field conditions. APLE is free to download at http://ars.usda.gov/Services/docs.htm?docid=21763.
Building on this work, Vadas, along with Mark Powell and Geoff Brink from the Dairy Forage Research Center and Dennis Busch from UW-Platteville, monitored phosphorus loss in runoff from grazed pastures and used APLE to predict phosphorus runoff from grazing farms. This research took place from 2010-2012 at the UW-Platteville Pioneer Farm and four Wisconsin grazing farms, and was funded by the WI DATCP Grazing Lands Conservation Initiative (GLCI). The researchers monitored phosphorous loss due to runoff from beef and dairy grazed pastures at the Pioneer Farm. They used this data to validate that APLE can reliably predict phosphorus loss from grazed pastures. They then used APLE to simulate phosphorous loss from the four farms, all of which use managed grazing. The focus of this brief is on the modeling results from these farms.
The researchers visited each farm three times in January, June and November 2011 to gather seasonal information about farm management. Questionnaires completed by each farm provided snapshot assessments of cattle, feed, fertilizer, manure and cropping management. Using this information, the researchers modeled year-round, whole-farm phosphorus losses under typical management for each farm.