MEASUREMENT OF ENDOGENOUS FECAL EXCRETION AND TRUE ABSORPTION OF SELENIUM IN DAIRY-COWS

A method combining balance techniques with an estimate of endogenous fecal selenium (Se) was evaluated as a measure of Se absorption in dairy cows. Endogenous fecal Se was estimated based on tracer enrichment in an index tissue (or fluid) that was assumed to represent enrichment of all endogenous sources of fecal Se. Two nonlactating Holstein cows fed a low-Se diet (0.035 mg Se kg-1 dry matter) were administered 4 mg Se-76, intraruminally, each day for 5 d (days 1-5). After a 10-d equilibration period, total collections of feces and urine were made atr 24-h intervals for two 5-d periods (days 16-20 and days 21-25). On day 26 the animals were sacrificed and samples of all major tissues were collected. Samples were analyzed for total Se and Se stable isotope ratios. Selenium-76 enrichment was similar in tissues considered to be contributors to endogenous fecal Se, which supported the assumption that an index tissue could be used to estimate endogenous fecal excretion of Se. Endogenous fecal Se estimated from tracer enrichment in serum and liver was 22-36% of the total fecal Se excreted. True Se absorption was 10-16% of the daily Se intake.