Greenhouse gases and of ammonia emissions from pork production will change when fattening of barrows switches towards to fattening of (intact) boars. The results of an accurate feeding experiment allow for the differentiation of the effects on emissions of gender (differentiating in boars, barrows and gilts) and of diet composition. The modified fattening pig module of the agricultural emission model GAS-EM was used to estimate emissions in 2020 when the fattening of barrows will no longer be common practice. The scenarios also reflect the effect of the expected increased weight gains and the related effect of increased numbers of animals produced. The fattening of intact boars as compared to barrows is associated with a reduction of emissions of greenhouse gases and of ammonia per animal. For ammonia, all scenarios result in reduced emissions, most markedly when this shift is combined with increased weight gains. To a lesser extent, this also applies to nitric and nitrous oxide emissions. Methane emissions are less affected; increased weight gains result in increased emissions. As the greenhouse gas balance is dominated by methane emissions, the overall emission of greenhouse gases (expressed as CO2 equivalents) is likely to increase slightly in 2020 despite the reductions in nitrous oxide emissions.