Influence of caecectomy and dietary protein concentration on apparent excreta amino acid digestibility in adult cockerels.

1. The influence of caecectomy and dietary crude protein content (50, 100, 150, 200 and 250 g/kg diet) on the apparent excreta amino acid digestibilities in adult cockerels fed on semi-purified diets containing soyabean meal (SBM) or cottonseed meal (CSM) as the sole source of protein was investigated. 2. Caecectomy had no influence on the apparent digestibilities of most amino acids in SBM, the exceptions being histidine, arginine and lysine, but lowered the apparent digestibility of amino acids in CSM. 3. The protein content in assay diets has a strong influence on the apparent amino acid digestibility values for poultry. Apparent values are lower when the dietary protein content is low. 4. The use of a single endogenous amino acid output value, generated by regression analysis or protein-free diets, for true digestibility adjustments across a wide range of protein intakes is questioned. In particular, such an application will penalise estimates of apparent digestibility when the dietary protein content is high.