Apparent and true amino acid digestibility of a crystalline amino acid mixture and of casein: comparison of values obtained with ileal-cannulated pigs and cecectomized cockerels.

Six ileal-cannulated pigs that averaged 100 kg BW and 16 adult cecectomized cockerels that averaged 2.35 kg BW were used to determine apparent and true digestibilities of amino acids (AA) in a complete crystalline AA mixture and in casein. A protein-free (PF) diet was included as a treatment to estimate endogenous AA losses. Fasted cockerels were compared to cockerels fed PF diets for calculation of true digestibility of AA in cockerels. For the AA diet, true digestibility of indispensable AA in the pig ranged from a low of 97.2% for leucine to a high of 100.5% for arginine (Arg). True digestibility of indispensable AA in casein ranged from a low of 93.5% for isoleucine (Ile) to a high of 99.9% for Arg. Correcting for endogenous losses increased digestibilities of lysine (Lys) and threonine (Thr) in pigs fed the AA diet by 2.4 and 7.1%, respectively, and increased Lys and Thr digestibility in pigs fed the casein diet by 1.8 and 6.1%, respectively. Feeding a PF diet to chickens to correct for endogenous losses resulted in higher true digestibility values for all AA with the exception of tryptophan, methionine, and Arg than those obtained using fasted animals. True digestibilities of Thr were 88.3 and 86.6% for AA and casein diets, respectively, using fasted controls but were 97.5 and 94.5% when the PF control was used. Proline digestibility was increased (P < .05) substantially in both pigs and cockerels when the PF control was used to correct for endogenous AA losses. Regardless of species, Ile in casein had a lower true digestibility value than any other indispensable AA. The results of this study indicated that true digestibilities of AA in a mixture of crystalline AA and in casein are essentially 100% in both pigs and cockerels.