The research was aimed at assessing the effect of complementary use of slurry with soil conditioners and with NPK fertilizers on the content of crude fibre fractions in the biomass of Lolium multiflorum. The studies were conducted on the basis of a two-year (2016-2017) field experiment established in three replications in a completely randomized design. The impact of slurry and its interaction with soil conditioners and with NPK fertilizers was tested on the Dukat variety of Lolium multiflorum forage grass. The results demonstrated that there was no significant effect of the treatments on the content of crude fibre and its fraction in the grass. However, ADF and ADL fibre content varied significantly across the growing seasons. In the same growing season crude fibre content as well as the content of the ADL fraction varied across three Lolium multiflorum harvests.