Yield and quality of cultivated forage crops are important determinants of animal production efficiency and both forage traits are sensitive to weather. Consequently, it can be presumed that these traits will respond to anticipated changes in the global environment associated with increases in [CO2] and temperature. Because of the large inter- and intra-annual weather variation that exists in most agricultural regions, it is not clear what the forage production response will be to predicted changes. This uncertainty in forage response to global environment change as compared to interannual weather variation was analysed using a mechanistic simulation model of plant development and growth. The results indicated that low soil-nutrient availability restricts forage production to such a degree that the global environment effects of higher [CO2] and temperature on quality are essentially irrelevant. When the nitrogen constraint was relaxed, higher [CO2] caused dry matter and digestibility to increase and protein concentration and leaf:stem ratio to decrease. Increased temperature had opposite effects. The combined effects of [CO2] and temperature were complex and annual weather variations were dominant, especially in rainfed situations. Limited reliability of predictions of crop response decades into the future indicates that it may be prudent to direct research effort to prepare for a wide range of climatic contingencies.