This study aimed to determine the effects of either dietary Se source or dose on dairy cow Se status, milk yield and subsequent milk characteristics. Forty lactating cows, offered the same basal diet, were blocked by DIM, milk yield and parity and then randomly allocated to 1 of 5 dietary treatments: negative control (CTRL; 0.098 mg of Se kg−1 DM), 2 Se yeast supplementation (SY03 and SY05, containing 0.31 and 0.50 mg of total Se kg−1 DM, respectively), and 2 sodium selenite supplementation (SS03 and SS05, containing 0.31 and 0.50 mg of total Se kg−1 DM, respectively). During the trial (lasted 140 d), whole blood, plasma, and milk were analyzed for total Se, and whole blood for glutathione peroxidase activity. The proportion of total Se as selenomethionine (SeMet) or selenocysteine (SeCys) in whole blood and milk samples was also determined. Milk constituent, SCC, technological properties and keeping quality were measured on milk samples. Neither source nor dose was seen to affect milk yield, SCC, or milk characteristics. Glutathione peroxidase activity was greater (Pb0.001) in Se supplemented cows than CTRL but no effect of source was observed. Total Se in whole blood, plasma and milk was greater (Pb0.001) in Se yeast than selenite, with higher asymptotic values in Se yeast, and also a greater ratio between Se in milk and Se in blood or plasma (Pb0.001). The time to asymptotic value of total Se in blood and milk was not affected by Se source, although time to asymptotic total Se in plasma was greater in Se yeast than selenite (Pb0.05). The 16.3% of supplemental Se was transferred into milk of SY05 animals whereas only 3.2% in SS05 supplemented animals. In blood SeCys was the main form of Se, and SeMet the main Se form in milk. In blood and in milk SeMet increased only in those animals supplemented with Se yeast, and at the end of the supplementation period values were greater in Se yeast animals than selenite (Pb0.001). These results demonstrate greater levels of Se in the blood and plasma of cows provided Se yeast compared with cows provided selenite, and a greater efficiency of transfer of Se from blood into milk resulting in a greater increase in milk Se concentration. Selenium source affected the proportion of total Se comprised as SeMet and SeCys in blood and milk, indicating further that different Se sources are metabolized by distinctly different mechanisms.
- Dairy cows