Dietary protein sources in concentrate supplementation influence growth performance by manipulating gut microbiota and serum metabolites in suckling Donkey foals.

Background: Protein is a primary nutrient in concentrate supplementation for donkey foals, and the source of this protein significantly influences their growth and development. Milk-derived protein sources, such as milk powder, casein, and whey protein, are widely used in milk replacers for donkey foals due to their balanced nutritional profiles, high digestibility, and high bioavailability. However, the increasing costs of milk powder and whey protein have prompted researchers to explore alternative protein sources, with soy protein being a particularly promising option. This study compared the effects of soybean meal and milk-derived ingredients as protein sources in concentrate supplementation on the growth performance, rectal microbiota, and serum metabolites of suckling donkey foals. A total of 42 Dezhou donkey foals, aged 10 days, were randomly assigned to three groups: SP (soybean meal as the main protein source in the diet), MP (milk-derived ingredients as the main protein source in the diet), and SMP (a combination of the SP and MP diets at a ratio of 6:4 used as the dietary component). Each group consisted of 14 replicates, with one donkey in each replicate. The foals were raised from 10 days of age to 130 days of age, and the entire experimental period lasted 120 days.

Results: The final body weight (at 130 days of age) and average daily gain (ADG) were significantly higher (P < 0.001) in the SP group compared to the MP and SMP groups. The feed conversion ratio (FCR) in the SP and SMP groups was significantly lower than that in the MP group (P < 0.05). Among the three groups, the serum levels of thyroxine, growth hormone (GH), insulin-like growth factor-І (IGF-І), and vitamin B6 were significantly enhanced (P < 0.05) in the SP group, whereas the cortisol levels were significantly decreased (P < 0.05). Rectal microbiota analysis further demonstrated that the SP intervention reshaped the gut microbial composition and enriched several genera, including Oscillospiraceae_UCG-005, Oscillospiraceae_NK4A214_group, Akkermansia, Porphyromonas, Streptococcus, Bacteroides, Fusobacterium, and Christensenellaceae_R-7_group. Metabolomic profiling identified 15 differential metabolites, which were considered the key differential metabolites in this study and were related to phenylalanine, tyrosine and tryptophan biosynthesis, phenylalanine biosynthesis, vitamin B6 metabolism, biotin metabolism, tryptophan metabolism, and some amino acid metabolic processes. Notably, the rectal microbial genera Akkermansia, Porphyromonas, Oscillospiraceae_NK4A214_group, Oscillospiraceae_UCG-005, and Streptococcus, which were most abundant in the SP group, showed significant positive correlations with ADG, serum concentrations of thyroxine, IGF-I, and vitamin B6, as well as with the levels of serum metabolites serotonin and pyridoxine.

Conclusions: Compared to the milk-derived protein in concentrate supplementation for donkey foals, soybean meal protein improved the abundance of beneficial gut microbiota, further affected serum hormones and metabolites, and probably resulted in improved body weight and feed efficiency. This study provides a new approach to modulating gut bacteria to enhance growth performance in donkey foals.