Varietal improvements in yield are often strongly associated with increases in the ratio of the harvested organ or product to total biomass, which itself has shown relatively little change. Thus the greatest opportunities for increasing yield have been for the cereal species, with root crops intermediate and forages the most difficult to improve. In favourable climates the yields of cereals are frequently double those obtained in 1950, improvements in varieties accounting for about half this increase. The work has been much less effective where growth is dominated by environmental stresses, especially water supply, and in these situations yields are inconsistent, with little average improvement. Breeding for resistance to the hazards of pests and disease, storms, temperature extremes and mineral deficiencies has had numerous successes. There remain considerable problems in the durability of resistance to pathogens, and in a number of cases there are no known sources of resistance that can be utilized by conventional breeding methods. Improvements in the quality of food crops are most notable for meeting technological requirements and consumer preferences; there are few examples of improvement in nutritional value other than in the elimination of nutritionally deleterious substances. In cereals and many other crops there are considerable limitations in protein content due to an inverse relation with yielding ability.