Comparison of chromic oxide, crude fibre, polyethylene and acid-insoluble ash as dietary markers for the estimation of apparent digestibility coefficients in rainbow trout

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A 14-day digestibility trial was conducted to compare the use of three external dietary markers (chromic oxide, polyethylene and acid-washed sand) and a natural internal dietary marker (crude fibre) for the estimation of apparent nutrient digestibility in rainbow trout fed a practical diet. The three external markers were evaluated at three dietary concentrations (0.5%, 1% and 2%). Faecal samples were collected by hand stripping individual fish and apparent dry matter, organic matter, crude protein and ash digestibility coefficients were calculated on the basis of the individual markers tested. Of the four dietary markers evaluated, chromic oxide and crude fibre were found to be the most reliable in terms of reproducibility of the observed nutrient digestibility coefficients within and between individual dietary treatments. However, although the crude fibre content of the experimental diets remained relatively constant, there was a significant increase (P<0.05) in nutrient digestibility at the 2% chromic oxide inclusion level. Both acid-insoluble ash and polyethylene displayed erratic and significantly reduced (P<0.05) nutrient digestibility coefficients over the range of dietary inclusion levels tested (with the exception of 1% polyethylene). The results are discussed in the light of previous investigations on the use of dietary markers within fish digestibility trials, and suggest that crude fibre may offer particular promise as a natural internal marker for use under practical farming conditions for the estimation of nutrient digestibility.