To reduce production costs, livestock producers have increased the use of agricultural by-products, including distillers dried grains with soluble (DDGS) of sorghum, wheat, corn, and rye, which can replace some conventional feedstuffs. The DDGS from six different dry grind ethanol plants contains various nutrients, including 7.89–15.10% oil, 28.01–30.03% crude protein (CP), 37.51–48.74% neutral detergent fiber (NDF), 13.92–18.71% acid detergent fiber (ADF), and 3.72–4.59% ash (Singh et al., 2002). Compared with corn and soybean, the DDGS has considerably more fiber which consists predominantly of 35% insoluble dietary fiber (Stein and Shurson, 2009) with low fermentation performance, including arabinoxylans, cellulose, and lignin (Gutierrez et al., 2014). Mao-tai liquor is famous due to its unique production process, impressive liquor quality, and complex aromas (Wang et al., 2016). The production of Mao-tai liquor was 50,235.17tons in Kweichow Moutai Co., Ltd. (2020), Guizhou, China (Annual Report, 2020), and the solid by-products (including lees) are estimated to be about 150,700tons. After the processing and production of liquor, a large amount of solid by-products remained unused, which can cause resource waste and environmental pollution. Therefore, the comprehensive utilization of Mao-tai lees (a common DDGS) in animal feed is conducive to the sustainable development of the wine industry and solves the problem of feedstuff shortage in animal husbandry.

Piglets are vulnerable to physiological, nutritional, and environmental weaning stressors due to the transportation, mixing with other littermates, separation from the sow’s milk, and changes to solid feed and ambient temperature (Tang et al., 2009; Azad et al., 2021). These abrupt changes often cause intestinal microbiota imbalance, which has been related to decreased growth rate, changes in gut morphology and microbial populations, and increased susceptibility to scouring and disease (Reilly et al., 2008). Previous studies indicated that increasing DDGS inclusion in diets resulted in the reduction of body gain to feed intake of weaned piglets, which might be due to the increasing unfermented DDGS inclusion reached to a physical limitation of the gut size, or by decreasing the density and altering the taste or smell of the diet, thus reduced the diet palatability (Avelar et al., 2010). Therefore, it is important to improve the digestibility and nutritional value of DDGS. Fermentation is a practical way to increase cellulose and hemicellulose degradation (Yang et al., 2012), lower phytic acid, and enhance protein digestibility, so fermentation of DDGS may facilitate higher inclusion levels in animal diets, especially for mono-gastric animals (Lamsal et al., 2012). Yeasts, such as Saccharomyces boulardii, have increasingly been used for fermenting DDGS, providing vital evidence of its efficacy as an adjuvant agent to treat diarrhea and prevent antibiotic-associated complications (Buts, 2009). Thus, microbial fermentation can improve the nutritional values and functional effects of DDGS.

Previous studies mostly focused on the effects of dietary supplementation with DDGS and fermented DDGS on the growth performance and digestibility of animals (Stein and Shurson, 2009; Gutierrez et al., 2014). However, the effects of dietary supplementation with DDGS on plasma metabolites and intestinal microbiota and their metabolites have not been elucidated yet. Our previous studies showed that supplementing 15% fermented Mao-tai lees (FML) to growing-finishing pigs’ diets could improve the gut health via increasing the abundance of putative beneficial microbiota and straight-chain fatty acid concentrations, and decreasing the abundances of potential pathogens in the colon (Li et al., 2019). Therefore, we hypothesized that the FML could be used as a cost-effective alternative for feedstuff of weaned piglets owing to its beneficial to plasma and intestinal metabolites and intestinal microbiota. Thus, the present study determined the effects of dietary FML supplementation on growth performance, plasma metabolites, and intestinal microbiota and metabolites of weaned piglets.

The nutritional values of DDGS may be improved by microbial fermentation or enzymatic degradation. Wiseman et al. (2017) reported that fermented DDGS is beneficial to improve the growth performance in weaned piglets, possibly due to nutrient breakdown and normal development of the gut microbiota. In the present study, dietary FML supplementation regulated plasma-free AAs metabolism and altered intestinal microbiota and metabolites of weaned piglets.

The growth performance of weaned piglets can reflect the subsequent performance and the production profitability to some extent. In the present study, although there were no statistical significances in the growth performance of piglets compared with the control group; however, the dietary supplementation with 4% FML had a higher ADG than the other three groups during days 1–14 of the trial. Several previous studies also showed that dietary DDGS supplementation with enzymes was not effective in improving the growth performance of nursery pigs (Jones et al., 2010; Kerr et al., 2013). However, other studies reported that multi-enzyme blends were tended to improve the growth performance in growing-finishing pigs fed diets with DDGS (Emiola et al., 2009; Li et al., 2012). Therefore, the inconsistency might have resulted from differences in the DDGS source, dietary level of the DDGS used, supplementing dose and types of the enzymes or microorganisms used, age of experimental animals, and the nutrient level of the experimental diet (Swiatkiewicz et al., 2016). The diarrhea rate during the nursery period is an essential indicator of the gut health status of piglets. Our results showed that the FML had no adverse side effects on the piglets’ diarrhea rate. Thus, these findings indicate that the dietary FML could be used in mono-gastric animals because microbial fermentation could improve the quality of protein and decrease the CF in the Mao-tai lees.

Plasma metabolites primarily originate from the digestion and absorption of the gut. The UN and TP play a critical role in transamination and reflect the status of protein synthesis and catabolism (Liu et al., 2015). The UN is the main nitrogenous product of protein, and its concentration reflects the efficiency of protein utilization (Chen et al., 2018). Lower UN concentration indicates a good balance of AAs (Hahn et al., 1995). In the present study, dietary FML supplementation decreased the plasma UN concentration on day 14 of the trial, suggesting that dietary FML improved the utilization of protein (Zhai et al., 2018). In addition, dietary 6% FML supplementation decreased the plasma TP concentration on day 42 of the trial, which may be due to that the relatively high fiber content decreased the digestion and absorption of dietary protein. The ALT and AST are transaminases which play crucial roles in protein and amino acid metabolism, and their plasma activities may increase when tissues damage and dysfunction, particularly in the liver (Canli and Canli, 2015). In the present study, dietary FML supplementation did not affect the plasma ALT and AST activities, suggesting that dietary FML had no detrimental effect on the liver tissue.

The TG plays an important role in nutrient metabolism as an energy source and transporter of dietary fat. There is no apparent relationship between the decrease in LDL-C and the increase in HDL-C (Barter et al., 2010). The LDL can antagonize the quorum sensing system that upregulates the gene expressions required for invasive Staphylococcus aureus infection (Peterson et al., 2008). In the present study, dietary 4% FML supplementation increased the plasma TG level on day 14 of the trial and LDL-C level on day 28 of the trial without decreasing the HDL-C level, which may be useful for promoting the ADG and inhibiting pathogenic infection.

Plasma proteins mostly exert their physiological functions in the form of AAs in the body. In the present study, dietary 4% FML supplementation increased the plasma concentrations of Arg, EOHNH2, His, Ile, Lys, Met, phenylalanine, Pro, Tau, Thr, and Tyr on day 14 of the trial. These findings indicated that the AA catabolism was improved by the FML supplementation, which may be due to high levels of acidic protein, cellulose, AAs, and organic acids in the FML. The AAs play important roles in regulating food intake and nutrient metabolism in animals (Wu et al., 2014). Several AAs (e.g., Arg, glutamic acid, glycine, Pro, and leucine) participate in cell signal transmission and metabolic regulation (Wu et al., 2014). The Tau, a sulfated compound synthesized by the eukaryotic host, could increase the ADG and hepatic antioxidant status, while relieve lipopolysaccharide-induced inflammation of broiler chickens (Han et al., 2020). A constant supply of sufficient AAs to living cells from the blood is required to ensure protein accretion in the skeletal muscle of growing pigs (Regmi et al., 2016). This may be one of the reasons why the 4% FML supplementation increased the ADG during days 1–14 of the trial.

In the gut microbiota of pigs, Firmicutes and Bacteroidetes are the most abundant bacteria (Guo et al., 2008). The piglets with a higher F/B ratio are easier to obtain energy from food (Turnbaugh et al., 2006). The present study showed that dietary 4% FML supplementation significantly increased the F/B ratio in the colon, which may increase the energy utilization of feed. Clostridium clusters IV produces butyrate as a result of carbohydrate fermentation (Duytschaever et al., 2012). Lactobacillus is a well-known probiotic offering diverse applications. Furthermore, Lactobacillus plantarum plays a role in preventing diarrhea, lowering cholesterol, reducing irritable bowel syndrome symptoms, and producing plantaricins (Seddik et al., 2017). In the present study, dietary 2–6% FML supplementation had no significant effect on the abundances of Clostridium clusters IV and Lactobacillus of the weaned pigs. However, our previous studies showed that dietary 15% FML supplementation could increase the abundance of Lactobacillus of the growing-finishing pigs (Li et al., 2019). This discrepancy might be related to the different doses of FML supplementation and the age of pigs.

E. coli includes harmless commensal and different pathogenic variants which can cause enteric diseases, such as diarrhea or dysentery, and even lead to extra-intestinal infections (Leimbach et al., 2013). Therefore, lower levels of pathogenic E. coli in the gut could reduce the incidence of diarrhea in piglets. Sulfate-reducing bacteria in the large intestine can produce volatile sulfur compounds, such as hydrogen sulfide and methanethiol, which are responsible for fecal odor (Ushid et al., 2002). Sulfate-reducing bacteria are also a potential player in the etiology of intestinal disorders, inflammatory bowel diseases, and colorectal cancer in particular (Carbonero et al., 2012). In the present study, dietary 2 and 4% FML supplementation could decrease amounts of E. coli and sulfate-reducing bacteria in the colon. Our previous studies also confirmed that dietary 15% FML supplementation could decrease the abundance of potential pathogen Escherichia spp. in pigs at different ages (Li et al., 2019). These findings suggested that dietary FML supplementation, particularly 4% FML, could regulate the balance of intestinal microflora.

The SCFAs, especially butyrate, are mainly produced by microbial fermentation of carbohydrates and non-starch polysaccharides in the large intestine, where the relative abundances of microbiota are greater than those in the small intestine (Le et al., 2016). Similarly, the concentrations of individual and total SCFAs were much lower in the ileum than those in the colon. The SCFAs absorbed in the colon not only serve as an energy source but also play roles in the regulation of the immune system, colonic gene expression, gut motility, and metabolic regulation (Puertollano et al., 2014). Acetate can protect gastrointestinal mucosa from ethanol-induced injuries via suppressing gastrointestinal oxidation, inflammation, and apoptosis (Liu et al., 2017). In the present study, dietary 2% FML supplementation improved the levels of acetate and straight-chain fatty acids in the colonic contents than the higher percentage of FML supplementation (6%), which possibly a lower percentage of FML benefits to intestinal health of weaned piglets.

Cadaverine is synthesized from the Lys through decarboxylation by lysine decarboxylase in microbiota and ornithine decarboxylase 1 in mammalian cells (Bekebrede et al., 2020), which is essential for the maintenance of cell growth and macromolecular biosynthesis by interacting with nucleic acids, proteins, and membranes (Millerfleming et al., 2015), and can protect cells from oxidative stress by scavenging superoxide radicals (Kang et al., 2007). In addition, cadaverine can regulate the gut physiology of animals in different ways (Wang et al., 2015). In the present study, dietary 4% FML supplementation increased the colonic cadaverine content of piglets. Bioamine level could be upregulated through the phosphorylation of related enzymes by activating mTORC1 (Bekebrede et al., 2020). Therefore, the present study results suggest that the increased colonic cadaverine level in the dietary 4% FML group may be via regulating the mTORC1 pathway. However, the underlying mechanism needs to be further clarified.

In summary, dietary FML supplementation (2 and 4%) could partly improve plasma-free AA levels of the weaned piglets without affecting the piglets’ growth performance. Furthermore, dietary 4% FML supplementation was more beneficial to intestinal health via decreasing the abundances of E. coli and sulfate-reducing bacteria, and increasing the concentration of cadaverine in the colon. This study also showed that dietary 4% FML supplementation had more distinct effects than the dietary 2 and 6% FML supplements. Thus, the FML could be used as an effective feed additive, which is not only beneficial to animal health but also resource-conserving and environment-friendly.

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

The animal study was reviewed and approved by the Animal Care and Use Committee of the Institute of Subtropical Agriculture, Chinese Academy of Sciences.

ZL and XK conceived and designed the experiment. ZL, QZ, PH, and HL performed the experiment. ZL, QZ, and MA processed the data. ZL prepared and drafted the manuscript. MA and XK revised the manuscript. All authors have read and approved the final manuscript.

The present work was jointly supported by the National Natural Science Foundation of China (31772613) and Special Funds for Construction of Innovative Provinces in Hunan Province (2019RS3022).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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