The caryopses of grasses have been harvested as human food since long before the advent of agriculture. Numerous species are still regularly harvested in Africa and Asia during times of scarcity. Among the many hundreds of species harvested as wild cereals, 33 species belonging to 20 genera were domesticated. Their cultivated cereals are dependent on humans for survival because they have lost the ability of natural seed dispersal and have become adapted to cultivated fields.

Cereals are grown on an estimated 730 million hectares and produce about 1,800 million metric tons of grain annually. Wheat, maize, and rice account for at least 80 percent of the annual world cereal production. Barley, sorghum, oats, rye, and pearl millet represent about 19 percent of cereal grains produced, and the remaining 1 percent of production comes from the other 19 grass species that are still grown as human food. These species are minor in terms of total world cereal production, but some are important components of agriculture in Africa and Asia (de Wet 1989).

Cereals that do not belong to the wheat (Triticum), barley (Hordeum), oats (Avena), maize (Zea), or rice (Oryza) genera are commonly referred to as millets (de Wet 1989).

American Millets

The first cultivated cereal in the Americas appears to have been a species of Setaria (Callen 1965, 1967). Archaeological records indicate that this millet was an important source of food in the Valley of Mexico and in northeastern Mexico before the domestication of maize. E. O. Callen (1967) demonstrated a steady increase in size of caryopses of this millet over 1,500 years of use as a cereal. The species, however, never lost the ability of natural seed dispersal. It was displaced by maize as a cereal during the fifth millennium B.C., but later enjoyed a temporary resurgence in importance when it was probably harvested from weed populations that invaded maize fields.