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True absorption of Se in dairy cows was determined using a method combining balance techniques with isotopic enrichment of body Se pools. Oral and intravenous routes of isotope administration and tracer enrichment in serum and liver were evaluated for determining endogenous fecal Se. The effect of supplemental dietary Cu on Se absorption was also determined. In two trials, five and six nonlactating Holstein cows were supplemented with 0 or 16 mg Cu kg −1  feed dry matter. The basal diet was cubed orchardgrass hay that contained 0.19 mg Se kg −1  (0.18 mg kg −1  as added selenite) and 14 mg Cu kg −1 . Each cow received Se-77 orally and Se-82 intravenously on day 1. Feces, urine and serum samples were collected during two 5-d balance periods (days 16–20 and days 21–25). Liver biopsies were taken on day 27. Endogenous fecal Se determined from tracer enrichment in serum and liver with oral isotope administration and from enrichment in serum with intravenous isotope administration were not different (P &gt; 0.05). It was concluded that any one of these approaches could be used to estimate endogenous fecal Se and true Se absorption. There was no effect of Cu on Se absorption (P &gt; 0.05). True absorption of Se was 11 ± 1.5% of the Se intake. Key words: Selenium, copper, stable isotope, endogenous excretion, absorption, dairy cows

True absorption of selenium in dairy cows: Stable isotope tracer methodology and effect of dietary copper