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 relative activity of a zinc-containing cyclic AMP phosphodiesterase towards the (Sp)- compared with the (Rp)-diastereoisomer of cyclic adensine phosphorothioate varied with the identity of the free bivalent metal ion from more than 35 to 0.074 along the series Mg2+ greater than Mn2+ greater than Co2+ greater than Zn2+ greater than Cd2+, showing that this ion, and not the tightly bound zinc, bonds to the phosphorothioate moiety of the substrate.

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