Animals are able to synthesize only about half of the 20 amino acids commonly found in proteins, the remainder being required in their diet and hence termed "essential." If only one of these is limiting, the remaining amino acids cannot be utilized but are broken down and excreted, leading to poor feed conversion and environmental pollution. This is not usually a problem with ruminants, in which the rumen bacteria provide all 20 amino acids, but is a problem for nonruminants such as pigs and poultry.
Comparison of the amino acid compositions of seeds with the requirements of essential amino acids for humans and nonruminant livestock shows that both legumes and cereals are deficient in one or more amino acids. Furthermore, these deficiencies are determined by the amino acid compositions of the storage protein fractions.
The prolamins of wheat, barley, and maize contain low levels of lysine (=1 mol % or less), and this is the first limiting amino acid in all three species. The prolamins (zeins) of maize also contain little or no tryptophan, which is the second limiting amino acid in this species, and all three also contain low levels of threonine. In contrast, the 7S and 11S globulins of legumes are severely deficient in cysteine and methionine, resulting in deficiencies in these amino acids in the whole seeds. Because cereal and legume seeds are largely complementary in their contents of essential amino acids, they are often used in appropriate mixtures to provide a balanced diet of essential amino acids. Nevertheless, there is considerable interest in producing high-lysine cereals and high-sulfur legumes to provide more flexibility in producing low-cost diets for livestock.
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