Comparison of selenium status in sheep after short-term exposure to high-selenium-fertilized forage or mineral supplement

Sheep grazed in Se-deficient areas without access to Se supplements may be Se deficient by the end of the grazing season. One limitation to feeding mineral mixes and feeds containing inorganic Se-supplements is the short duration of Se storage in the animal. Another is that Se supplementation may be limited by country-specific regulations. However, the use of feedstuffs naturally high in Se to deliver surpranutritional levels of Se is not regulated. The purpose of this study was to evaluate Se status in sheep after short-term exposure to high-Se-fertilized forage vs. a commonly used mineral supplement containing inorganic sodium selenite. Selcote Ultra^® was mixed with nitrogen fertilizer in the form of urea and applied to pasture at a rate of 3.4 kg Selcote Ultra^®/ha and 67.4 kg nitrogen/ha. Thirty ewes were randomly divided into two groups. One group of 15 ewes grazed Se-fertilized pasture for 40 days and had no mineral supplementation. The other group of 15 ewes grazed a non-Se-fertilized pasture and received a custom made mineral supplement containing 200 mg/kg sodium selenite for 40 days. Subsequently the two groups of ewes were combined and grazed a non-Se-fertilized pasture and had no mineral supplementation. Sheep were bled pre- and post-treatment and then every 2-4 weeks thereafter for approximately 9 months to assess whole-blood Se concentrations. Whole-blood Se concentrations were higher (P < 0.0001) immediately post-treatment in sheep grazing Se-fertilized forage (573 ± 20 ng/ml) compared to sheep receiving mineral supplement containing Se (286 ± 20 ng/ml), and were still higher (P < 0.0001) at the end of 9 months (97 ± 7 ng/ml vs. 61 ± 7 ng/ml, respectively). Whole-blood Se concentrations were within the normal reference interval (150-500 ng/ml) for a longer period of time in sheep grazing Se-fertilized forage (7 months) compared to sheep receiving mineral supplement containing inorganic Se (4 months). No sheep showed clinical signs of ill-effects from Se supplementation. In conclusion, short-term exposure of sheep to Se-fertilized forage results in whole-blood Se concentrations sufficient to maintain adequate concentrations throughout grazing periods when there is limited access to Se supplements.