Heidi Waldrip1, Richard Todd2, N. Andy Cole2, Changsheng Li3, William Salas4 and C. A. Rotz5, (1)USDA-ARS Conservation and Production Research Lab, Bushland, TX (2)Conservation and Production Research Laboratory, USDA-ARS, Bushland, TX (3)Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, NH (4)Applied Geosolutions, LLC, Durham, NH (5)USDA-ARS, Pasture Systems and Watershed Management Research Unit, University Park, PA
Ammonia (NH3) emissions from beef cattle feedyards result in loss of nitrogen (N) that could be useful for crop production, and this emission can have negative environmental consequences. Both empirical and process-based models have been developed to estimate NH3 emissions from various livestock production systems; however, little work has been conducted to assess their accuracy for large open-lot feedyards. To validate two process-based models, Manure-DNDC and the Integrated Farm System Model (IFSM), model-simulated NH3 emissions were compared to data collected from two commercial feedyards in the Texas High Plains from 2007 to 2009. Model predictions by both Manure-DNDC and IFSM were in good agreement with observed data from both feedyards (p < 0.001), and reflected high sensitivity to seasonal variations in air temperature and dietary crude protein level. Manure-DNDC predicted mean daily NH3 flux densities for the two feedyards of 43.6 and 55.7 kg NH3 ha-1 d-1, and regression analysis showed good agreement with observed data (p < 0.001). With the addition of an open-lot emission routine, IFSM predicted mean per capita emission rates of 72.8 and 60.7 g NH3 head-1 d-1 for the two feedyards, which also agreed well with observed data (p < 0.001). Furthermore, despite differences in the core models, Manure-DNDC and IFSM predicted values were highly correlated (r = 0.74, p < 0.001). This evaluation indicates that both IFSM and Manure-DNDC provide useful tools for predicting NH3 emissions from commercial feedyards, offering accurate information for legislators and policy makers and methods for evaluating the effects of specific management practices and mitigation strategies.