1. Introduction
Weaning is a critical period in commercial pig production systems. Due to the nutritional, immunological, and psychological disruptions, nursery pigs may face with reduced feed intake, increased incidence of diarrhea, body weight loss, and higher mortality. Nutritional strategies were usually used to improve performance, and gut health in nursery pigs [
1]. The use of different additives such as natural extracts are largely adopted in swine livestock [
2], which have been widely studied for their promising antioxidant, anti-inflammatory and antibacterial properties [
3]. Antibiotics have been used to enhance growth performance and to reduce the abundance of harmful bacteria in the intestine of nursery pigs. However, the overuse of antibiotics might lead to problems related with the bacterial resistance. Therefore, many countries (including China, European Union, Philippines, South Korea, United States of America and Vietnam) have banned the use of antibiotics in animal feeds. Zinc oxide (ZnO) is often used to decrease the occurrence of diarrhea in nursery pigs, whereas its excretion is a possible threat to the environment. Many international organizations, such as the European Food Safety Authority and World Health Organization, have recommended decreasing the use of copper and ZnO due to the suspected resistance to certain bacteria and risks associated with the impact of these heavy metals on the environment [
2]. Probiotics can be used to replace ZnO [
1] and antibiotics [
4] in order to improve the structure and functions of the gastrointestinal tract. Moreover, probiotics could also be useful to improve intestinal development and immune functions in nursery pigs [
5].
Yeast is one of the most commonly used probiotics replacing the use of antibiotics to reduce post-weaning diarrhea [
6,
7]. The yeast additives have been used to improve gut integrity in nursery pigs [
8] and alleviate the negative effects on the growth and health caused by mycotoxins in nursery pigs [
9,
10]. Moreover, yeasts, especially live yeast (LY), could enhance antioxidative capacity and gut immunity by increasing secretory IgA level in mucosa and reducing harmful microbiota community in the intestine of nursery pigs [
11]. Previous studies have shown that LY
Saccharomyces cerevisiae (
S. cerevisiae) could improve beneficial gut microbiota community in suckling or nursery piglets [
12,
13]. The yeast extract complex derived from the culture of
Kluyveromyces maxianus and
S. cerevisiae could also improve nutrient digestibility in nursery pigs [
14]. Moreover,
S. cerevisiae have been widely used to reduce diarrhea and to modulate gut health in animals and humans [
15,
16], which were also used to improve the immunity, gut development, microbiota community, and thus alleviate weaning stress in nursery pigs [
7,
17,
18].
Previously, it was found that
S. cerevisiae could replace chlortetracycline on improving immune function, antioxidant capacity, nutrient digestibility, gut morphology and performance in broilers [
19]. Supplemental effects of
S. cerevisiae might be further enhanced with a combinational use of ZnO. Based on previous findings, it is hypothesized that dietary supplementation with LY or LY combined with ZnO could improve performance, nutrient digestibility, antioxidative status, immune function and gut health in nursery pigs. Therefore, the objective of the current study was to test this hypothesis and find effective feed additives combination to alleviate weaning stress in pigs.
4. Discussion
The performance of piglets in the first week after weaning might play an important role in the whole production performance. Currently, antibiotics have been widely used to enhance performance and alleviate weaning stress in pigs, while the abuse of antibiotics might do harm to the environment and human health [
21]. High dose of inorganic Zn (2000–4000 mg/kg of Zn as ZnO) has been widely used in the diets of piglets in the swine industry, due to its effects on increasing growth performance and alleviating post-weaning diarrhea [
27]. An increased use of zinc oxide at a pharmacological level (ZnO, 2000–3000 mg/kg) was also observed as an alternative to antibiotics. However, the widespread use of pharmacological levels of ZnO has raised concerns related to environmental issues and the potential increase in the prevalence of antibiotic resistant bacteria [
2]. However, the overuse of ZnO could pollute the environment and could lead to increased antibiotic resistant bacteria since the zinc could not to be fully utilized by pigs. The current study aimed to investigate the effect of LY or LY combined with ZnO as antibiotics and ZnO substitute on performance and gut health in nursery pigs. Mathew et al. [
28] reported that yeast extract could improve ADG and reduce FCR in nursery pigs, indicating that yeast could be probiotic for enhancing performance in pigs. According to the current result, we found pigs fed LY or LY-ZnO had similar effect as antibiotics and ZnO on increasing ADG and decreasing FCR in the first, second, and fourth weeks compared with CTR. One of the reason for this finding might be that
S. cerevisiae could improve nutrient utilization and gut morphology [
19]. Another possible mechanism of this finding might be that the effect of β-glucan and mannan oligosaccharides in the cell wall of LY (
S. cerevisiae) could improve immunity and small intestinal development in nursery pigs [
17,
29,
30]. In the present study, we also found pigs fed LY-ZnO had increased ADFI in the first week compared with CTR and CTC-ZnO, which might be due to that LY could improve the flavor of diet, enhance intestinal peristalsis, and facilitate digestion and absorption in nursery pigs [
31].
The incidence of diarrhea during weaning is a great economic issue in production stage. Nutritional strategies (such as dietary supplement of probiotics) can be used to promote intestinal development of nursery pigs and to improve diarrhea caused by dysbacteriosis of piglets. Pan et al. [
5] reported that probiotics had significant effects on the prevention and treatment of diarrhea in nursery pigs. In the current study, pigs fed a CTR diet showed higher diarrhea rate in phase 1 and overall compared with other treatments, which agreed with the study of Shen et al. [
6], who reported that yeast could reduce the incidence of diarrhea in nursery pigs. One of the possible reasons for the current finding might be that yeast could help to increase beneficial microbial populations and decrease the populations of pathogens in piglets [
32]. Moreover, Kim et al. [
33] also reported that the nucleotide-rich yeast extract could improve gut health, which could reduce the diarrhea in nursery pigs
After weaning, the nutrient utilization was decreased since digestive organs were not fully developed in nursery pigs. In the current study, we found pigs fed LY or LY-ZnO showed increased ATTD of GE, CP, and EE compared with CTR in phase 1, indicating that LY or LY-ZnO could help improve the development of digestive organs and nutrient utilization. This finding was partly agreed with Shen et al. [
6], who reported yeast culture could enhance the digestibility of DM, CP and GE, and thus decrease the FCR in nursery pigs. Moreover, the current study showed pigs fed LY had increased ATTD of CP, while pigs fed LY or LY-ZnO also showed increased ATTD of OM and NDF compared with CTR in phase 2. The reason for this finding might be that the
S. cerevisiae could enhance the development of small intestine [
19].
For nursery pigs, the immunoglobulins are important for the development of immune system. Shen et al. [
34] reported that maternal yeast supplementation could enhance the IgG contents in sow milk, which could help to increase litter weight and weaning piglet‘s weight [
34,
35], indicating yeast had positive effects on increasing immunoglobulins levels in animals. The current study found that the concentration of both IgA and IgG in serum of pigs fed LY or LY-ZnO were increased compared with CTR and CTC-ZnO on d 7 and d 14. The mannanoligosaccharides and β-glucans were the main components of LY cell wall and LY [
19]. The current finding was in agreement with Wang et al. [
36] and He et al. [
19], who also reported that the Ig levels were enhanced by
S. cerevisiae, which might be due to the β-glucan in LY also has anti-inflammatory and immune regulatory functions [
37]. Besides, the nucleotide in yeast might also help to enhance immune response via increasing inflammatory cytokines in ileum and modulate proliferation of beneficial gut bacteria in nursery pigs [
38].
After weaning, pigs were faced with severe oxidative stress, which might cause lipid peroxidation and disruption of DNA replication and protein synthesis. Lipid peroxidation transforms reactive oxygen species into active chemicals, which could cause cell metabolism and dysfunction, or even death. The MDA was a biomarker, reflecting the rate and intensity of lipid peroxidation in tissue. Therefore, the measurement of MDA could reflect the degree of lipid peroxidation and indirectly measure the degree of cell damage. In the current study, the serum MDA level in pigs fed CTR was higher compared with other treatments on d 28, indicating LY could alleviate the lipid peroxidation in nursery pigs. He et al. [
19] also reported that
S. cerevisiae could improve serum SOD level and decrease MDA level in broilers, which demonstrated that
S. cerevisiae could potentially enhance antioxidant capacity via improving antioxidant enzyme activities. The SOD was one of the antioxidant enzymes which help alleviate oxidative stress after weaning. In the current study, the serum SOD content in pigs fed LY was increased on d 7, 14, and 28 compared with CTR, which might be the polysaccharide in LY could enhance the antioxidant activity [
39]. The mechanism of LY on alleviating oxidative stress (reducing MDA level) might be related to the beneficial effect of β-glucan in LY [
40]. Moreover, the β-glucan in LY could also enhance antioxidant capacity and reduce oxidative damage of lymphocytes through different enzymatic and non-enzymatic systems [
37].
In order to figure out the possible mechanism of LY on modulating gut health in nursery pigs, we investigated the fecal microbiota community and fecal VFA contents. The VFA produced by microbial fermentation of non-digestible carbohydrates in large intestine is beneficial to animals [
41]. The addition of yeast extract to diet could utilize carbohydrate to produce VFA, which had a positive effect on the growth epithelial cells in pigs [
42]. In the current study, we found that the concentration of propionic acid, butyric acid and TVFA in feces of pigs fed LY or LY-ZnO were increased on d 14 compared with CTR. On d 28, the concentrations of butyric acid and TVFA in feces of pigs fed LY or LY-ZnO were increased compared with CTR and CTC-ZnO. These results are in agreement with a previous study which reported that fecal samples from yeast fed sows increase fermentation and VFA production when incubated in vitro with different sources of fiber [
43]. The VFA are the gut microbiota metabolites, which play an important role on solving gut diseases. The VFA produced by bacterial fermentation was an important component of colonic contents, which could be rapidly absorbed by colonic epithelial cells, participate in metabolism and supply energy [
41], while the organic acids in large intestine could also enhance the immunity and regulate the micro-ecological environment in the digestive tract of piglets [
21,
44]. Therefore, the trend for the increased TVFA content in the large intestine was beneficial for the immunity and gut health of nursery pigs in the current study.
In this study, we found the relative abundance of Firmicutes on phylum level in feces of pigs fed LY were higher than pigs fed LY-ZnO. While Kiros et al. [
13] reported that the enhanced Firmicutes family by
S. cerevisiae might contribute to regulate intestinal homeostasis and to improve the performance of piglets. Therefore, LY alone might be more effective in regulate intestinal homeostasis than LY combined with ZnO, but its mechanism still remained to be investigated. It has been reported that the LY
S. cerevisiae was effective in alleviating the negative effect by
E. coli on jejunal mucosa, which indicated that
S. cerevisiae could modulate the gut microbiota community in pigs [
45]. In this study, we found the relative abundances of
Faecalibacterium,
Prevotellaceae_NK3B31_group,
norank_f__Prevotellaceae, and
Anaerovibrio at the genus level were higher in feces of pigs LY or LY-ZnO compared with CTR. Villot et al. [
46] reported that
S. cerevisiae could increase the level
Lactobacillus and tended to have higher
Faecalibacterium prausnitzii content in the jejunum, which might help stimulate IgA in the gut of newborn calves. Besides, dietary supplementation with LY
S. cerevisiae could also improve the relative abundance of
Prevotella in cecum, which was positively correlated with the increased ADG in piglets [
12]. The positive correlation between
Prevotella abundancy and the piglets’ growth performance has been attributed to its ability to process complex dietary saccharides of the diet and favoring monosaccharide uptake by the host [
47]. The
Prevotella could also use monosaccharide to produce VFA [
48], which was conducive to the digestion and absorption of nutrients in nursery pigs. The production of VFA was one of the mechanisms by which intestinal microbiota could influence and promote the host metabolism and physiology [
49] and might explain the positive correlation between
Prevotellaceae_NK3B31_group, norank_f__Prevotellaceae, and performance by LY or LY-ZnO observed in our study.