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Ammonia (NH 3) emissions originate predominantly from manure. In outdoor pig production, excretory behaviour creates nitrogen (N) point loads. This study examined ammonia losses from pigs on grassland at two farms by calculating N field balances per pen and sub-areas preferred (P) and not preferred (NP) for excretion. Spatial variation in ammonia losses was measured with an equilibrium concentration method in one pen per farm during two years at the end of the fattening period. Cumulative ammonia losses during a fattening period were measured using a micrometeorological mass balance method. P sub-areas had 10-to 100-fold higher amounts of excreted N than NP sub-areas. Ammonia losses were higher from P sub-areas (0.056-1.843 g NH 3 ha-1 hr-1) than from NP sub-areas (0.001-0.332 g NH 3 ha-1 hr-1). Ammonia losses from NP sub-areas varied more widely. Cumulative NH 3 losses were 28 kg pen-1. Gross NH 3 losses were 0.66 kg N pig-1 , representing 14% of excreted N.