At seven days, the ratio T3/T4 was equal in foals from both groups. After drinking milk with higher selenium levels, the foals from the mares receiving selenomethionine, showed an increased serum triiodothyronine to thyroxine (T3/T4) ratio but this level was not significant. In further research, it would be interesting to investigate if this increase in T3/T4 ratio would have a significant effect in the long term.
Osteocalcin and bone specific alkaline phosphatase (bone- ALP) are markers for the bone formation in young animals. Although the osteocalcin levels were not different in foals at 30 days of age, bone-ALP was significantly higher in the foals from the mares fed with selenomethionine. Interestingly, in an earlier trial it was shown that supplementation of selenium in combination with iodine to mares increased as well the bone- ALP as osteocalcin in their foals. These results suggest that selenium, probably in combination with iodine, has a positive influence on bone development in growing foals.
Selenium in the metabolism
In the animal metabolism there is a difference between selenomethionine and all other forms of selenium. All selenium compounds are recognised for the supply of selenium and all can be used for selenoproteins (selenoenzymes) synthesis. Beside this general pathway, selenomethionine is utilised in the body as an amino acid and can be built into body proteins instead of methionine. Via this specific pathway, selenomethionine is able to build up selenium reserves in the body and is able to transfer selenium to the offspring through colostrum and milk. The selenium reserve can be mobilised for later selenoprotein synthesis. An adequate selenium reserve ensures an optimal selenium and antioxidant status at all times, even in times of stress or low feed intake. The beneficial effect of higher milk selenium is shown in the horse trial discussed above. Compared to inorganic sodium selenite, supplementation of mares with selenomethionine improved selenium and antioxidant status of their foals and seems to have a positive influence on bone development.