Leucine and DDGS in Pig Diets – Benefits of Tryptophan?

Research Summary

Many of the ingredients commonly used in swine diets contain greater amounts of leucine than required by the pig. Previous research has suggested that feeding elevated amounts of leucine can depress intake and performance in pigs. Fortunately, researchers have now started investigating formulation strategies to counter some of these potential negative effects.

Researchers from South Dakota State University presented some data at the 2020 Animal Science meeting that looked at the effects of increasing tryptophan in grow-finish pig diets. They observed a linear improvement in both body weight gain and average daily gain from 0 to 98 days of growth as the SID tryptophan:lysine ratio increased.


The amino acid leucine comprises a large percentage of the amino acid profile for ingredients like DDGs, soybean meal and corn (Figure 1). Unfortunately, feeding excessive leucine can sometimes reduce performance in pigs because of reduced intake and effects on serotonin.

To counter this effect, nutritionists have started to look at the ratio between leucine and tryptophan. Feeding additional tryptophan may provide benefits because leucine competes with tryptophan for absorption.

This relationship becomes even more important when feeding diets with greater inclusion of DDGS. Although both DDGS and soybean meal contain a significant amount of leucine (SID of 2.73 and 3.19%, respectively), DDGS contain much less tryptophan. As a result, feeding greater amounts of DDGS can reduce the ratio of these 2 amino acids.

*These results are not a guarantee of nutritional value, as laboratory results are influenced by factors beyond the control of POET feed ingredients.

(Figure 1) Standard ileal digestibility of ingredients used in pig formulations.