Extensive breeding of farmed cervids, similarly to other livestock, affects the vegetation of grasslands in different seasons. For this reason, the impact of the rotational grazing of fallow deer on the chemical and species composition of the pasture sward was assessed, along with the possibility of using these animals for grasslands conservation. The species composition of the pastures was analysed through the botanical-weight method. A quality index and mineral concentration test by inductively coupled plasma mass spectrometry were used to evaluate the feed. The highest proportion of valuable grasses, such as Dactylis glomerata, Poa pratensis and Lolium perenne, was recorded in the summer pens (65.7–66.1%), while the smallest proportion was recorded in the control area (46.1%). The estimated yield potential was relatively large, from 5.74 to 7.02 t ha−1 dry matter. The lowest total protein content occurred in the control area in the spring and autumn. The summer pens, including the sown one, had a better fodder quality, depending on the species composition. All pens were characterised by a high production potential and similar floristic composition, without the participation of undesirable plant species, which confirms the hypothesis that, under extensive grazing conditions, fallow deer can be used for grassland conservation.