Re-evaluation of crude fibre and acid-insoluble ash as inert markers, alternative to chromic oxide, in digestibility studies with rainbow trout ( Oncorhynchus mykiss)

Abstract

The validity of crude fibre and acid-insoluble ash as inert markers as alternatives for chromic oxide was tested for digestibility studies in rainbow trout. Six practical diets were assayed: one of fish meal (C), and five in which 40% of the fish meal protein was replaced by corn gluten meal (CGM), cottonseed meal (CSM), lupine seed meal (LSM), soybean meal (SBM) or sunflower meal (SFM) on a crude protein basis. The apparent digestibility coefficients for crude protein, NFE, dry matter and gross energy were calculated using crude fibre, acid-insoluble ash and Cr 2O 3 as inert markers. For all diets, when acid-insoluble ash was used as the marker, results for the apparent digestibility coefficients for crude protein, NFE, dry matter and gross energy were higher ( P<0.05) than those based on Cr 2O 3. All apparent digestibility coefficients for the control, corn gluten meal, cottonseed meal and lupine seed meal diets calculated with crude fibre as the marker were similar to those based on Cr 2O 3, whereas all results based on crude fibre for the soybean meal and sunflower meal diets, compared with those obtained with chromic oxide, were lower ( P<0.05). Based on these results, if one considers chromic oxide as the reference marker, then acid-insoluble ash is not a suitable marker for digestibility studies in rainbow trout. With respect to crude fibre, our results suggests that this substance can be an effective endogenous marker, although attention should be paid to the type of crude fibre present in the diets.