The Effect of Lentinus edodes Stem Powder on the Growth Performance Immune Responses and Gut Microbiota of Sika Deer Fawns

As the main byproduct of edible mushroom processing, mushroom stems are often discarded due to their rough texture and poor palatability, resulting in resource waste. However, as a ruminant animal, sika deer has good fiber digestion ability, providing the possibility for the resource utilization of mushroom stems. This study evaluated the effects of dietary supplementation with Lentinus edodes stem powder (LESP) on growth performance, nutrient digestibility, serum biochemical parameters, immune responses, and gut microbiota composition in sika deer fawns. A total of 120 weaned fawns (120 ± 10 days old; 35.0 ± 1.0 kg body weight) were randomly assigned to four dietary treatments, receiving a basal diet supplemented with 0% (control), 2.5% (LE1), 5% (LE2), or 7.5% (LE3) LESP for 35 days. Growth performance did not differ significantly among treatments (p > 0.05). However, apparent digestibility of ether extract and crude protein was significantly higher in the LE1 group (p < 0.05), while LE3 exhibited reduced digestibility of organic matter, ether extract, and crude protein (p < 0.05). Serum total protein (TP) and albumin (ALB) levels increased with LESP inclusion, with the highest values observed in LE3 (p < 0.05). Serum glucose (GLU) decreased dose-dependently (p < 0.05). Immunoglobulin A (IgA), G (IgG), and M (IgM) concentrations were significantly elevated in LE2 and LE3 groups (p < 0.05). Gut microbiota analysis revealed that Firmicutes, Bacteroidetes, and Actinobacteria were the dominant phyla. LE3 supplementation altered beta diversity and reduced microbial evenness. At the genus level, LE2 and LE3 increased the abundance of beneficial taxa such as Planomicrobium and Psychrobacter, while decreasing Ruminococcus and Clostridiaceae_Clostridium. In conclusion, adding 5% LESP to the diet can increase the digestibility of nutrients, enhance humoral immunity, and regulate the composition of the gut microbiota. This study promotes the transformation of mushroom stems from agricultural waste into functional feed additives, which helps reduce breeding costs, improve animal health, and provide a practical path for the coordinated and sustainable development of the edible mushroom industry and animal husbandry.