Evaluation of selected trivalent metal oxides as inert markers used to estimate apparent digestibility in salmonids

Summary

Trivalent oxides of yttrium and rare earth metals were evaluated as inert markers in apparent digestibility studies with salmonids in four experiments. Tn Experiment 1, 100 mg kg(-1) of each of 15 oxides (Dy2O3, Er2O3, Eu2O3, Gd2O3, Ho2O3, La2O3, Lu2O3, Nd2O3, Pr2O3, Sc2O3, Sm2O3, Tb(III and IV) oxide, Tm2O3, Y2O3 and Yb2O3) were included in a feed fed to rainbow trout. The ratio between each marker and Yb2O3 in stripped faeces was used as an indicator of recovery, Only Er2O3, Ho2O3 and Tm2O3 had lower recoveries than the other markers. Experiment 2 compared the excretion rates of Cr2O3 and of the selected alternative markers (La2O3, Y2O3 and Yb2O3). A feed with 7.5 g kg(-1) of Cr2O3 and 750 mg kg(-1) of each of the other markers was fed to Atlantic salmon for a period of 1 week. Thereafter, the fish were fed with a marker-free feed, and gastro-intestinal evacuation was evaluated by comparing the marker ratios in the feed and in the faeces sieved from the outlet water of the tanks. The results did not reveal any systematic differences in evacuation among the various markers. Experiment 3 compared in vitro solubility of Cr2O3, Dy2O3, La2O3, Y2O3 and Yb2O3 in weak acid (HCl, pH 3 as in stomach contents of Atlantic salmon), weak acid neutralised with NaOH, and in water. Cr2O3 was not dissolved. Only 1.3% of Yb2O3, 22% of Y2O3, 31% of Dy2O3, and 96% of La2O3 was soluble in weak acid, but more than 99% of the dissolved markers precipitated when neutralised, and none of the markers were soluble in water. Experiment 4 compared the estimates of apparent digestibility coefficients (ADCs) of nitrogen and fat: in rainbow trout when using Cr2O3, La2O3, Y2O3 and Yb2O3 as markers. The feed contained 10 g kg(-1) Cr2O3 and 100 mg kg(-1) of each of the other markers. Markers in feeds and stripped faeces were dissolved for the analysis with both HCl:HNO3 and H3PO4:MnSO4. Except for Cr2O3, the markers gave similar ADCs within each acid solubilisation procedure. The ADCs of fat were similar with both procedures, but the ADCs of nitrogen were 0.2% lower with HCl:HNO3 than with H3PO4. Cr2O3 was incompletely dissolved in HCl:HNO3, resulting in low ADCs. With H3PO4, no differences were seen among the ADCs obtained with Cr2O3 and the other markers. In conclusion, trivalent metal oxides, such as La2O3, Y2O3 and Yb2O3, can substitute Cr2O3 in digestibility studies with salmonids, and can be used at lower concentrations without affecting accuracy. (C) 2000 Elsevier Science B.V. All rights reserved.