The recent increase of red deer Cervus elaphus population and consequent damage caused by their herbivory impact increasingly concern foresters and farmers in Slovakia as well as in other European countries. Thus, the topic of vegetation—deer interactions with focus on forage production is especially relevant for developing adequate management guidelines. Using data from 320 sampling plots, we estimated the overall availability of all forage items seasonally consumed by red deer in commercial temperate forests and identified the main factors affecting forage availability in summer and winter. We found that cutblocks were the most productive habitats throughout the year irrespective of the site quality. Summer forage biomass peaked at ≈8 years and winter forage biomass at ≈10 years following felling, then slowly declined as the cutblocks aged and the canopy increased. Understorey vegetation production in mature forests was determined primarily by light availability, as the major driving factor of vegetation growth in the closed-canopy forest ecosystems, and to a lesser extent by a site quality. We suggest that the site quality index that is traditionally used in forestry is not an efficient predictor of the forage availability for red deer, and estimations of the forage potential of hunting grounds should incorporate more complex models to evaluate carrying capacity of the landscape.