Comparison of internal and external markers in digestibility studies involving the Australian freshwater crayfish, Cherax destructor Clark (Decapoda, Parastacidae)

Apparent dry matter and protein digestibility coefficients (DMD and PD, respectively) were determined using several different marker compounds [Cr 2 O 3 , ash, crude fibre (CF), hydrolysis resistant ash (HRA) and hydrolysis resistant organic matter (HROM)] for nine different pelleted diets fed to adult Australian freshwater crayfish, Cherax destructor Clark. For a soybean-meal-based diet (Soy-60), HRA resulted in the highest DMD and PD coefficients (98.6 and 99.6%, respectively), while the lowest values were obtained by using CF (DMD, 74.3%; PD, 91.6%). For the series of diets tested, digestibility coefficients determined using Cr 2 O 3 were significantly higher than those obtained using ash or CF, and lower than those obtained using HRA and HROM. Using ash as the marker resulted in a mean reduction in DMD and PD of 12.4% and 5.5%, respectively, compared to Cr 2 O 3 . On average, coefficients obtained using CF were 11.8% and 4.2% lower, respectively, than the values obtained using Cr 2 O 3 . Despite an overall reduction in the digestibility coefficients estimated using ash, the ash-ratio technique provided a reasonable estimate of nutrient digestibility for some diets. Based on the ease and speed of determination, it is proposed that dietary ash is suitable for use as a digestibility marker in field situations.