Wetland ecosystems are highly productive and valued for numerous reasons including wildlife habitat, biodiversity, water quantity and quality, and human uses. Grazing livestock on wet grasslands can sometimes be controversial due the humidity of the habitat, but on the other hand, it plays an important role in grassland preservation. Therefore, we evaluated the impact of Polish white-backed cattle grazing on changes in the vegetation (13 phytosociological relevés taken in years 2016–2019) of wet meadows as well as forage quality based on the species composition. Biodiversity was estimated based on species richness, Shannon–Wiener diversity and evenness indices, and Rényi diversity profiles. The peatland featured mostly high-sedge and wet meadows communities of the Magnocaricion and the Calthion alliances. The species and biodiversity indices demonstrated significant rising trends. Extensive grazing resulted in the decreased cover of the dominant species of rush meadows, e.g., the common reed, acute, and tufted sedge. The gaps that had emerged thanks to the reduced cover of the dominant species were filled by meadow plants, which led to increased biodiversity. During the four years of grazing, the cattle obtained satisfactory weight gains, particularly in 2019, which indicates that wet meadows are suitable for grazing and can provide sufficient feed for cattle.