The project assessed the feasibility of application of urease inhibitor (UI) to cattle pens and manure stockpiles, as a strategy for reducing ammonia (NH3) and nitrous oxide (N2O) emissions. The study included a combination of atmospheric dispersion modelling, mineral nitrogen analysis and laboratory incubations. UI-application to cattle pens was found to have a significant effect on urea content in manure but, even after treatment, retained urea was rapidly depleted within the first days after pen clearing and manure stockpiling, and UI-treatment could not be reliably linked to reduced NH3 emissions from manure stockpiles. Sustained retention of urea in the manure as it is removed, stockpiled and ultimately incorporated into agricultural soils remains an operational challenge, because of the transient effect of the UI, and pen-access difficulties in wetter months. Moreover, even if practicable, the additional cost of implementing UI-application, at the label rate, was estimated at $38 per turned-out-steer, or $459 per tonne of mitigated CO2-e. In conclusion, costeffectiveness of UI-application for mitigation of ammonia and greenhouse gas emissions seems doubtful, however recommendations to progress this work include more resilient additives, cheaper and more reliable application methods, and improved emissions measurement within the pens.