Selenium uptake by ruminal microorganisms from organic and inorganic sources in dairy cows

The objectives of this study were (i) to determine the amount of time required for microbial Se concentration to reach a plateau and the half-life of Se in the ruminal microorganisms (exp. 1) and (ii) to compare Se uptake by ruminal microorganisms from organic and inorganic sources (exp. 2). In exp. 1, four dry ruminally cannulated Holstein dairy cows (863 ± 98 kg of BW, mean ± SD) housed in a tie-stall facility were used in a completely randomized design with a 14-d adaptation and 9-d measurement period. Cows were offered 10 kg of timothy hay (divided in 12 equal portions and delivered at 2-h intervals) supplemented with 500 g of dry-cow mineral and vitamin premix at 0800 daily. The treatments were inorganic and organic Se that were each enriched with Se stable isotopes (inorganic 82 Se and organic 77 Se Sel-Plex ® ). The Se treatments were supplemented at 0.3 mg kg -1 DM and administered directly into the rumen under the rumen mat every 6 h. Rumen samples (both fluid and solid) were collected before treatment administration every 6 h and the microbial fraction were isolated. On day 9 after the administration of the last dose of tracer, 600 mL of ruminal content was collected every hour for 24 h to determine Se half-life in the ruminal microorganisms. In exp. 2, treatments were administered for only 5 d, i.e., 2 d to reach a plateau in Se concentration and 3 d of collection and measurement. The cows were supplemented with the control premix (no added Se) during the adaptation period and with the organic and inorganic Se premixes during the treatment and measurements period. In exp. 1, microbial 77 Se concentration reached a plateau in 2 d, whereas 82 Se reached a plateau in 1 d. The half-life of Se in the ruminal microorganisms was 16 and 48 h, for 77 Se and 82 Se, respectively. In exp. 2, 77 Se uptake by ruminal microorganisms was higher (P < 0.05) than that for 82 Se and averaged 330 ± 7.7 and 125 ± 3.9 µg kg -1 for 77 Se and 82 Se, mean ± SEM, respectively. The results show that inorganic Se has a lower ruminal microbial uptake than organic Se sources.