1. Introduction
Selenium (Se) is considered a necessary micronutrient element for almost all life forms. In recent decades, the response of broilers to dietary selenium supplementation levels and sources has received considerable attention. In fact, the grains used in the actual feed contain Se, 50% of which is in the form of selenomethionine (Se-Met); however, as the Se content of plant grains is affected by the Se content of the soil, the poultry industry needs Se supplements to avoid nutritional deficiencies [
1]. It is a crucial trace element necessary for animal growth, playing key roles in animal development, immunity, reproduction, and antioxidation [
2,
3]. Research indicates that Se, after being absorbed, exists in the form of selenocysteine in the body, which serves as the active center in 21 essential amino acids and 25 Se-containing proteins [
4]. Selenocysteine is also a vital component of enzymes such as glutathione peroxidase, thioredoxin reductase, and thyroglobulin deiodinase. Se deficiency may cause many diseases in livestock, such as poor growth, liver necrosis, pancreatic fibrosis, leucomyopathy, exudative quality disorders, hypoimmunity, hypothyroidism, and reduced fertility and hatchability [
5]. Therefore, selenium is indispensable in animal diets. The efficiency of Se depends on the level and form of dietary Se. For instance, organic Se sources are more effective as antioxidant modulators compared to sodium selenite [
1].
Se yeast is a microbial fermentation product created by adding sodium selenite to the yeast cultivation process, which is a superior product of an organic Se source [
6]. It was reported that Se from Se yeast exhibited greater bioavailability than Se from inorganic Se sources, and increased Se levels are maintained for a longer period after supplementation has ceased [
7,
8]. Additionally, Se present in the selenium yeast is an organic structure which is less toxic, easier to digest, and easier to be absorbed and used by the body [
9] than inorganic selenium. Organic selenium supplements are more effective than inorganic selenium supplements at improving broiler growth performance [
10], because selenium yeast is more likely to promote GSH-Px activity and selenium deposition in the muscle of broilers [
11,
12,
13,
14]. The bioefficacy of different Se additives may be assessed by the deposition of Se in different animal tissues. It has been shown in various studies that the Se deposition in the muscles could be significantly enhanced by using organic Se forms [
15,
16,
17]. However, there were only a few systematic studies on organic Se in broiler production. The purpose of this study was to investigate the effects of different levels of Se yeast in grain feed on the growth performance, slaughter performance, antioxidant capacity, and relationship between Se content in feed and Se deposition in broiler tissue. Se yeast in the form of organic matter in grains used to feed broilers was provided by the Beijing University of Chemical Technology. This study provides a theoretical basis for the rational use of Se yeast and technical support for the production of functional Se-enriched meat.
4. Discussion
Se plays a vital role in animal growth as it is involved in the enzyme group of iodothyroninedeiodinases, which are responsible for the metabolism of thyroid hormones necessary for normal growth and development [
22]. In the present study, no significant differences were found in ADG, BW, ADFI, and F/G of broiler chickens between the control and test groups. However, broilers’ BW and ADFI tended to go up with larger doses of yeast Se treatment. These results are in line with what other studies have reported [
23,
24,
25]. However, Marković et al. found that supplementation of Se improved BW, WG, and FCR in the broilers [
26]. This improvement could be attributed to the enhanced activation of thyroid hormones due to increased Se content [
27]. Therefore, increasing the Se supplementation levels in our experiments may yield similar results.
Our study yielded no significant differences in the majority of the carcass features that were investigated between the control and experimental groups, which aligns with the findings reported by Payne and Southern [
23] that carcass traits were not affected by the Se source or level of supplementation. In contrast, Upton et al. [
28] showed that Se-enriched yeast supplementation (0 to 0.2 mg/kg) boosted various characteristics of cutup yields of high-yielding broilers. The abdominal fat weight were significantly lower in broiler chickens fed the diet containing nano-Se than those chickens fed the control diet [
29]. In our study, compared to the control group, the abdominal fat ratio (AFR) of the treatment groups was significantly decreased, and the 0.1 mg/kg treatment group reduced it effectively. The lower the AFR, the better the carcass quality. It is suggested that an appropriate amount of Se supplementation should be added to the feed, which can promote the digestion and absorption of fat in the intestinal tract and reduce the accumulation of fat in the body, effectively improve slaughtering performance.
Water-holding capacity, pH value, and color are the significant assessment indicators of meat quality. After the animals were slaughtered, the blood circulation in the muscle stopped, resulting in a large accumulation of lactic acid, thus reducing the value of the pH. The delayed decrease in pH value will lead to the decrease in protein denaturation, thus improving the water-retention capacity of the muscle [
30]. The experiment’s findings showed that there were no notable variations between the control and experimental groups in terms of drip loss, cooking loss, pH, or meat color at 45 and 24 h following slaughter. However, the drip loss tended to decrease, and the effect of the 0.4 mg/kg addition group was the best. Payne and Southern [
18] also proved that the 24 h breast moisture loss of broilers was not affected by Se supplementation sources of yeast Se. However, broilers ingesting organic Se in their diets showed a lower drip loss, according to Wang et al. [
31]. And it was also reported that organic selenium might improve meat quality by reducing the drip loss from poultry meat [
32]. The disagreement in results may be due to the differences in animal species, Se sources, and addition level. According to several reports, adding nano-Se could lessen the loss of chicken muscle drip [
32,
33]. The ability of muscle proteins to attract water and hold it within the cells is of great importance to meat quality. Selenium is essential to the body’s intracellular and extracellular antioxidant systems [
34,
35]; the improved antioxidant status may promote the maintenance of cell membrane integrity [
36]. M.J.A. et al. also found that a lower pH also decreases the muscle protein ability to bind to water, causing shrinkage of the myofibrils [
37], which could ultimately result in reduced drip loss [
38].
Antioxidant capacity is an important determinant of animal health and the quality of animal products. The antioxidant capacity of chickens is regulated by the antioxidant system and a multitude of important enzymes, including T-AOC, GSH-Px, SOD, CAT, and MDA [
39,
40]. GSH-Px is a Se-dependent enzyme that catalyzes the reduction of H
2O
2 and organic peroxides to H
2O and the corresponding stable alcohol, thus inhibiting the formation of free radicals [
41]. SOD is a crucial antioxidant enzyme in organisms that helps superoxide anion dismutate into H
2O
2 and O
2 [
42]. MDA is one of the metabolic products of lipid peroxides, and it is negatively correlated with the GSH-Px activity [
11]. Together with other compounds like glutathione peroxidases, CAT can split H
2O
2 into safe H
2O and O
2, blocking that pathway and protecting the organism [
43]. The addition of selenium yeast reduces the production of oxidized products and thus improves the antioxidant capacity of broiler muscle. Our findings showed that adding yeast Se to the diet greatly raised the GSH levels in broilers. By adding different levels Se to diets, there was a tendency for T-AOC, SOD, and CAT levels to rise while MDA levels fell. And the 0.2 mg/kg treatment group had the best effect, which is consistent with the conclusion above. Our findings supported other studies that found birds fed yeast enhanced with selenium had higher antioxidant capacities than birds fed alternative sources of selenium [
44].
In Se-enriched yeast, Se mainly exists as selenomethionine (SeM) and is incorporated non-specifically into peptide chains. Furthermore, it exhibits a high digestibility [
45].
It is generally accepted that organic selenium has a better bioavailability and tissue retention than inorganic selenium. Therefore, selenium accumulation in tissues is a very important criterion for the use of the mineral. Testing of the selenium in chicken muscle confirmed that organic selenium (selenium yeast) has a higher absorption rate [
46,
47].
In the present study, the dietary supplementation of yeast Se at varying levels resulted in a significant increase in selenium contents in the breast muscle, leg muscle, liver, and kidney of broiler chickens. These findings align with several previous reports on the subject [
23,
48]. Payne and Southern [
23] proved that broilers fed yeast Se had increased breast Se concentrations. Konkol et al. [
48] tested the content of selenium in the muscles of chickens and confirmed that selenium in the organic form (selenium yeast) is better absorbed. Se and S (sulfur) have extremely similar atomic characteristics, which suggests that SeM might be integrated into proteins at a rate similar to Methionine (Met). The increase in breast Se in broilers fed Se-enriched yeast (SY) could be explained by the ability of Se to be regarded as S and the ability of SeM to replace Met so that it can be integrated into protein when digested [
23]. Gul et al. [
44] showed that compared to other sources of Se, birds fed Se-enriched yeast had more Se deposition. Deng et al. [
49] and Bauché et al. [
39] reported that dietary organic Se increased Se concentrations in the liver, breast muscles, thigh muscle, and kidney of broilers. Our study also found that the Se deposition efficiency was different between the early stage and late stage. The deposition rate of Se in broiler tissues was kidney > liver > pectoral muscle > leg muscle for the early stage (1–21 days of age) and kidney > liver > leg muscle > pectoral muscle for the late stage (22–42 days of age).
In our study, the Se content of broiler tissue presented a significant linear and secondary dose effect; of those, a quadratic regression equation simulated the results better. Such outcome supports previous research finding that Se contents in breast and thigh meat increased significantly (
p < 0.01) with the level of Se supplementation [
26]. The study found that the mRNA levels of the methionine (Met) metabolism gene glycine N-methyltranserfase (GNMT) were markedly upregulated (
p < 0.05) in the Se-enriched yeast group, which may reveal that Se from yeast Se is deposited more efficiently than Se from sodium selenite or nano-selenium, probably via enhancing the route of Met metabolism [
50].