Nowadays, the researches of antibiotic substitutes, such as probiotics (1), antimicrobial peptides (2), Chinese herbal medicine additives (3), plant-derived phytochemicals (4), functional amino acids (AAs) (5), organic acids (6), and other functional additives (7, 8) without potential threats have become a hot issue in the field of animal nutrition. Microbial fermented feed (MFF), including liquid fermentation feed and solid-state fermentation feed, is an important part of feed industry, which has become an effective strategy to reduce or no use of antibiotics in animal feed (9). Previous studies have demonstrated that MFF not only can improve growth performance, but also can improve animal health (9–11). For example, Chen et al. (10) reported that Landes geese fed with fermented liquid feed had a positive effect on growth performance, relative organ weight, intestinal microflora, and organ antioxidant status. However, previous studies were mainly focused on the liquid fermentation feed (10, 12) and solid-state fermentation of single feed ingredients, such as cottonseed meal (13), soybean meal (14), and cassava residue (15); little research has focused on the solid-state fermentation of complete feed, especially the effect of fermented complete feed (FCF) on growth performance, and intestinal microbiome composition of pigs has not been investigated in detail.

At present, because of the excessive attention to the breeding of growth rate, back-fat thickness, and lean meat percentage in the long-term breeding process, the quality of pork has been declining. Thus, improving meat quality by means of nutrition is a current hot topic in animal research. Previous studies have found that many strategies can improve meat quality (16–18). Among them, nutritional strategies are an effective way to improve meat quality. For example, Ornaghi et al. (19) reported that beef cattle fed with a mixture of natural additives significantly influenced the pH, shear force, tenderness, and lipid oxidation of meat. Cheng et al. (20) reported that intrauterine growth–retarded pigs fed with resveratrol could improve meat quality, muscular antioxidant capacity, lipid metabolism, and fiber-type composition of meat. MFF as an important part of feed industry can also improve meat quality. Zhu et al. (21) showed that pigs fed with a mixture diet containing 80% basal diet and 20% fermented feed can increase the meat color and tenderness of longissimus dorsi and improve the meat quality of finishing pigs. However, the effect of FCF on meat quality of pigs has not been investigated in detail. Therefore, the present study aimed to study the effects of FCF on growth performance, biochemical profile, carcass traits, meat proximate composition, AA and fatty acid profile, and gut microbiome composition of finishing pigs, thereby providing references for the application of FCF in animal production.

MFF has been widely investigated to reduce the use of antibiotic growth promoters in pig production (25, 26). Previous studies had confirmed the growth-promoting effects of fermented liquid feed in pig production (27, 28). However, literature on solid-state FCF is lacking. In the present study, a mixed culture containing Lactobacillus, C. utilis, B. subtilis, and A. niger was used for producing FCF. Previous studies have demonstrated that nutritional components were changed before and after fermentation (29, 30). In the present study, the nutritive values were also changed in a certain extent, for example, the CP content increased to 16.92% (basal feed is 15.89%), and crude fiber decreased to 3.27% (basal feed is 3.45%). The results from the present study showed that the ADG and ADFI were significantly increased after pigs received FCF, and the F/G ratio was significantly decreased compared with pigs fed with basal diet, which consist of, in previous studies, pigs that received fermented liquid feed (27, 28). It indicated that solid-state FCF has potential to be efficient in promoting animal growth without antibiotic use. The reasons of FCF to promote growth performance of pigs in this study may be due to the following characteristics gained from solid-state fermentation. First, A. niger has strong enzyme (cellulase, protease, and amylase)–secreting ability during fermentation (13), after fermentation, the macromolecules were decomposed into small molecules, which improves the palatability and nutrient digestibility of feed. Second, FCF contains Lactobacillus (1.5 × 10⁸ CFU/g), C. utilis (2.1 × 10⁶ CFU/g), and B. subtilis (3.0 × 10⁸ CFU/g), which can function as probiotics to promote growth performance (1, 31, 32).

Evidence shows that serum biochemical parameters reflect comprehensive functions of body organs and nutritional metabolism, which could indirectly response the health status of pigs (33, 34). Serum ALP and GLU index can reflect the growth and health of the animals (35–38). In the present study, pigs fed with FCF had higher ALP, GLU, IgG, IgM, SOD, and T-AOC levels compared with pigs that received basal feed, which demonstrated that FCF has benefit for animal growth, nutrition absorption, and animal health. Similar results have been obtained in other studies using fermented feed (39–41). Health benefit effects of FCF may be related to probiotics contained in fermented feed. Because probiotics such as B. subtilis has earned many benefit claims, including the immune modulation, promotion of nutrients, and digestibility, along with improvements of intestinal health and growth performance in animals (42, 43). Therefore, FCF may be an important nutritional strategy to maintain the health status of pigs.

Microbial fermentation has been recognized as an effective way to improve nutritional composition and decrease the antinutritional effects of feed ingredients (44–46), which had a benefit to improve carcass traits and meat quality. In the present study, the fermented diet significantly increased carcass length and significantly decreased the average back-fat thickness. Similar results were obtained by Qiu et al. (47), who reported that the fermented diet significantly increased the loin eye area and lean mass percentage of finishing pigs. However, the mechanisms underlying this observation remain unknown. The possible reasons of FCF to promote carcass traits may be related to increased AA profile of the feed after fermentation (47), which may play an important role in muscle accretion through increased protein synthesis signaling and suppressed degradation signaling in LDM in finishing pigs (47, 48). The pH value, drip loss, CP content, and fat content are commonly used indices for evaluating meat quality of pork (34, 49, 50). pH is an important indicator of meat quality as it is related to the shelf life, color, and water-holding capacity of the meat (51). In the present study, we found that FCF has no effects on pH value, which indicated that FCF would not affect the shelf life, color, and water-holding capacity of meat. Drip loss reduces meat nutritional value by carrying away some nutrients in exudates and leading to drier and tougher meat and with a worse flavor (49). The results of this study showed that FCF would not cause muscle drip loss, which means that pigs fed with FCF would not affect meat nutritional value. Meat is the main protein source for human beings with high economic value. In the present study, the CP content was increased significantly after the pigs received FCF, which indicated that FCF had a beneficial effect on meat quality. The intramuscular fat content is also an important index for evaluating meat quality and is important for tenderness, succulence, and flavor (34, 50). In the present study, there was no difference of fat content between the CN and FCF groups, which meant that there was no adverse effect on fat content in pork when pigs were fed with FCF. These results likely were because Lactobacillus, C. utilis, and B. subtilis had prebiotic effects on improving pork quality such as pH value, drip loss, CP content and fat content.

The AA profile in pork is important to evaluate meat quality. The type, content, and proportion of AAs in pork are related to meat quality and meat flavor. For example, essential AA (EAA) determines the quality of muscle protein quality, and the delicious AA, such as Ala, Gly, Glu, Asp, and Ser, may affect the delicate flavor of pork (34, 51). In the present study, the higher Lys and Glu content in pork from pigs fed with FCF indicated that FCF could improve meat quality as well as enhance meat flavor. Because Lys is not only an EAA in the human body, but also an important substance to improve the texture and color and enhance the product yield of meat products (52), and Glu is one of the most important AAs, which is the primary flavor molecule and functions in meat freshness and buffering salty and sour tastes (50).

Fatty acids, including monounsaturated fatty acids (MUFAs), polyunsaturated fatty acids (PUFA), and SFA, are directly related to the nutrient value as well as the basis that constitutes the characteristic flavor of meat (53). MUFA comprised the largest proportion of UFA, with C18:1n9c being the most abundant, and this is consistent with the previous studies in pork (54) and cattle (55). C18:1n9 is suggested to be positively associated with the softness of fat, which could improve the sensory quality of meat (55, 56). PUFA plays a regulatory role on human hormone metabolism and the activity of many enzymes and plays a wide range of roles in regulating lipid metabolism, preventing cardiovascular and cerebrovascular diseases, and delaying the decline of immune function (53). C18:2n6c and C20:4n6 are two important PUFA considered good for human health (57). In the present study, the MUFA (C18:1n9c), PUFA (C18:2n6c, C20:4n6), and total UFA in LDM were enhanced by FCF, which indicated that fermented feed can improve the pork quality by increasing the content of UFA. Excessive intake of SFA would increase the risk of cardiovascular diseases and easily cause insulin resistance (58, 59). Thus, SFA can be considered as an important index of pork quality. The present results showed that pigs that received FCF significantly decreased the C18:0 and the total SFA content. These data suggested that FCF could improve the composition of muscle fatty acids by increasing the content of UFA and decreasing the content of SFA, thus improving pork quality. The composition of the fatty acid profile significantly affects the fatty acid composition of the meat (60). Thus, the mechanisms of FCF, which improved fatty acid composition of pork, are likely because the composition of fatty acids in feed was improved after fermentation.

Previous studies have reported the beneficial effects of fermented feed on the swine gut microbiota during different growth states (61–63). Alpha diversity has been considered to be a comprehensive indicator of species richness and evenness (64). In the current study, the ACE index, Chao1 index, Shannon index, and Simpson index were calculated to assess the diversity, richness, and phylogenetic diversity of the bacterial community of jejunum and cecum content. The results showed that FCF significantly increased the Shannon index and Simpson index in jejunum and significantly increased the ACE index, Chao1 index, Shannon index, and Simpson index in cecum. Bata diversity analysis showed that FCF had a scattered distribution structure in both jejunum content and cecum content. It indicated that FCF could increase the diversity of gut microbes. It not only influenced the diversity of gut microbes, but also FCF affected the gut microbiome composition. Generally, Firmicutes, Bacteroides, Proteobacteria, and Fusobacteria are the dominant phyla of mammalian gut (65, 66). In the present study, Proteobacteria, Firmicutes, Tenericutes, and Bacteroidetes were the dominant phyla in jejunum and cecum contents. Pigs fed with FCF, the Proteobacteria phylum was significantly decreased in jejunum and cecum, Firmicutes and Tenericutes were significantly increased in jejunum, and Bacteroidetes and Tenericutes were significantly increased in cecum. Similarly, Xie et al. (67) showed that weaned pigs fed with fermented soybean meal significantly increased the proportion of Firmicutes but decreased the proportion of Proteobacteria. Proteobacteria mainly includes gram-negative bacteria and composed of many pathogenic microorganisms, such as Escherichia, which is associated with intestinal inflammation (67, 68). Firmicutes mainly includes gram-positive bacteria, such as Lactobacillus and Clostridium, which is benefit for animal health (69, 70). Accompanying the higher proportion of Firmicutes and lower proportion of Proteobacteria, the pigs fed FCF showed a better growth performance, indicating a possible relationship between the composition of bacteria and BW gain. At the genus level, pigs fed with FCF significantly increased Clostridium genus abundance and significantly decreased Escherichia–Shigella genus abundance. Many bacteria in Clostridium genus are thought to be probiotics, such as Clostridium butyricum, which had been indicated to improve growth and maintain intestinal barrier function of pigs (71, 72). Some bacteria in Escherichia–Shigella are known to cause intestinal pathology in pigs; the increased Escherichia–Shigella abundances may cause diarrhea of pigs (70, 73), which may cause adverse effects on animal health. These results are likely because Lactobacillus, C. utilis, and B. subtilis had prebiotic effects on improving intestinal microbiome composition.

The current study showed that FCF could improve the pig health status by modulating gut microbiota, whereas the growth of pigs and pork quality are closely related to pig health. Gut microbiota plays a vital role in host health, which is thought to tightly associate with the intestinal barrier function including physical barrier, chemical barrier, immune barrier, and microbial barrier (74). Thus, we can speculate that FCF promotes the growth performance, carcass performance, and meat quality of finishing pigs, maybe through modulating gut microbiome composition. Of course, this hypothesis needs further confirmation.

The current study showed that the ADG and ADFI of pigs significantly increased and the F/G ratio significantly decreased after the pigs received FCF; the ALP, GLU, IgG, IgM, SOD, and T-AOC content significantly increased after the pigs received FCF; pigs that received FCF had a higher carcass length, crude protein content, lysine content, Glu content, C18:ln9c, C18:2n6c, C20:4n6, and unsaturated fatty acids and had a lower average back-fat thickness, C18:0, and saturated fatty acids; FCF significantly reduced the abundances of pathogenic bacteria Proteobacteria phylum and Escherichia–Shigella genus and enhanced the abundances of beneficial bacteria Firmicutes phylum and Clostridium genus (summarized in Figure 7). In summary, FCF had a certain effect on the improvement of growth performance serum biochemical profile, carcass traits, meat proximate composition, AA and fatty acid composition, and gut microbiome composition of finishing pigs. The results of the present study thereby can provide some references for the application of FCF in animal production.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found below: https://www.ncbi.nlm.nih.gov/, PRJNA725094.

The animal study was reviewed and approved by the Animal Welfare Committee of the Shanxi Agricultural University (Taigu, China). The ethic approval number is SXAU-2018-0093.

XT and KZ performed the experiments, contributed to experimental concepts and design, provided scientific direction, and finalized the manuscript. XT, XL, and KZ performed the statistical analyses and wrote the manuscript. All authors read and approved the final manuscript.

This research was funded by grants from the World Top Discipline Program of Guizhou Province (No. 125 2019 Qianjiao Keyan Fa); the Key Science and Technology Program of Guizhou Provence (No. 5411 2017 QianKehe Pingtai Rencai); China Overseas Expertise Introduction Program for Discipline Innovation (D17016); the Key Research and Development Program of Shanxi Province (201603D221026-4); the Central Guide Local Science and Technology Development Special Fund (2017GA630002); the Shanxi Academy of Agricultural Sciences Director and Youth Guidance Special Item (yydzx14); the Natural Science Research Project of Education Department of Guizhou Province (Qianjiaohe KY Zi [2021] 294); and the Doctoral Launched Scientific Research Program of Guizhou Normal University (GZNUD [2018]26).

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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