ABSTRACTIncreasing atmospheric [CO2] is stimulating photosynthesis and plant production, increasing the demand for nitrogen relative to soil supply with declining global foliar nitrogen concentrations as a consequence. The effect of such oligotrophication on the forage quality of sweetveld, mixedveld, and sourveld grasslands in South Africa, which support livestock production and native ungulates, are unknown. Soil characteristics and the herbage quality of an abundant grass are described from baseline historical (mid 1980s) data collected across a sweet-mixed-sour grassland gradient in KwaZulu-Natal. Sourveld occurred on the most acidic, dystrophic soils and exhibited a pronounced decline in leaf N, digestibility, and other macronutrients during winter, in sharp contrast to sweetveld, on nutrient-rich soils, where forage quality varied little seasonally. In a carbon enriched, warmer, and most likely drier future climate, we predict that forage quality will be little altered in sweetveld where soil nutrients and temperature are not limiting but that sourveld could become ‘sourer’ because soil nutrients will be inadequate to match higher plant production promoted by elevated [CO2] and warmer and longer growing seasons. Reassessing historical data and seasonal and spatial monitoring of forage quality will enable past and future impacts of climate change on grassland forage quality to be assessed.SignificanceGrassland forage quality will likely decline with elevated [CO2] and warming, particularly in sourveld.Climate change could deepen and widen the sourveld winter forage bottleneck, necessitating greater supplementary feeding of livestock.