Comparison of Amino Acid Digestibility Assays

New Zealand researchers compared assays based on the collection of digesta from the terminal ileum to an excreta-based assay for evaluating the apparent and true amino acid digestibility of several common feedstuffs used in formulating diets for broiler chicks. Apparent amino acid digestibility values of the assay diets at ileal and excreta levels were calculated using chromic oxide as the indigestible marker. True digestibility values were calculated using endogenous outputs determined by feeding a protein-free diet. Significant differences were found between ileal and excreta-based digestibility of certain amino acids in some ingredients, with the excreta values being usually higher than the ileal values, indicating a net catabolism of amino acids in the large intestine. The net amino acid disappearance was found to be variable among amino acids and ingredients. In general, threonine has the lowest digestibility at the ileal level compared to the other amino acids and greater degradation during passage through the hindgut. These results indicate that amino acid digestibility based on excreta collection will overestimate the update for some amino acids in feedstuffs. The research group concludes that the digestibility values measured at the terminal ileum is a more reliable measure of amino acid availability than those measured in the excreta.

Kadim, I.T., P.J. Moughan and V. Ravindran. 2002. Ileal amino acid availability assay for the growing meat chicken comparison of ileal and excreta amino acid digestibility in the chicken. Br. Poultry Sci. 43(4):588-597.