The aim of study was to compare the effect of additives on changes in fractions (A, B and C in The Cornell Net Carbohydrate and Protein System CNCPS) during the fermentation of alfalfa silage. Alfalfa from the 1st growth cut was ensiled (under anaerobic conditions in plastic mini-silos) without additive, or treated with chemical additive (formic acid, 85 %), or with a bacterial inoculant (Lactobacillus plantarum, Pediococcus pentosaceus, Pediococcus acidilactici, lxlO11 CFU/g) at 1 g/t fresh weight. Silages were analysed after 2, 14 and 60 days of fermentation (6 replications from each treatment). During fermentation the proportion of non-protein fraction A increased, whereas the proportion of soluble protein fraction B decreased. After 60 days of fermentation the proportion of fraction A of silage treated with formic acid was numerically 5.0 % lower compared to silage treated with the inoculant, and 5.5 % lower than the untreated control.