Piglet diets with a low crude protein (CP) concentration are recommended when in-feed antibiotics are not used and diarrhea is likely to occur. A normal piglet diet with about 22 percent crude protein can be formulated to contain as little as 18 percent crude protein with the use of crystalline amino acids. Of the latter, lysine, methionine, threonine and tryptophan are widely available, whereas valine is also commercially available.

Reducing crude protein below four percentage points, in any piglet diet, is likely to create amino acid deficiencies that cannot be remedied as such amino acids do not exist in commercially available forms. Such is the case for Tyrosine (Tyr) and Phenylalanine (Phe).

Standardized ileal digestible requirements


The standardized ileal digestible (SID) Phe:Lys and Tyr:Lys requirements and the possibility to substitute Tyr by Phe have never been studied in piglets. Thus, it was the objective of a recent study to estimate these requirements in 10- to 20-kg pigs as well as to determine the extent to which Phe can be used to cover the Tyr requirement. Therefore, in three dose–response studies, six pigs within each of 14 blocks were assigned to six low CP diets (14.5 percent CP) sub-limiting in Lys at 1.00 percent SID.

In experiment 1, the SID Phe:Lys requirement estimate was assessed by supplementing a Phe-deficient diet with different levels of L-Phe to attain 33 percent, 39 percent, 46 percent, 52 percent, 58 percent and 65 percent SID Phe:Lys. Because Phe can be used for Tyr synthesis, the diets provided a sufficient Tyr supply.


A similar approach was used in experiment 2 with six levels of L-Tyr supplementation to attain 21 percent, 27 percent, 33 percent, 39 percent, 45 percent and 52 percent SID Tyr:Lys. Phenylalanine was supplied at a level sufficient to sustain maximum growth (estimated in experiment 1). The SID Phe:Lys and SID Tyr:Lys requirements for maximizing daily gain were 54 percent and 40 percent using a curvilinear-plateau model, respectively. A 10 percent deficiency in Phe and Tyr reduced daily gain by 3.0 percent and 0.7 percent, respectively.

In experiment 3, the effect of the equimolar substitution of dietary SID Tyr by Phe to obtain 50 percent, 57 percent, and 64 percent SID Phe:(Phe+Tyr) was studied at two limiting levels of Phe+Tyr. From 57 percent to 64 percent SID Phe:(Phe+Tyr), performance was slightly reduced.

Ideal amino acid profile

In conclusion, it is recommended not to use a Phe+Tyr requirement in the ideal amino acid profile but rather use a SID Phe:Lys of 54 percent and a SID Tyr:Lys of 40 percent to support maximal growth. These results verify currently available data based on total levels for these amino acids.