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Despite the availability of selenium (Se)-enriched trace mineral supplements, we have observed low Se status in cattle and sheep offered traditional inorganic Se supplements. Reasons for this may include inadequate intake or low bioavailability of inorganic Se sources. The objective of this study was to determine whether rumen microorganisms (RMO) alter the bioavailability of Se sources commonly used in Se supplements. Rumen microorganisms were isolated from ewes (n = 4) and incubated ex vivo with no Se (control), with inorganic Na selenite or Na selenate, or with organic selenomethionine (SeMet). Total Se incorporated into RMO and the amount of elemental Se formed were determined under equivalent conditions. Incorporation of Se from Na selenite, Na selenate, or SeMet into RMO was measured as fold change compared with control (no added Se). Incorporation of Se into microbial mass was greater for SeMet (13.2-fold greater than no-Se control) compared with inorganic Se supplements (P = 0.02); no differences were observed between inorganic Na selenate (3.3-fold greater than no-Se control) and Na selenite (3.5-fold greater than no-Se control; P = 0.97). Formation of non-bioavailable, elemental Se was less for RMO incubated with SeMet compared with inorganic Se sources (P = 0.01); no differences were observed between Na selenate and Na selenite (P = 0.09). The clinical importance of these results is that the oral bioavailability of organic SeMet should be greater compared with inorganic Se sources because of greater RMO incorporation of Se and decreased formation of elemental Se by RMO.

White snakeroot ( Ageratina altissima ) and rayless goldenrod ( Isocoma pluriflora ) can cause "trembles" and "milk sickness" in livestock and humans, respectively. Tremetol, a complex mixture of sterols and derivatives of methyl ketone benzofuran has been extracted from white snakeroot and rayless goldenrod and is reported to be the toxic substance in plant material. In this study, the three major benzofuran ketones, tremetone, dehydrotremetone, and 3-oxyangeloyl-tremetone, were isolated from rayless goldenrod. Using these compounds as standards, a quantitative high-performance liquid chromatography (HPLC) method was developed to measure these compounds in white snakeroot and rayless goldenrod. Concentrations of tremetone, dehydrotremetone, and 3-oxyangeloyl-tremetone were found to vary considerably among the different white snakeroot and rayless goldenrod plant collections. Differences in concentrations of tremetone, dehydrotremetone, and 3-oxyangeloyl-tremetone in white snakeroot and rayless goldenrod plants may explain the historical sporadic and unpredictable toxicity of these plants to livestock and humans.