The research objective was to determine the effect of the additive Sill-All application on ad libitum intake, in vivo digestibility and nitrogen balance of alfalfa haylage. About 40 % alfalfa plants was ensiled at the phonological flowering stage into plastic foil-wrapped bales without or with additive (2 L t-1 plant mass). No statistically significant differences were found between alfalfa ensiled without additive and alfalfa ensiled with additive for the content of dry matter (DM) (632 g and 631 g DM kg-1 fresh sample, respectively). Alfalfa ensiled without additive contained 921 g organic matter (OM) kg-1 DM, which was significantly higher (P<0.001) compared to alfalfa ensiled with additive (902 g OM kg-1 DM). Alfalfa ensiled without additive contained 141 g crude proteins (CP) kg-1 DM, which was significantly higher (P<0.001) compared to alfalfa ensiled with additive (139 g CP kg-1 DM). Alfalfa ensiled with additive contained significantly less acid detergent fibers (ADF) (P<0.001) compared to alfalfa ensiled without additive (445 g kg-1 DM and 456 g kg-1 DM, respectively) and had a lower pH value (P<0.001) (5.29 and 5.56, respectively). No statistically significant differences were found between the studied feeding treatments for ad libitum intake of fresh ration and DM ration, for the measured parameters of digestibility and N balance. It was concluded that addition of the additive to alfalfa haylage led to significant changes in chemical composition; however, changes in chemical composition had no impact on measured biological parameters (ad libitum intake, in vivo digestibility and nitrogen balance).