The gut microbiome of animals varies by species, age, sex, and nutrition. The content of nutrients in the diet as well as feed additives (synbiotics, eubiotics, phytobiotics) determine the quantity and quality of the intestinal flora. The ban on the use of antibiotic growth promoters (AGP) has led to the search for various effective alternatives that affect the gut microbiome, animal health and production results. It has been shown that the use of fermented feeds, beneficially modifying the composition of the intestinal flora, can be a significant alternative to AGP in the production and breeding of monogastric animals. The use of fermented feeds in wet nutrition or in dry mixtures contributes to the improvement of the nutritional value of the diet by increasing the content of protein and exogenous amino acids and polyunsaturated fatty acids, reducing the content of anti-nutritional substances (glucosinolates, phytates, β-glycinates) in feeds (rapeseed, soybean, lupins, horse bean) and increasing the concentration of beneficial microorganisms (Bacillus and Lactobacillus) and their products (bacteriocins, organic acids)