The aim of this study was to analyze changes in botanical and chemical composition, as well as in vitro rumen fermentation characteristics of an upland grassland exposed to climate changes in controlled CO2 concentration, air temperature and precipitation conditions. Grassland was exposed to a future climate scenario coupled with CO2 treatments (390 and 520 ppm) from the beginning of spring. During summer, an extreme climatic event (ECE; 2 weeks of a +6 ∘C increase in temperature, together with severe drought) was applied and then followed by a recovery period. Three cutting dates were considered, i.e. in April, June and November. The results indicate that increases in greenness, nitrogen (N) content and changes in water-soluble carbohydrate profile in association with botanical composition changes for the November cut lead to higher in vitro dry matter degradability (IVDMD) in the rumen. The neutral detergent fiber : nitrogen (NDF:N) ratio appeared to be a key driver of forage quality, which was affected in opposite ways by elevated CO2 and ECE, with a strong impact on rumen fermentation. Atmospheric CO2 concentration in interaction with ECE tended to affect IVDMD, indicating that the effects of elevated CO2 and ECE may partly offset each other. Our findings indicate that the various factors of climate change need to be considered together in order to properly characterize their effects on forage quality and use by ruminants.