Results and Discussion

Each pasture sample was a composite of 25 subsamples. Forage Se concentration (on a dry matter basis), on samples collected the day the sheep began grazing the pasture (58 days following fertilization), was 1.46 ppm, and on day 40, the day the sheep were removed from the pasture, it was 2.02 ppm. On grazing day 10, pasture Se content was 0.712 ppm. Variation in the measured pasture Se content could be attributed to laboratory error, sampling technique, or errors in dry matter determination.

In this study, whole blood Se concentration was influenced by treatment (sheep grazing Se-fertilized forage 319 ng/ml vs sheep consuming mineral supplement 172 ng/ml, P< 0.0001), collection date (P<0.0001) and their interaction (P<0.0001).

Sheep grazing Se-fertilized forage received 52.4 μg organic Se/kg BW/day. Sheep consuming the mineral supplement received 19.5 μg inorganic Se/kg BW/day. Whole-blood Se concentrations were higher (P<0.0001) immediately post treatment in sheep grazing Se-fertilized forage (573 ± 20 ng/mL) compared with sheep consuming the mineral supplement containing Se (286 ± 20 ng/mL), and remained higher for approximately 9 months (97 ± 7 vs. 61± 7 ng/mL, respectively).  (See figure one).

Whole blood Se concentrations were within the normal reference interval for a longer (P<0.0001) period of time in sheep grazing Se-fertilized forage (243 ± 44 d) compared with sheep that received the mineral supplement containing Se (163 ± 55 d).

The maximum tolerable level for dietary Se in nonruminant (such as swine) diets is considered 2 ppm . In accounting for lower Se bioavailability in ruminants (sheep, etc) compared to nonruminants, the maximum tolerable level is defined as 5 ppm (NRC, 2005). However, some studies indicate that higher dietary Se concentrations can be fed without toxic effects.

Based on the amount of Se safely consumed in these toxicology studies, there appears to be a wide margin of safety between the concentrations of Se applied as fertilizers and subsequently consumed by ruminants in fertilized forage in our study, and the maximum tolerable Se intake for ruminants. In our study, sheep feet were monitored during periodic hoof trimming sessions. There were no obvious clinical signs of selenium toxicity, such as lameness or cracked hooves, indicating that the Se-fertilized forage was non toxic for short- term exposure. Additionally, ewes were evaluated during blood sampling for other signs of selenium toxicity and no symptoms were observed.