Influence of differential movement of the marker chromic oxide and nutrients on digestibility estimations in the Australian freshwater crayfish Cherax destructor

Faeces were obtained from the Australian freshwater crayfish C. destructor (commonly called the yabby) using a faecal collection system. Daily samples were obtained using 2 different protocols: an 8 h feed immediately followed by a single 4 h collection period, and a 2 h feed followed by 2 separate faecal collections at 10 and 14 h thereafter. Faecal samples using the latter protocol were either analysed separately (as early and late collections) or pooled prior to analysis. The composition of the faeces varied according to the time of deposition. The faecal chromium, protein and ash content were 788, 133 and 244% higher, respectively, in early deposited material compared to faeces collected 4 h later. In contrast the late faecal sample contained about 46% more fibre. The faecal collection period (early versus late) and the collection protocol (single versus pooled) strongly influenced the digestibility estimations. Dry matter (DMD) and protein (PD) digestability coefficients were 14.1 and 61.1% higher, respectively, for the early deposited faeces compared to the late samples. Ash (AD) and crude fibre (CFD) digestibility coefficients were negative for the late deposited faeces and highly positive for the early samples. The chromium and nutrient content of pooled (early + late) samples, and the resultant digestibility coefficients, were arithmetically located between those values for the early and late collections. The single collection protocol resulted in significantly lower dry matter, protein, ash and crude fibre digestibility coefficients (10.1, 2.8, 13.1, and 54.1%, respectively), than for the pooled collection protocol. C. destructor may be capable of selectively processing certain dietary components whereby low nutrient value foods are rapidly transported through the gut. Despite the limitation of using Cr 2 O 3 it is proposed that the errors associated with its use are small providing the majority of the deposited faecal matter is collected.