Agricultural nutrient management continues to be an important area of research and policy due to concerns of phosphorus (P) loss in runoff and water quality impacts. For dairy and beef farms, outdoor cattle lots (feedlots, barnyards, exercise lots, over-wintering lots) can be significant sources of P loss (Koelsch et al., 2006). There is a need to assess P loss from lots, especially relative to other farm areas (cropland, pastures), to see if alternative lot management is needed and cost-effective. Computer models can be effective tools to help quantify P loss from cattle lots. Despite quite a bit of physical monitoring research on P loss from lots since the 1970’s, there has been little development of models to predict P loss from these areas. To our knowledge, the only two examples of runoff and P loss models for cattle lots are in the AGNPS model (Young et al., 1989) and the APEX model (Gassman et al., 2010; Williams et al., 2006). Barnyard runoff models such as BARNY in Wisconsin and MinnFarm in Minnesota use the same approach as AGNPS. Both AGNPS and APEX have had only minimal testing for P loss from lots (Kizil et al., 2006; Williams et al., 2006), so it is not clear if they are reliable across a range of cattle lot managements, conditions, and locations. Our objectives were to:
1.Develop a relatively simple, annual model to estimate P loss in runoff from cattle lots
2.Test the model with data available in the published literature
3.Compare the new model to BARNY and MinnFarm.