The aim of the studies was to determine the effect of slurry supplemented with UGmax and Humus Active soil conditioners and with NPK fertilizers on the content of structural and non-structural carbohydrates and lignin in a forage grass species of Sulino variety [Festulolium braunii (K. Richt. A. Camus]. The 2-yr field experiment with three replicates and a completely randomized design was established on the experimental facilities of the University of Natural Sciences and Humanities in Siedlce, Poland. Cellulose, hemicellulose, lignin, total protein, crude ash, and crude fat in plant material were determined by near-infrared reflection spectroscopy (NIRS). The amount of non-structural carbohydrates was calculated by deducting the weight of other components from 1,000 g of dry matter. The applied fertilizer significantly affected the content of structural and non-structural carbohydrates, but not lignin. The highest amounts of cellulose (283.0 g kg -1 ) and hemicelluloses (220.2 g kg -1 ) were recorded in plants treated with both slurry and Humus Active and UGmax, while the highest content of non-structural carbohydrates (220.2 g kg -1 ) was found in Festulolium braunii treated with slurry only. Cellulose and hemicellulose content significantly varied across growing seasons, while the amounts of cellulose, lignin, and non-structural carbohydrates varied across harvests. The interaction of slurry with soil conditioners, unlike only slurry treatment affected the formation of hemicellulose and non-structural carbohydrates. The combined use of Humus Active and slurry increased hemicellulose content by approximately 23%, while the content of non-structural carbohydrates decreased by about 35% in comparison to plants treated with slurry only.