Protein and amino acids quality in feed ingredient is most correctly evaluated by determining the standardized ileal digestibility (SID) of crude protein (CP) and amino acids. Experiments to determine the SID of CP and amino acids in feed ingredients often utilize semi-synthetic diets that contain the test ingredient as the sole source of CP and amino acids. Therefore, the CP and amino acid supply from experimental diets do not always meet the requirement for standardized ileal digestible amino acids by pigs and, therefore, pig growth is often limited in these experiments.

Crystalline amino acids are believed to be rapidly absorbed and 100% digestible. Addition of crystalline amino acids to diets during the adaptation or collection phase is, therefore, believed not to influence values for SID of amino acids in the test feed ingredient, but this has not been experimentally verified. Therefore, an experiment was conducted to test the hypothesis that addition of crystalline amino acids to diets during the adaptation or collection periods of digestibility studies will not influence calculated values for SID of CP and amino acids in corn and soybean meal.