Estimation of Ammonia Emissions From Beef Cattle Feedyards in the Southern High Plains with Process-Based Modeling.

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.