The seasonal variation in diet selection and herbage intake of cattle and sheep and the effects on animal performance at various research centres are put into perspective, taking into consideration the annual rainfall and its distribution. The most important constraint on animal production from pastures, appears to be an insufficient intake of digestible nutrients in relation to animal requirements, which may at times be aggravated by deficiencies of specific nutrients in the herbage. Supplementary feeding may increase the nutrient intake of grazing ruminants and correct deficiencies in pastures. However, despite considerable costs incurred, animal response to supplementation can at best be described as unpredictable and far less than might be expected from feeding standards. Renewed efforts towards gaining a more comprehensive understanding of, and explanation for, the interactions between the integrated animal/plant biological system of grazing conditions, are discussed.