Comparison of yttrium and chromic oxides as inert dietary markers for the estimation of apparent digestibility coefficients in mirror carp Cyprinus carpio fed on diets containing soybean-, maize- and fish-derived proteins

The apparent digestibility coefficients (ADC) of protein, lipid and energy components within three selected plant protein by‐products, i.e. solvent‐extract soybean meal, full‐fat soybean meal and maize gluten meal were evaluated for mirror carp (Cyprinus carpio L.). In a series of feeding and digestibility experiments, the ingredients were incorporated at the rate of 50% within a basal diet formulation, at the expense of a high quality fish‐meal (LT94) source using both yttrium oxide, Y₂O₃ and chromic oxide, Cr₂O₃ as inert markers in test diets. The ADC of nutrients obtained from Cr₂O₃ closely matched those obtained from Y₂O₃. The ADC of protein were generally high, with >91% for both the fish‐meal reference diet and solvent‐extract soybean meal, 86% for full‐fat soybean meal and 79% for maize gluten meal. The ADC by yttrium oxide gave values of 75–90% for protein. Similarly close values were obtained for both markers with respect to lipid ADC (77–91% with chromic oxide and 74–89% with yttrium oxide). It was concluded that the use of Y₂O₃ as an inert marker in digestibility studies with carp ought to be suitable for carp following further validation. This should be considered in view of predicting the digestibility of feeds for this fish species before inclusion in practical diets.