Selenium is an essential micronutrient in sheep. Adequate Se transfer from ewes to lambs is important to prevent Se-responsive diseases in lambs such as nutritional myodegeneration and Se-responsive unthriftiness (Muth et al., 1958). Current FDA regulations limit Se supplementation to 0.3 mg Se/kg diet (as fed), which is equivalent to 0.7 mg·ewe−1·d−1 (FDA, 2009). The most common inorganic Se sources are Na-selenite and Na-selenate, which are usually provided in mineral premixes or are 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 organic Se to the dam is an effective method to meet Se requirements of newborn lambs because Se crosses the placental barrier into fetal tissues and enters mammary secretions with greater transfer efficiency over a wider supplementation range for organic Se vs. inorganic Na-selenite (Taylor et al., 2009; Stewart et al., 2012). Selenium supplementation may improve reproductive efficiency (Balicka-Ramisz et al., 2006; Koyuncu and Yerlikaya, 2007; Muñoz et al., 2009) and lamb survival and growth (Langlands et al., 1991b; Gabryszuk and Klewiec, 2002; Muñoz et al., 2008). There are concerns, however, that Se supplementation around conception reduces embryonic survival (van Niekerk et al., 1996; Sánchez et al., 2008). Little is known about how different chemical sources of Se given at comparative dosages to ewes affect ewe and progeny performance. The objective of this study were to evaluate how different chemical forms of Se (Se-yeast and Na-selenite) administered at comparative dosages (4.9, 14.7, and 24.5 mg/wk) to mature ewes reared in Se-deficient regions affect ewe and progeny performance. We hypothesized that supplementing ewes with organic Se-yeast above current FDA-allowed levels improves lamb growth and survival with no detrimental effects on ewe reproduction.
MATERIALS AND METHODS
Experimental Design and Treatments
Experimental procedures used in this study were approved by the Institutional Animal Care and Use Committee of Oregon State University and have been described in detail previously (Hall et al., 2012). Briefly, 240 mature ewes were randomly assigned to 8 treatment groups (n = 30 each) based on Se supplementation rate and source: Na-selenite, Na-selenate (both from RETORTE Ulrich Scharrer GmbH, Röthenbach, Germany), and Se-yeast (Prince Se Yeast 2000, Prince Agri Products Inc., Quincy, IL). One group received no Se (depletion group); 1 group received Na-selenite at 8.95 mg Se/wk. Three groups received Na-selenite at 4.9, 14.7, or 24.5 mg Se/wk, and 3 groups received Se-yeast at 4.9, 14.7, or 24.5 mg Se/wk. The treatment period started approximately 2 wk before breeding and lasted for 62.5 wk for ewes receiving inorganic Se source. Treatments were continued for another 21 to 24 wk in remaining ewes of the no-Se and Se-yeast groups until they lambed a second time. All dosages were less than the maximum tolerable level (5 mg/kg as fed) for small ruminants (NRC, 2007). Treatment groups were blocked for age of ewe and footrot (FR) incidence and severity. Ewes were from 3 genotypes (Polypay, Suffolk, and Suffolk × Polypay) 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.
Treatments were administered individually once weekly by oral drench (with the calculated weekly amount of Se supplement being equal to the summed daily intake). The Se dose (0, 4.9, 14.7, or 24.5 mg Se) 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 Se solutions, respectively). Solutions were prepared weekly and administered with a dose syringe. 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 aliquots of inorganic Se solutions 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 calculated the molecular weight for 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 Se, respectively) were within expected analytical variance of their targeted concentrations. Concentrations of Se in the pasture forage from Sheep Center pastures ranged from 0.12 to 0.14 μg/g DM.
Ewes were naturally mated with a ram:ewe ratio of 3:100 starting in wk 3 and 62.5 of the supplementation period. Rams were with ewes the equivalent of 3 estrous cycles. Ewes were fed on pasture, except for a 3-mo period around lambing when ewes were housed in the barn. Ewes on pasture were supplemented with grass hay and later with alfalfa hay when grass was scarce. 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. Ewe feed sources and management details have been previously described (Hall et al., 2012). Sheep were fed to meet or exceed NRC (2007) recommendations.
Ewe and Progeny Performance
Reproductive performance was assessed by calculating the number of lambs born, nursed, or weaned per ewe (to 120 d of age in yr 1 and to 60 d of age in yr 2). For each treatment group, we also calculated percentage of ewes that lambed (% lambing; ewes that died before lambing were not included), that nursed lambs (% nursing; if insufficient milk production or mastitis was present at lambing, lambs were removed from the ewe), and that raised lambs (% raising; to 120 d of age in yr 1 and to 60 d of age in yr 2). The latter calculations did not account for the number of lambs per ewe. In yr 2, we recorded an additional variable (i.e., % assistance; percentage of ewes requiring assistance at birth for ewes in each treatment group).
Ewe health was evaluated for ewes in each treatment group by calculating the number of ewes that completed the trial in yr 1 and ewes that started and completed the trial in yr 2 (% survival). At the end of yr 1, ewes were weighed and BCS were assigned on a 1 to 5 scale (1 = emaciated; 5 = severely obese) by two independent evaluators (ASIA, 2002). Ewes with low BCS at the beginning of yr 2 were culled by the flock manager at the time the rams were introduced (wk 62.5). Thus, the numbers of ewes in each group at the beginning of yr 2 were 25, 20, 18, and 27 for the no-Se, 4.9, 14.7, and 24.5 Se-yeast groups, respectively. Based on necropsy results of ewes that died, culled ewes most likely also had severe intestinal parasitism with anthelmintic reisistance.
To assess lamb performance, lambs were weighed at birth and at 90 and 120 (weaning) d of age in yr 1 and at birth and at 10, 20, and 60 d of age in yr 2. Bodyweights (except for birth weights) were adjusted for lamb age at weighing because not all lambs were weighed at the same age, using the American Sheep Industry Association formulas (ASIA, 2002). To assess lamb health, the percentage of lambs that died before weaning (to 120 d of age in yr 1 and to 60 d of age in yr 2) was calculated for each treatment group. In yr 2, lamb vigor was scored from 0 to 15 min and from 15 to 30 min after lambing on a 1 to 7 scale: 1 = dead; 2 = hypothermic (recorded rectal temperature); 3 = weak and lethargic; does not attempt to hold up head or stand; 4 = holds up head; 5 = holds up head and attempts to stand; 6 = stands for over 20 sec; 7 = vigorous (stands immediately and attempts to nurse).
Statistical analyses were performed using SAS, Version 9.1 (SAS Inst., Inc., Cary, NC) software. Body weights and vigor scores for lambs from the same ewe were averaged because ewe was the experimental unit. PROC GLIMMIX was used to analyze data for % lambing, % nursing, and % weaning, assuming a binomial distribution. Count data (lambs per ewe born, nursed, and weaned) were analyzed in PROC GLIMMIX assuming a negative binomial distribution. Fisher’s exact test was used for assessing % ewe survival and % lamb survival (from nursing to weaning). The remaining data (BW, BCS, and vigor scores) were analyzed with PROC GLM assuming a normal distribution. Fixed effects in the model were treatment (no Se, 8.95 mg/wk Na-selenate, 4.9 mg/wk Na-selenite, 14.7 mg/wk Na-selenite, 24.5 mg/wk Na-selenite, 4.9 mg/wk Se-yeast, 14.7 mg/wk Se-yeast, and 24.5 mg/wk Se-yeast), ewe FR-status at the beginning of the treatment period (yes or no), ewe breed (Polypay, Suffolk, or Suffolk × Polypay; Suffolk and Suffolk × Polypay ewes were combined into one group because the animal numbers for both groups were small and phenotypically the crossbreds more closely resembled the Suffolks in size), ewe age at lambing (3 to 6 or ≥6 yr), sex of lamb (male or female; only available for yr 1; used only for weights and weight gains of lambs), number of lambs born (0, 1, and ≥2; used only for lamb data), number of lambs reared (0, 1, and ≥2; used only for lamb growth data). To evaluate whether FR-status, ewe breed, number of lambs born, and number of lambs reared modified the treatment effect, data were checked for interactions and additionally stratified by FR-status, ewe breed, number of lambs born, and number of lambs reared, respectively.
To test the effect of chemical form and Se dosage on ewe and progeny performance, orthogonal contrasts were constructed using the ESTIMATE statement in PROC GLIMMIX, PROC GLM, and PROC MIXED. The effect of Se depletion on ewe and progeny performance was evaluated (no-Se vs. 7 Se groups). The effect of Se dosage for Na-selenite (only for yr 1) or for Se-yeast was evaluated by comparing the 3 dosages through linear (24.5 vs. 4.9 mg Se/wk) and quadratic (14.7 vs. 4.9 and 24.5 mg Se/wk) contrasts, respectively. The no-Se group was not included in testing the effect of Se dosage because the purpose of the no-Se group was to measure the effect of Se depletion. During depletion, Se values continued to decrease (Hall et al., 2012); thus, Se values were unstable in the no-Se group (unlike what is expected of a true control group). The control groups are those sheep groups receiving the maximum FDA-allowed Se amount (4.9 mg Se/wk). The effect of chemical form of Se was evaluated (3 Na-selenite groups vs. 3 Se-yeast groups; only for yr 1). 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 (only for yr 1). Data are reported as least squares means ± SEM except for % ewe survival and % lamb survival; the latter are means ± SEM. Statistical significance was declared at P ≤ 0.05, and a tendency at 0.10 ≤ P < 0.05.
RESULTS AND DISCUSSION
Supplementation of pregnant ewes with Se is an effective method to improve Se status of their lambs; however, producers are primarily interested in how Se supplementation affects ewe and lamb performance. The objective of this study was to evaluate how inorganic (Na-selenite) and organic forms (Se-yeast) of Se administered at FDA-allowed (4.9 mg Se/wk) or supranutritional dosages (14.7 and 24.5 mg Se/wk) to mature ewes affect ewe and lamb performance. Our primary findings are that chemical form or dosage of Se did not significantly affect reproductive performance. However, BCS of ewes and growth of lambs were or tended to be better when ewes received Se-yeast at 24.5 mg Se/wk vs. 4.9 mg Se/wk. We conclude that Se-yeast at 24.5 mg Se/wk may improve lamb performance, compared with FDA-allowed dosages of 4.9 mg Se/wk in regions with Se-deficient soils.
Effect of Selenium on Ewe Reproductive Performance and Health
Chemical form and dosage of Se, and Se depletion, did not significantly affect the proportion of ewes that lambed or needed assistance at birth, or the number of lambs born, nursed, or weaned per ewe in yr 1 (Table 1) or yr 2 of the study (Table 2). Similar results, albeit with smaller animal numbers and fewer treatment groups, have been reported previously (Rock et al., 2001; Rodinova et al., 2008). Results from other studies show that testing for an effect of Se supplementation on reproductive performance depends upon the Se status of the control group. In Se-deficient ewes, Se supplementation improves reproductive performance (Hemingway, 2003; Muñoz et al., 2009), but not always in Se-replete ewes (Whanger et al., 1977; Langlands et al., 1991a), as shown in our study (Tables 1 and 2).
|Oral selenium drench
|Se dosage, mg Se/wk||0||8.95||4.9||14.7||24.5||4.9||14.7||24.5||SEM2||Overall|
|Ewe Reproduction||n = 30||n = 30||n = 30||n = 30||n = 30||n = 30||n = 30||n = 30|
|Ewe Health, after 60 wk|
|Individual BW, kg4|
|At 90 d of age||33.2||32.8||32.6||32.4||34.0||31.9||33.0||33.2||1.8||0.75|
|At 120 d of age|
|Single lambs7||n = 7||n = 13||n = 7||n = 8||n = 7||n = 14||n = 15||n = 10|
|Multiple lambs7,8||n = 20||n = 15||n = 19||n = 18||n = 21||n = 13||n = 11||n = 13|
|Polypay ewes9||n = 17||n = 16||n = 14||n = 14||n = 16||n = 13||n = 16||n = 17|
|Suffolk ewes10||n = 10||n = 12||n = 12||n = 12||n = 12||n = 14||n = 10||n = 6|
|Sum BW/ewe, kg11|
|At 120 d of age||47.4||46.5||45.2||44.7||46.7||44.7||44.4||47.1||3.2||0.79|
|% Survival from onset of nursing to 120 d||100||100||100||100||98||100||100||98||3||NA|
|Se dosage, mg Se/wk||0||4.9||14.7||24.5||SEM1
|Ewe reproduction||n = 25||n = 20||n = 18||n = 27|
|% Survival, overall3||92||100||100||100||6||0.07||1.00||1.00|
|Individual BW, kg4|
|At 10 d of age||7.28||6.92||7.31||7.50||0.47||0.94||0.32||0.84|
|At 20 d of age||10.22||9.79||10.73||10.38||0.55||0.88||0.39||0.30|
|At 60 d of age|
|Single lambs6||n = 8||n = 3||n = 5||n = 5|
|Multiple lambs6,7||n = 10||n = 9||n = 9||n = 17|
|Polypay ewes8||n = 11||n = 6||n = 10||n = 15|
|Suffolk Ewes8,9||n = 7||n = 6||n = 4||n = 7|
|Vigor score, 15 min10||4.87||4.70||4.92||4.94||0.26||0.94||0.45||0.74|
|Vigor score, 30 min10||6.37||5.97||5.90||6.52||0.25||0.32||0.08||0.21|
|% Survival from onset of||64||75||75||90||9||0.06||0.16||0.54|
|nursing to 60 d11|
There are some concerns that additional Se supplementation in ewes with adequate Se status may negatively affect embryonic survival shortly after conception through an unknown mechanism (van Niekerk et al., 1996; Sánchez et al., 2008; Palmieri and Szarek, 2011). Davis et al. (2006a,b) addressed these concerns by feeding ewes throughout 2 lambing seasons with up to 20 mg Se/kg of diet as fed of Na-selenite, dietary Se concentrations much greater than in our study, and did not detect detrimental effects on ewe conception rates. Similarly, Glenn et al. (1964) did not observe detrimental effects on fertility or early embryonic development in ewes treated daily with subtoxic to toxic (50 mg Se/d) doses of Na-selenate for extended periods. The previously reported negative effects of Se supplementation on ewe reproduction may be because van Niekerk et al. (1996) used a very high dosage of Ba-selenite (one time injection of 50 mg Se as Ba-selenite; no significant effect of injecting 1 mg Se as Na-selenite) administered 18 to 35 d after mating to achieve the decrease in conception rates. Sánchez et al. (2008) observed that estrus synchronized ewes receiving 0.125 mg Se/d as SeMet in the diet had lower conception and lambing rates than estrus synchronized ewes receiving no Se supplement, although naturally mated and Se-supplemented ewes had similar conception and lambing rates, compared with non-Se-supplemented ewes. Therefore, we conclude that there is little evidence to suggest a detrimental effect of supranutritional Se supplementation at dosages up to 24.5 mg Se/wk. However, ours as well as many of the other studies, have the limitation that several hundred animals per treatment group are needed to detect differences of even 10% with binary data. Therefore, an even larger-scale study to evaluate the effect of supranutritional Se supplementation with organic Se-yeast on reproductive efficiency is warranted.
Ewes receiving Se-yeast at 24.5 vs. 4.9 mg Se/wk were in better body condition at the end of yr 1 (P = 0.05; Table 1). A similar effect was not found with supranutritional dosages of Na-selenite. Muñoz et al. (2009) reported that Se supplementation increased BW and BCS in ewes. Meyer et al. (2010) reported that although dietary Se supply had no effect on ewe BW, it did affect ADG during pregnancy such that ewes fed high Se had a greater BCS increase during midgestation and a lesser BCS loss during late gestation than those fed adequate Se. In most other studies, inorganic or organic dietary Se supplementation above the maximum FDA-allowed level was not associated with changes in BW (Davis et al., 2006b; Neville et al., 2008; Swanson et al., 2008).
There are at least 3 groups of selenoproteins: gluthathione peroxidase family of enzymes, responsible for reduction of hydroperoxides; iodothyronine deiodinases, responsible for metabolism of thyroid hormones; and thioredoxin reductases, which reduce thioredoxin and are thus involved in many cell functions including cell growth, control of apoptosis, and maintenance of cellular redox status, often through regulation of transcription factors (Rooke et al, 2004). An understanding of the role of Se in immune function and disease resistance led Finch and Turner (1996) to summarize that Se deficiency could compromise the immune system and result in a decline in production and performance before gross effects became apparent. Selenium supplementation might thereby improve feed efficiency by reducing the incidence of subclinical disease. We conclude that supranutritional Se supplementation with Se-yeast at 24.5 mg Se/wk vs. 4.9 mg Se/wk improves the overall fitness of ewes.
Effect of Selenium on Lamb Performance
The dosage of Se administered to ewes as Se-yeast, but not Na-selenite or Se depletion, affected birth weights of their lambs in yr 1 (Table 1). There were both linear and quadratic effects of Se-yeast dosage (4.9, 14.7, and 24.5 mg Se/wk). Lambs from ewes receiving Se-yeast at 4.9 mg Se/wk had the lightest birth weights, compared with any of the other groups (P ≤ 0.05). There was no effect of maternal Se-yeast dosage on lamb birth weights in yr 2 (Table 2). However, lambs from ewes receiving Se-yeast at 24.5 vs. 4.9 mg Se/wk tended to have higher vigor scores 30 min after lambing in yr 2 (P = 0.08). Some studies have shown a significant increase in fetal weight with supranutritional Se supplementation of ewes during pregnancy (Reed et al., 2007); others have not (Neville et al., 2008). Some studies have shown decreased lamb BW at birth (Gabryszuk and Klewiec, 2002); others have shown no effect on birth weight (Muñoz et al., 2008; Swanson et al., 2008) in lambs from ewes consuming either organic or inorganic Se sources at various dosages.
Ewes receiving Se-yeast at 24.5 vs. 4.9 mg Se/wk had heavier lambs at weaning (120 d of age) in yr 1 (P = 0.05; Table 1) and tended to have heavier lambs in yr 2 (P = 0.09; Table 2). In addition, compared with ewes receiving no Se supplement, lambs from ewes receiving Se-yeast at 24.5 mg Se/wk were heavier (P = 0.04) and had a higher survival rate at 60 d of age in yr 2 (P = 0.01; Table 2). The effect was more pronounced in ewes of heavier breeds (Suffolk or Suffolk × Polypay) vs. lighter breeds (Polypay), and in ewes having multiple vs. single lambs; however, the sample size got very small and the interaction was not always significant (Tables 1 and 2). The level of selenium supplementation needed for adequate performance may be less under optimum conditions; however, in situations of raising multiple lambs and in breeds with faster growth rates, Se requirements may be increased for optimum performance.
Previous studies have reported that Se supplementation of ewes during pregnancy improves lamb growth in the first 2 wk (Abd Elghany et al., 2008), 4 wk (Gabryszuk and Klewiec, 2002), 6 wk (Muñoz et al., 2009), and at 60 d (Koyuncu and Yerlikaya, 2007). Similar to our study, Muñoz et al. (2008) reported reduced perinatal lamb mortality, yet overall mortality from birth to weaning was unaffected when dams were supplemented with Se. Muñoz et al. (2008) also reported heavier weaning weights at 16 wk in lambs from ewes receiving 0.5 mg Se/d as Se-yeast from −14 to 90 d after mating. Kott et al. (1983) reported improved preweaning lamb survival in ewes receiving monthly injections of 4 mg Se as Na-selenite during mating and pregnancy. It is well known that Se supplementation of Se-deficient lambs results in significant BW gain responses (Grace and Knowles, 2002; Rooke et al., 2004). We have no explanation why the lower level of Se supplementation had no beneficial effect, compared with Se depletion. One potential reason could be that animals in the Se depletion group were not Se deficient. We did not observe any overt clinical signs of Se deficiency in ewes or their progeny. Another possibility is that the lower level of Se supplementation was not adequate to see beneficial effects.
In summary, supranutritional Se supplementation with Se-yeast at 24.5 mg Se/wk compared with the maximum FDA-allowed levels of 4.9 mg Se/wk resulted in increased BW of weaned lambs and improved overall health of ewes and lambs in a region with Se-deficient soils. Supranutritional Se supplementation of pregnant ewes with Se-yeast may be an effective management strategy for meeting the Se requirements of growing lambs, especially when providing Se supplementation to lambs is a complicated endeavor.