The paper evaluates the effect of short-term air contamination on the fermentation profile and aerobic stability of alfalfa silage. Seven silage additive variants (formic acid, biological inoculant, Safesil Challenge, Silafor 2000 plus) applied at two dose levels plus an untreated control were tested. After 28 days of anaerobic fermentation, half of the samples were punctured for 24 hours to simulate cover film damage, then re-sealed anaerobically. Aeration stress deteriorated the lactic acid to volatile fatty acid ratio (LA/VFA) in most variants and promoted undesirable secondary fermentation. The best resistance to aeration stress was shown by Silafor 2000 plus (5 l/t) and Safesil Challenge (3 l/t). Results emphasise the importance of immediate repair of damaged cover film and the selection of an appropriate silage additive.