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Zearalanol Metabolism in Steers^1,2

Washington State University, Pullman 99163

Abstract

A tracer study was conducted with tritium-labeled zearalanol to study the pathway and rate of excretion of zearalanol from implanted pellets. Samples of the feces, urine and plasma were taken at 48-hr, intervals from five steers implanted with 72 mg of labeled zearalanol (186 µCi). Tissue samples from 12 different sites were taken from animals sacrificed at intervals of 65, 80, 95, 110 and 125 days post-implantation.

Zearalanol was released slowly from the pellets and was rapidly cleared from the plasma. No residual label was found in any of the major edible tissues at any of the intervals studied. Approximately 10% of the implanted drug was excreted through the urine, 45% was eliminated in the feces, and 10% remained as residual pellets. The liver appears to remove the compound from the blood after which the bile excretes it into the gut.

Residual compact pellets were encapsulated by non-vascular tissue which apparently reduced the release rate of the residual zearalanol during the latter stages of the experiment.

¹ Scientific Paper No. 3653, College of Agriculture, Washing State University, Project 1217.

² Appreciation is expressed to Commercial Solvents Corporation for partial support of this study.

³ Present address: Division of Biological Science, Fort Hays State College, Fort Hays, Kansas.