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Journal of Animal Science - Article

 

 

This article in JAS

  1. Vol. 90 No. 2, p. 577-584
     
    Received: Mar 18, 2011
    Published: January 20, 2015


    3 Corresponding author(s): Jean.Hall@oregonstate.edu
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doi:10.2527/jas.2011-4076

Organic and inorganic selenium: II. Transfer efficiency from ewes to lambs1

  1. W. C. Stewart*22,
  2. G. Bobe*†,
  3. W. R. Vorachek,
  4. G. J. Pirelli*,
  5. W. D. Mosher*,
  6. T. Nichols*,
  7. R. J. Van Saun§,
  8. N. E. Forsberg* and
  9. J. A. Hall 3
  1. Department of Animal Sciences, College of Agriculture,
    Linus Pauling Institute, and
    Department of Biomedical Sciences, College of Veterinary Medicine, Oregon State University, Corvallis 97331; and
    Department of Veterinary and Biomedical Sciences, College of Agricultural Sciences, Pennsylvania State University, University Park 16802

ABSTRACT

Adequate Se transfer from ewes to lambs is important to prevent Se-deficiency diseases. To evaluate how different chemical forms of Se administered at comparative dosages to mature ewes affect Se status of their lambs, 240 ewes were divided into 8 treatment groups (n = 30 each) and drenched weekly (at an amount equal to their summed daily intake) with no-Se (controls); at recommended amounts (4.9 mg of Se/wk) with inorganic Na-selenite, inorganic Na-selenate, or organic Se-yeast; or at supranutritional amounts (14.7 and 24.5 mg of Se/wk) with Na-selenite or Se-yeast for 1 yr. Weekly drenching of Se was effective at increasing (P < 0.002) Se concentrations in ewe colostrum and milk at 30 d of lactation and in improving (P < 0.001) the Se status of lambs (whole-blood and serum-Se concentrations at birth, and skeletal-muscle Se concentrations at 14 d of age). Selenium concentrations in lacteal secretions were greater in ewes drenched with Se-yeast (colostrum: 374, 436, and 982 ng/mL at 4.9, 14.7, and 24.5 mg of Se/wk, respectively; milk: 26, 39, 64 ng/mL) compared with ewes drenched with Na-selenite (colostrum: 204, 334, 428 ng/mL; milk: 16, 21, 24 ng/mL), and were also greater (P < 0.001) in their lambs. Selenium concentrations continued to increase (P < 0.001) in lamb whole blood (558 and 695 ng/mL at 14.7 and 24.5 mg of Se/wk, respectively), serum (126, 183 ng/mL), and skeletal muscle (991, 1,696 ng/mL) with supranutritional concentrations of Se-yeast, whereas Se concentrations did not differ in whole blood (304, 332 ng/mL), serum (77, 85 ng/mL), or skeletal muscle (442, 482 ng/mg) of lambs from ewes drenched with 14.7 or 24.5 mg of Se/wk of Na-selenite. We conclude that weekly oral drenching of ewes during gestation and lactation with organic Se-yeast results in a more efficient transfer of Se (over a wide range of supplementation rates) from ewe to lamb than does inorganic Na-selenite.



INTRODUCTION

Selenium is an essential micronutrient in sheep, and adequate Se transfer from ewes to lambs is important to prevent Se-responsive diseases such as nutritional myodegeneration and Se-responsive unthriftiness. Current FDA regulations limit the amount of dietary Se supplementation to 0.3 mg of Se/kg of diet (as fed), which is equivalent to 0.7 mg per animal per day (FDA, 2009). Current recommendations do not account for the chemical form of Se and its effect on Se bioavailability, which may change with supplementation rate. Selenium sources can be classified into 2 categories: inorganic and organic. The most common inorganic Se sources are Na-selenite and Na-selenate, which are usually provided to sheep in mineral premixes or injected. Organic Se sources are seleno-AA [e.g., selenomethionine (SeMet) and selenocysteine (SeCys)], which are found in Se-yeast or in feeds grown on Se-rich soils.

Provision of Se to the dam during gestation and early lactation is an effective method to meet Se requirements in newborn lambs because Se efficiently crosses the placental barrier into fetal tissues and enters colostrum and milk (Rock et al., 2001; Abd El-Ghany et al., 2007). Several studies have examined the effect of different chemical forms and dosages of Se on Se status of ewes (reviewed in Hall et al., 2012); however, few studies have evaluated the transfer efficiency of dietary organic Se-yeast vs. inorganic Na-selenite from ewes to lambs (Rock et al., 2001; Taylor et al., 2009), and both of these studies were limited to 1 organic-Se treatment and less than 8 ewes per group. The objective herein was to compare the transfer efficiency of Se-yeast and Na-selenite, dosed at the allowed rate (0.3 mg of Se/kg diet = 0.7 mg of Se/d or 4.9 mg of Se/wk) and at supranutritional concentrations (14.7 and 24.5 mg of Se/wk), from ewes to lambs. Our hypothesis was that Se-yeast is transferred more efficiently over a wide range of supplementation rates from ewe to lamb than inorganic Na-selenite.


MATERIALS AND METHODS

The experimental protocol used in this study was approved by the Institutional Animal Care and Use Committee at Oregon State University.

Experimental Design and Treatments

The study design has been described in detail in a companion manuscript (Hall et al., 2012). Briefly, 240 mature ewes were randomly assigned to 8 treatment groups (n = 30 each) based on Se supplementation rate (0, 4.9, 14.7, and 24.5 mg of Se/wk) and source: Na-selenite (Retorte Ulrich Scharrer GmbH, Röthenbach, Germany), Na-selenate (4.9 mg of Se/wk dosage only; Retorte Ulrich Scharrer GmbH), and Se-yeast. All dosages were below the maximum tolerable level (5 mg/kg as fed) for small ruminants (NRC, 2007). The organic Se-yeast (Prince Se Yeast 2000, Prince Agri Products Inc., Quincy, IL) had a guaranteed content of 2,000 mg/kg of organically bound Se, which was verified by analysis (1,959 mg/kg). Treatment groups were blocked for footrot (FR) incidence and severity, and age of ewe. Ewes were from 3 genotypes (Polypay, Suffolk, and crossbred), and 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, OR.

Selenium treatments were administered individually by oral drench once weekly, at amounts equivalent to their summed daily supplementation rates, for a total of 52 wk. The (no-Se) control ewes received water. The Se dose was suspended in a reasonable volume of water [5 mL for inorganic Se with more water required for the organic solutions (i.e., 11, 30, and 48 mL for the 4.9, 14.7, or 24.5 mg of Se solutions, respectively)]. Solutions were made up fresh each week and administered with a dose syringe as sheep moved through a cutting chute. To ensure a homogeneous dosage, the solution was stirred each time before being drawn into the dose syringe. Lambs did not receive any additional Se supplementation after birth.

Individual drenches of inorganic Se were submitted for Se analysis to the Center for Nutrition, Diagnostic Center for Population and Animal Health, Michigan State University (East Lansing). 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) were within expected analytical variance of their targeted concentrations.

Blood Collection and Se Analysis of Ewe and Lamb Samples

Blood samples (20 mL) were collected from the jugular vein of all ewes at the start of Se treatment, which was approximately 2 wk before breeding. Whole-blood (WB) and serum-Se concentrations were measured every 3 mo in all ewes (0, 3, 6, 9, and 12 mo from the start of Se treatment) directly before the weekly oral drenching. Immediately after parturition and before lambs had nursed, jugular venous blood (5 mL) was also collected from lambs. Blood samples were processed as described previously (Hall et al., 2012).

Colostrum samples were collected immediately after parturition. Milk samples were collected again at 30 d of lactation. Ewes were milked by hand, and samples (10 mL) were collected into 15-mL centrifuge tubes (ISC BioExpress, Kaysville, UT) and stored at −20°C. Skeletal-muscle samples were collected from lambs at 14 d of age coinciding with tail docking. Tails were docked using a hot-docking method. Once docked, skeletal muscle from the discarded tail was scraped off with a scalpel, separating muscle tissue from vertebrae and fat tissue. Muscle tissue from each lamb was then placed in a screw-top microtube (1.5 mL, ISC BioExpress) and stored at −20°C. Selenium concentrations in WB, serum, colostrum, milk, and skeletal-muscle samples were determined by a commercial laboratory (Center for Nutrition, Diagnostic Center for Population and Animal Health), as described previously (Hall et al., 2012).

Se Transfer Efficiency Ratios

Selenium transfer efficiency ratios were calculated as the ratio of ewe-colostral Se to ewe-WB Se, ewe-colostral Se to ewe-serum Se, lamb-WB Se to ewe-WB Se, lamb-serum Se to ewe-serum Se, lamb-skeletal-muscle Se to ewe-WB Se, lamb-skeletal-muscle Se to ewe-serum Se, lamb-skeletal-muscle Se to lamb-WB Se, lamb-skeletal-muscle Se to lamb-serum Se, and lamb-skeletal-muscle Se to ewe-colostral Se. Values for multiple lambs from the same ewe were averaged because ewe was the experimental unit. To have ewe WB- and serum-Se concentrations, which were measured every 3 mo, assessed as close as possible to collection times for colostrum, milk at 30 d of lactation, and lamb samples, we used the 6-mo ewe WB- and serum-Se concentrations in these calculations. More than 95% of these ewe WB- and serum-Se samples were collected within 30 d of lambing. Lamb WB- and serum-Se samples were collected at birth before lambs nursed, and lamb skeletal-muscle-Se samples were collected at 14 d of age.

Statistical Analysis

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 = 3 ewes; 4.9 mg of Se/wk Na-selenate = 2 ewes; 4.9 mg of Se/wk Na-selenite = 4 ewes; 14.7 mg of Se/wk Na-selenite = 4 ewes; 24.5 mg of Se/wk Na-selenite = 2 ewes; 4.9 mg of Se/wk of Se-yeast = 3 ewes; 14.7 mg of Se/wk of Se-yeast = 4 ewes; 24.5 mg of Se/wk of Se-yeast = 7 ewes). 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 of Se-yeast, 14.7 mg of Se/wk of Se-yeast, 24.5 mg of Se/wk of Se-yeast) on Se concentrations of WB (6 mo of supplementation), serum (6 mo of supplementation), colostrum, and milk (at 30 d of lactation) in ewes; and on WB and serum in lambs at birth, and muscle at 14 d of age were analyzed using PROC GLM. Besides treatment, fixed effects in the model were FR-status (yes, no), breed (Polypay, Suffolk, or crossbred), and number of lambs born (1, >1). To evaluate whether FR status, breed, number of lambs born, and number of lambs reared (1, >1) modified the effect of treatment on WB- and serum-Se concentrations, data were checked for interactions and additionally stratified by FR status, breed, number of lambs born, and number of lambs reared, respectively. Data are reported as least squares means ± SEM. Statistical significance was declared at P ≤ 0.05.


RESULTS

Effect of Dietary Se Depletion on Se Transfer Efficiency

Ewes receiving no Se supplementation had reduced Se concentrations in WB and in serum collected within 30 d of lambing and decreased Se concentrations in colostrum compared with ewes receiving 4.9 mg of Se/wk (all P < 0.001; Table 1). The fold-decrease in WB Se was greater than in serum Se (−3.2 vs. −2.5; P < 0.001). Changes in 30-d milk Se concentrations were similar in direction, but smaller in magnitude (P = 0.003; Table 1). Ewes receiving no Se were the only treatment group with similar Se concentrations in colostrum and serum (P = 0.64). All other Se-treatment groups had greater Se concentration in colostrum than in serum (P < 0.005). The Se transfer efficiency ratios calculated for ewe-colostral Se to ewe-WB Se and ewe-colostral Se to ewe-serum Se were less in ewes receiving no Se supplementation (both P < 0.001; Table 2).

Changes in WB- and serum-Se concentrations in response to no Se supplementation were similar in ewes and their offspring (Table 1). Lambs from ewes receiving no Se supplementation had decreased WB-Se concentrations (P < 0.001; Table 1). Changes in serum-Se concentrations were similar in direction, but smaller in magnitude (P < 0.001; Table 1). Changes in lamb skeletal-muscle-Se concentrations at 14 d of age followed similar trends as other Se measures in ewes and lambs, with decreased concentrations in lambs from ewes receiving no Se supplementation (P < 0.001; Table 1).

Effect of Dietary Se Source, Orally Drenched at 4.9 mg of Se/wk on Se Transfer Efficiency

Ewes receiving 4.9 mg of Se/wk as Se-yeast had greater WB-, serum-, and colostral-Se concentrations than ewes receiving Na-selenite (all P < 0.001; Table 1). Changes in milk-Se concentrations were similar in direction but smaller in magnitude in ewes receiving Na-selenite vs. ewes receiving Se-yeast (P = 0.01; Table 1). The Se transfer from ewe blood to colostrum, calculated from Se transfer efficiency ratios, was more efficient in ewes receiving Se-yeast than in ewes receiving Na-selenite (WB: P = 0.003; serum: P < 0.001; Table 2).

Similar changes in WB- and serum-Se concentrations were observed in lambs as in their mothers (Table 1). Lambs from ewes receiving Se-yeast had greater WB-Se concentrations than lambs from ewes receiving Na-selenite (P < 0.001; Table 1). Changes in serum-Se concentrations were similar in direction but smaller in magnitude (P < 0.001; Table 1) in lambs from ewes receiving Se-yeast vs. lambs from ewes receiving Na-selenite. In addition, Se transfer from ewe WB to lamb WB and ewe serum to lamb serum was more efficient in lambs from ewes receiving Se-yeast (both P < 0.001; Table 2).

Lamb skeletal-muscle-Se concentrations at 14 d of age reflected other Se measures in ewes and lambs (Se-yeast vs. Na-selenite: P < 0.001; Table 1). Selenium transfer was more efficient in lambs from ewes receiving Se-yeast compared with Na-selenite (transfer from ewe WB or serum to lamb skeletal muscle: both P < 0.001; Table 2), and tended to be more efficient for transfer from ewe colostrum to lamb skeletal muscle (P = 0.08; Table 2). The chemical form of Se did not change the calculated Se transfer efficiency ratio within lamb (lamb skeletal muscle to lamb WB ratio; P = 0.17; Table 2).

When the 2 inorganic Se sources were compared, ewes receiving Na-selenate had greater Se concentrations in WB (P = 0.009), serum (P = 0.001), and colostrum (P = 0.05) as well as greater WB-Se concentrations (P = 0.05) in their lambs (Table 1), which can be partly explained by the 82% greater Se dosage. Even though the Se dosage administered was greater for Na-selenate than for Na-selenite, there were no differences in lamb serum-Se (P = 0.21) or skeletal-muscle-Se (P = 0.47) concentrations (Table 1).

Effect of Supranutritional Dietary Se Supplementation on Se Transfer Efficiency

When ewes received supranutritional supplementation amounts of Se-yeast, ewe WB-Se concentrations increased (all P < 0.003) linearly (Table 1). Serum-Se concentrations increased in a similar pattern (Table 1; Figure 1). In contrast WB-Se concentration plateaued in ewes receiving more than 14.7 mg of Se/wk of Na-selenite (P = 0.18). Supplementing ewes with Se-yeast at 4.9 mg of Se/wk resulted in similar WB-Se concentrations, as did 3 (P = 0.41) or 5 (P = 0.61) times this dosage of Na-selenite (Table 1). None of the ewes receiving supranutritional Se supplementation showed clinical signs of Se toxicity at any time during the study.

Figure 1.

Effect of the FDA-recommended dietary Se concentration (4.9 mg of Se/wk) and supranutritional dietary Se supplementation (14.7 and 24.5 mg of Se/wk) with either Na-selenite (4.9 mg of Se/wk: n = 26; 14.7 mg of Se/wk: n = 26; 24.5 mg of Se/wk: n = 27) or Se-yeast (4.9 mg of Se/wk: n = 27; 14.7 mg of Se/wk: n = 26; 24.5 mg of Se/wk: n = 23) on Se concentrations in A) ewe whole blood (within 30 d of lambing), B) ewe colostrum, C) lamb whole blood (at birth), and D) lamb skeletal muscle (at 14 d of age). Values are least squares means ± SEM. Bars without a common letter (a–e) differ (P < 0.05).

 

Similar changes were observed in colostral-Se (P < 0.001; Table 1) and 30-d milk Se concentrations (P < 0.001; Table 1; Figure 1) when ewes received supranutritional dietary amounts of Se-yeast. The Se transfer from ewe blood to colostrum, calculated from Se transfer efficiency ratios, was most efficient in ewes receiving Se-yeast at 24.5 mg of Se/wk (both ewe colostrum-to-ewe WB and ewe-colostrum-to-ewe serum ratios; P < 0.001; Table 2). Only minor changes in colostral-Se (P = 0.02) and milk-Se (P = 0.43) concentrations were observed in ewes receiving 14.7 and 24.5 mg of Se/wk as Na-selenite, respectively. Supplementing with 4.9 mg of Se/wk as Se-yeast resulted in similar colostral-Se (Table 1) and 30-d milk-Se concentrations (Table 1), and similar Se transfer efficiency ratios (ewe colostrum-to-ewe WB or ewe colostrum-to-ewe serum ratios; Table 2) as did 3 (colostrum: P = 0.33; milk: P = 0.19; colostrum to ewe WB ratio: P = 0.47; colostrum to ewe serum ratio: P = 0.10) or 5 times (colostrum: P = 0.18; serum: P = 0.58; colostrum to ewe WB: P = 0.27; colostrum to ewe serum: P = 0.61) this dosage of Na-selenite.

The dosage range over which supplementation of ewes with Na-selenite was able to increase Se concentrations in lamb WB and serum was limited. The WB-Se (P = 0.17) and serum-Se concentrations (P = 0.20) did not differ between lambs from ewes receiving 14.7 and 24.5 mg of Se/wk of Na-selenite, respectively (Table 1). In contrast, lamb WB- and serum-Se concentrations continued to increase as supplementation rates of Se-yeast to ewes increased (both P < 0.001; Table 1; Figure 1). Selenium was more efficiently transferred from ewe WB to lamb WB and from ewe serum to lamb serum with Se-yeast (both P < 0.001; Table 2) than with Na-selenite. In addition, WB-Se (P < 0.001) and serum-Se concentrations (P = 0.02) were greater in lambs from ewes receiving 4.9 mg of Se/wk of Se-yeast compared with 5 times this dosage of Na-selenite (Table 1).

Changes in lamb skeletal-muscle-Se concentrations followed similar trends as other Se measures in ewes and lambs. Muscle Se concentrations were greater in lambs from ewes receiving 14.7 vs. 24.5 mg of Se/wk as Se-yeast (P < 0.001), but not different in lambs from ewes receiving 14.7 vs. 24.5 mg of Se/wk as Na-selenite (P = 0.66; Table 1; Figure 1). Furthermore, muscle-Se concentrations were greater in lambs from ewes receiving 4.9 mg of Se/wk as Se-yeast compared with 5 times this dosage of Na-selenite (P = 0.01; Table 1). Selenium was more efficiently transferred from ewe WB to lamb skeletal muscle (P < 0.001; Figure 2), from ewe serum and colostrum to lamb skeletal muscle (both P < 0.001; Table 2), and from lamb WB to lamb skeletal muscle (P < 0.001; Table 2) in ewes receiving Se-yeast.

Figure 2.

Effect of the FDA-recommended dietary Se concentration (4.9 mg of Se/wk) and supranutritional dietary Se supplementation (14.7 and 24.5 mg of Se/wk) with either Na-selenite (4.9 mg of Se/wk: n = 26; 14.7 mg of Se/wk: n = 26; 24.5 mg of Se/wk: n = 27) or Se-yeast (4.9 mg of Se/wk: n = 27; 14.7 mg of Se/wk: n = 26; 24.5 mg of Se/wk: n = 23) on Se-transfer efficiency ratio of lamb skeletal muscle to ewe whole blood. Lamb skeletal muscle Se samples were collected at 14 d of age. Values are least squares means ± SEM. Bars without a common letter (a–c) differ (P < 0.05).

 


DISCUSSION

The purpose of this study was to evaluate the effects of Se source and supplementation rate in ewes on transfer efficiency of Se from ewes to their lambs. Dietary Se depletion in no-Se control ewes decreased blood- and colostral-Se concentrations, with a 1.3-fold greater decrease in WB-Se compared with serum-Se. As a result, skeletal-muscle-Se concentrations were reduced in lambs from no-Se control ewes. Inorganic Se supplementation with Na-selenite increased WB-Se, serum-Se, and colostral-Se concentrations in ewes, and increased WB-Se and serum-Se concentrations in their lambs. However, the effectual supplementation range for Na-selenite, and thus its bioavailability to the lamb, was limited. Selenium-yeast supplemented at the 4.9 mg of Se/wk dosage was more effective at improving Se status of lambs than Na-selenite at the 24.5 mg of Se/wk dosage, highlighting the superior bioavailability of Se-yeast. Thus, organic Se is transferred over a wide range of supplementation rates more efficiently from ewe to lamb than inorganic Se.

Se Transfer Across the Placenta

Others have shown that Se status of newborn lambs is closely correlated to Se status of their mothers (Abd El-Ghany et al., 2007). Thus, transplacental transfer of Se is the primary source of Se in newborn lambs before ingestion of colostrum. This fact was reinforced in our study by comparing WB- and serum-Se concentrations in lambs at parturition with Se concentrations in their mothers, which were measured within 30 d of lambing. In our study, transplacental transfer of Se from ewe to lamb was affected by both the source of Se (chemical form) as well as the dose of Se administered. This was best illustrated in WB- and serum-Se measurements of lambs born to ewes receiving supranutritional doses of Se-yeast. Lamb WB- and serum-Se concentrations in our study are also consistent with findings of Davis et al. (2006a) and Juniper et al. (2008), although our study is unique in that different sources and rates of Se supplementation were assessed. Additionally, our study compared supranutritional Se dosages to ewes that were closer to the current 0.7 mg/d NRC recommendations (i.e., 2.1 and 3.5 mg/d) compared with other studies that were aimed at determining maximum tolerable concentrations (Davis et al., 2006b; Tiwary et al., 2006).

Our results showed differences in the transplacental transfer efficiencies for Na-selenite vs. Se-yeast. At equivalent ewe dosing concentrations (4.9, 14.7, or 24.5 mg of Se/wk), we observed that lambs from ewes receiving Se-yeast had greater WB- and serum-Se concentrations compared with lambs from ewes receiving Na-selenite. Failure of Na-selenite to increase newborn Se status when supplemented at increasing concentrations agrees with conclusions of Behne and Kyriakopoulos (2001) that concentrations of inorganic Se sources are homeostatically controlled and blood concentrations cannot be significantly increased with increasing dosages. Our study findings are also consistent with those of Davis et al. (2006a), who found that lambs from ewes fed Na-selenite at 0.2, 4, 8, 12, and 16 mg/kg (as-fed basis) did not have significantly different plasma-Se concentrations. In contrast to Na-selenite, the transplacental transfer of Se from yeast increased as ewe supplementation rates increased from 4.9 to 14.7 mg of Se/wk, and from 14.7 to 24.5 mg of Se/wk, based on WB- and serum-Se concentrations of newborn lambs. Taylor et al. (2009) also observed differences in the transferability between inorganic and organic Se sources (i.e., ewes fed Se from Se-enriched grain more efficiently incorporated Se into fetal tissue than Se fed as Na-selenate). 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%; in our study at least 85%) and it is primarily responsible for the more efficient Se transfer (Whanger, 2002). The biochemical similarity between SeMet and Met allows nonspecific substitution of SeMet for Met. In other words, the placenta cannot distinguish between SeMet and Met, and consequently, SeMet is transported to the fetus via an AA transporter rather than by a Se-selective transporter (Hawkes et al., 1994). For this reason, the capacity to accumulate Se in WB is greater in lambs from ewes supplemented with Se-yeast compared with lambs from ewes supplemented with inorganic Se sources (e.g., Na-selenite and Na-selenate).

Tiwary et al. (2006) has shown that most of the Se from ingested Na-selenite is present as glutathione peroxidase (75 to 85%) in red blood cells, or in the selenoprotein P fraction of serum. In contrast, when SeMet is consumed, it is either nonspecifically incorporated into hemoglobin in red blood cells as SeMet, or it follows a specific metabolic pathway leading to hydrogen selenide, similar to selenite metabolism. Excess SeMet that is not immediately metabolized is incorporated nonspecifically into proteins in place of Met as SeMet (Tiwary et al., 2006). In our study, lambs from ewes fed 24.5 mg of Se/wk as Se-yeast achieved WB-Se concentrations at birth of ~700 ng/mL, which was similar to those attained in lambs in the Juniper et al. (2008) study that consumed 6.30 mg/kg of DM of Se-yeast for 91 d. Our study results show that the developing fetus accumulates Se relative to the amount of Se the dam consumes. Lamb WB- and serum-Se measurements suggest that supranutritional Se supplementation to ewes throughout gestation is an effective method of increasing newborn lamb Se status and is best achieved with Se-yeast.

Se Transfer to Colostrum and Milk

Results from our study emphasize the importance of colostrum as a concentrated source of Se for lambs. Regardless of the source of Se administered to ewes, Se was transferred to colostrum and 30-d milk, and more so when ewes were supplemented with Se-yeast at the 24.5 mg of Se/wk dosage. These findings are in agreement with those of Ortman and Pehrson (1999), who found that Se from Se-yeast was more efficiently transferred to milk of dairy cows compared with Na-selenite when fed at equal concentrations. We speculate that greater Se concentrations in the milk of ewes in the 24.5 mg of Se/wk as Se-yeast group are the result of SeMet being nonspecifically incorporated into milk proteins because of the relatively high Met content of milk (Weiss, 2005).

Milk-Se concentrations, even after Se supplementation, were more than 10-fold less in milk from ewes after 30 d of lactation than in colostrum, which indicates the limited ability to increase Se concentration in milk. A similar observation was made by Davis et al. (2006a) in milk samples collected at 28 d of lactation from ewes consuming dietary Na-selenite at supranutritional concentrations (e.g., 0.2, 4, 8, 12, 16, and 20 mg/kg, as-fed basis). Based on data from Wohlt et al. (1984) and current NRC recommendations, it is improbable that sufficient Se can be supplied in milk to satisfy Se requirements at this stage of lamb growth.

Se Transfer into Lamb Skeletal Muscle

Similar to WB-Se concentrations, Se concentrations in skeletal muscle increased in lambs from ewes supplemented with increasing Se dosages, with the greatest concentrations in lambs from ewes supplemented with 24.5 mg of Se/wk of Se-yeast. Lambs from ewes receiving Se-yeast at 14.7 and 24.5 mg of Se/wk had 29 and 59% greater Se concentrations in skeletal muscle, respectively, compared with lambs from ewes supplemented at the NRC-recommended rate of 4.9 mg of Se/wk. Ewes receiving supranutritional dosages of inorganic Se were not as effective at raising lamb skeletal-muscle-Se concentrations. In fact, the lamb skeletal-muscle-Se response to Na-selenite supplementation remained relatively constant. Although Juniper et al. (2008) showed that lambs fed supranutritional amounts of Se-yeast (e.g., at 6.3 mg/kg of DM) for a period of 91 d achieved Se-skeletal-muscle concentrations of 7.82 mg/kg, our lambs from ewes receiving 24.5 mg of Se/wk of Se-yeast had skeletal-muscle-Se concentrations of 1.70 mg/kg of DM. Based on the fact that the ~4-times greater supplementation rate in Juniper et al. (2008) achieved 4 times greater skeletal-muscle-Se concentration compared with our results, it is likely that lambs from ewes supplemented with Se-yeast accumulate SeMet in a dose-dependent manner in skeletal muscle. An abundance of SeMet in skeletal muscle could serve as a Se storage pool to be drawn upon as needed for the synthesis of specific selenoproteins when dietary Se intake is low.

Conclusions

This study has 2 major conclusions. First, weekly drenching of ewes with Na-selenite, although relatively cheap, is an ineffective method to improve Se status of newborn lambs. This is likely because of its low bioavailability and limited half-life (Whanger, 2002). Second, Se-yeast weekly drenching of pregnant ewes, although more expensive, provides an effective method to improve Se status of newborn lambs. The benefits for lambs born with adequate concentrations of Se are enhanced absorption of maternal antibodies in colostrum (Rock et al., 2001) and modulation of immune function, including improved immune responses to vaccination (reviewed in Rooke et al., 2004). In addition, organic Se stored as SeMet in skeletal muscle provides a slow-release reserve pool from normal protein turnover such that Se-yeast drenching before the lambing period may eliminate the need for costly injections or mineral Se premixes for lambs marketed within 210 d of birth (Juniper et al., 2008; Hall et al., 2009). Oral Se-yeast drenching would be especially useful for producers who raise animals in Se-deficient regions.

 

References

Footnotes