Significance of amount and form of dietary selenium on blood, milk, and casein selenium concentrations in grazing cows

Organic selenized yeast enriched with selenoamino acids or inorganic sodium selenate (Na2SeO4) was administered per os three times weekly as a drench for 133 d to previously unsupplemented cows that were grazing low Se pastur es. Treatment groups received the equivalent of 2 or 4 mg of Se/d of either supplement form. Control cows did not receive a drench. Samples of blood a nd milk were collected regularly throughout the trial. Selenium concentrati ons in blood, milk, casein, and liver and glutathione peroxidase activity i n blood and liver are reported as responses per milligram of Se intake. Mea n blood Se concentrations in treated cows increased steadily and, by d 133, were 4.7 to 8.8 times that in controls. Selenized yeast was 2 to 3 times m ore effective than was Na2SeO4, and low Se intakes were 27% more efficient per milligram of Se administered than were high Se intakes at increasing mi lk Se concentration. Casein Se content mirrored that of milk; among all tre ated and control cows and throughout the trial, the molar ratio of Se in ca sein as a percentage of the Se in whole milk was constant at 71 +/- 1.2%. T he Se concentration in liver biopsies taken on d 133 was indicative of tota l Se intake during the trial and ranged from 920 to 3920 nmol of Se/kg of f resh weight. These results demonstrate the differing efficacy of organic an d inorganic Se dietary supplements to increase dairy cow Se status and to e nhance Se content of milk and casein.