Summary The dairy cows at the Estonian Agricultural University appeared to have an extremely low selenium status. The selenium level was 5.6 μg/l in whole blood and 3.2 μg/l in milk, on average. The blood glutathione peroxidase was consequently extremely low. The effects of organic selenium (selenized yeast) and sodium selenite were compared in a feeding experiment on 100 dairy cows. Selenium incorporation, udder health and the in vitro function of blood neutrophils were monitored. Supplementation of the feed either with 0.2 ppm organic selenium or sodium selenite for 8 weeks, increased the blood selenium level (geometric mean) within this period from the background level (about 5.6 μg/l) to 167 (Se‐yeast) and to 91 μg/l (selenite). The respective change in whole blood glutathione peroxidase (GSH‐PX) was from 0.22 to 3.0 (Se‐yeast) and to 2.3 (selenite) μKat/g Hb. Blood GSH‐PX continued to increase up to 10 weeks after the supplementation was stopped. The bioavailability of yeast selenium was superior to selenite: the relative bioavailability (selenite = 1) of yeast selenium was 1.4 if blood GSH‐PX, 1.9 if blood selenium, and 2.7 if milk selenium was used as the response criterion. Selenium‐supplementation showed a positive effect on udder health. The percentage of quarters harbouring mastitis pathogens dropped from 22.9 to 13.0 in the Se‐yeast group and from 18.4 to 7.4 in the selenite group during the supplementation period. The effect of selenium on mastitis was also reflected as a decrease in the output of milk somatic cells and N‐acetyl‐β‐D‐glucosaminidase (NAGase). The time‐luminescence profile of zymosan‐induced activity of blood neutrophils became skewed to the left in Se‐supplemented cows.

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The Se content of aquatic organisms was lowest in aquatic plants, varying from 0.02 to 0.14 ppm (dry wt). Plankton samples contained from 1.1 to 2.4 ppm, fishes cultured in ponds 0.5 to 0.9 ppm, and fishes from natural environments 1.0 to 2.9 ppm. The phytoplankter, Scenedesmus dimorphus, actively concentrated 75Se‐selenomethionine, but neither actively nor passively concentrated inorganic selenite. The zooplankton, consisting mostly of Daphnia pulex, absorbed 75Se from selenite. In aquariums fish concentrated only a small amount of organic or inorganic Se directly from water, but did concentrate Se from food.

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