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Social, economic, and environmental factors have prompted the desire to reduce global atmospheric ammonia emissions. A research project was conducted to assess the efficacy of the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) for reducing ammonia emissions from simulated open-lot beef cattle feedyard surfaces. A mixture of beef cattle feces and urine (manure) was placed into small emission chambers (167 167 170 mm deep). A urea solution was added every 2 days to simulate continual urine deposition in the feedyard. Clean air (1.4 L min-1) was passed over the manure surface, and ammonia was trapped in an acid solution. The six treatments (three replications per treatment) included combinations of NBPT application rate with or without simulated rainfall. NBPT was applied at zero, steady (5 kg ha-1 every 4 days), or increasing (5 kg ha-1 initially, doubled every 4 days up to 40 kg ha-1) rates. Rainfall treatments received 6 mm every 4 days. For all treatments, mean ammonia emissions from the manure were lower (p < 0.05) when simulated rainfall was added. Mean ammonia emission rates for the NBPT treatments were 26% to 33% of the non-NBPT treatments, demonstrating that NBPT was effective at reducing emissions from the manure surfaces in both wet and dry conditions. There were no statistical differences in mean ammonia emission rates for the steady and increasing NBPT application rates, showing that a steady NBPT application of 5 kg ha-1 every 4 days was effective in reducing ammonia emissions from the manure. The use of NBPT appears promising for reducing ammonia emissions at beef cattle feedyards. Additional research is warranted to study the effectiveness under long-term conditions in an outdoor feedyard setting.