The goal of the investigations was to determine the boundary conditions of compactness and hermetic level of covering material (permeability of the covering material) to inhibit mould growth during silage storage. The influence of air exclusion on fermentation quality, mould growth and mycotoxin content in the silage should be tested. To get a sufficient fermentation quality, as well as an inhibition of mould growth in the silages, necessary boundary conditions of air exclusion were: a compactness of at least 200-210 kg/DM m(3) and a maximum permeability of the covering material of 1,7 l/m(2) in 24 hours (six numbers of wraps of the used ensiling stretch film). Summarised, the measured amounts of <1,0 mg roquefortine C/kg in the silages is considered to be relatively low. Within the actual state of knowledge of toxicology, it is not considered dangerous to feed ruminat animals with silages containing these low amounts of roquefortin C.