Selenium Supplementation Efficacy in Small Ruminants: A Review

Iranian Journal of Applied Animal Science

دوره 12، شماره 4، اسفند 2022، صفحه 633-645   

نوع مقاله: Review Article

نویسندگان

A.B. Amin^{* 1، 2}؛ R. Audu¹؛ A.A. Ibrahim¹؛ M. Dalha¹؛ M.T. Aleem^{* 3}؛ A.I. Abdullahi⁴

¹Department of Animal Science, Federal University Dutse, P.M.B 7156 Dutse, Jigawa State, Nigeria

²College of Animal Science and Technology, Nanjing Agricultural University, Nanjing 210095, China

³MOE Joint International Research Laboratory of Animal Health and Food Safety, College of Veterinary medicine, Nanjing Agricultural University, Nanjing 210095, China

⁴National Productivity Centre, Abuja, Nigeria

چکیده

This review is aimed at highlighting the role of selenium (Se) in enhancing the antioxidant status in the blood, immune response, reproductive performance, rumen fermentation, as well as feed efficiency, and weight gain in small ruminants. Selenium is supplied in small ruminant’s diet in organic form, inorganic form, or selenium nanoparticles (nano-Se). Each form of Se is metabolized differently and exhibits different levels of bioavailability with nano-Se having the highest bioavailability followed by organic Se. A sufficient supply of Se in small ruminant’s diet is important to maintain the proper functioning of the antioxidant defense system which reduces the detrimental effects of free radicals in cells thereby preventing the incidence of metabolic diseases. The ability of Se to improve the antioxidant capacity of the cells also helps to strengthen the immune response of animals and ameliorate the detrimental effects of heat stress. Selenium has also been proven to modify rumen fermentation and microbiota which translated to improved feed efficiency and enhanced weight gain. However, the activities and bioavailability of Se are limited by several factors including the rumen microbiota, the form of Se, and competition with other minerals. Further studies need to investigate the effects of Se on rumen microbiology and heat stress in small ruminants.

کلیدواژه‌ها

antioxidant؛ glutathione peroxidase؛ immunity؛ rumen fermentation؛ selenium

اصل مقاله

INTRODUCTION

Selenium (Se) is an essential trace mineral in livestock due to its role in health and the prevention of nutritional disorders. The supplementation of Se is practiced worldwide in ruminants kept under an intensive system of production (Ahmadi et al. 2018; Ahmadi et al. 2019; Pan et al. 2021). Moreover, several farmers in developing countries are becoming aware of the need to include exogenous Se sources in livestock diets (Suganthi et al. 2019). This has been attributed to the low level of Se in soils resulting in Se deficiency in available grains and forages (Suganthi et al. 2019; Mojapelo et al. 2021). The concentration of Se in forages and grains varies widely depending on the soil’s Se content and geographical location (Zhang et al. 2018). In ruminants, Se is required in minute amounts but it plays a key role as a constituent of about 30 seleno-proteins which are responsible for its biological functions (Niwinska and Andrzejewski, 2017). Selenium is a major component of many enzymatic structures and seleno-proteins like glutathione peroxidase 1, glutathione peroxidase 4, iodothyronine-5-deiodinase, and thioredoxin reductase 1 (Kachuee et al. 2019; Lee et al. 2019). Glutathione peroxidases (GSH-Px) and Seleno-protein W1 are seleno-proteins that play a vital role in the antioxidant defense system of the animal (Rezaei and Dalir-Naghadeh, 2009) by protecting immune cells from oxidative damage (Carr et al. 2020; Ghaderzadeh et al. 2020; Wang et al. 2021a). Seleno-protein W1 is also used as a biomarker to measure the efficiency of Se as a source of seleno-protein production (Kipp et al. 2013). The majority of the seleno-proteins identified in livestock were found to be associated with stress responses, antioxidant system, immune system, and inflammatory processes in the body (Guo et al. 2018; Taheri et al. 2018; Kachuee et al. 2019; Lee et al. 2019; Surai et al. 2019; Ianni et al. 2020; Zhuang et al. 2020; Libera et al. 2021). Probably due to its role in several physiological processes in the animal’s body (Taheri et al. 2018; Zhang et al. 2018; Mousaie, 2021), the supplementation of Se was reported to improve the rate of maturity in kids (Mojapelo and Lehloenya, 2019), increase the milk yield in does (Rashnoo et al. 2020), enhance feed efficiency in lambs (Mousaie, 2021), and improve semen quality in bucks (Mojapelo et al. 2021). Inadequate supply of Se in small ruminants can result in several metabolic diseases such as retarded growth, weakened immune system, low feed efficiency, and reproductive failures, (Ianni et al. 2019; Sun et al. 2019; Lizarraga et al. 2020). On the other hand, the increased concentration of Se beyond the required amounts leads to Se toxicity in livestock (Taheri et al. 2018). In livestock, Se deficiency is more common than Se toxicity (Ensley, 2020), but for normal physiological processes, an optimal level of Se should always be maintained (Sun et al. 2019). However, Se has a very narrow safety margin, thus making it the only element with a legal limit, above which Se toxicity occurs (Ensley, 2020). This review is therefore aimed at summarizing the roles of Se on blood antioxidant status, immune response, reproductive performance, rumen fermentation, as well as feed efficiency, and weight gain in small ruminants.

 

Forms of Se

Selenium is usually included in the diets of sheep and goats in the form of organic and inorganic Se (Table 1 and Table 2). Examples of organic Se used in small ruminants include selenomethionine (Davis et al. 2017; Erdogan et al. 2017; Kachuee et al. 2019; Paiva et al. 2019; Suganthi et al. 2019) and selenium yeast (Se-yeast) (Czauderna et al. 2017; Paiva et al. 2019; Mousaie, 2021). A common source of inorganic Se in sheep and goats is sodium selenite and sodium selenate (Czauderna et al. 2017; Davis et al. 2017; Kachuee et al. 2019; Paiva et al. 2019; Khalili et al. 2020). In the body system, both organic and inorganic Se sources are converted to selenide which is later transformed into Se-cysteine and this is an important constituent of seleno-proteins (Ullah et al. 2020). However, these two forms of Se exhibit different metabolic potentials in ruminants (Khalili et al. 2020). The organic Se is reported to have a higher absorption rate and bioavailability, increased tissue storage, and less toxicity in the animal’s body when compared to the inorganic forms (Groce et al. 1973; Ortman and Pehrson, 1997; Danielsson et al. 2012; Davis et al. 2017). This could be attributed to the slow absorption of organic Se in the body, which is similar to that of amino acids. In addition, some organic Se such as Se-yeast has a bioavailability of about 120% to 200% when compared with the inorganic Se sources (Muegge et al. 2017). However, the inorganic Se is usually absorbed and metabolized in a similar rapid pattern as other trace mineral elements (Groce et al. 1973) and it is reported to be more toxic than the organic Se. Some of the soluble Se salts like Na₂SeO₃ and Na₂SeO₄ are considered highly toxic while the poorly soluble Se salts such as elemental Se, SeS₂, diphenyl selenide, and Na₂Se are less toxic (Ullah et al. 2020). The efficacy of Se nanoparticles (nano-Se) on the performance of small ruminants has been investigated (Kachuee et al. 2019; Lee et al. 2019; Ghaderzadeh et al. 2020; Nateq et al. 2020) and the results seemed promising. Their mode of action is via stimulating the activities of selenoenzymes (Kachuee et al. 2019). This technology enhances the bioavailability of Se by increasing its surface area (Arshad et al. 2021). Nanoparticles occur in the natural environment and range between 1 to 100 nm in size (El-Naby et al. 2020). Due to their small sizes, nano-Se have higher absorption rates in the gut which improved their biological activities within the animal’s system (Patra and Lalhriatpuii, 2020). The smaller the nano-Se the higher the activity (Hosnedlova et al. 2018). The smaller sizes and increased surface area allow nano-Se to exhibit greater mucosal permeability and higher intestinal absorption (Libera et al. 2021). Selenium nanoparticles have also been reported to possess higher bioavailability, increased tissue storage, higher surface charge, and reactivity, enhanced antioxidant effect, increased binding properties reduced antagonism with other minerals, and less toxicity than the organic and inorganic sources of Se (Hosnedlova et al. 2018; Khalil et al. 2019; Patra and Lalhriatpuii, 2020; Han et al. 2021; Libera et al. 2021). These distinctive characteristics of nano-Se make them the preferred form of Se in many biomedical applications (El-Naby et al. 2020).

 

Effects of Se blood antioxidant status

Antioxidants are important in scavenging free radicals and inhibiting oxidative damage in the cells (Sun et al. 2019; Ghaderzadeh et al. 2020). Inadequate production of antioxidants or excessive production of free radicals can cause oxidative stress in livestock (Gong and Xiao, 2018; Sun et al. 2019). Reactive oxygen species (ROS) are free radicals that are usually produced in the body during energy production or increased activities of NADPH oxidase can damage body cells when they accumulate (Gong and Xiao, 2018).

 

Table 1 Performance of sheep supplemented with Se

 

Continuation of Table 1 Performance of sheep supplemented with Se

 

Examples of ROS include hydroxyl radicals (OH-), superoxide anion radicals (O^-₂), and hydrogen peroxide (H₂O₂). During oxidative metabolism, H₂O₂ is produced and it infiltrates the cells to cause oxidative stress. The excessive production of ROS can also activate cellular apoptosis which could destroy nucleic acids, proteins, and lipids within the cell (Surai et al. 2019; Wang et al. 2021b). Therefore, the accumulation of ROS is seen as the cause of several metabolic diseases and there is a need to increase the synthesis of radical scavenging enzymes capable of neutralizing the excess ROS within the cell (Ullah et al. 2020). Research has shown that an exogenous supply of Se is important to prevent the buildup of these ROS in animal cells (Muegge et al. 2017).

 

Table 2 Response of goats supplemented with Se

 

This action is due to the production of some enzymes that participates in the defense system such as coenzyme Q, GSH-Px, and thioredoxin (Surai et al. 2019). These enzymes eliminate excess ROS thereby maintaining a balance between the synthesis and elimination of free radicals (Mudron and Rehage, 2018; Surai et al. 2019; Libera et al. 2021). Glutathione peroxidase is an enzyme that protects the intracellular fluids from damage caused by free radicals which makes it a great antioxidant. It safeguards the development and structure of the mitochondrial capsule by forming cross-links between itself and other proteins (Khalil et al. 2019). Glutathione peroxidase also reduces H₂O₂ to H₂O (Han et al. 2021) and destroys lipid peroxides that are synthesized by free radicals and other products of oxidative metabolism (Lizarraga et al. 2020). The seleno-cysteine present in GSH-Px 1 is utilized for decreasing the reactive oxygen species and intracellular hydrogen peroxide produced by the cells during oxidative respiration (Ghaderzadeh et al. 2020). Hence, due to the role of GSH-Px in the antioxidant system, it is widely recognized as a biomarker for oxidative stress in ruminants (Han et al. 2021) and GSH-Px activity in the whole blood is a reliable index to predict long-term Se status in livestock (Juniper et al. 2019). In addition to the GSH-Px, serum malondialdehyde (MDA) is used to estimate the level of lipid peroxidation (Khatti et al. 2017). Malondialdehyde is produced from lipid peroxidation in the cells and is used as a marker to indicate oxidative stress in cells. An increased concentration of MDA suggests a high level of oxidative stress (Gaweł et al. 2004). Studies investigating the efficacy of Se administration of blood on blood glutathione peroxidase status are highlighted below.

 

Effects of Se on blood glutathione peroxidase status

The concentration of GSH-Px in the blood is a good indicator of the Se status of the animal and can be a cheaper alternative to assessing blood Se levels (Juniper et al. 2019; Diyabalanage et al. 2020). It can also be used to assess the level of oxidative stress in cells (Han et al. 2021). The administration of Se in sheep and goats has been effective in stimulating the activities of glutathione peroxidase in the blood. In goats, the administration of Se resulted in increased activity of GSH-Px in the serum of the treated groups when compared to the unsupplemented group (Rashnoo et al. 2020; Mojapelo et al. 2021). Similarly, in sheep, the supplementation of organic and inorganic Se (Paiva et al. 2019) increased the activity of GSH-Px (Paiva et al. 2019; Suganthi et al. 2019; Mousaie, 2021), in addition to iodothyronine deiodinases, seleno-protein P, and seleno-protein W1 (Suganthi et al. 2019) in the blood of the treated lambs when compared to the unsupplemented group. In another study on lambs, the addition of nano-Se in their diets increased the activities of superoxide dismutase and GSH-Px in the blood in addition to an upregulation in the expression of seleno-protein W1 and GSH-Px 1 in the liver when compared to the unsupplemented group and lambs fed conjugated linoleic acid (Ghaderzadeh et al. 2020). The efficacy of vitamin E (Vit E), and sodium selenite administration intravenously was investigated and the results revealed increased activity of GSH-Px in the blood of the treated rams (Mahmoud et al. 2013).

 

Effects of Se on heat stress

The supplementation of Se in ruminants is known to ameliorate the detrimental effects of heat stress on the animal. Heat stress accelerates the accumulation of free radicals in the cells thereby resulting in the destruction of DNA, lipids, and proteins including those cells associated with immune response in the body (Patra and Kar, 2021). The negative effects of heat stress on the humoral and cellular immune response are linked to the alteration of cytokine expression and increased production of cortisol during a severe incidence of heat stress thereby causing oxidative stress as well as immunosuppression (Patra and Kar, 2021; Zheng et al. 2021). In a study on lambs exposed to heat stress, the supplementation of Se and vitamin E improved the feed intake and weight gain of lambs when compared to the unsupplemented group (Chauhan et al. 2016). The administration of Se injection in sheep under heat stress decreased the rectal temperature by 0.3 ˚C in the treated group when compared to the unsupplemented group. In addition, the animals in the treated group recorded an increased blood eosinophil count and decreased weight loss as compared to the unsupplemented group (Alhidary et al. 2012). In a related study, Alhidary et al. (2015) supplemented Se and vitamin E to heat-stressed sheep and reported a decreased weight loss in the treated group in addition to increased antioxidant status. This suggests that the amelioration of the negative effects of heat stress on the animals is due to an increased antioxidant defense system stimulated by Se supplementation.

 

Effects of Se on the immune system and health of small ruminants

A sufficient supply of Se in small ruminants is important to maintain the proper functioning of the antioxidant defense system (Muegge et al. 2017). An enhanced antioxidant system in animals will reduce the detrimental effects of ROS thereby preventing the incidence of metabolic diseases, especially during the periparturient period in female animals (Gong and Xiao, 2018). Oxidative stress can weaken the host immune system and trigger inflammatory responses thereby resulting in clinical symptoms such as mastitis, embryonic degeneration, encephalomalacia, liver necrosis, cardiomyopathy, and myopathy (Gong and Xiao, 2018; Mudron and Rehage, 2018; Libera et al. 2021). Selenium is absorbed as seleno-protein (mainly selenocysteine) which is important for the synthesis of GSH-Px by the body (Muegge et al. 2017). The importance of GSH-Px in neutralizing the activities of ROS in the cells and ensuring a balance between the synthesis and elimination of free radicals cannot be overemphasized (Mudron and Rehage, 2018; Surai et al. 2019; Libera et al. 2021). Hence, it is imperative to maintain a sufficient supply of Se to ensure continuous synthesis of seleno-proteins and GSH-Px (Muegge et al. 2017). Selenium plays a key role in the function of the immune system and its shortage will negatively affect the animal’s immune defense system (Prince et al. 2017). Selenium improves the immunity of animals via both humoral and cellular immune activities (Khalili et al. 2020; Wang et al. 2021a). This is achieved through stimulation of the cytokines secretion and enhancement of the proliferation as well as differentiation of lymphocytes (Wang et al. 2021a).

 

Effects of Se on rumen fermentation, feed intake, and weight gain

The rumen harbors several species of beneficial microorganisms (Archaea, Bacteria, Protozoa, Fungi, and Viruses) that are responsible for the breakdown of fibrous feeds. These rumen microbiotas interact with each other and their activities are linked to the host’s metabolism and performance (Amin et al. 2022). In the rumen, Se is either utilized by the microbiota for the synthesis of seleno-amino acids for their use or reduced to elemental Se salts with poor solubility and hence cannot be absorbed by the animal (Niwinska and Andrzejewski, 2017; Ferreira and Petzer, 2019). The metabolism of Se in the rumen environment depends on the form of Se given to the animal. Although both organic and inorganic Se could be metabolized by rumen microbes into selenite which is further transformed into bacterial protein (seleno amino acids) (Ferreira and Petzer, 2019), the absorption of organic Se by the rumen microbiota is greater than inorganic sources of Se. Due to the powdery form of most inorganic Se, they provide a higher surface area for the action of rumen microbiota and conversion into poorly soluble elemental Se (Niwinska and Andrzejewski, 2017). The action of the rumen greatly reduces the bioavailability of Se in ruminants (about 34%) when compared with non-ruminants like swine (about 85%) (Galbraith et al. 2016). The encapsulation of inorganic Se salts has proven to be effective in protecting them from the action of rumen microbiota until they reach the small intestine (Niwinska and Andrzejewski, 2017). The efficacy of supplementing Se on rumen fermentation and microbiota has been investigated. In a study on goats by Tian et al. (2022), the application of metagenomics revealed enrichment in the carbohydrate-binding module and glycosyl transferase pathway in the group supplemented with Se-yeast when compared to the unsupplemented group. The goats supplemented with Se-yeast also had a higher concentration of propionic acid, isobutyric acid, caproic acid, and isovaleric acids when compared to the unsupplemented group. In addition, the relative abundances of the genera Sarcina and Methanobrevibacter were found to be reduced by Se supplementation. In another study, the supplementation of sheep with Se-yeast increased the ruminal concentration of total volatile fatty acids and propionate in the treated group as compared to the unsupplemented group. In addition, gene function prediction showed that carbohydrate metabolism and metabolism of other amino acids were upregulated in the rumen of sheep supplemented with Se (Cui et al. 2021). Naziroglu et al. (1997) studied the influence of Se and vitamin E supplementation on rumen fermentation parameters in lambs and they reported an increased concentration of acetic acid, butyric acid, propionic acid, and total volatile fatty acids in the rumen of the treated groups when compared to that of the unsupplemented group. In addition, the total protozoa counts, and the relative percentage of Diplodinium were also found to be significantly higher in the group supplemented with Se and vitamin E. However, the rumen pH and abundance of Dasytricha ruminantium were decreased by the treatment. Similarly, Xun et al. (2012) reported a higher concentration of total volatile fatty acids (VFA) and propionate production and a decreased ruminal pH in sheep supplemented with Se as compared to the unsupplemented group. Hendawy et al. (2022) added that Se encourages the growth of cellulolytic bacterial species in the rumen thus resulting in higher VFA production and consequently decreasing ruminal pH. The positive effects of Se on rumen fermentation as evidenced in these studies are expected to translate to improved growth performance in the animals (Figure 1). In some studies, investigating the effects of Se on the performance of sheep, the results showed an improved dry matter intake (Maraba et al. 2018; Mousaie, 2021; Pan et al. 2021), dry matter digestibility (Pan et al. 2021), body weight gain (Maraba et al. 2018), and feed conversion ratio (Mousaie, 2021) in the treated group when compared to the unsupplemented group. Likewise in goats, pregnant does given Se supplements in their diets recorded an improved weaning weight and average daily gain of their kids (Rashnoo et al. 2020; Zarbalizadeh-Saed et al. 2020) as well as increased dry matter intake and crude fat digestibility (Taheri et al. 2018) when compared to the group that did not receive Se supplements.

 

Figure 1 Summary of the role of Se administration in sheep and goats

 

Effects of Se on milk quality

Similar to the function of Se in the antioxidant defense of cells, selenoproteins also protect polyunsaturated fatty acids particularly linoleic acid, an important component of milk fat, from peroxidation (Ling et al. 2017). The activities of GSH-Px were confirmed by an upregulation in the expression of mRNA GPx in the mammary glands thereby suggesting increased antioxidant protection (Han et al. 2021). In a study by Pulido et al. (2019), the oral administration of α-tocopherol (1000 IU/day) and sodium selenite (0.4 mg/day) to lactating ewes increased proportions of long-chain fatty acids while the concentrations of short-chain fatty acids and medium-chain fatty acids decreased in the milk as compared to that of the unsupplemented group. Similarly, Mitsiopoulou et al. (2021) added organic Se (0.1 mg Se-yeast/kg) and vitamin E (60 mg/kg) into the diets containing sesame seed oil and reported an increased proportion of long-chain fatty acids, unsaturated fatty acids, and medium-chain fatty acids in the milk of goats when compared to the unsupplemented group. The inclusion of Se-yeast into the diets of lactating goats increased the milk yield, milk fat yield, and milk protein yield while the somatic cell count decreased when compared to goats supplemented with sodium selenite (Reczyńska et al. 2019). In the same way, the inclusion of organic (Se-enriched yeast) and inorganic (sodium selenite) Se into the diets of goats increased the milk yield. Interestingly, organic Se was found to be more effective than inorganic Se at 0.4 mg/kg diet (Zhang et al. 2018). The efficacy of slow-releasing Se boluses was investigated in lactating goats and the results showed an improved milk yield, milk fat percentage, milk Se, and iodine concentration as well as milk component yields in the treated groups when compared to goats in the unsupplemented group (Rashnoo et al. 2020). Although the mechanism of action is unclear this suggests that Se administration in lactating goats could enhance milk yield and yield of milk components.

 

Effects of Se on reproductive performance in ewes and does

The nutrient requirements, including trace elements, in pregnant animals increase as the gestation stage progresses until it reaches its peak around the late pregnancy period. Hence, there is a need to increase the supply of trace minerals to ewes and does in late pregnancy (Zarbalizadeh-Saed et al. 2020). The appropriate concentration of Se in animal diets is determined by the physiological stage of the animal, the Se status of the animal, and the form of Se (Arshad et al. 2021). When Se supplement is given to pregnant ewes, it helps to maintain the Se concentration in the blood and colostrum concentration. The increased Se concentration in the blood also reaches the developing fetus through the placenta and improves the level of Se in the milk (Erdogan et al. 2017). Insufficient amount of Se in the animal’s diet results in deficiency symptoms which causes great economic losses to farmers. Common symptoms of Se deficiency in ewes and does includes reduced fertility, abortion, and retained placenta (Novoselec et al. 2017). In most cases, oxidative stress leads to the incidence of retained placenta, and a sufficient supply of micro minerals including Se and vitamins A, and E have been reported to prevent its occurrence in ruminants (Khalili et al. 2020). In support of this, Erdogan et al. (2017) supplemented pregnant ewes with selenomethionine and recorded a higher concentration of Se in the placenta, serum, and colostrum than the unsupplemented. In another study, pregnant and lactating ewes supplemented with both organic (Se-yeast) and inorganic (sodium selenite) Se recorded higher levels of Se in the blood when compared to the unsupplemented group (Novoselec et al. 2017).

 

Effects of Se on reproductive performance and semen quality

Both organic and inorganic Se sources have been used to improve the quality of semen and the overall reproductive performance of rams and bucks. During the cryopreservation of semen, the activities of free radicals cause oxidative stress and eventually reduce the viability of the sperm cells as well as their fertilization ability after thawing (Khalil et al. 2019). Interestingly, the quality of frozen semen can be enhanced by the addition of Se to the semen extender. In an in vitro study on the post-thawing quality of ram semen, the inclusion of nano-Se into semen extender decreased lipid peroxidation and resulted in a lower percentage of abnormal sperms while sperm viability and motility were higher when compared to the unsupplemented group (Nateq et al. 2020). Compelling evidence from several studies on goats revealed that Se supplementation has improved semen volume, and semen quality (Shi et al. 2010; Lukusa and Lehloenya, 2017; Mojapelo and Lehloenya, 2019; Mojapelo et al. 2021), and increased the levels of luteinizing hormone and testosterone (Lukusa and Lehloenya, 2017; Mojapelo and Lehloenya, 2019) in the treated group when compared to the unsupplemented group. This could be attributed to the ability of Se to enhance spermatogenesis via an association with glutathione peroxidase in the testis (Jamali et al. 2019). Similarly, in rams, the administration of Se and vitamin E increased ejaculate volume in the treated group as compared to the unsupplemented group (Baiomy et al. 2009; Mahmoud et al. 2013). In another study on rams, the efficacy of sodium selenate on reproductive performance was investigated by Marai et al. (2009). The results revealed an increased libido, higher ejaculate volume, and improved sperm motility as well as the sperm-cell concentration in the treated group when compared to the unsupplemented group. Selenium plays a key role in the spermatogenesis and synthesis of testosterone which are vital components of the reproductive life of a male animal (Badgar and Prokisch, 2020). Selenium acts directly on the interstitial cells of the testes or indirectly via the hormones of the anterior pituitary gland to enhance semen volume and sperm quality (Yousef et al. 1990). This explains the increased semen volume and higher concentration of testosterone reported in the studies above.

 

Factors limiting the activities of Se in ruminants

The rumen environment is one of the major factors that limit the activity of Se in ruminants. When sheep were supplemented with dietary Se, the Se in the rumen fluid was found to be insoluble (Badgar and Prokisch, 2020). This could be due to the action of microbes on the Se and converting it to the poorly soluble elemental Se or selenides thus decreasing its bioavailability (Spears, 2003). Another factor that affects the bioavailability of Se in ruminants is the form of Se. Organic, inorganic, and nano-Se are metabolized differently in ruminants. Organic Se is slowly absorbed in the body and has a high bioavailability, increased tissue storage, and less toxicity than inorganic Se (Groce et al. 1973; Ortman and Pehrson, 1997; Fairweather-Tait et al. 2010; Danielsson et al. 2012; Davis et al. 2017). However, when compared with the organic and inorganic sources of Se, nano-Se has proven to exhibit higher bioavailability, increased tissue storage, enhanced surface charge, and reactivity improved antioxidant effect, increased binding properties as well as reduced antagonism with other minerals, and less toxicity (Hosnedlova et al. 2018; Khalil et al. 2019; Patra and Lalhriatpuii, 2020; Han et al. 2021; Libera et al. 2021). Due to the similar chemical properties to sulfur, Se has been reported to compete with sulfur. This means that a diet with high sulfur will decrease the bioavailability of Se. In addition, some anti-nutritional factors such as cyanogenic glycosides in forages might limit the availability of Se (Spears, 2003).

 

Recommendation

The effect of Se on heat stress and immunity is worth investigating. This is due to the relationship between oxidative stress and heat stress. Prospective researchers can compare the effects of different forms and doses of Se on small ruminants exposed to heat stress. In addition, with recent advances in metagenomics and metabolomics, future researchers could employ metagenomics and metabolomics to explore the effects of Se on rumen fermentation and microbiology, as well as the interaction between rumen microbiota and Se supplementation in small ruminants.

 

CONCLUSION

The supplementation of Se in sheep and goats has been shown to improve blood antioxidant status, immune response, reproductive performance, rumen fermentation, as well as feed efficiency, and weight gain. The influence of Se on heat stress and immune response were all associated with its ability to neutralize the free radicals in cells thereby protecting the cells from oxidative stress. This is due to the action of many enzymatic structures and seleno-proteins, especially glutathione peroxidase which is responsible for the reduction of H₂O₂ to H₂O and the elimination of lipid peroxides that are synthesized by free radicals in the cells. Due to the positive correlation of oxidative stress with heat stress, prevention of oxidative stress will automatically translate to a reduced risk of heat stress. Similarly, oxidative stress can weaken the host immune system and activate inflammatory responses thereby resulting in several metabolic diseases. Selenium helps to provide an antioxidant defense to the immune system, stimulates the secretion of cytokines, and enhances the differentiation of lymphocytes. However, factors limiting the bioavailability of Se include the rumen microbiota, the form of Se, and competition with the other minerals.

 

ACKNOWLEDGEMENT

The authors acknowledge the support of friends and colleagues whose advice was valuable during the preparation of this manuscript. The figure used was created using https://biorender.com.

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