Evaluation of different markers to determine apparent nutrient digestibility coefficients of feed ingredients for South African abalone ( Haliotis midae L.)

Chromic oxide (Cr 2O 3), acid-insoluble ash (AIA) and crude fibre were evaluated as inert markers in digestibility studies with Haliotis midae by calculating apparent dry matter, organic matter, energy, protein and amino acid availability for fish meal, soybean meal and cottonseed meal included at 30% into a reference diet. AIA was the only marker that yielded consistent realistic apparent digestibility coefficients (ADCs), which were comparable ( P>0.05) to values derived through total collection. Both Cr 2O 3 and crude fibre in faeces were either lower or similar to their respective levels in feed, resulting in negative ADCs. Leaching of markers from the diets did not have an effect ( P>0.05) on digestibility coefficients. No difference ( P>0.05) was found in apparent dry matter digestibility between faeces voided either during the night or daytime. Washing of faeces with distilled water after collection had no influence ( P>0.05) on AIA content. In all test ingredients, mean apparent amino acid availability reflected apparent protein digestibility. Apparent digestibility of protein in fish meal (76%) by H. midae was within the range of values reported for fish species (62–91%). Soybean meal is a promising feed ingredient in abalone feeds due to the high apparent digestibility of its organic matter (79%), energy (83%), protein (85%) and both essential and non-essential amino acids (mean of 86%).