Sheep are traditionally reared by grazing, but with improved raising of mutton sheep and people’s attention to the safety of livestock products, farm feeding is gaining popularity. The availability of feed nutrients is one of the major issues in captive animal production, and feeding only native forage lacks the essential minerals needed to maintain optimal animal performance (1). In this case, supplementation with trace minerals is necessary to maximize animal performance. Trace minerals, such as copper (Cu), zinc (Zn), manganese (Mn), cobalt (Co), selenium (Se), and iodine (I), are essential for animals (2). Although these minerals constitute <0.01%, they are essential in maintaining normal physiological activities, such as hematopoiesis, immune response, energy metabolism, enzyme activity, and reproductive functions, in an animal’s body (3, 4). In addition, most trace minerals are involved in the composition of oxidoreductases and other enzymes that participate in the redox reactions or bind to oxygen radicals to alleviate oxidative stress in the body. Trace mineral deficiencies in livestock are known to negatively affect various biological functions, growth performance, and health status of the animals (5, 6). Therefore, trace mineral supplements are usually added to the water and feed for animals or supplemented through oral formulations, rumen instillation, or direct injection (7). The most common method used in livestock production is to supplement feed with trace inorganic salts, such as sulfates, oxides, carbonates, and chlorides. However, in ruminants, the ionic bonds in inorganic salts dissociate as they flow through the digestive tract, interacting with other components of the gut to form complexes and prevent absorption, thus reducing the bioavailability and absorption efficiency of trace minerals (8, 9). To ensure adequate intake, trace element minerals are usually over-added to feed, which increases the residual components, resulting in accumulation and toxicity in the animal’s body. In addition, the excretion of excess inorganic salts through feces and urine adds to the environmental burden (10). Conversely, organic trace minerals can avoid the adverse effects of interfering components and gastric acid in the digestive tract of animals, such that the metal ions can reach the absorption site, improving the absorption and utilization of trace mineral elements, such as Cu, Zn, Fe, and Mn, and reducing the environmental pollution and wastage of resources (11).

Oligosaccharides can promote the growth performance of animals, improve the immune capacity improve meat quality and other effects (12–14). In recent years, oligosaccharides amino acids and other advanced chelating technology in mineral additives have been widely used, chelating organic trace minerals because of its small antagonism in the stomach and other dietary components, can improve immunity, promote productivity and affect meat quality and other effects, has been confirmed in many tests (15–17).

Oligosaccharide-chelated organic trace minerals (OTMs) are the products of a new chelating technology developed in recent years. It is composed of molasses and trace minerals, such as Fe, Cu, Mn, Zn, Co, I, and Se. The OTMs are lipophilic and do not need ligands to enter the animal’s body as they can be directly absorbed by the animal through multiple channels of glycolipids, glycoproteins, and oligosaccharides on the cell surface. So, it is expected to have higher absorption and utilization efficiency and better production performance. Some studies have shown that OTMs are effective in pigs and poultry (18, 19), but only a few studies have been reported in ruminants. Therefore, the present study aimed to investigate the effects of OTM on growth performance, digestive enzyme activity, blood parameters, slaughter performance, and meat quality indexes of mutton sheep to provide an opportunity for OTM in ruminant production.

In large-scale animal breeding, trace minerals play a critical role in weight gain and FCR, as well as in body structure, physiological functions, and hormone regulation (22). Previous studies have shown that trace minerals, such as copper and zinc, regulate animal appetite and improve animal feed intake. For example, copper stimulates the hypothalamus to secrete neuropeptide γ (a hormone that stimulates food intake), thus improving the feed intake of pigs (23). Zinc stimulates the regeneration of animal taste bud cells, increases animal appetite, improves intestinal flora, elevates FCR (24), and increases DMI in dairy cows (25). Compared to inorganic trace elements, organic chelated trace elements have the advantages of good palatability, high absorption efficiency, and small antagonism between minerals, which can improve feed intake and ADG of Angus calves before weaning (26). However, some studies have shown that organic trace elements do not exert a regulatory effect on animal feed intake. For example, cows fed lick blocks containing cobalt, iron, selenium, manganese, and zinc have no significant effect on DMI (27). Also, ewes-fed lick blocks containing iron, copper, zinc, manganese, cobalt, and selenium did not show any significant change in feed intake (28). Organic trace minerals have no significant effect on animal feed intake but improve FCR (29). This phenomenon could be attributed to the fact that animal appetite is affected by various factors, such as environment, gastrointestinal content, blood glucose, mental status, and neurological function (30), such that the effect of trace mineral supplementation on an animal’s appetite and feed intake may depend on the factors mentioned above. In this study no significant change was detected in ADG, DMI and FCR of mutton sheep in the OTM group.

The digestive enzyme activities of the gastrointestinal tract reflect the digestive and absorptive ability of nutrients, which has a close correlation with the animal’s production performance. The jejunum is the main organ of nutrient digestion and absorption. It is reported that increasing supplementation of Zn, and Cu and can increase trypsin, lipase amylase activity (31, 32). In this study, the trypsin, lipase, and amylase activities in the jejunum chyme of the OTM group were significantly increased, which indicated that trace mineral supplementation is more efficient. Furthermore, it was expected to improve the digestibility of feed CP, EE, and energy by the micronutrients (33). However, the FCR showed no significant difference, it may be related to ruminant digestion or the consumption of animals, which needs more research to confirm. On the other hand, OTM increased the immunoglobulin levels in the blood but decreased the inflammatory response indicators and the IFN-γ level, which indicated the potential increase of the humoral and innate immunity of the animals (34, 35).

Antibodies (immunoglobulins) are major indicators of the animal’s immune capacity. Immunoglobulins (IgA, IgM, and IgG) are associated with blood diseases, infections, and autoimmune diseases (36). Among these, IgG constitutes the largest proportion and is vital in maintaining the body’s immune barrier (37) and in the early defense of the body. IgA has anti-viral and bacteriostatic immunological activities, is the main effector molecule of humoral and mucosal immunity, and directly participates in immune responses. IgM is a high-performance antibody in the body’s defense and immune process, with bactericidal function and antiviral (38). Previous studies have shown that the addition of zinc glycinate chelate to diets increases the serum IgA, IgM, and IgG levels (39). Supplementation of manganese and selenium in the diets of goats and lambs increases the serum IgA, IgM, and IgG content (40, 41). It was reported that replacing 1/3 of the trace element inorganic salt in the diet with chelated organic trace elements can significantly increase the serum IgG content of piglets (42). IgA, IgM, and IgG were significantly higher in the serum of mutton sheep in the OTM group in this experiment, which was consistent with the previously reported results, indicating that OTM enhances the immune competence and health of the animals. This phenomenon may be related to the fact that organic trace minerals promote the proliferation and differentiation of lymphocytes and increase the number of plasma cells, reducing inflammation and enhancing the immune capacity of the body (43). Additionally, trace minerals, such as manganese, zinc, copper, and selenium enhance humoral and cell-mediated immune responses, maintain oxidative balance in the body, reduce morbidity and treatment costs in cattle, and positively affect breeding (44).

Furthermore, the increase of immunoglobulins in the OTM group may be also relate to the immune stimulation of oligosaccharides (45, 46). Some oligosaccharide (such as mannan) itself is involved in the formation of immunoglobulins (glycoproteins) (47). Therefore, the OTM may provide oligosaccharides for immune synthesis. Blood biochemical indices reflect the nutritional metabolism of mutton sheep (48). The protein content in the serum is closely linked to protein uptake and metabolism, as many proteins in the serum act as molecular carriers of nutrients, hormones, or metals and are extensively involved in a wide range of functions in the organism (49). The TP content is related to the nutritional status blood capacity and stress of animals (50), etc. The lower TP in the CON group indicates poorer health condition of mutton sheep, which increases the proportion of protein dealing with harsh environments or immune depletion and it is not conducive to improving nutrient utilization. On the other hand, ALB in sheep blood is correlated significantly and positively with TP (51). It is a marker of liver function, and higher values of ALB imply improved liver function (52). The liver plays a vital role in the digestion of nutrients, which might explain the improved feed efficiency of the OTM group. Moreover, serum GLB mainly includes IgG, IgA, and IgM. These immunoglobulins regulate the immune system of the animals, and the increased GLB level enhances humoral immune responses against pathogenic viruses and microorganisms (53). Blood TP, ALB, and GLB were significantly increased in the OTM group, indicating that OTM provides an immune-enhancing environment that is beneficial to improving the performance of mutton sheep (54). This may be also related to increased activity and functions of rumen microorganisms by organic selenium (55). This further improves the utilization of ammonia in the rumen, and increases the blood urea nitrogen that can be used to synthetic proteins.

The level of cytokines in serum is a major indicator for evaluating the immune function of the body, i.e., immune regulation, inflammatory response, and neuroendocrine function. Among these, IL-10 regulates the anti-inflammatory cytokines and immune responses (56). Some studies have shown that IL-10 inhibits the production and expression of IL-2, IL-6, and IFN-γ (57). It also promotes B cell proliferation, differentiation, and antibody production while inhibiting the secretion of pro-inflammatory factors, such as IL-6 via macrophages (58). and IL-10 inhibits the synthesis of Th1 cells (59). Since IL-2 and IFN-γ are secreted by Th1 cells (60), IL-10 inhibits the production and expression of pro-inflammatory factors, such as IL-2, IL-6, and IFN-γ. IL-2 is a crucial Pro-inflammatory factor that regulates T cell proliferation and secretion (61). It also stimulates the production of IFN-γ by Th1 cells, which in turn affects the IFN-γ-mediated immune response. Interestingly, IL-6 has a dual role in the animal’s organisms: as a pro-inflammatory factor that judges the inflammatory situation in the organism (62), and as a factor that promotes the production of IgM, IgG, and IgA through mature B cells (63). In this study, the serum levels of IL-10 in mutton sheep were significantly higher, while IL-2, IL-6, and IFN-γ were significantly lower in the OTM group than in the CON group. Thus, we speculated that OTM promotes the proliferation and differentiation of B cells, which was also evident from the content of IgA, IgM, and IgG antibodies in the blood of mutton sheep, while the production of IL-2, IL-6, and IFN-γ was suppressed; this finding was in agreement with the results reported previously and could be related to the modulation of cytokines by certain mineral elements. For example, zinc induces the proliferation of T lymphocytes by inhibiting IL-2 factor (64). and decreases the levels of pro-inflammatory cytokines, such as IL-6 (65). Selenium reduces the IL-2 levels in mouse serum (66). Fe increases the bactericidal and phagocytic capacity of neutrophils, improves the proliferation and differentiation of T and B lymphocytes, and promotes the production of antibodies (67). Copper has a dual effect on cytokines in rat kidney tissues, promoting cell proliferation at low doses and inhibiting proliferative activity at high doses (68). Moreover, the cytokines in animal organisms are affected by a single trace mineral and the combined effects of multiple trace minerals. For example, on the basal diet of high-sugar rats, a mixed solution containing 11 trace minerals (B, V, Cr, Mn, Fe, Co, Cu, Zn, Se, Sr., and Mo) was instilled, which increased the IL-10 but significantly decreased the IL-6 level in rat serum. This phenomenon could be attributed to the fact that supplementation of multiple trace minerals upregulates the expression of IL-10 and downregulates the expression of IL-6, thus reducing the inflammatory response in rats fed a high-sugar diet (69). This, influencing the secretion of cytokines by immune cells is the result of a comprehensive effect of a variety of trace minerals. In this study, OTM increased the IL-10 content in the blood of mutton sheep and decreased the content of IL-2, IL-6, and IFN-γ, suggesting that OTM promotes the proliferation and differentiation of the organism’s B cells. In addition, the synthesis of Th1 cells was inhibited, which might be related to the provision of organic trace minerals and the enhancement of the secretory function of B cells as well as immune cell activity. However, further immunological experiments are needed to substantiate this finding.

Slaughter performance is a major index to measure the economic performance of meat livestock; the items include live weight before slaughter, carcass weight, dressing percentage, and eye muscle area. Studies have shown that supplemental feeding of trace mineral lick bricks significantly improves the dressing percentage and eye muscle area of semi-fine wool sheep in Guizhou and thus, the economic benefits of farming (70). Tibetan sheep supplemented with lick bricks containing manganese, iron, copper, iodine, zinc, selenium, cobalt, and other composite trace mineral nutrients show a significantly increased dressing percentage, net meat rate, eye muscle area, and carcass quality (70, 71). In this trial, the slaughter differences and the body weights of the mutton sheep were similar between the two groups, which was different from previous reports. This might be because the licked bricks in the previous studies were an additional supplement and exhibited an incremental effect whereas in the present trial, the supplementation was rationed according to the nutrient needs of the goats. Typically, the intake of microminerals of the mutton sheep in the two groups was adequate, such that the OTM group did not reflect an incremental effect; however, this finding needs to be confirmed by further investigation. Meat quality reflects the taste and food value of mutton. Interestingly (72), the meat will have a better flavor and color and will be more palatable to the consumer at pH 5.3–5.8 than at pH 6.6–7.0. In the present study, the pH of the longest muscle of the back of the two groups of mutton sheep decreased from 6.4–6.6 at 45 min of slaughter to 5.6–5.8 after 24 h of acid drainage, indicating that the meat of the test goats underwent a desirable maturation process, which is consistent with the results of the previous study. Muscle tenderness is also a critical indicator for evaluating the texture of meat, which is measured by shear force: the lower the shear force, the finer the muscle fibers, the more tender the meat, and the better the texture. A study on whether it is acceptable for people to pay a premium for buying more tender meat showed that meat can be categorized as very soft (2.27–3.58 kg), medium soft (4.08–5.40 kg), and mildly soft (5.90–7.21 kg) based on the shear force profile of the meat (73), with the more tender meat as preferable. In this study the shear force of 5.41 kg in the OTM group was classified as medium soft meat and 6.86 kg in the CON group was classified as light soft meat. In comparison, the lamb meat in the OTM group was softer and easier to chew, which was in line with people’s tendency to choose tenderness. In addition, drip loss and cooking loss are indicators of muscle tethering force, which affect the color, flavor, tenderness, and nutritional value of meat. Meat with high hydraulics tends to be juicy, tender, and dry on the surface. On the other hand, meat with low hydraulics suffers from surface water exudation, loss of soluble nutrients, and flavor changes, the muscle becomes dry and tough, and the quality of the meat decreases. Also, high cooking losses make the meat tough and intolerant to chewing after cooking, which reduces the flavor (74). Herein, we did not detect any significant difference between drip loss and cooking loss between the two groups of lamb, indicating that OTM does not alter the cooking quality.

In terms of ether extract, the breed of sheep exerts a significant influence. Merino sheep have a lower ether extract content and a high protein content, with nine amino acids higher than that of the small-tailed Han sheep (75). In addition, nutrition affects the deposition of ether extract. Some studies have shown that supplementation with Cu (40 mg/kg DM) and Se (2 mg/kg DM) reduces cholesterol deposition in the longest muscle of the back of Brangus cattle, possibly due to the alteration of the ratio between reduced and oxidized glutathione, which in turn affects lipid metabolism of the cattle (76). Adding Cu to the diet of Angus beef cattle reduces the ether extract content of the longissimus muscle of the back (77). Selenium regulates lipid metabolism and ether extract accumulation in muscle by affecting thyroid hormones (78). High zinc increases the activity of rat liver cells and upregulates fat synthesis genes, which in turn promotes lipid metabolism in dirty cells (79). In this experiment study, the content of EE in meat was significantly increased, while the content of CP was significantly increased, indicating that the addition of organic trace minerals changed the composition of meat. However, the raised protein content did not increase the shear force, which might be related to factors such as muscle fiber diameter or connective tissue content; nonetheless, these findings need to be confirmed by further studies.

The CP content of meat is the basis for measuring meat quality, and its level is related to the amino acid content and composition of the meat. Based on the nutritional aspect the amino acid content of the meat determines the flavor and nutritional value of meat and a high content of essential amino acids can satisfy human nutritional requirements (80). The analyses revealed that the content of all amino acids was higher in the OTM group than in the CON group. Among these, the essential amino acids Thr, Val, Leu, and Lys and the total number of EAA were significantly higher in the OTM group than in the CON group, which increased the relative protein content in the meat. Therefore, the ability of OTM to promote amino acid synthesis in muscle was superior to that of inorganic trace minerals. This phenomenon may be related to the high absorption and utilization of organic minerals. Previous studies have shown that the addition of yeast selenium to the diet promotes the synthesis of selenate amino acids by rumen bacteria, which in turn, increases the amino acid content of goat meat (81).

Regarding the balance of amino acids and ideal protein, FAO/WTO suggested that the ratio of EAA/TAA is 40%, and EAA/NEAA >60% is considered high-quality protein. In the present study, the EAA/TAA in the CON and OTM groups of lamb was 30.72% and 34.65%, respectively, wherein the ratio of the OTM group was 3.93% higher than that of the CON group and almost 40%. On the other hand, the ratios of EAA/NEAA in mutton of both CON and OTM groups were higher than the standard 60%; the OTM group was 12.51% higher than the CON group and was above the standard. Therefore, it could be deduced that the protein and amino acid quality of the OTM group was better and the nutritional value was richer than that of the CON group. Furthermore, flavor and taste are subjective sensations. Some amino acids are either flavor substances or precursors for the formation of flavor substances that can be used to evaluate the flavor of the meat. Previous studies have shown that Glu and Asp are the characteristic amino acids for fresh flavor, Ser, Ala, Gly, Thr, and Pro are responsible for sweetness, and Leu and Phe form the bitterness (82–84). Other studies have shown that trace mineral supplementation modulates the content of umami amino acids (UAA). For example, yeast Se supplementation reduced the dissipation of fresh flavor in lamb meat and increased the content of five umami amino acids (UAA; Ala, Asp, Glu, Phe, and Tyr) in lamb meat (85). Taken together, the current results showed that OTM increases the content of various fresh amino acids and improves the flavor of lamb meat.

Oligosaccharide-chelated organic trace minerals supplemented in the diet of mutton sheep could increase the jejunal digestive enzyme activity, the immune level in the blood, and improve the meat quality, but had no significant effect on growth performance, feed conversion ratio and slaughter parameters.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

The animal studies were approved by Laboratory Animal Ethics Committee of Inner Mongolia Minzu University College of Animal Science and Technology. The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent was obtained from the owners for the participation of their animals in this study.

RZ: Writing – original draft, Writing – review & editing. MW: Writing – original draft, Writing – review & editing. JZ: Writing – original draft. ZY: Writing – original draft, Supervision. MX: Writing – original draft, Data curation. LD: Writing – original draft. MB: Writing – original draft. JJ: Writing – original draft. CD: Writing – original draft. YZ: Writing – original draft. HB: Writing – original draft.