Feeding cereal diets rich in water-soluble non-starch polysaccharides, such as wheat, barley and rice, increases the intestinal chyme viscosity and reduces the contact area between chyme and digestive enzymes in the gastrointestinal tract of animals. However, the utilization efficiency of some nutrients can be improved along with the appropriate supplementation of enzyme preparations. Nasir et al. [
8] found that barley partially replacing corn had no negative impact on the growth performance of piglets. Bruneau [
9] and Mavromichalis et al. [
10] also found that wheat has high palatability to pigs. When phytase is added to the wheat diet, more energy and protein can be released in wheat under the action of phytase. This allows growing pigs fed with barley and wheat to achieve the same growth performance as those fed with corn. This study also showed that barley and wheat diets had no significant effects on the average daily gain, average daily feed intake and feed conversion rate of growing pigs. Therefore, under the condition of meeting the amino acid and protein requirements of growing-finishing pigs, adding an appropriate level of barley or wheat to the diet does not affect the growth performance of pigs.
Phytase and xylanase are typically added to wheat-based diets because combining phytase and xylanase significantly improves pig growth performance and nutrient digestibility [
11]. It has been reported that the digestibility of crude protein and amino acids is higher in wheat-type diets. However, the digestibility of nitrogen and dry matter is no different from that of corn [
12]. In this study, dietary treatment did not significantly affect the apparent digestibility of dry matter, crude protein and the gross energy of growing pigs. The results of this experiment are inconsistent with those of previous studies, which may be caused by the difference between barley and wheat feed materials selected in this experiment.
The integrity of intestinal morphology is crucial for the growth of pigs, and the complete intestinal morphology is more conducive to the digestion and absorption of nutrients by pigs [
13]. This study showed that the partial substitution of corn by wheat and barley had no significant effect on the integrity of the intestinal morphology of growing pigs. Willamil et al. [
14] also found that compared with the corn-soybean meal diet, piglets fed barley and wheat diets supplemented with 0.01% complex enzymes (xylanase and β-glucanase) tended to have a higher average daily gain, healthier intestinal morphology and better nutrient utilization efficiency.
Microorganisms ferment in the cecum and colon of pigs and can produce a variety of volatile fatty acids, which plays an important role in protecting the intestinal health of pigs [
15]. A resistant starch diet not only significantly increased the concentration of short-chain fatty acids in the cecum and colon, but also regulated the composition of intestinal microflora and affected the expression of short-chain fatty acid-related genes [
16]. Diebold et al. [
17] found that xylanase supplementation in wheat diets increased the amounts of acetic acid and propionic acid in the intestines of piglets. Yin et al. [
18] also found similar results. Adding xylanase to the wheat-based diet of finishing pigs increased the content of intestinal volatile fatty acids. This study also showed that phytase and xylanase supplementation in barley and wheat diets produced more volatile fatty acids in the cecum and colon of pigs, which is highly significant for the improvement of the intestinal health of pigs.
The microbiome in the gut plays a vital role in maintaining animal nutrition and immunity. Microorganisms can help animals digest and utilize nutrients in the diet, assist animal metabolism, provide nutrients for intestinal epithelial cells and help hosts resist the invasion of pathogenic bacteria by strengthening the immune and disease resistance function of intestinal cells [
19]. There is a wide variety of microorganisms in the gastrointestinal tract of monogastric animals, including bacteria, fungi, viruses and parasites, all of which play a critical role in maintaining the host’s normal metabolism and immune health [
20]. This study found that the microbial community in pig feces had changed. This may be due to barley and wheat containing more crude fiber; crude fiber could be directly used as microbial energy material. Therefore, different fiber types and contents will change the species and relative abundance of microorganisms [
21]. In this experiment, through the test and analysis of the significance of differences between the microbiome, it was found that at the genus level, the abundance of harmful bacteria such as
Turicibacter and
Escherichia-Shigella in barley and wheat diet groups was significantly decreased, while the abundance of beneficial bacteria such as
Bifidobacterium,
Lactobacillus,
Prevotella and
Roseburia was significantly increased, compared to the corn group. Drew et al. [
22] also showed that compared with the corn-fed diet, the relative abundance of lactobacillus in the chyme of piglets fed with wheat feed increased, while the relative abundance of
Escherichia coli decreased, and the abundance of
Bifidobacteria in cecum also significantly increased.
Rosebaria is a bacterium that produces butyric acid. Butyric acid is the primary energy substance for colon cells, providing energy for colon epithelial cells and preventing colon inflammation [
23]. Donohoe et al. [
24] also found that butyric acid functions as an amino acid transporter, which can transport all kinds of amino acids but does not participate in amino acid catabolism. Moreover, butyric acid can inhibit the proliferation of intestinal pathogenic microorganisms and harmful bacteria and promote the colonization and development of beneficial bacteria such as
Bifidobacteria,
Lactobacillales and
Clostridiales, and
Bifidobacteriales can produce conjugated linoleic acid in the metabolic process [
25]. Conjugated linoleic acid can improve the content of immunoglobulin in animal serum and the activity of antioxidant enzymes in the liver [
26,
27]. Therefore,
Lactobacillales,
Clostridiales and
Bifidobacteriales play an important role in the immune process of animals. In addition to producing lactic acid and acetic acid [
28],
Bifidobacteriales can also promote the production of antibodies and cytokines, thereby enhancing the disease resistance of animal bodies [
29]. Liu et al. [
30] also showed that
Lactoales acid bacteria can improve the intestinal health of piglets. Therefore, partially replacing corn with barley and wheat can effectively improve intestinal health by increasing the relative abundance of beneficial bacteria in growing pigs, enabling barley and wheat to achieve the same growth performance as corn when fed to growing pigs. This study is significant in reducing the waste of feed resources and saving production costs.