The aim of this study was to evaluate the effects of three strategies of protein feeding (12%, 16% and 20%), supplying adequate levels of all essential amino acids, on finishing pig performance, nitrogen excretion and ammonia emission in the laboratory and in the field. The animals (15 castrated males) were housed individually in metabolism cages and were fed one of the three diets. The experiment lasted 21 days and urine and faeces were collected separately. Samples of each type of manure (urine alone, fresh slurry and 18-day slurry) were placed for 6 days in a laboratory pilot scale system designed to measure ammonia emission. In the field, measurements of ammonia volatilization were made during storage and surface application of slurry. Lowering dietary crude protein, while maintaining similar levels of essential amino acids, reduced total amount of manure, because of a lower water consumption, without any negative effect on animal performance. Total ammoniacal nitrogen content in the slurry was reduced from 4.32 g N kg−1 to 3.13 and 1.92 g N kg−1 when dietary CP decreased from 20% to 16% and 12%, respectively (the corresponding figures for total nitrogen content were 5.48, 4.30 and 3.05 g N kg−1, respectively). The pH of slurry decreased by 1.3 units when dietary crude protein decreased from 20% to 12%. These changes in slurry composition and pH resulted in a reduction of ammonia emissions at each stage of manure management. Over the whole process of the slurry, ammonia emission was decreased by 63% when dietary protein decreased from 20% to 12%.