1st Page

Journal of Animal Science - Article



This article in JAS

  1. Vol. 90 No. 2, p. 568-576
    Received: Mar 18, 2011
    Published: January 20, 2015

    2 Corresponding author(s):


Organic and inorganic selenium: I. Oral bioavailability in ewes1

  1. J. A. Hall 2,
  2. R. J. Van Saun,
  3. G. Bobe‡§,
  4. W. C. Stewart33,
  5. W. R. Vorachek*,
  6. W. D. Mosher,
  7. T. Nichols,
  8. N. E. Forsberg and
  9. G. J. Pirelli
  1. Department of Biomedical Sciences, College of Veterinary Medicine, Oregon State University, Corvallis 97331;
    Department of Veterinary and Biomedical Sciences, College of Agricultural Sciences, Pennsylvania State University, University Park 16802; and
    Department of Animal Sciences, College of Agriculture, and
    Linus Pauling Institute, Oregon State University, Corvallis 97331


Although the essentiality of dietary Se for sheep has been known for decades, the chemical source and Se dosage for optimal health remain unclear. In the United States, the Food and Drug Administration (FDA) regulates Se supplementation, regardless of the source of Se, at 0.3 mg of Se/kg of diet (as fed), which is equivalent to 0.7 mg of Se/d or 4.9 mg of Se/wk per sheep. The objectives of this study were to evaluate the effects of Se source (inorganic vs. organic) and supplementation rate (FDA vs. supranutritional rates of 14.7 and 24.5 mg of Se/wk) on whole-blood (WB) and serum-Se concentrations. Mature ewes (n = 240) were randomly assigned to 8 treatment groups (n = 30 each) based on Se supplementation rate (4.9, 14.7, and 24.5 mg of Se∙wk−1∙sheep−1) and source [Na-selenite, Na-selenate (4.9 mg/wk only), and organic Se-yeast] with a no-Se control group (0 mg of Se/wk). Treatment groups were balanced for healthy and footrot-affected ewes. For 1 yr, ewes were individually dosed once weekly with 0, 4.9, 14.7, or 24.5 mg of Se, quantities equivalent to their summed daily supplementation rates. Serum- and WB-Se concentrations were measured every 3 mo in all ewes; additionally, WB-Se concentrations were measured once monthly in one-half of the ewes receiving 0 or 4.9 mg of Se/wk. Ewes receiving no Se showed a 78.8 and 58.8% decrease (P < 0.001) in WB- (250 to 53 ng/mL) and serum- (97 to 40 ng/mL) Se concentrations, respectively, over the duration of the study. Whole-blood Se decreased primarily during pregnancy (−57%; 258 to 111 ng/mL) and again during peak lactation (−44%; 109 to 61 ng/mL; P < 0.001). At 4.9 mg of Se/wk, Se-yeast (364 ng/mL, final Se concentration) was more effective than Na-selenite (269 ng/mL) at increasing WB-Se concentrations (P < 0.001). Supranutritional Se-yeast dosages increased WB-Se concentrations in a dose-dependent manner (563 ng/mL, 14.7 mg of Se/wk; 748 ng/mL, 24.5 mg of Se/wk; P < 0.001), whereas WB-Se concentrations were not different for the Na-selenite groups (350 ng/mL, 14.7 mg of Se/wk; 363 ng/mL, 24.5 mg of Se/wk) or the 4.9 mg of Se/wk Se-yeast group (364 ng/mL). In summary, the dose range whereby Se supplementation increased blood Se concentrations was more limited for inorganic Na-selenite than for organic Se-yeast. The smallest rate (FDA-recommended quantity) of organic Se supplementation was equally effective as supranutritional rates of Na-selenite supplementation in increasing WB-Se concentrations, demonstrating the greater oral bioavailability of organic Se.


The bioavailability of Se is not straightforward because of large variations in Se content of foods (determined by a combination of geographical and environmental factors) and chemical forms in which it may be absorbed and metabolized (Fairweather-Tait et al., 2010). Selenium is normally present in the diet in organic forms [e.g., as selenomethionine (SeMet) or selenocysteine (SeCys; Whanger, 2002)]. Inorganic Na-selenite and Na-selenate are present in the diet in very small amounts. In general, organic forms are absorbed and retained more readily by ruminants than inorganic forms (Qin et al., 2007).

Current FDA (2009) regulations allow Se to be added to sheep diets as Na-selenite, Na-selenate, or Se-yeast (FDA, 2005) in complete feeds not to exceed 0.3 mg of Se/kg of diet (as fed), or in supplements for limit feeding not to exceed 0.7 mg of Se/d per sheep. There is interest in supranutritional supplementation (nontoxic doses greater than that required to support the maximal expression of selenoenzymes; above 0.7 mg of Se/d but less than 5 mg/kg of diet as fed) relative to health, performance, and disease prevention in animals and humans (Rayman, 2008; Zeng and Combs, 2008; Fairweather-Tait et al., 2010).

Several studies have examined how dietary Se alters blood Se concentrations in ewes using various chemical sources and Se dosages (Davis et al., 2006; Steen et al., 2008; Taylor et al., 2009). Less is known how different chemical forms of Se at comparative dosages alter blood Se concentrations and whether production stage may modify blood Se concentrations. Our hypothesis was that organic Se-yeast has greater oral bioavailability than inorganic forms of Se, dosed at the allowed rate (0.3 mg of Se/kg of diet = 4.9 mg of Se/wk) and at supranutritional concentrations (0.9 mg of Se/kg of diet = 14.7 mg of Se/wk; and 1.5 mg of Se/kg of diet = 24.5 mg of Se/wk), as determined by changes in WB- and serum-Se concentrations in sheep grazing in a region known to have soils deficient in Se.


The experimental protocol was reviewed and approved by the Oregon State University Animal Care and Use Committee.

Animals and Study Design

This was a prospective, placebo-controlled clinical trial of 12-mo duration involving 240 mature ewes from 3 genotypes (136 Polypay, 67 Suffolk, 37 crossbred). Ewes ranged in age and BW from 2 to 6 yr and 51 to 93 kg, respectively. The experiments were conducted at the Oregon State University Sheep Center, Corvallis.

Ewes in treatment groups were stratified by age and footrot (FR) status (120 healthy and 120 with FR; the latter also were stratified by FR severity) and were randomly assigned to 1 of 8 treatment groups (n = 30 each) based on Se supplementation rate: 0 (no-Se), 4.9 mg/wk (1× the FDA allowed supplementation rate), 14.7 mg/wk (3× the FDA allowed supplementation rate), or 24.5 mg/wk (5× the FDA-allowed rate) and Se source: organic Se (Se-yeast), inorganic Na-selenite, or inorganic Na-selenate (4.9 mg/wk only). All dosing levels were less than the maximal tolerable level (5 mg/kg) for ruminants (NRC, 2005). The proportion of sheep with FR was intentionally set at 50% because the study was designed, in part, to examine the effect of various chemical forms and Se dosages on FR incidence and immune function. Those results, including the effects of Se concentrations in blood on FR incidence, will be presented in a companion manuscript.

Treatments were administered individually once per week (with the calculated weekly amount of Se supplement being equal to the summed daily intake) by an oral drench. The oral Se drench, prepared fresh each week, consisted of 5 mL of water for no-Se control sheep; the appropriate Se dosage (4.9, 14.7, or 24.5 mg of Se) dissolved in 5 mL of water for sheep receiving inorganic Se; and 4.9, 14.7, and 24.5 mg of Se suspended in 11, 30, and 48 mL of water, respectively, for sheep receiving Se-yeast (greater amounts of water were required to suspend Se-yeast in water). The Se drench was administered with a dose syringe as ewes, identified by color-coded ear tags and ear tattoos (in case eartags fell out), moved through a cutting chute. The treatment period started approximately 2 wk before breeding and lasted for 12 mo.

Ewes were fed on pasture, except for a 3-mo period around lambing (wk 15 through 28), when ewes were housed in the barn (Figure 1). Ewes on pasture were supplemented with grass hay (wk 0 through 12) and later (wk 13 through 15) with alfalfa hay. In the barn, sheep were fed alfalfa hay and shelled corn, except for 2 d in the lambing jug when ewes were fed alfalfa pellets. Sheep were fed to meet or exceed NRC (2007) recommendations.

Figure 1.

Location of sheep (pasture or barn) and diets provided over the duration of the study. Ewes, regardless of group, were on pasture from September 1 to January 13 and consumed grass. In addition, from September 1 to December 14 ewes received grass hay and after that alfalfa hay ad libitum. Between January 13 and March 26, ewes were housed in the barn and lambed from February 4 to March 24. For 2 d during lambing, ewes received alfalfa pellets. Otherwise, ewes in the barn received alfalfa hay and corn. Starting on March 26, ewes were returned to pasture and consumed grass (no supplemental hay or corn was provided).


Concentrations of Se in the forage from the sheep center pastures ranged from 0.12 to 0.14 μg/g of DM. The Se concentrations of the grass hay, alfalfa hay, alfalfa pellets, and whole corn were 0.02, 0.05, 0.06, and 0.01 μg of Se per g of DM, respectively. A custom-made mineral supplement (OSU Sheep Mineral Premix, Wilbur-Ellis Company, Clackamas, OR) was provided free choice to all ewes throughout the duration of the study. The mineral supplement contained 8.0 to 9.5% Ca, 6.0% P, 33.5 to 37.5% NaCl, 2.7% Mg, 60 mg/kg of Co, 1,700 mg/kg of Mn, 210 mg/kg of iodine, 1,350 mg/kg of Fe, 7,700 mg/kg of Zn, 116,120 IU/kg of vitamin A, 14,515 IU/kg of vitamin D, 25.4 IU/kg of vitamin E, and no added Se. The measured Se concentration of the mineral supplement was 0.44 μg of Se per g of DM. Assuming pasture DMI of 2% of BW, ewes would consume between 0.12 and 0.26 mg of Se/d. For an average mineral intake of 8 g/d, an additional 3 µg of Se would be consumed. Other feed ingredients would contribute less than 20 µg of Se/d. Thus, the majority of Se intake (4.9, 14.7, or 24.5 mg of Se/wk) was provided by the oral Se drench. Routine farm management practices were not altered.

Se Sources

Two inorganic Se sources were used: Na-selenite and Na-selenate (both from Retorte Ulrich Scharrer GmbH, Röthenbach, Germany). Sodium selenite (Na2SeO3) was 456 g/kg of Se or 45.6% Se. Sodium selenate (Na2SeO4) was 418 g/kg of Se or 41.8% Se (NRC, 2001). The organic Se source (Se-yeast, Prince Se Yeast 2000, Prince Agri Products Inc., Quincy, IL) had a guaranteed analysis of 2 g/kg of organically bound Se with 78% being SeMet. Calculated amounts of Se delivered in the 0.7, 2.1, and 3.5 mg/d weekly drenches were 4.9, 14.7, and 24.5 mg of Se per dose, respectively. Each composited drench was submitted for Se analysis (Center for Nutrition, Diagnostic Center for Population and Animal Health, Michigan State University, East Lansing) to verify the desired Se concentration. Except for the 4.9 mg weekly dose of Na-selenate (sheep received 8.95 vs. 4.9 mg because we accounted for the molecular weight of Na-selenate decahydrate instead of Na-selenate anhydrous), the 4.9, 14.7, and 24.5 mg weekly doses of Na-selenite (4.85, 14.85, and 24.6 mg of Se, respectively) and organic Se-yeast supplements (4.80, 14.40, and 24.0 mg of organically bound Se, respectively) were found to be within expected analytical variance of their targeted concentrations.

Se Assay

Pasture forage samples were obtained once using a systematic grid pattern with 1 sample generated from 25 subsamples to obtain representative samples for Se analysis. Mineral supplement, grass hay, alfalfa hay, alfalfa pellets, and corn were also submitted for Se analysis. Twenty core samples from random bales in each grass hay or alfalfa hay lot were combined into 1 composite sample each for grass hay or alfalfa hay and submitted for Se analysis. Twenty subsamples of alfalfa pellets or corn were also taken from each storage bin and mixed together to make 1 composite sample for each.

To assess the effect of Se supplementation on WB-Se status, blood samples were collected from all at 0 mo (study initiation), which was approximately 2 wk before breeding. Whole blood- and serum-Se concentrations were measured every 3 mo in all ewes immediately before the time of weekly oral drenching. In addition, WB-Se concentrations were measured monthly in 16 ewes from these 4 groups of sheep: no-Se control, 4.9 mg/wk of Na-selenite, 4.9 mg/wk of Na-selenate, and 4.9 mg/wk of Se-yeast. The 16 ewes in each of these groups were selected at the start of the experiment based on FR status and age, and the same ewes were sampled each time. Jugular venous blood was collected into evacuated EDTA tubes (2 mL; final EDTA concentration 2 g/L; Becton Dickinson, Franklin Lakes, NJ) and stored on ice until it could be frozen at −20°C. Jugular venous blood was also collected into evacuated tubes without EDTA (10 mL; Becton Dickinson) for subsequent harvesting of serum. The latter tubes were centrifuged at 850 × g for 10 min at 20°C; serum was collected, centrifuged again at 16,300 × g for 1 min at 20°C in a microcentrifuge to remove any remaining red blood cells (RBC), transferred into 2.0-mL screw-cap self-standing micro tubes (ISC BioExpress, Kaysville, UT), and stored at −20°C.

Selenium concentrations in WB, serum, pasture forage, and all dietary components were determined by a commercial laboratory (Center for Nutrition, Diagnostic Center for Population and Animal Health, Michigan State University, East Lansing) using an ionized coupled plasma mass spectrometry method (Wahlen et al., 2005) with modifications. Two hundred microliters of each WB or serum sample was diluted 1:20 with a solution containing 0.5% EDTA and Triton X-100, 1% ammonia hydroxide, 2% propanol, and 20 µg/kg of scandium, rhodium, indium, and bismuth as internal standards. All samples were analyzed on an Agilent 7500ce ionized coupled plasma mass spectrometer. Selenium, at mass 78, was analyzed in hydrogen mode to reduce spectral interference (Agilent Technologies, Santa Clara, CA).

Statistical Analyses

Statistical analyses were performed using SAS (SAS Inst. Inc., Cary, NC) software. Ewes that did not give birth or rear a lamb were excluded from the statistical analysis (no-Se control group, 1 ewe died before lambing, 1 ewe was not pregnant, and 1 ewe did not rear a lamb; 4.9 mg of Se/wk Na-selenate, 1 ewe was not pregnant and 1 ewe did not rear a lamb; 4.9 mg of Se/wk Na-selenite, 4 ewes were not pregnant; 14.7 mg of Se/wk Na-selenite, 2 ewes were not pregnant and 2 ewes did not rear a lamb; 24.5 mg of Se/wk Na-selenite, 1 ewe died before lambing and 1 ewe did not rear a lamb; 4.9 mg of Se/wk of Se-yeast, 1 ewe died before lambing and 2 were not pregnant; 14.7 mg of Se/wk of Se-yeast, 2 ewes died before lambing and 2 ewes did not rear a lamb; 24.5 mg of Se/wk of Se-yeast, 1 ewe died before lambing, 4 ewes were not pregnant, and 2 ewes did not rear a lamb). No significant differences (P > 0.10) in reproductive variables were observed. Detailed results of reproduction and production performance will be presented in a companion manuscript.

The effect of treatment (no-Se, 4.9 mg of Se/wk Na-selenate, 4.9 mg of Se/wk Na-selenite, 14.7 mg of Se/wk Na-selenite, 24.5 mg of Se/wk Na-selenite, 4.9 mg of Se/wk Se-yeast, 14.7 mg of Se/wk Se-yeast, 24.5 mg of Se/wk Se-yeast) on WB- and serum-Se concentrations were analyzed as a repeated-measures-in-time design using PROC MIXED. Besides treatment, fixed effects in the model were FR status (yes, no), breed (Polypay, Suffolk, or crossbred), number of lambs born (1, >1), month of sampling, and the interaction between month of sampling and treatment. To evaluate whether FR status, breed, number of lambs born, and number of lambs reared (1, >1) modified the treatment effect, data were checked for interactions and additionally stratified by FR status, breed, number of lambs born, and number of lambs reared, respectively. A completely unrestricted variance-covariance structure was used to account for repeated measures taken on individual ewes across time. To obtain the correct degrees of freedom, the KENWARDROGER option was invoked. The KENWARDROGER option consists of the Satterthwaite adjustment for degrees of freedom with a Kenward-Roger adjustment on SE, which can be used for repeated measures studies.

Contrasts were constructed using the ESTIMATE statement in PROC MIXED. The effect of Se depletion was evaluated by comparing the no-Se control group with the 3 groups receiving 4.9 mg of Se/wk. The effect of Se source was evaluated by comparing the 3 groups of ewes receiving Na-selenite with the 3 groups receiving Se-yeast. The effect of Se dosage for Na-selenite or Se-yeast was evaluated by comparing the 3 dosages through linear and quadratic contrasts, respectively. The interaction between Se source and Se dosage was evaluated by constructing orthogonal contrasts between groups of ewes receiving different Se sources and Se dosages of Na-selenite and Se-yeast. Data are reported as least squares means ± SEM. Statistical significance was declared at P ≤ 0.05.


Although the essentiality of Se has been known for 5 decades (Muth et al., 1958), the most effective method of Se delivery to sheep, as well as Se requirements for optimal health, remains unclear. In this study, we evaluated the effect of Se source (organic vs. inorganic) and dosage (0, 4.9, 14.7, or 24.5 mg of Se/wk) on WB- and serum-Se concentrations. Our main findings were that Se concentrations decreased primarily in the last third of pregnancy and during peak lactation, suggesting increased Se requirements of ewes during late pregnancy and at peak lactation. At the FDA-allowed rate (4.9 mg of Se/wk), Se-yeast was more effective than inorganic Se, either as Na-selenate or Na-selenite, at increasing WB- and serum-Se concentrations. Administration of Se-yeast at supranutritional concentrations (14.7 and 24.5 mg of Se/wk) increased WB- and serum-Se concentrations in a dose-dependent manner, whereas blood Se concentrations reached a maximum with 14.7 mg of Se/wk of Na-selenite. The Se-yeast supplemented at 4.9 mg of Se/wk resulted in similar WB-Se concentrations as Na-selenite supplemented at 14.7 and 24.5 mg of Se/wk, which suggests that Se-yeast is more efficiently retained than inorganic Se. None of the ewes receiving supranutritional Se supplementation showed clinical signs of Se toxicity at any time during the study.

Regarding chemical forms of Se and Se distribution in blood, WB-Se is mainly found in hemoglobin, in the RBC-bound selenoprotein glutathione peroxidase, and in the plasma as selenoprotein P, glutathione peroxidase, and albumin (Deagen et al., 1993; Whanger et al., 1996; Finley, 1998). Selenoprotein-P is involved in Se homeostasis and transport of Se to tissues (Burk and Hill, 2009; reviewed in Fairweather-Tait et al., 2010). The 2 major chemical forms of Se found in WB and serum are SeCys and SeMet. The SeCys is inserted co-translationally at loci encoded by a specific codon to produce selenoproteins, whose concentrations in the body are highly regulated. On the other hand, dietary SeMet is incorporated into many proteins in place of Met; the concentration of SeMet is not regulated and ultimately reflects dietary intake (reviewed by Whanger, 2002; Fairweather-Tait et al., 2010). Selenomethionine acts as a storage form of Se in body proteins, including hemoglobin and albumin, from which it is slowly released by protein catabolism. The SeMet can then be trans-selenated into SeCys and subsequently used to provide Se requirements for selenoprotein synthesis (Rayman, 2008).

Effect of Dietary Se Depletion on WB- and Serum-Se Concentrations

In ewes receiving no Se, WB-Se concentrations decreased during pregnancy by 57% and again during peak lactation by 44% (Table 1), reflecting increased Se requirements. No significant changes were observed during other production periods. Selenium losses during pregnancy and lactation are accounted for by additional coefficients in the net Se requirements for sheep (NRC, 2007).

Across the 12-mo treatment period, serum-Se concentrations decreased less than WB-Se concentrations (Figure 2); as a result, a greater proportion of WB-Se was in the serum fraction (increased from 40 to 77%; P < 0.001; Table 2). The greater Se decline in the nonserum fraction of WB (i.e., the cellular fraction) suggests that proteins containing SeMet in the nonserum fraction of WB serve as Se-reservoirs that can be used as a source of Se during negative Se balance (e.g., during fetal growth and peak lactation). We propose that changes in the nonserum fraction of WB serve as an indicator of Se balance. Serum-Se concentrations will decrease during negative Se balance to a much smaller extent than WB-Se, with the exception of when WB SeMet reserves are depleted; at this point serum-Se concentrations also decrease. In support, Juniper et al. (2008) reported that WB SeMet but not SeCys concentrations are decreased 42 d after the withdrawal of feeding Se-yeast to lambs. In addition, greater nonserum WB-Se concentrations may indicate increased Se storage at similar Se-supplementation rates. This could explain why Polypay ewes had greater WB-Se, but not serum-Se concentrations for the first 6 mo of the study compared with the larger framed Suffolk or crossbred ewes (results not shown).

Figure 2.

Effect of dietary Se depletion on whole-blood and serum-Se concentrations in ewes. The vertical lines represent the start and end of pregnancy and lactation. Values are least squares means ± SEM.


After approximately 7 mo in our study, WB-Se concentrations reached a plateau, indicating that Se reserves in WB were probably depleted. A similar time length was reported for ewes and cattle after feeding low-Se forage (Nicholson et al., 1991; Hall et al., 2009). The remaining Se in WB is most likely in the form of SeCys in glutathione peroxidase, an important antioxidant in blood, and selenoprotein P, the plasma Se transporter. Any further decrease in WB-Se concentration will most likely affect animal performance, because the function of selenoproteins will be affected.

Effect of Dietary Se Source, Orally Drenched at 4.9 mg of Se/wk, on WB- and Serum-Se Concentrations

At 4.9 mg of Se/wk, dietary supplementation with Se increased WB-Se concentrations, but concentrations were dependent on Se source and ewe production stage (Table 1). Supplementation with organic Se as Se-yeast was more effective than inorganic Na-selenate or Na-selenite at increasing WB- and serum-Se concentrations (Tables 1 and 2). Therefore, we conclude that Se-yeast is more bioavailable than inorganic Se.

Similar findings have been reported for WB-Se concentrations after administering organic vs. inorganic Se sources in studies of other sheep (Tiwary et al., 2006; Qin et al., 2007; Steen et al., 2008), cattle (Awadeh et al., 1998; Ortman and Pehrson, 1999), humans (Butler et al., 1991), and broiler chickens (Yoon et al., 2007). There are several reasons why inorganic Se is less bioavailable than organic Se in ewes: 1) inorganic Se may be partially reduced to elemental Se in the rumen. A freshly prepared oral bolus drench may attenuate this limitation, and 2) inorganic Se has a shorter half-life than organic Se because SeMet can be incorporated into proteins in place of Met, sequestering Se until the protein is turned over and Se is either excreted or converted into SeCys. In contrast, inorganic Se can only be incorporated into SeCys. Thus, urinary Se losses are likely greater when ewes are supplemented with inorganic vs. organic Se.

Our results are specific for Prince Se Yeast 2000, the source of organic Se-yeast we tested. Yoon et al. (2007) showed differences in bioavailability between the 2 organic Se-yeast sources used in their study. Admittedly, there is considerable variation in products described as Se-yeast, which is reflected in the Se-species composition. The percentage of Se that is organically associated should ideally be greater than 90%, and more than 80% should be associated with yeast proteins (Rayman et al., 2008). Although at least 15 species of Se have been documented in Se-yeast (Rayman et al., 2008), the predominant form is SeMet (60 to 80%; Whanger, 2002).

Irrespective of chemical form of Se administered, WB-Se concentrations decreased in the last third of pregnancy and at peak lactation, which was similar to what we observed in our non-Se-supplemented group (Table 1). These results reflect increased Se transfer from ewe to growing fetus and lamb, respectively, and, thus, the increased Se requirements of pregnancy and peak lactation. During lactation, SeMet is utilized for the synthesis of milk proteins as well as hemoglobin, which could explain the decrease in the nonserum fraction of WB-Se during lactation.

The differences in blood-Se concentrations among ewes fed organic vs. inorganic Se were similar in direction but smaller in magnitude for serum than for WB (Table 2). Oral Se drenches increased serum-Se concentrations over baseline values irrespective of the chemical form of Se, suggesting that oral drenching of Se was more effective at providing Se to ewes than the mineral mixture that was fed ad libitum before our study. Variable mineral intake and degradation of Se in mineral mixtures (J. A. Hall, unpublished results) likely explain the greater blood-Se concentrations after oral drenching. In late gestation, serum-Se concentrations decreased despite Se supplementation, whereas WB-Se concentrations and the nonserum fraction of Se in WB increased (Table 2). Decreased glutathione peroxidase activity in late pregnancy (Lacetera et al., 1999) may explain the decline in serum Se concentrations at this time.

Our results are consistent with other studies that reported increased WB-Se concentrations as well as increased serum- or plasma-Se concentrations after administering organic vs. inorganic Se sources (Butler et al., 1991; Ortman and Pehrson, 1999; Tiwary et al., 2006). However, some studies have reported increased WB-Se concentrations, but no change in either plasma- (van Ryssen et al., 1989) or serum- (Awadeh et al., 1998) Se concentrations after administering organic vs. inorganic Se sources. For example, in a previous study in sheep (van Ryssen et al., 1989), the Se content of WB, but not plasma, was significantly greater after consuming high-Se wheat compared with selenite when both were fed at 1.0 mg of Se/kg of diet. In addition, other studies that did not report WB-Se concentrations found no change in either serum- or plasma-Se concentrations after administering organic vs. inorganic Se (Mateo et al., 2007; Arthington, 2008; Taylor et al., 2009). In general, changes in WB-Se concentrations are much more pronounced than changes in serum or plasma, such that changes in serum or plasma may be undetectable with small numbers of study animals.

We measured both WB- and serum-Se concentrations as indicators of Se status. The belief that serum-Se concentrations provide short-term feedback on Se status, whereas WB-Se concentrations represent longer-term Se status (Stowe and Herdt, 1992), may not be completely accurate. Whole blood remains the sample of choice for Se-bioavailability studies because SeMet is incorporated into nonfunctional structural proteins as a direct Met replacement. Selenite can be effectively used for selenoprotein synthesis, but it cannot be stored in the body for later use. Plasma Se concentrations reflect dietary exposure to most forms of Se, but in the absence of a well-described homeostatic regulation there is no absolute plateau, although the concentration will reach a steady state at any constant intake after approximately 10 to 12 wk of supplementation (Fairweather-Tait et al., 2010). The plasma response to dietary Se is Se-species dependent, so consumption of 2 different forms may result in different plasma Se concentrations (Fairweather-Tait et al., 2010). In addition, the use of plasma Se as an indicator of Se status in studies with a treatment period less than the average RBC lifespan (125 to 160 d in sheep) may underestimate the true Se status of the animal because the 2 pools accumulate Se differently depending on the dietary form of Se (Butler et al., 1991). In our study, serum concentrations were always less than WB-Se concentrations, but changed in parallel. Because our sampling period was longer than the turnover time for RBC, we did not see a lag effect based on RBC-Se content. Together, WB- and serum-Se concentrations can be used to calculate nonserum WB-Se concentrations and, thus, provide information about Se balance.

We found no differences between WB- or serum-Se concentrations in ewes receiving inorganic Na-selenate vs. Na-selenite (Tables 1 and 2), even with significant differences in target concentrations for Na-selenate. Our results are consistent with other comparisons of Na-selenate and Na-selenite as dietary supplements in sheep (Podoll et al., 1992; Serra et al., 1994; Ortman and Pehrson, 1999). We therefore agree that Na-selenite and Na-selenate are equally available to sheep at the FDA-allowed rate.

Effect of Supranutritional Dietary Se Supplementation on WB- and Serum-Se Concentrations

The effect of Na-selenite at 4.9, 14.7, or 24.5 mg of Se/wk on WB-Se (Figure 3) and serum-Se concentrations plateaued at concentrations above 14.7 mg/wk (Table 2). Supplementation with Na-selenite at 4.9, 14.7, or 24.5 mg/wk increased WB-Se concentrations in the first 3 mo of the trial by 20.1, 43.0, and 51.7%, respectively, after which WB-Se concentrations remained constant (Table 2). Based on our results, there are limitations for inorganic Se supplements to further increase WB- and serum-Se concentrations, with a maximum plateau effect for selenite at ≤14.7 mg of Se/wk. This is because selenite and selenate are first reduced to hydrogen selenide (H2Se) and then used in selenoprotein synthesis, or excreted if availability exceeds this requirement (Fairweather-Tait et al., 2010).

Figure 3.

Effect of Se supplementation at the US Food and Drug Administration-allowed rate of 4.9 mg of Se/wk vs. supranutritional dietary Se supplementation at 14.7 and 24.5 mg of Se/wk with a) Na-selenite (Retorte Ulrich Scharrer GmbH, Röthenbach, Germany), or b) Se-yeast (Prince Agri Products Inc., Quincy, IL) on whole-blood Se concentrations in ewes. The vertical lines represent the start and end of pregnancy and lactation. Values are least squares means ± SEM.


Supranutritional dosages of Se-yeast linearly increased both WB-Se (Figure 3) and serum-Se concentrations (Table 2). In the first two-thirds of pregnancy, 4.9, 14.7, or 24.5 mg/wk of Se-yeast increased WB-Se concentrations by 43.8, 112, and 179%, respectively, from baseline values (Table 2). In the last 3 mo of the supplementation period, 4.9, 14.7, or 24.5 mg/wk of Se-yeast increased WB-Se concentrations by 8.1, 16.4, and 23.0%, respectively. During other production stages, WB-Se concentrations were not affected by Se-yeast dosage (Table 2). Our results suggest that SeMet in supranutritional dosages of Se-yeast can be retained as protein-bound SeMet and subsequently used to maintain Se requirements after Se-yeast supplementation ceases (Juniper et al., 2008).

Similar dose-response findings have been reported with WB-Se concentrations after administering organic (Tiwary et al., 2006; Yoon et al., 2007; Juniper et al., 2008) vs. inorganic Se sources (Davis et al., 2006; Tiwary et al., 2006). In addition, plasma- or serum-Se concentrations after administering organic- (Taylor, 2005; Tiwary et al., 2006; Juniper et al., 2008) vs. inorganic-Se sources (Davis et al., 2006; Tiwary et al., 2006; Taylor et al., 2009) have also increased.

We observed similar WB- and serum-Se concentrations in sheep receiving supranutritional quantities (14.7 and 24.5 mg of Se/wk) as inorganic Na-selenite (Table 2). Furthermore, WB- and serum-Se concentrations were the same for sheep receiving these greater quantities of inorganic Se supplements as for the 4.9 mg of Se/wk organic Se-yeast supplement (Table 2).


In conclusion, weekly drenching of ewes with Se is an effective method to improve Se status of ewes in Se-deficient regions. Organic Se-yeast has greater bioavailability over a wider dosage range than inorganic Na-selenite; similar blood-Se concentrations were achieved with 14.7 and 24.5 mg of Se/wk as Na-selenite and 4.9 mg of Se/wk as Se-yeast. Thus, choosing an organic-Se source to increase blood-Se concentrations could be a more predictable means to achieve Se concentrations that support optimum immune function.