Sixteen of approximately 500 yearling steers died of acute selenium (Se) toxicosis after grazing on a Se-contaminated range for only a few days. Field studies and chemical analyses identified the predominant toxic plant as western aster (Symphyotrichum ascendens, previously Aster ascendens), which contained over 4,000 ppm Se (dry weight). Several dead animals that were necropsied had acute severe myocardial necrosis characterized by edema and myocyte swelling, with hypereosinophilia, clumping, and coagulation of myocardial proteins. Whole blood from 36 surviving steers was collected and analyzed, and 10 steers with elevated Se concentrations were selected for close monitoring and clinical evaluations. Each steer was weighed, and serum, blood, liver, skeletal muscle, and hair were regularly collected after removal from the Se-contaminated range. One animal that died 18 days after exposure was necropsied and exhibited severe multifocal myocardial fibrosis with extensive hepatic congestion, degeneration, and hemosiderosis. At 180 days postexposure, 2 of the 10 steers were euthanized, and tissue samples were collected. Both steers had rare, small fibrotic foci in their hearts. The Se elimination half-lives from serum, whole blood, liver, and muscle of the recovering steers were 40.5 ± 8.2, 115.6 ± 25.1, 38.2 ± 5.0, and 98.5 ± 19.1 days, respectively. The Se concentration in hair reached a peak of 11.5 ± 5.3 ppm at 22 days postexposure. The findings indicate that cattle are sensitive to acute Se toxicosis caused by ingestion of Se-accumulator plants, with myocardial necrosis as the primary lesion. Some poisoned animals may develop congestive heart failure weeks after the toxic exposure, and in the current study, Se was slowly excreted requiring a relatively long withdrawal time.

Background and Aims Selenium accumulation in vegetation has resulted in toxicity in livestock grazing on phosphate mine soils in Southeastern Idaho. Methods Plant and soil samples were collected from sites located near phosphate mines. Soil physicochemical properties, Se speciation, and Se distribution from a sequential extraction procedure (SEP) were examined in relation to bioavailability in the Se-hyperaccumulator, western aster (Symphyotrichum ascendens Lindl.). Selenium-hyperaccumulators are plants that can absorb over 1000 mg Se kg⁻¹ _DM (Dry Matter). Results Chemical analyses revealed that western aster contained Se exceeding 6000 mg kg⁻¹ _DM. Soil speciation results indicated that selenite (SeO₃ ²⁻) was dominant with lower levels of selenate (SeO₄ ²⁻) present. This was expanded using an SEP that accounted for six fractions. Regression analyses indicated a strong relationship for western aster Se and the water-soluble and phosphate-extractable SEP fractions combined (R ² = 0.85). Once carbonate, amorphous Fe-oxide, organic, and residual Se fractions were factored into the analysis, the relationship decreased. A strong relationship between selenate and the water-soluble Se fraction was also observed (R² = 0.83). Conclusions Soluble and phosphate-extractable Se were determined to be “bioavailable fractions” for western aster. Thus, simple water extractions can be used for quick assessment of Se bioavailability and provide a means to identify potentially hazardous areas locations.