Selenium supplementation and selenium status of dairy cows fed diets based on grass, grass silage or maize silage.

In three separate trial series (TS) the effect of diet composition on selenium (Se) status of dairy cows were investigated. Diets were formulated based mainly on grass (TS1), grass silage (TS2) or maize silage (TS3) with different levels of Se supplementation. Each TS comprised a total of 30 dairy cows and contained one treatment group without Se supplementation (control) and two groups with increasing levels of Se supplementation (levels 1 and 2). Selenium was administered as Na-selenite. The control groups of the different TS showed a very low Se supply of 38-54 microg Se/kg DM. At level 1 the Se supply was increased to 102-165 microg Se/kg DM and at level 2 was 294-373 microg Se/kg DM. After completion of the 6-week trials the average plasma Se concentration of the control cows (without Se supplementation) across all TS was 21.5 microg/l; this increased significantly following Se supplementation, to 37.7 microg/l at level 1 and 61.5 microg/l at level 2. The plasma glutathione peroxidase (GSH-Px) activity of the control cows averaged 67 U/l, rising considerably after supplementation at level 1 to a value of 101 U/l, but showed little further increase at level 2 with a mean value of 120 U/l. By contrast, the average Se content of the milk was unchanged in the control and level 1 groups at 10.5 microg/kg and 10.9 microg/kg, respectively, and only increased markedly after supplementation at level 2 to a mean value of 15.1 microg/kg. The diet based on maize silage, while having a similar Se content as the grass and grass silage-based diets, resulted in a slightly improved Se status, which is due to a higher Se intake from soybean meal.