Limiting amino acids
When an essential amino acid is not provided in adequate amounts in the diet, protein synthesis is limited to the rate at which the essential amino acid is available. Essential amino acids then become limiting amino acids. A way of describing a limiting amino acid is using the concept of a rain barrel. The protein is the rain barrel and the amino acids are the individual staves that make up the barrel. When one stave is shorter than the others, the barrel can only be filled to the level of the shortest stave. In other words, when one amino acid is deficient, proteins can only be synthesized to the level of availability of that amino acid.
The first-limiting amino acid is the essential amino acid that first become deficient in the diet. Lysine is typically the first-limiting amino acid in most swine diets because the requirement of lysine is greater relative to the amount provided in the diet.