Natural pastures in Montenegro, together with natural meadows, are the most important forage source and comprise 95% of the used agricultural area (UAA). The aim of this study was to estimate the changes in grassland biomass, chemical and floristic composition and fatty acid composition in milk of cows grazed onAgrostis stoloniferasemi-natural wet grassland in the Central region of Montenegro. Investigation was conducted during three phenological phases: early vegetative, optimal vegetative and full reproductive phase. The chemical composition of the grassland biomass was changed through the phenological phases. A significant increase during the experimental period in the content of dry matter and crude fibers and a decrease in crude protein content were determined. The pasture community consisted of 52 species, belonging to 19 families. The total amount of plant species classified as excellent, very good and good quality was 7.69%, while medium quality plants amounted to 13.46%, low quality plants to 19.23%, valueless plants to 34.62% and toxic plants to 25%. The total quality index was 4.166. The highest quality index of this community was in the second phenological phase, then in the third phase the lowest quality was in the first phenological phase. According to the floristic composition and biomass quality, this lowland pasture can be qualified as being of moderate to good quality. The content of milk fat, protein and solid non-fat (SNF) was increased from the first to the third vegetation phase, but the differences between the phases were not significant (p> 0.05). The content of a majority of saturated fatty acids (SFAs) also increased, while the total content of monounsaturated fatty acids (MUFAs) was high in the second phase and decreased significantly in the third phase. The content of polyunsaturated fatty acids (PUFAs) was constant in the first two phases, while it significantly decreased in the third phase. The highest content of most of MUFAs and PUFAs was obtained in the second phenological phase. The content of fatty acids in the third phase significantly affected (p< 0.05) by the phenological phase. This research contributes to the improvements in the grazing management of wet grasslands as well as the animal performances (milk yield and quality of milk).
