1. Introduction
The alternance between dynamism and stability in the gut microbial ecosystem impacts the animal host, and the type of microorganisms in the gut is closely associated with the host’s metabolism, immunity, as well as tissue and intracellular homeostasis [
1]. A stable gut microbiota can regulate animal health by promoting nutrient digestion and absorption, maintaining intestinal barrier integrity, and enhancing organism immune function [
2]. On the contrary, disruptions to the gut microbiota can cause metabolic disorders, such as diabetes and non-alcoholic fatty liver, which hinders animal growth and development [
3,
4]. Therefore, maintaining a stable gut microbiota, being beneficial to animal growth and development, is one of the focuses of animal husbandry practices.
Zinc (Zn) is a component of more than forty different enzymes and two-hundred enzyme activators, and thus plays a key role in animal growth and development [
5]. Inorganic zinc (IOZ) is the main form of Zn added to feed. However, due to its low absorption rate, unstable chemical properties, and environmental pollution capacity, IOZ has been gradually replaced by organic zinc (OZ), which has good palatability, high chemical stability, lipophilicity, and bioavailability [
6,
7]. Previous studies have shown that the addition of OZ to feed (protein zinc, amino acid chelated/complexed zinc, and polysaccharide zinc [
8,
9]) can alleviate diarrhea [
10], improve immune function [
11] and antioxidant capacity [
12], promote intestinal health [
13], and enhance reproductive performance [
14], thus supporting the healthy growth of animals.
Different sources and doses of Zn affect the gut microbiota of animals. The appropriate amount of Zn can promote the growth and proliferation of beneficial bacteria while inhibiting harmful bacteria in the gut. Yu et al. found that the addition of 120 mg/kg of OZ to the diet increased the relative abundance of Firmicutes, while decreasing the relative abundance of Proteobacteria in the intestinal tract of fattening pigs [
15]. Xie et al. found that the addition of 100 mg/kg of zinc chitosan to the diet of weaned piglets increased the number of
Lactobacillus in the cecum and colon, while decreasing the number of
Escherichia coli and
Salmonella [
16]. Broom et al. and Højberg et al. also showed that a zinc-rich diet decreased the number of anaerobic bacteria and
Lactobacillus in the ileum of piglets, but increased the number of
Escherichia coli [
17,
18]. Few studies have been conducted on the application of OZ on calves, especially considering its effects on gut microorganisms.
Therefore, the aim of this study was to analyze the effects of supplementation with Zn amino acid complexes at different concentrations on the immunity, antioxidant capacity, and gut microbiota composition of calves. High-throughput second-generation 16S rRNA sequencing was used for assessing bacterial community composition in the guts of calves. The findings discussed herein provide a theoretical basis for investigating the appropriate level of Zn amino acid complex dietary supplementation to calves.
4. Discussion
Zinc is a crucial component for animal growth and development, being also crucial for preserving proper metabolism [
23]. Recent studies have demonstrated the value of dietary supplementation with adequate levels of zinc to support animal development performance [
24]. Pei et al. demonstrated that weaned pigs fed with 450 mg/kg of ZnO nanoparticles had increased growth performance [
25]. In addition, Graget et al. reported that adding 20 mg/kg of ZnO and Zn methionine to feed promoted the growth of lambs, the Zn-meth group having considerably greater daily weight gain than the ZnO group [
26]. Chang et al. showed that neonatal calves receiving 80 mg of Zn methionine daily had increased daily weight gain after 14 days of supplementation [
27]. Moreover, the average daily weight gain of bull calves was greatly increased by daily supplementation with 0.45 g of Zn methionine complexes (80 mg of Zn) [
28]. Thus, the results of the current study are consistent with those of previous studies, in that the average daily weight gain of calves increased as the level of dietary zinc amino acid supplementation increased; in particular, group C outperformed group A in terms of weight gain. Subsequently, we investigated the effect of Zn amino acids at different levels on the immune function and antioxidant capacity of calves to better determine the impact of Zn supplementation on calves’ health.
Animal health and metabolic state are reflected in blood markers. The levels of IgA, IgM, and IgG are significant markers of immune response, and the trace element Zn is intimately involved in the immune response [
29,
30,
31,
32,
33]. The levels of IgM and IgG in the blood of calves increased after dietary supplementation with 80 mg/day of ZnO [
27]. Wei et al. described that the levels of IgA, IgM, and IgG in the blood of calves showed an upward tendency with the increase in ZnO concentration (40–120 mg/kg) in the diet [
34]. In the present study, no discernible effect was observed as a result of dietary supplementation with Zn amino acids on the levels of IgA and IgG in blood; in contrast, the level of IgM in group B was higher than that in groups A and C, which was inconsistent with the results of the above studies. This could be likely due to different sources of Zn used in this study, which led to differences in actual concentrations of Zn supplemented. IL-2 is a T-lymphocyte growth factor that plays an important role in the immune response [
35]. Chen et al. revealed that feeding dairy cows with 60 mg/kg of Zn methionine dramatically raised the levels of IL-2 in the blood [
36]. In the present study, the levels of IL-2 in the blood of calves increased initially and then decreased as the amount of Zn amino acids in the feed increased, thus suggesting that increasing dietary supplementation with 80 mg/kg DM of Zn amino acids will benefit the immune system of calves.
Antioxidative compounds are produced by the body in response to harmful external stimuli to withstand challenges [
37]. The body’s potential for antioxidant defense is indicated by levels of T-AOC, which is composed of numerous antioxidative compounds and enzymes. SOD can neutralize a great number of free radicals generated in the body as a result of stressors and scavenge reactive oxygen species (ROS) [
38]. Moreover, the level of cell damage is indicated by MDA concentration, which is a byproduct of lipid peroxidation in cell membranes. Several studies have demonstrated that Zn can increase the body’s capacity for antioxidant response, which is directly linked to the equilibrium of the body’s redox system [
39,
40]. Yu et al. revealed that serum T-AOC levels during the fattening stage were considerably greater in pigs receiving 90 mg/kg, 120 mg/kg, and 150 mg/kg of cysteamine chelated Zn compared to pigs receiving 60 mg/kg [
15]. The addition of protein Zn to the diet at concentrations greater than 80 mg/kg dramatically raised serum SOD levels, as demonstrated by Kannan et al. [
41]. In the present study, the levels of T-AOC and SOD in the blood of calves tended to increase and then decrease as the concentration of dietary Zn amino acids increased, whereas the serum MDA levels increased. Thus, these findings suggest that, under the conditions employed in the current experiment, moderate addition of Zn amino acids to the diet improved the antioxidant capacity of calves, while an excess of Zn decreased antioxidant capacity. Subsequently, we carried out correlation analysis of the gut microbiome of calves following the addition of Zn amino acids at different concentrations to further investigate the ideal amount of Zn amino acids to be supplemented in calf diets.
In order for calves to digest and absorb nutrients, the gut microbiota is essential, as it directly impacts calves’ health. When the gastrointestinal microbiota is unbalanced and prone to invasion by pathogenic bacteria, diarrhea and other intestinal disorders can occur [
42]. Herein, the alpha-diversity of the gut microbiota of calves in group B was substantially greater than in groups A and C, showing that the addition of 80 mg/kg DM of Zn amino acids to the diet improved the variety of the gut microbial composition. Jensen et al. showed that the variety of the rectal microbiota was not affected by the addition of 2500 mg/kg of ZnO to the diet of weaned pigs [
43] which was not in accordance with the results of the present study, possibly due to the differences in feed composition, nutrition level, feeding environment, and Zn source.
Firmicutes play a significant role in how well proteins and carbohydrates are absorbed [
44]. In contrast,
Bacteroides primarily target non-fibrous materials for digestion, but species within this phylum can also colonize the gut to reduce the adherence of invasive pathogens [
45,
46]. It is known that obesity level is reflected in the ratio of
Firmicutes and
Bacteroides in the gut microbiota [
47]. Tian et al. found that the dominant phyla in the gut microbiota of calves were
Firmicutes and
Bacteroidetes [
48]. In the present study, the dominant microbial phyla and genera in the fecal samples of calves receiving Zn supplementation were not impacted by dietary levels of Zn amino acids. In addition,
Ruminococcus and
Bacteroides were the dominant genera in the gut microbiota of calves in all sample groups. The phylum
Bacteroides is primarily involved in the hydrolysis of proteins and the degradation of carbohydrates [
46]. According to Chang et al., the relative abundance of
Bacteroides in the gut microbiota of calves increased considerably after receiving daily supplementation with 457 mg (80 mg/day) of Zn methionine [
27]. In the present study, the relative abundance of Bacteroides in calves of group C was considerably higher than in group A. Moreover, supplementation with 120 mg/kg DM of Zn amino acids increased the utilization of protein and carbohydrate in the feed, thus promoting the growth of calves, based on average daily weight gain data.
The immune system and
Lactobacillus are closely associated [
49]. The production of lactic acid prevents the growth and multiplication of pathogens such as
Colidextribacter, and it has been demonstrated that
Lactobacillus can lower local pH in the gastrointestinal tract [
50]. To maintain the environmental acid and prevent the growth of infective microbial species in the gut,
Lactobacillus can also convert lactic acid to butyrate. Hou et al. found that feeding weaned piglets with chitosan-chelated zinc (100 mg/kg zinc) increased the relative abundance of
Lactobacillus in the cecum [
13]. In addition,
Faecalibaculum is a probiotic species that produces a natural antibiotic that can improve health in the colon by generating metabolites, such as butyric acid and short-chain fatty acids, among others [
51]. In the present study, the relative abundances of
Lactobacillus and
Faecalibaculum were considerably higher in group B than in group A, suggesting that feeding calves with 80 mg/kg of Zn amino acids was more effective in improving intestinal health.
Prevotella degrade starch into monosaccharides and other non-cellulose polysaccharides to provide energy to the body [
52] as well as to boost nutritional digestion and absorption [
53,
54,
55]. However, this degradation might promote inflammation [
56]. In addition,
Bifidobacterium is another physiologically significant intestinal bacterium that can interact with immune cells and control the immune system [
57,
58]. In another study, compared to the group receiving no dietary addition of Zn methionine, it was revealed that the addition of 70 mg/kg and 140 mg/kg of Zn methionine considerably increased the abundance of
Bifidobacterium in the cecum of laying hens [
59]. In the present study, a considerably higher relative abundance of
Prevotella-UCG-003 and
Bifidobacterium was found in group B compared to group C. Thus, the addition of Zn amino acids to the feed boosted the immune system of calves, while it also promoted nutrient absorption. Therefore, it was established that the optimal amount of Zn amino acids to be added to the feed given to calves was 80 mg/kg DM.
Furthermore, using LEfSe analysis, significant differences in the abundance of
Spirochaetacene,
Treponema, Bacteroides, Prevotellaceae-UCG-003, and other species were found in group B. Among these,
Spirochaetacene and
Treponema are Gram-positive intestinal pathogenic bacteria [
60,
61]. By interacting with the immune system,
Bacteroides stimulate T-cell-mediated responses and prevent potentially harmful bacteria from colonizing the gut [
62,
63]. Chang et al. demonstrated that 104 mg/day of ZnO supplementation decreased the incidence of diarrhea in calves and raised the relative abundance of
Bacteroides in seven-day-old calves [
27]. In the present study, compared to group A, a higher relative abundance of
Bacteroides and a lower relative abundance of
Spirochaetaceae and
Treponema were found in group B, which suggests that including Zn amino acids in the diet of calves will improve the body’s immune system.
Finally,
Prevotellaceae-UCG-003 was first described by Koh et al. as having the capacity to control intestinal inflammation by triggering dendritic cells through succinate synthesis [
64]. In the present study,
Prevotellaceae-UCG-003 was found in greater relative abundance in group B than in group C, showing that dietary addition of the Zn amino acids at the levels used in group B was more effective in enhancing host immune function. Collectively, it can be stated that the addition of 80 mg/kg DM of Zn amino acids to feed improved intestinal health and led to an increase in the relative abundance of beneficial microorganisms in the guts of calves.