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

Trivalent oxides of yttrium and rare earth metals were evaluated as inert m arkers 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) oxi de, Tm2O3, Y2O3 and Yb2O3) were included in a feed fed to rainbow trout. Th e ratio between each marker and Yb2O3 in stripped faeces was used as an ind icator of recovery, Only Er2O3, Ho2O3 and Tm2O3 had lower recoveries than t he 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 comp aring the marker ratios in the feed and in the faeces sieved from the outle t water of the tanks. The results did not reveal any systematic differences in evacuation among the various markers. Experiment 3 compared in vitro so lubility 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, a nd 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 t he dissolved markers precipitated when neutralised, and none of the markers were soluble in water. Experiment 4 compared the estimates of apparent dig estibility coefficients (ADCs) of nitrogen and fat: in rainbow trout when u sing 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 an d stripped faeces were dissolved for the analysis with both HCl:HNO3 and H3 PO4:MnSO4. Except for Cr2O3. the markers gave similar ADCs within each acid solubilisation procedure. The ADCs of fat were similar with both procedure s, 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 H 3PO4, no differences were seen among the ADCs obtained with Cr2O3 and the o ther markers. In conclusion, trivalent metal oxides, such as La2O3, Y2O3 an d Yb2O3, can substitute Cr2O3 in digestibility studies with salmonids, and can be used at lower concentrations without affecting accuracy. (C) 2000 El sevier Science B.V. All rights reserved.