CH such as dandelion have received widespread attention for their beneficial uses, including promoting growth and improving immunity and intestinal microbiota [24, 25]. CH have demonstrated positive effects on growth performance, such as increasing ADG and decreasing ADFI . It is accepted that feed costs represent approximately about 70% of the cost of poultry production, making a bird’s ability to use feed efficiently particularly important . Therefore, having a lower F/G is of great significance for poultry production. Numerous studies have reported that dietary supplementation with CH can enhance the growth performance of broilers, indeed, dandelion leaf powder and licorice essential oil have been shown to have a positive effect on improving the growth performance, in which 2.0 g/kg dandelion leaf powder treatments obtained the best feed conversion factor, and 200 mg/kg licorice essential oil showed greater weight gain and better feed conversion ratio compared to diets containing 400 mg LEO/kg . Similar to the above results, our study showed that dandelion is beneficial for improving the growth performance of broilers at both the starter and overall phases, as indicated by the decreased ADFI and F/G. This result demonstrated that diets supplemented with dandelion could enhance feed utilization rate of broilers, thus reducing some feed costs during production. The improved growth performance of broilers in the current study may be related to the better digestion and absorption function, possibly due to the beneficial changes in intestinal barrier function regulated by tight junction proteins, intestinal immunity and intestinal microbiota [28, 29].
The physical, chemical, immunological and microbiological barriers in the intestinal tract constitute the complete intestinal barrier, which plays an important defensive role against the invasion of harmful substances from the intestines . Research has shown that the intestinal barrier can prevent the invasion of pathogens and food-borne antigens through the interaction between the four barriers, thereby regulating the homeostasis of the intestinal environment, which is essential for maintaining the homeostasis of health and disease . Therefore, a study on intestinal barrier function is crucial for the continued success of poultry production.
The physical barrier is a tightly connected intestinal epithelial structure, which is composed of intestinal mucosal epithelial cells and their tight junctions . Claudins, occludin and zonula occludens are the main proteins forming tight junctions, and the dysfunction of these proteins is harmful to the physical barrier function . Increasing evidence indicates that CH improves the physical barrier by enhancing the expression of tight junction proteins [33, 34]. Consistently, this research showed that broilers fed a dandelion-supplemented diet exhibited significant up-regulation of claudin and occludin-1 gene expression in the ileum at 21 days old compared to CON and CTC birds. Moreover, the gene expression of ZO-1 in HD birds was significantly increased at 21 days old. At 42 days old, the gene expression of claudin and occludin-1 in LD and ZO-1 in HD were significantly increased, resulting in improved physical barrier function. Tan et al. found that dandelion extract significantly increased intestinal ZO-1 and occludin mRNA levels . Therefore, dandelion can improve physical barrier function by increasing various tight junction proteins, which may eventually promote the integrity of the intestinal barrier.
The chemical barrier is mainly composed of mucus covering the intestinal epithelium, including digestive fluid, lysozyme, mucin and other substances in mucus. Mucins are secreted by intestinal goblet cells, which are an important component of the intestinal chemical barrier and are of great significance to the integrity of the intestinal barrier [35, 36]. Mucins include secreted mucins [mucin-2, mucin-5AC, mucin-5B, mucin-6 and so on] and cell surface mucins [MUC1, mucin-3A, mucin-3B, mucin-4, mucin-12], in which cell surface mucins must prevent enteric pathogens from penetrating the inner mucus layer, to protect the mucosa from infection . In this study, dietary supplementation with two dose of dandelion both enhanced the gene expression of MUC1 at 21 days old, while 500 mg/kg dandelion increased the gene expression of MUC1 at 42 days old, which resulted in better chemical barrier function and promote the integrity of the intestinal barrier.
The intestinal immunological barrier is mainly composed of gut-associated lymphatic tissues. SIgA is the primary component of the immunological barrier and acts as the first line of defense of the intestinal lumen from antigens and pathogens by preventing their access to epithelial cell receptors [38, 39]. In addition, the equilibrium among pro- and anti-inflammatory cytokines is necessary for gut immunological homeostasis . However, in contrast to previous studies, we observed no effects of dandelion on inflammatory cytokines and SIgA in the current study [40, 41], indicating that dandelion failed to induce any notable effects in the immunological barrier.
The microbiological barrier is a microecosystem formed by intestinal microbial flora in the intestinal cavity environment, which is an important component of the intestinal barrier . The intestinal microbiota has co-evolved with the host to form a stable intestinal micro-environment, which provides the host a broad range of bio-functions, such as inhibiting pathogen infection, promoting nutritional metabolism, regulating the immune response, and improving the barrier function of the body [43, 44]. Alpha diversity is used to analyze the complexity of species diversity and beta diversity is to evaluate differences of species complexity among different treatment . CH have been shown to affect the alpha diversity indices such as increasing Shannon index and decreased Simpson index, indicating that CH can improve the diversity of intestinal microbiota . However, in contrast to previous studies, our results showed that dandelion decreased the diversity of intestinal microbiota, which may be due to the antibacterial activity of dandelion [47, 48]. The results of PCoA shown that these two doses of dandelion have no effects on beta diversity, which difference with the result of Tan et al.  in golden pompano Trachinotus ovatus. This may indicate that dandelion has little effect on beta diversity in broilers.
In this study, dandelion enhanced the relative abundance of Firmicutes, decreased the relative abundance of Bacteroidetes at the phyla level, increased the relative abundance of Lactobacillus, and decreased the relative abundance of Bacteroides at the genera level at 21 days old. Consistent with the findings of a previous study, the abundance of Firmicutes was gradually increased while the abundance of Bacteroidetes was decreased . This may indicate that Firmicutes is a more effective energy source than Bacteroidetes, thus promoting more efficient absorption of carbohydrate and increased weight gain. Overall, dandelion was beneficial for improving the feed utilization efficiency and the improved growth performance of broilers; this may be mediated by the change in the relative abundance of Firmicutes and Bacteroidetes. However, at 42 days old, dandelion had no significant effect on the intestinal microbiota at the levels of phyla and genera. Moreover, the relative abundance of Escherichia-Shigella in the LD group was the lowest at the genus level, indicating that a sufficient concentration of dandelion could decrease harmful microbiota and improve intestinal microbiota structure. The above results showed that dandelion could improve the structure of the intestinal microbiota, although the appropriate concentration needs to be determined. SCFAs are the main metabolic end products of intestinal microbiota, which are not only an important energy source for the gut microbiota itself and intestinal epithelial cells, but also play important roles in balancing intestinal homeostasis, and inhibiting growth and colonization of pathogens . Butyric acid, as the main energy material of epithelial cells, is mainly produced by the metabolism of Firmicutes . Butyric acid affects the proliferation and differentiation of intestinal epithelial cells, which helps to strengthen intestinal tight junctions . This study showed that dandelion was increased the butyric acid content to a certain extent, which may lead to the enhancement of tight junction connections.