ABSTRACT
Humans and other monogastric animals cannot synthesize the complete set of 20 amino acids that are needed for building proteins; therefore, they must obtain several of the amino acids from their diet. Unfortunately, crop plants, which represent the largest food sources worldwide. contain low levels of some of these essential amino acids. Grains of most cereal species. such as wheat and maize, that represent the world's largest providers of food and, consequently, important economic commodities (Tables I and 2). contain inadequate levels of some essential amino acids, particularly lysine and threonine (Table 3). Indeed, cereal-based diets for livestock are routinely supplemented with lysine and, in some instances, also threonine; these supplements are highly beneficial for livestock growth (Bright and Shewry 1983; Cuaron et al. 1984; Fuller et al. 1979). Over 200.000 tons of lysine. produced by microbial fermentation, are used annually to enrich cerealbased diets for animals. resulting in increased costs of these feeds (Falco et al. 1995. 1997). Cereal-based diets are also routinely supplemented with legume grains because the latter are relatively rich in lysine. However, legume grains are also more expensive and contain very low levels of methionine, another important essential amino acid.
