Effects of selenium source and dose on selenomethionine and selenocysteine in milk of dairy cows.

The aim of this study (research funded by Alltech EU Regulatory Affairs Department, trial ref. SEL/DAI/EFF/04/0107/IT; the authors would also like to give special thanks to Ryszard Lobinski, UT2A laboratories, for performing the selenium analyses) was to determine the effect of dietary Se source and dose on selenomethionine (SeMet) and selenocysteine (SeCys) content of milk in dairy cows. The study involved 40 Italian Friesian lactating cows in mid lactation in a randomized complete block design (5 treatments with 8 animals per group): during the 140-day experiment all animals received the same basal diet, which differed only in Se source and dose: SY3 and SY5 (0.31 and 0.50 mg total Se/Kg DM respectively) containing organic Se (Se yeast) produced by Saccharomyces cerevisiae CNCM I-3060 (Sel Plex®); SS3 and SS5 (0.31 and 0.50 mg total Se/Kg DM respectively) containing inorganic Se obtained from sodium selenite; CTRL as negative control (0.09 mg Se/Kg DM). Prior to the start of the study (T0), at 112 (T112) and 140 (T140) d, samples of milk were analyzed (one pooled milk of 8 samples per treatment at T112; two pooled milk of 4 samples per treatment at T0 and at T140) to determine the proportion of total Se comprised as SeMet and SeCys. At T140 the SeMet content of milk was greater (P<0.001) in Se yeast supplemented animals (234 and 366 ng Se/g DM in SY3 and SY5 respectively) than CTRL (93 ng Se/g DM), and than those receiving comparable doses of selenite (96 and 112 ng Se/g DM in SS3 and SS5 respectively). A dose effect with Se yeast was also observed. Conversely, the SeMet content of milk from selenite treatments at T140 did not differ from CTRL. Consequently the proportion of total Se present as SeMet was greater (P<0.05) in Se yeast (56 and 60% of total Se in SY03 and SY05 respectively) than selenite (37 and 40% in SS03 and SS05 respectively) and CTRL (44%). At T140 the SeCys content of milk was greater in Se yeast supplemented animals (44 and 51 ng Se/g DM in SY3 and SY5 respectively) than selenite supplemented (32 and 36 ng Se/g DM in SS3 and SS5 respectively). At T140 the proportion of total Se present as SeCys was, on average, 13% of total Se, without significant differences between treatments. These results indicate that increases in the total Se content of milk obtained from Se yeast supplemented animals when compared to selenite supplemented ones was principally a consequence of the increase in the proportion of total Se present as SeMet, which accounted for 60% of total Se increase.