During the process of alfalfa wilting on the field and during the subsequent fermentation under anaerobic conditions there are changes not only in the DM content but also in percentages of individual nutrients, above all of total crude protein and fibre (as well as in their fractions). Wilting and especially fermentation increase the percentages of soluble non-protein fraction (A) to the detriment of soluble protein fractions (B). Although the differences between contents of NDF in wilted and ensiled alfalfa were statistically insignificant, the content of hemicelluloses was significantly higher (P < 0.05) in wilted material than in silage. This indicates the fermentation activity of lactic acid bacteria, which use hemicelluloses for their growth and propagation.